the uv peak in active galactic nuclei: a false continuum from blurred reflection?

Mon. Not. R. Astron. Soc. (2012) doi:10.1111/j.1365-2966.2012.20889.x

The UV peak in active galactic nuclei: a false continuumfrom blurred reflection?

A. Lawrence�Institute for Astronomy, SUPA (Scottish Universities Physics Alliance), University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ

Accepted 2012 March 8. Received 2012 March 2; in original form 2011 July 18

ABSTRACTI summarize and analyse key problems with observations of the UV bump in active galacticnuclei (AGN), and especially the accretion disc interpretation – the temperature problem, theionization problem, the time-scale problem and the co-ordination problem – and suggest thatall these problems can be solved if, in addition to the accretion disc, there is a population ofcold, thick clouds at approximately 30RS which reprocess the intrinsic continuum. Exploringcloud parameter space, I find that clouds with density n ∼ 1012 cm−3 and column NH > 4 ×1024 cm−2 reflect most of the intrinsic continuum, but convert a substantial fraction of theEUV luminosity into lines, dominated by Lyβ and He II Lyα. When velocity-blurred, thismakes a false continuum peak at ∼1100 Å which fits the observed spectral energy distribution(SED) well, but turns back up in the FUV to make a hard EUV SED, as required by ionizationmodels. I argue that the observed UV variability is dominated by this component of fixed shape,possibly due to changes of covering factor. The amount of mass required is small, so it is notnecessary to disrupt the disc, but only to make an unstable and inhomogeneous atmosphere.The proposed clouds may be related to those suggested by several X-ray phenomena (X-rayreflection components, high-velocity outflows, Compton thick partial covering) but are not thesame, leading to a picture with a wide range of inhomogeneous structures at different radii.

Key words: accretion, accretion discs – galaxies: active – quasars: emission lines.

1 IN T RO D U C T I O N

The most prominent feature in the spectral energy distribution(SED) of active galactic nuclei (AGN) is the so-called ‘Big BlueBump’ (BBB). This broad feature peaks in the UV (Sanders et al.1989; Elvis et al. 1994), roughly consistent with the expectationof thermal emission from gas accreting on to a black hole masswith a mass of 107 −9 M�. The BBB is, however, broader than asingle blackbody, as one would expect if accretion takes place in adisc with a range of temperatures at different radii (Shields 1978;Malkan & Sargent 1982). In more detail, modellers have found ithard to reproduce the shape of the SED, especially near the UVpeak, and through the soft X-ray region (e.g. Blaes et al. 2001;HaroCorzo et al. 2007).

Some observations provide generic support for the idea of a disc-like structure in the centre of AGN. (i) The strong Fe line and hardX-ray flattening seen in the X-ray spectra of AGN are often in-terpreted as being due to reflection by ‘cold’ material that coversa solid angle of ∼2π as seen by the X-ray source (Pounds et al.1990). (ii) In a handful of quasars, optical spectra in polarized light,removing contaminating emission, show a broad dip possibly corre-

�E-mail: [email protected]

sponding to the Balmer edge absorption expected from an accretiondisc atmosphere (Kishimoto, Antonucci & Blaes 2003). Extend-ing the polarized continuum into the near-IR reveals the classiclong wavelength ν1/3 spectrum expected from simple accretion discmodels (Kishimoto et al. 2008). (iii) For radio-loud objects, andespecially superluminal objects, where we can have some indica-tion of the orientation of the sources, AGN show velocity widthand surface brightness correlations with viewing angle (Brotherton1996; Rokaki et al. 2003; Jarvis & McLure 2006). (iv) At opticalwavelengths, over a very broad range of luminosities AGN showcolour–luminosity trends consistent with the generic expectationfrom multi-temperature black bodies (Lawrence 2005).

It is notable that all the above successes concern optical or near-IR wavelength emission. In the UV, however, a number of observa-tions disagree badly with at least the simplest accretion disc models.These issues can be characterized as the temperature problem, theionization problem, the time-scale problem, the co-ordination prob-lem and the size problem. Some of these problems, along with otherkey issues such as polarization predictions, have been discussed byAntonucci (2002) and references therein. In the next section, I lookat the current state of each of these key problems in turn. In thefollowing section, I consider an idea that has often been seen asa rival to accretion disc models – emission from dense clouds. Infact, a combination of accretion disc and dense inner clouds may

C© 2012 The AuthorMonthly Notices of the Royal Astronomical Society C© 2012 RAS

2 A. Lawrence

be exactly what is needed to solve the problems of accretion discmodels, as large velocity blurring can produce a localized false peakin the SED. Finally, I briefly discuss implications of this idea, andhow it can be tested.

2 PRO BLEMS WITH THE BIG BLUE BUMP

Here I summarize a number of problems with our current under-standing of the BBB.

2.1 The temperature problem

AGN seem to be cooler than they ought to be. The SEDs of AGNseem to show a universal near-UV shape, reaching a maximumin νSν at a wavelength of around 1100 Å, more or regardless ofluminosity or redshift (Zheng et al. 1997; Telfer et al. 2002; Scottet al. 2004; Binette et al. 2005; Shang et al. 2005). Such a peaksuggests a characteristic temperature of T ∼ 30 000K, and indeedthis is similar to the inner temperature of accretion disc fits eversince Malkan & Sargent (1982). However, for any thermal model,the characteristic temperature should be roughly

Tch = 95 000L1/4E M

−1/49 R

−1/25 ,

where LE = L/LEdd is the luminosity in units of the Eddington lumi-nosity, M9 is the mass of the central object in units of 109 M� andR5 is radius in units of 5RS, where RS is the Schwarzschild radius.Here R5 is taken to be the radius of an optically thick sphericalsurface from which blackbody radiation at temperature T emerges.

Other estimates, such as the radiation–density equivalent tempera-ture, will be similar, and other possible characteristic temperatures,such as the Compton heating temperature, will be hotter, and rathermodel dependent (Ferland & Rees 1988). The maximum temper-ature of a simple accretion disc extending to the last stable orbitis substantially hotter than the naive temperature above. Discs inwhich one imposes the zero-stress condition at the inner boundarycan be cooler in the middle and hotter towards the outer radii, but itis far from clear whether this applies to black hole accretion discs(see e.g. Frank, King & Raine 2002, p. 90). That detailed accretiondisc models do not overcome this problem is clear, for example,from the attempts to fit the SED of 3C 273 by Blaes et al. (2001).Smaller black holes are hotter, but objects radiating at much lessthan the Eddington limit are cooler, so for local Seyferts, as opposedto luminous quasars, we might expect some diversity; but these alsoseem to show the universal knee.

Fig. 1 shows the SED of the nearby quasar 3C 273, plotted inlinear units, compared to a blackbody of temperature T ∼ 32 000 Kwhich is a good first approximation to the observed SED. Accordingto Peterson et al. (2004) the mass of the black hole in 3C 273 is8.87 × 108 M�. [Note that Kaspi et al. (2000) and Paltani & Turler(2005) find values roughly a factor of 2 on either side of this value.)Integrating over the SED shown in Fig. 1, and extrapolating the FUVwith α = −1.8, the total luminosity is approximately LBB ∼ 4 ×1039 W, so that LE ∼ 0.356. If this luminosity were radiated from5RS, the expected blackbody temperature would be T ∼ 76 000 K.This is clearly discrepant with the observed SED, as also shown inFig. 1.

Figure 1. Comparisons between various observed SEDs and simple theoretical curves. Note that both axes are linear, which helps to emphasize how stronglythe SED is dominated by the NUV peak. The dotted lines show the SED of 3C 273 taken from Kriss et al. (1999), kindly supplied by G. Kriss, using nearlysimultaneous data from various instruments. The data points have been de-reddened assuming E(B − V) = 0.032 (see Kriss et al. 1999). The circles showcontinuum from two quasars analysed by Binette et al. (2005), estimated from their figures. The dashed line labelled ‘T02’ shows the composite SED of Telferet al. (2002), represented as a power law with index −1.8. The blackbody curve with T = 32 000K is chosen as an illustration of how a temperature in thisregion dominates the SED. The blackbody curve with T = 76 000 K represents the temperature that would naively apply given the observed luminosity andmass of 3C 273 (see text). The dotted line labelled ‘K97’ represents the ionizing continuum used by Korista et al. (1997). The normalizations of all componentsare arbitrary; they are scaled for comparison purposes.


AGN UV peak: blurred reflection? 3

One could fix the temperature problem by making AGN radiatewell below the Eddington limit, but only by making their blackholes almost two orders of magnitude more massive. Alternatively,one could require that the radiation emerges from a larger radius– of the order 50RS. Of course, most of the energy generation hasto take place interior to this radius, or once again an unreasonablylarge mass is required. One is led therefore to the possibility thatinterior energy is absorbed and reprocessed, and that the peak wesee corresponds to the reprocessed rather than the original emission.

The FUV SED shortwards of the 1100 Å peak is more contro-versial. Zheng et al. (1997) and Telfer et al. (2002), by compilinga composite SED using Hubble Space Telescope data spanninga large range of redshifts, find that the FUV falls steeply all theway to 300 Å as Sν ∝ να with α ∼ −1.8. Constructing such ahigh-z composite requires careful correction for Lyα forest absorp-tion by the intervening Intergalactic Medium (IGM). On the otherhand, Scott et al. (2004) use FUSE data for relatively low redshiftAGN to construct a composite reaching 650 Å and find a mean slopeof α ∼ −0.6. They find considerable FUV diversity, and a tendencyfor high-luminosity AGN to be steep, and low-luminosity AGN tobe flat. Binette et al. (2005), Shang et al. (2005) and HaroCorzoet al. (2007) examine the FUV SEDs of individual AGN, up to z ∼1, where the IGM correction is still small, and confirm the diversityof FUV shapes. In particular, Binette et al. (2005) stress that whilesome quasars continue to decline in νLν out to 600 Å others turnback up at short wavelengths. Fig. 1 shows examples of these twocases taken from their paper.

Another way of altering the expected emission is by absorption.Various possibilities are reviewed by Binette et al. (2008). Theobserved knee is close to the Lyman edge, but the shape is notlike an edge, and it is definitely at somewhat longer wavelength.It is also possible that dust reddening has altered the shape – de-reddening the 3C 273 SED by AV ∼ 0.35 over and above the lineof sight Galactic extinction of AV ∼ 0.1 makes the match withthe expected temperature much better. Binette et al. (2005) explorethe possibility that crystalline dust can produce a broad absorptiontrough matching that seen in PG 1008+1319 and similar objects.Overall, absorption models remain plausible, but I do not considerthem further in this paper.

2.2 The ionization problem

The ionization problem is a corollary of the temperature problem:an SED falling in νLν terms does not seem capable of producing thebroad emission lines observed in AGN. This is especially true if thehigh-redshift composites derived by Zheng et al. (1997) and Telferet al. (2002) are correct; these show a very soft FUV continuum, withα ∼ −1.8, whereas the observed emission lines in the optical andnear-UV seem to indicate a much harder continuum, with α ∼ −0.5.Fig. 1 contrasts the observed SED with the ionizing continuum usedin the comprehensive Broad Line Region (BLR) study of Koristaet al. (1997) – a power law with index α = −0.5 and an exponentialcut off at kT = 44 eV – which does a good job of reproducing theobserved ratios of broad emission lines in AGN. The ionizationproblem has been known for many years (Netzer 1985; Collin-Souffrin 1986; Korista, Ferland & Baldwin 1997; Dumont, Collin-Souffrin & Nazarova 1998). Netzer’s (1985) argument concernedthe ratio of the total BLR emission to Lyα. Roughly speaking, ifLyα tells us the continuum near the Lyman edge, then extrapolatingwith a steep slope does not provide enough energy to produce allthe BLR emission. Netzer argued that the problem could be solvedif the BLR (but not the continuum) is reddened by AV ∼ 0.25–0.5,

but Collin-Souffrin (1986) shows that the observed continuum thenfails to produce the reddening-corrected line ratios. Korista et al.(1997) concentrate on the He II lines, especially He II 1640, and showthat a steeply extrapolated continuum in the manner of Zheng et al.(1997) fails to produce the observed He II 1640 equivalent width byat least a factor of 5, and probably considerably more. Korista et al.consider several possible solutions. One is that the UV-EUV bumpis double peaked. This is the possibility that I pursue in this paper.

Fig. 1 illustrates the ionization problem, contrasting the ob-served SED of 3C 273, the Telfer et al. (2002) composite and theKorista et al. (1997) ionizing continuum. Interestingly, althoughthe observed SED seems inconsistent with both the required ioniz-ing continuum and the naively expected temperature, the ionizingcontinuum and the naively expected temperature are roughly con-sistent with each other. Furthermore, the FUV upturn shown byPG 1008+1319 is roughly consistent with the slope expected fromthe naively expected temperature, suggesting that this very hot emis-sion is present, but somehow masked in the NUV region.

2.3 The time-scale problem

Relatively few AGN have well-sampled UV light curves, but thosethat do show fairly well defined ups and downs. The situation issummarized by Collier & Peterson (2001), who derive UV structurefunctions for four AGN (NGC 3783, NGC 7469, NGC 5548 andF9) and optical structure functions for six other AGN. They showcharacteristic time-scales (defined by the flattening of the structurefunction) of between 5 and 94 d. Collier and Peterson show evidencethat this characteristic time-scale is proportional to black hole mass,which for these UV-studied local AGN is of the order of 107–108 M�. This is roughly confirmed by the fact that much moreluminous quasars seem to show time-scales of the order of years(Giveon et al. 1999; Turler et al. 1999; Trevese, Kron & Bunone2001; Trevese & Vagnetti 2002; Hawkins 2007).

A time-scale of the order of tens of days is inconsistent withsimple quasi-steady accretion disc models, as the viscous (radialdrift) time-scale is far longer, as was first recognized by Alloin et al.(1985). Following Frank et al. (2002), chapter 8, and scaling tothe time expected for a radius appropriate to a standard thin discradiating at a characteristic NUV temperature of T = 30 000K, theviscous time-scale is

tvisc = 12.6 yr × L−3/10E M

6/58 R

5/430 α

−4/50.1 μ

3/100.1 ,

where α and μ are the usual viscosity and efficiency parameters,scaled to their expected values. Acoustic modes also do not work.For T = 30 000 K, assuming the gas is largely ionized, the soundspeed is ∼22 km s−1, so that the sound crossing time-scale is tsound =12.6 yr × R30M8. On the other hand, the light crossing time-scale,tlight ∼ 8.3 h × R30M8, is much too short, although it may be rele-vant to the co-ordination of variability at different wavelengths, asdiscussed below.

There are two time-scales that may work. The first is somekind of dynamical time-scale. The free-fall time-scale from 30RS

is tff ∼ 5.9 d × R3/230 M8. This is slightly on the fast side, but very

much in the right ball park, suggesting that accretion flow may be dy-namically unstable. The second possibility is some kind of coolingtime-scale, which will for example determine the time in which in-ternally stored energy could be released as radiation in a flare. This ismuch more model dependent, depending on the total mass involved,its temperature and whether it is optically thick or thin. Collier &Peterson (2001) state that for a standard accretion disc model the


4 A. Lawrence

thermal or cooling time-scale is tthermal ∼ 27.5 d × R3/230 M8α

−10.1

where α is the standard disc viscosity parameter.If either a dynamical time-scale or a thermal time-scale is the cor-

rect answer, the variability cannot be modelled by a quasi-steadyaccretion disc – the physical structure of the disc is changing fasterthan the gravitational energy is being dissipated. However this prob-lem could be removed if what we see is reprocessed emission – onlythe reprocessor has to change fast, not the disc itself. In Section 4.3we will look at whether dense reprocessing clouds can provide theright time-scale.

2.4 The co-ordination problem

The next serious problem for accretion disc models is that varia-tions at different UV-optical wavelengths occur in phase – i.e. thepeaks and troughs line up, so that any delay or smearing is smallcompared to the variability time-scale itself, which is very hard toachieve if the radiation at different wavelengths comes from dif-ferent parts of the disc. The best evidence comes from a handfulof local Seyfert galaxies with well-sampled UV light curves (NGC5548: Clavel et al. 1991; Korista et al. 1995; NGC 4151: Edelsonet al. 1996; Crenshaw et al. 1996; NGC 7469: Kriss et al. 2000;NGC 3516: Edelson et al. 2000). For more luminous quasars, thereare few UV data sets, but fairly well sampled long-term opticallight curves presented by Giveon et al. (1999) and Hawkins (2003)show changes at different wavelengths that are also aligned to muchless than the typical variability time-scale. The lack of wavelength-dependent delays led to the suggestion that the inner accretion discis heated at least in part by the central X-ray source (rather thanlocally by viscous torques), so that the UV variations are actu-ally a reprocessed version of variations in the central X-ray source(Krolik et al. 1991). The expected delay between different wave-lengths is then the light travel time between different regions, whichis of the order of hours. Comparing UV wavelengths, several authorshave claimed a measurement of such a short delay for NGC 7469(Wanders et al. 1997; Collier et al. 1998; Kriss et al. 2000), but forother AGN (NGC 5548, 3516, 3783, 4151 and Fairall 9), attemptshave only resulted in limits of the order of 0.1 to 0.3 d (Crenshawet al. 1996; Edelson et al. 1996, 2000; Peterson et al. 1998).Korista & Goad (2001) have claimed that the NGC 7469 effectis an artefact due to contamination of the continuum by diffuseemission from the BLR. Comparing different optical wavelengths,a number of authors have shown marginally significant delays ofthe order of days in several different AGN (Collier et al. 2001;Oknyanskij et al. 2003; Doroshenko et al. 2005; Sergeev et al.2005; Breedt et al. 2009).

A prediction of the ‘disc heated by X-ray source’ picture is that theUV and optical emission should lag the X-ray emission. However,simultaneous monitoring campaigns show that any such delay is lessthan 0.15 d in both NGC 4151 (Edelson et al. 1996) and NGC 3516(Edelson et al. 2000), and in the case of NGC 7469, if anything theUV leads the X-ray emission by ∼4 d. Breedt et al. (2009), from5 years of monitoring MKN 79, find that overall, X-ray and opticalemission are well correlated, with an optical versus X-ray lag of lessthan ∼2 d, but also note differences. The X-ray variability has moreshort time-scale power, as might be expected, but when smearedwith the expected disc transfer function it fails to reproduce theobserved optical light curve (see their fig. 7) – the predicted lightcurve matches some but not all of the observed flaring on time-scales of tens of days, and does not produce the long-term trendson time-scales of hundreds of days. Similar problems were notedby Kazanas & Nayakshin (2001).

A further problem is that the X-ray luminosity in AGN is muchsmaller than the UV luminosity. For relatively low luminosity AGN,the total X-ray luminosity is sensitive to how the observed 2–10 keVspectrum is extrapolated, so that X-ray luminosity may in fact becomparable to the UV luminosity (Chiang & Blaes 2003). How-ever, this cannot be a universal explanation, as the X-ray/UV ratiois an order of magnitude smaller for the most luminous quasars(Strateva et al. 2005). Overall, there is clearly a close connectionbetween UV-optical emission and X-ray emission, but the relationis a complicated one. One possibility is that, rather than the X-rayemission being primary and the UV a reprocessed version, the realprimary emission is the FUV peak at ∼300 Å which is predicted bysimple accretion disc theory and photoionization calculation. Wedo not have FUV light curves, but one AGN, NGC 5548, has beenmonitored the other side of the possible peak, at ∼80 Å with EUVE.Marshall et al. (1997) showed that over 10 days the variations at76 Å and 1350 Å are well correlated, with any lag less than 0.25 d,but that the amplitude of variations was several times larger in theEUV. In a later EUVE observation made simultaneously with RXTEand ASCA, Chiang et al. (2000) showed that the 76 Å emission leadsthe X-rays by 0.4 d. There is a case to be made then that both theUV and X-ray emission are substantially reprocessed.

2.5 The size problem

Sizes deduced from the fluctuations of gravitationally lensedquasars seem to be discrepant with expectation. The light from eachlensed component can suffer additional micro-lensing by stars, orpossibly by dark matter substructure. The expected optical depth isclose to unity, so that each component has ongoing irregular fluc-tuations that are uncorrelated with each other, and can therefore bedistinguished from intrinsic variability common to all components(see e.g. the review of Wambsganss 2006). The magnification de-pends on source size, as the expected size, while small comparedto the Einstein radius of the lenses, is not point like – i.e. a largersource has more washed out fluctuations. A clear qualitative resultis that component-to-component anomalies are significantly biggerin X-rays than in the optical-UV, showing that the X-ray source ismore compact, by a factor of several (Chartas et al. 2010; Dai et al.2010) and possibly that the hard X-ray size is smaller than the softX-ray size (Chen et al. 2012). Likewise, broad emission lines showlittle or no fluctuation, and therefore the BLR size is substantiallybigger than the continuum source size (Lewis et al. 1998). Withinthe optical-UV range, fluctuations are larger at shorter wavelengths(e.g. Eigenbrod et al. 2008; Mosquera et al. 2011), strongly support-ing the general idea of a distributed source with a colour gradient.The slope is plausibly consistent with the radial temperature gradi-ent expected from an accretion disc, but with a rather large error.This is an area that will hopefully improve strongly in coming years.

Along with these striking relative results, lensed quasar moni-toring can give us information on the absolute source size. This issomewhat model dependent as it requires understanding the macro-scopic lensing, the mass function of the microlenses and the relativetransverse motions. Early results claimed either consistency with ex-pected source sizes (Wambsganss, Schneider & Paczynski 1990) orthat observed sources’ sizes were smaller than expected accretiondiscs (Rauch & Blandford 1991). More recent and substantial stud-ies have consistently concluded that observed sizes are a factor ofseveral larger than expected from simple thin accretion disc modelsof the same objects (Pooley et al. 2007; Morgan et al. 2010; Jimenez-Vicente et al. 2012), while correlating clearly with the black holemass estimated from line widths (Morgan et al. 2010). Suggested



solutions to this problem could include much lower accretion effi-ciency, or a flatter temperature profile (Morgan et al. 2010) or aninhomogeneous accretion disc (Dexter & Agol 2011). The centralreprocessing region discussed later in this paper is another possiblesolution to this problem.

2.6 Amplitude effect and mixing model

Related to the co-ordination problem is the fact that the amplitudeof variability for both local AGN and luminous quasars is stronglywavelength dependent (Cutri et al. 1985; Clavel et al. 1991; Wilhiteet al. 2005). This is most clearly demonstrated in fig. 13 of Wilhiteet al. (2005), which shows the ratio of the mean difference spectrumto the mean spectrum for 2500 Sloan Digital Sky Survey quasars.This variability index increases steeply at wavelengths shorter than2500 Å. A generic way to explain the amplitude and co-ordinationeffects at the same time is by mixing a constant red component witha variable blue component (Lawrence 2005). However, only somesuch models work. Fig. 2 shows two deliberately simple toy modelsapplied to the NGC 5548 campaign data of Clavel et al. (1991). Theconstant component is modelled as a power law with slope α andnormalization NPL, and the variable component as a blackbody at asingle temperature T with normalization NBB.

In the first model, the temperature T of the blackbody was fixed,and flux-ratio values were calculated for a sequence of values ofthe blackbody normalization, NBB, to produce predicted tracks intwo diagrams – the ratios F1337/F1825 and F1337/F2670 versusthe flux F1337, where F1337 refers to the flux at 1337 Å. The otherparameters (NPL, α and T) were adjusted to produce a good simulta-neous (eyeball) fit between these tracks and the data. The best-fittingtemperature for the variable blackbody was T = 27 500 K, with thepower law of the fixed component having slope α = −2.0.

In the second model, the emitting area of the blackbody was heldfixed, and the temperature T varied to produce the predicted tracks.This of course produces variation in both colour and normalization.As with the first model, the free parameters (NPL, α and blackbodyemitting area) were adjusted to try to achieve a simultaneous fit to

Figure 2. Mixing models for the colour–amplitude trend in the variabilityof NGC 5548. The data are from Clavel et al. (1991). The open circles showthe flux ratio F1337/F2670, and the filled circles show F1337/F1825, whereF1337 refers to Fλ at 1337 Å. The model tracks show the effect of a fixedpower law and a variable blackbody. In the left-hand panel, the temperatureof the blackbody is fixed at T = 27 500 K, with only its normalizationvarying. In the right-hand panel, the emitting area of the blackbody is fixedand its temperature varied.

the two flux ratios, but no fit could be found. As an illustration, Fig. 2shows parameters adjusted to fit the run of the ratio F1337/F1825,which results in a prediction for F1337/F2670 which is a long wayoff.

Of course, both models are physically unrealistic – the variablecomponent is unlikely to be a single blackbody, the red componentmay not be well represented by a power law, and the variable com-ponent may vary in both area and temperature. Nonetheless, thedifference between the two simple extremes is striking. It stronglysuggests that the way to explain the variability of NGC 5548 is bythe variability of a component with fixed shape.

3 R EPRO CESSI NG BY DENSE INNER C L O UDS

3.1 Introduction

The temperature problem (i.e. the premature turnover of the SED)and the variability behaviour of AGN suggest that the observedUV continuum represents re-processed emission. The earlier ideathat the UV represents reprocessed X-rays seems untenable becausethere is not enough X-ray luminosity, and because in at least somecases the X-ray emission lags the UV emission. The primary radi-ation could instead be the unseen peak EUV emission – the innerdisc heating the outer disc. Against this idea is the observation ofsteeply declining FUV continuum. In support of the idea is the factthat emission lines suggest that there is indeed a hard EUV contin-uum, and the fact that at least some high-redshift quasars show aturn up shortwards of 800 Å. Perhaps, as suggested by Korista et al.(1997), the continuum is double peaked.

However, there remains the puzzle that the wavelength of the νLν

peak (∼1100 Å) seems to be universal (Shang et al. 2005), even ifthe continuum a few hundred Å shortwards of this is not (Scott et al.2004; Binette et al. 2005; HaroCorzo et al. 2007). Furthermore,within individual objects, it seems that multi-wavelength changesmay be explained by the variability of a component of a fixed shape.Where a spectral shape is relatively constant across a large rangeof conditions, this is suggestive of a solution dependent on atomicphysics rather than macroscopic physics. Perhaps what we see as theUV bump is actually made of emission lines. The features that werecognize as the ‘broad emission lines’ come characteristically froma radius of ∼1000RS. Clouds at say ∼30RS would produce emissionlines six times broader, which would be seen as a false continuum.A similar logic has led some previous workers to consider opticallythin (primarily free–free) emission from hot (105 −6) K gas as analternative to the accretion disc (Ferland, Korista & Peterson 1990;Barvainis 1993), but my aim here is to discuss the role of opticallythick clouds in conjunction with the accretion disc. In the nextsubsection, I discuss some general issues concerning dense innerclouds. Following this, I show model results for emission fromsuch clouds, how they could produce a false local continuum peakat 1100 Å, and discuss the prospect for more detailed models. InSection 4, I discuss whether we expect such clouds to exist, andhow this relates to other issues such as the X-ray spectra of AGN.

3.2 Cold dense clouds

Cold dense clouds in AGN were a minor fashion for just over adecade (Ferland & Rees 1988; Guilbert & Rees 1988; Lightman& White 1988; Celotti, Fabian & Rees 1992; Sivron & Tsuruta1993; Collin-Souffrin et al. 1996; Kuncic, Blackman & Rees 1996;Kuncic, Celotti & Rees 1997; Czerny & Dumont 1998; Celotti& Rees 1999). In all these papers, it was assumed that the primary


6 A. Lawrence

energy source was either non-thermal (with power-law slope −1) ora very hot X-ray plasma, with the issue being whether ‘cold’ thermalmaterial could survive within the non-thermal plasma, and whetherthe reprocessed thermal emission could be a viable alternative toan accretion disc for explaining the optical-UV emission in AGN.These ideas seem to have gone out of fashion because it has becomewidely accepted that an accretion disc is the primary mode of energygeneration. An overlapping series of papers has considered the effectof such cold clouds on the X-ray spectrum, especially the Fe lineand the Compton reflection hump (Lightman & White 1988; Sivron& Tsuruta 1993; Karas et al. 2000; Malzac & Celotti 2002; Merloniet al. 2006), and also the possibility that variable partial coveringexplains X-ray variability (Abrassart & Czerny 2000). However,this has been a minority industry, as most workers have followedPounds et al. (1990) in assuming that X-ray reflection is from thesurface of the accretion disc.

My approach in this paper differs from the above-mentionedpapers in two respects. First, rather than trying to produce an alter-native to the accretion disc, I will examine how cool dense cloudscan modify the accretion disc SED. Secondly, I will concentrate ona somewhat different region of parameter space. Most of the abovepapers consider very dense clouds in the very central regions (3RS <

R < 10RS) where most of the energy is generated, arguing that theycan be confined by the strong magnetic fields that may be present.Kuncic et al. (1996) argued that such clouds must be very smalland dense (hydrogen density n ≥ 1014 cm−3R−2

3 M−18 ) if they are to

cool in less than the free-fall time. The radiation from such cloudswill be close to blackbody at around 105 K, as shown by detailedcalculations in nearly all the above papers, and as such would behard to distinguish from other forms of cold material, such as anaccretion disc, and indeed this emission would suffer from many ofthe same problems previously discussed in Section 2. However, atsomewhat larger radial distances, say 30RS, the density required tocool in a reasonable time is much less, n ∼ 1012. Such clouds willradiate mostly in atomic transitions, but they will still be movingfast enough that the Doppler blurred emission will produce a falsecontinuum. If the accretion disc itself is the source of such material,then there is no confinement issue, and the starting temperature ofthe clouds (∼104 K) will be similar to the expected photoioniza-tion equilibrium temperature when exposed to the ∼105 K radiationfrom the inner region. I will consider the origin of the clouds a littlemore closely in Section 4. For now, I will assume that moderatelydense clouds exist at R ∼30RS, and consider the consequences. Inote that Czerny & Dumont (1998) have also examined n ∼ 1012

clouds, but they consider clouds at rather higher ionization param-eter, and assume a non-thermal primary continuum.

3.3 Emission from cold dense clouds

I examined the properties of clouds ionized by a hot central source ata 3C 273-like luminosity, using version C08.00 of the well-known‘CLOUDY’ software package (Ferland et al. 1998). Except wherenoted below the ionizing source was a blackbody with temperatureT = 100 000 K and (total) luminosity L = 4 × 1046 erg s−1, and thedistance of the clouds from the source was R = 1016 cm, correspond-ing to 30RS for MH = 109 M�. The calculations were performedassuming an open geometry. A variety of densities and column den-sities were examined, as described in the following subsections.The temperature of 100 000 K could be seen as an approximationto the disc emission interior to the clouds, or if perhaps the disc isdisrupted, it could be thermal emission from the very dense interiorclouds discussed in Section 3.2. In many of the cases described be-

low, I tried varying the temperature over the range 75 000 to 200 000,at a fixed total luminosity, and found no qualitative difference. In afew cases, I also tried a continuum which added a two-componentX-ray continuum to the 100 000 K blackbody, based on the data inKriss et al. (1999). I used a soft X-ray power law with slope α =−1.5, a low energy break at 7.5 Rydberg, and luminosity L = 5 ×1045 erg s−1; and a hard X-ray power law with α = −0.6 and lumi-nosity L = 1.25 × 1046 erg s−1. The main effect of such an X-raycontinuum was to turn a back-end neutral zone into an extended par-tially ionized zone. The effect on the reflected spectrum describedbelow was significant but not large – around 20 per cent. Finally,the choice of 30RS is guided by the logic described in Section 3.2,but I also considered some clouds at other distances, as describedbelow.

In the subsections given below I refer to log n and log (NH) wherethe hydrogen number density n is in units of cm−3 and the hydrogencolumn density NH is in units of cm−2.

3.3.1 BLR-like clouds, log n ∼ 10

Clouds with density log n = 10, such as are often assumed in BLRmodels, can be considered for column densities log NH < 24.5.Above this NH value, the cloud size is larger than the whole region.Throughout this column density range, the clouds are fully ionizedin H and He. At low columns, log NH < 23, the clouds are essentiallytransparent. Above this range, the electron scattering depth becomessignificant, and an increasingly large fraction of the continuum isreflected back. The total emission, which is what one would see ifequal numbers of front faces and rear faces are visible, is not verydifferent from the input continuum. There are significant emissionlines, but not strong enough to produce the effect we are lookingfor here. Clouds at this density may, of course, exist in the innerregion, and produce observable absorption and emission lines, butwill not produce a gross modification of the SED.

3.3.2 Dense clouds, log n ∼ 12

Clouds with density log n = 12 are much more promising. At lowcolumn densities, they are ionized, with the results being fairlysimilar to lower density clouds and with little net effect on the SED.However at log NH = 24.55 and above the rear part of the cloudbecomes neutral. (With X-ray continuum added, this rear zone ispartially ionized, but the transition is in the same place.) As the totalcolumn increases above this value, the ionized column stays thesame and the neutral column simply increases. The ionized columncorresponds to the point where the electron scattering optical depthis 2.3. Varying the density from log n = 11 to 13 changes thetransition optical depth a little, from 4.0 to 0.9.

The emission from the cloud is more or less the same for allcolumns log NH > 24.6, and is very asymmetric. Seen from the farside, the cloud is essentially opaque at all wavelengths. However, thecloud is a strong reflection source in both continuum and lines. Theresulting spectrum is shown in Fig. 3. The electron scattering depthof the ionized zone is of the order of unity, so that a large fractionof the continuum is reflected back. However, around 8 per cent ofthe incident EUV continuum is reprocessed into emission lines, thestrongest lines being He II Lyα at 304 Å and H I Lyβ at 1025 Å. Thesummed luminosity of the lines in the 1000 Å region is ∼30 percent of the incident luminosity at <1 Rydberg. This amount of lineemission, together with the marked drop shortwards of the Lyman



Figure 3. The emitted spectrum from dense inner clouds at a distance 1016 cm from a blackbody radiation source of temperature 100 000 K with luminosity4 × 1046 erg s−1. The vertical units are erg s−1 cm−2. Only the incident and reflected spectra are shown, but the transmitted and diffuse-out components arevery small. The line-to-continuum contrast is approximately what would be seen at velocity resolution 1000 km s−1.

Figure 4. Dependence of reflected line luminosity on column density. Inall cases the other parameters are log n = 12, T = 100 000 K, log Ltotal =46.6 and log R = 16. The luminosities are calculated on the basis of a 4π

covering factor.

edge, is sufficient when velocity-blurred to make a substantial butlocalized false continuum, as discussed in Section 3.4.

Over the range 22 < log NH < 24.5 the luminosity in Lyβ in-creases by more than two orders of magnitude, as shown in Fig. 4,while the ratio Lyβ/Lyα increases from ∼0.1 to ∼8, and the ratioLyβ/He II Lyα increases from ∼0.1 to ∼1.4. From log NH = 24.5to the maximum plausible column of log NH = 27, the luminosityin Lyβ is constant, and the ratios Lyβ/Lyα and Lyβ/He II Lyα areconstant at 8.51 and 1.38, respectively. If we fix the column densityat log NH = 25 and vary the temperature of the ionizing source fromT = 75 000 to T = 200 000, the ratio Lyβ/Lyα changes little (from8.71 to 8.32), but the ratio Lyβ/He II Lyα changes from 4.27 to 0.45.This last sensitivity may give a way to test models of this kind.

Figure 5. Dependence of reflected line luminosity on density. In all casesthe other parameters are log NH = 25, T = 100 000 K, log Ltotal = 46.6 andlog R = 16. The luminosities are calculated on the basis of a 4π coveringfactor.

Clouds with log n = 11 give a similar result. Fig. 5 shows howthe luminosity in various lines changes as the density is varied, forlog NH = 25 and T = 100 00 K. The Lyβ effect is significant over arange of density, but strongly peaked around log n ∼ 12.

Similar clouds much closer in give a somewhat different result.At 5RS, a cloud with log n = 12 and log NH = 25 still has a neutralback end, and so is opaque when seen from the rear, but the largerionized column results in larger electron scattering depth, τ es = 5.3,so that the reflected continuum is little different from the incidentcontinuum, and the reflected lines are an order of magnitude smallerthan the 30RS case. Further out, similar clouds at 100RS have τ es =0.7 and give a result similar to the 30RS case, with Lyβ in fact severaltimes stronger, although of course these lines will be distinctly lessbroad than those at 30RS. By 300RS, line reflection is still strong,but now dominated by Lyα rather than Lyβ.


8 A. Lawrence

3.3.3 Very dense clouds, log n ∼ 14

What about clouds with even higher density, say log n > 14? Thisstrays into the region which CLOUDY was not designed for, and so is-sues warnings, but the rough results are clear. At low columns, suchclouds are ionized, but at log NH > 23 they are mostly neutral with athin ionized skin. There is little continuum reflection; the reflectionspectrum is dominated by H I Lyman continuum emission, withHe I Lyman continuum and He II Lyα also strong, and significantH I Balmer continuum. The relative strength of these componentsseems to be roughly constant at log NH > 24. I do not explore thesedensities in more detail, because the results from CLOUDY may beunreliable and also because they do not seem to give the 1100 Åpeaking that observations require.

At even higher densities, clouds will of course start to look likeblack bodies, and emit at a temperatures in the range 104 −5 K, de-pending on radial distance, as discussed by several previous authors(see discussion in Section 3.2).

3.4 Prospect of dense cloud reflection models

Adding velocity blurring to the reflected spectrum of a single cloudcan produce a false continuum of roughly the right kind. What aboutan ensemble of clouds surrounding the accretion disc – will theemission be dominated by the intrinsic continuum or the reflectedspectrum? This will depend on both the covering factor of theclouds, and the amount of blurring. I blurred the spectrum shownin Fig. 3 with a Gaussian of full width at half-maximum (FWHM)75 000 km s−1, roughly what one might expect for clouds at 30RS,depending on the kinematic model. The reflected quasi-continuumνSν at 1100 Å is then roughly twice the incident continuum at that

wavelength. If the covering factor is C then the amount of leaked-through intrinsic continuum is proportional to 1 − C and the fractionof the total due to reflection, ignoring self-occultation effects, is thenapproximately 2C/(1 + C). For the reflection spectrum to dominatetherefore requires C > 1/3. The fraction changes quite quicklyaround this value, with C = 0.1 giving 18 per cent, and C = 0.5giving 67 per cent. Larger covering factors require self-occultationto be taken into account. There is therefore only a small rangeof covering factors which produces a strong reflection spectrumwithout self-occultation.

However self-occultation, with large covering factor, may be ex-actly what we need. Note that the reflected spectrum from a singlecloud, like the incident continuum, is dominated by emission atseveral Rydbergs. Clouds which see only other clouds, rather thanthe disc surface, will also produce a spectrum like that of Fig. 3. Thecloud spectrum bounces from one cloud to another and emerges atthe edge of the system of clouds at a kind of quasi-photosphere. Thecentral disc will be completely occulted, so the SED that we seeis composed of the pure cloud spectrum plus the un-occulted outerpart of the disc. Fig. 6 shows a toy model of this kind comparedto the combined data for 3C 273 and PG 1008+1319. The modelshows a blurred version of the Fig. 3 reflection spectrum, added toa ν1/3 power law with an exponential cut-off at 0.4 Rydberg, meantas an approximation to an accretion disc truncated at 30RS. Therelative strengths of the two components are roughly correct, butnote that this is not a full or self-consistent model. It should be seenas a plausibility demonstration rather than a real model. Significantextra work, and decisions on model assumptions, is needed beforeit makes sense to attempt a proper fit.

(i) There will very likely be a wide range of densities and columndensities at different radii. To some extent, the SED modification

Figure 6. An illustrative model, compared to the combined SED of 3C 273 (Kriss et al. 1999) and PG 1008+1319 (Binette et al. 2005). The data points forPG 1008+1319 are shifted vertically to match on to the 3C 273 data. The black hole is assumed to have mass M9 = 0.89 and to be radiating with LE = 0.36.The reflecting clouds are assumed to have log n = 12 and log NH = 25, to be located at 30RS and to be ionized by an intrinsic continuum with temperatureT = 100 000 K. The reflection spectrum is smeared with a Gaussian of FWHM 75 000 km s−1. The power law follows ν1/3 with an exponential cut-off at 0.4Rydberg, roughly corresponding, for our parameters, to the expected accretion disc occulted at 30RS.



may naturally be dominated by clouds in a particular range – bycomparison with these just-right clouds, clouds of lower densityor column may be transparent, and clouds that are closer in mayprimarily reflect an unaltered continuum. However, a prediction ofthe actual range of cloud parameters could make a big difference.

(ii) The shape of the SED modification will be sensitive to theradial distribution of velocity and cloud density. In this paper, Imake basic arguments about the likely density and velocity regimeconcerned. Going beyond this needs a physically motivated modelwhich makes a prediction of the velocity and density distributions.

(iii) A realistic model for the intrinsic continuum is also needed,both for the interior region, which provides the continuum whichionizes the clouds, and for the exterior region, which produces theoptical SED. The behaviour of the inner accretion disc is clearlycrucial.

(iv) Geometrical considerations could also make a big difference.For example, it may be that clouds lifted out of the disc in a narrowrange of radii will form a kind of funnel, somewhat akin to the modelof Elvis (2000) but much closer in. In this case, the continuum thatwe see will be inclination dependent.

(v) To fit the whole optical-UV-FUV SED, we may need to takeinto account a more continuous system of clouds, between mysuggested ∼30RS region and the traditional ∼1000RS BLR region.

4 D ISCUSSION

Do we expect that such dense thick clouds will exist in the innerregions of quasars? Here I briefly discuss the mass of the possiblecloud system, its origin, the lifetime of the clouds, the origin of UVvariability and the implications for AGN spectra.

4.1 Size of clouds and overall cloud system

Clouds with density NH = 1012 cm−3 and column NH = 1025 cm−3

will have thickness D ∼ 1013 cm. If spherical, they will have in-dividual masses of the order of 10−6 M�, but of course it is quitepossible that the material is the form of much more extended sheetsor filaments. The total mass of the system of clouds can be estimatedby considering a spherical sheet at radial distance R with coveringfraction C, which gives

ms = 8.3 M� × CM29 R2

30N25,

where as usual M9 is black hole mass in units of 109 M�, R30 is theradial distance in units of 30RS and N25 is in units of 1025 atomscm−2. How does this compare with the mass likely to be involved inthe accretion flow on to the black hole? Let us first consider simpleuniform spherical accretion. Assuming steady flow in free-fall thedensity at R is

n = 2.2 × 108 cm−3 × μ−10.1M

−19 LER2

30,

where LE = L/LEdd, and μ is the usual radiative efficiency scaled toan expected value of 0.1. The accreting mass interior to R is

ma = 1.1 M� × μ−10.1M

29 LER

3/230 .

The mass available is therefore an order of magnitude less than therequired mass of clouds, and a very large degree of clumping wouldbe needed to make clouds of the necessary density, so it seems thata simple spherical accretion model does not work.

There are two ways to increase the mean density and mass avail-able. The first is for accretion to be inefficient (μ � 1), as in Ad-vection Dominated Accretion Flow (ADAF) models, where mostof the energy generated by infall is advected into the black hole.

Potentially a clumpy ADAF model could be constructed (Celotti& Rees 1999), but I do not consider this further here as it seemsunlikely to be relevant to luminous quasars.

The second way to increase the mass available is to slow downthe infall and so increase density by a factor K = vff /vinf . In standardaccretion disc models K ∼ α−1(R/H)2 where H is the characteristicscale height of the disc and α is the usual viscosity parameter (Franket al. 2002, chapters 5 and 8). The density is further increased bythe vertical compression factor R/H. For an AGN disc, assumingα ∼ 0.1 and R/H ≥ 10, then the mass available within R is at leastma ∼ 103 M�M−1

9 LER3/230 and the mean density in the disc at R is

at least n ∼ 1012M−19 LE cm−3R

−3/230 , with each of these quantities

likely to be orders of magnitude larger. In conclusion there is aplentiful supply of material in the disc. We look at disc models alittle more closely in the next section.

4.2 Possible disc origin of clouds

Detailed disc models (e.g. Hubeny et al. 2000) confirm the aboverough calculation. From the figures in Hubeny et al. (2000), I es-timate that for a disc radiating at 15 per cent of Eddington arounda black hole of mass 109 M�, the mass interior to 30RS is 7 ×103 M�, so only a very small fraction of the disc mass is needed.The density varies greatly, from ∼1015 cm−3 in the midplane to∼1010 cm−3 at the disc surface. Likewise the total column densityis of the order 1028 cm−2 or more, depending on radius, and thinningout towards the surface. Production of clouds then does not requirecomplete disruption of the disc, but just a clumping of the upperparts. It has been argued that in the inner regions discs become un-stable to clumping (Krolik 1998) but in fact only a modest degreeof clumping would be required to produce the necessary clouds.

To provide the reflection medium we are considering, the cloudsalso need to be lifted out of the disc, and to be cool enough tobe not already completely ionized, i.e. a few times 104 K. Discsare expected to have a significant vertical temperature gradient, soagain this means that the upper portions would be the source ofcloud material, rather than the disruption of the whole disc. In discwind models (see e.g. Proga 2007 and references therein), upliftand gas temperature are closely linked. Radiation pressure comesfrom absorption of resonance lines as well as electron scattering,so that even well below the nominal Eddington limit gas can belifted above the disc surface, where it is then hit by the central UVsource and driven out. In the very inner disc, it is generally arguedthat the material is too highly ionized for this effect to work, so thatwinds are launched at ≥100RS, and only for high-mass and high-luminosity systems (e.g. Shlosman, Vitello & Shaviv 1985; Murrayet al. 1995; Proga, Stone & Kallman 2000). However, Proga (2005)shows that in the inner regions a ‘failed wind’ or ‘puffed up disc’can be produced, which may be exactly the kind of structure weneed. Note that local radiation force is very sensitive to clumping.If clumps can form within the upper disc with the kind of parameterswe have considered here, log n = 12 and log NH = 25, they will beoptically thick to the Lyman continuum, so that the ‘force multiplier’will be extremely large. Such clumps are then very likely to beejected from the disc when formed. A reasonable guess is that atlarge radii (>100RS) gas can be uplifted without clumping, andso form a smooth wind, whereas at very small radii, (<10RS) thematerial is too dense and hot to either feel uplift forces from heavyelement absorption lines or make the cool thick clouds needed toget Lyman uplift. The result may be that dense clouds are formedand uplifted at a characteristic radius around 30RS. More detailedcalculations are needed to see if this is indeed the case.


10 A. Lawrence

Alternatively, clouds may be uplifted by magneto-centrifugal ef-fects, as in the models of Konigl & Kartje (1994), de Kool &Begelman (1995) and Everett (2005). This idea is usually consid-ered at much larger radii, but may also be relevant in the innerregions.

4.3 Cloud time-scales and variability

Do we expect such clouds to survive? Unless confined (e.g. byan external medium or magnetic field) they would disrupt on thesound crossing time-scale. Assuming that the gas is mostly ion-ized and at a temperature T30 = T/30 000 this gives tdisrupt =52.0 d×N25n

−112 T

−1/230 , which is very similar to the dynamical time-

scale at 30RS. Because there is a plentiful supply of material, wedo not require the clouds to be long lived. The cloud sound cross-ing time-scale will also be (very roughly) the time-scale on whichclumps could grow inside the disc, so we may expect that cloudsare repeatedly forming and being destroyed.

How quickly would such clouds be expelled? If the clouds origi-nate in the disc, they will initially be moving at orbital velocity, butbecause they are optically thick to Lyman continuum and electronscattering, they will be driven radially outwards with a greatly super-Eddington force. Given that they have mass per unit area NHmH, thetime to reach escape velocity will be Tesc = 124 d×N25M9R

3/230 L−1

E .It seems therefore that clouds will be destroyed faster than they areexpelled, and so we do not necessarily expect a wind to form, butrather a kind of broiling cloudy atmosphere .

The covering factor of clouds may well vary significantly onthe cloud formation/destruction time-scale, leading to a natural ex-planation of UV variability. The intrinsic UV continuum, and theoptical continuum, could vary quite slowly; it is just the reflectedcontinuum that varies rapidly. The time-scale is about right for theobserved UV variability, and the fixed shape of the reflection spec-trum is consistent with the strongly wavelength-dependent variabil-ity, and the lack of delays between wavelengths. There could stillbe hours-time-scale light travel time delays, just as in the disc re-processing model. A possible objection to the idea that the intrinsicdisc emission varies slowly is that the BLR emission lines, whichtrack the observed UV continuum with a time lag, must surely beresponding to the EUV continuum, which then must also be varyingrapidly. However the dense inner clouds also reflect EUV contin-uum; the BLR clouds will see a mixture of slowly varying intrinsicEUV continuum and rapidly varying reflected EUV continuum.Whether this works quantitatively should be explored.

4.4 Effect on X-ray spectrum and variability

Much has been written about absorption and reflection effects inX-ray spectra (see the review by Turner & Miller 2009). Ultimately,a self-consistent model for both UV and X-ray emission is needed,which is beyond the scope of this paper. I restrict myself to a fewgeneral points.

(i) In the picture emerging in this paper, the inner accretion discis assumed to be present, so blurred ionized disc reflection willcertainly occur. However, the clouds we have proposed will alsoproduce blurred X-ray reflection effects, albeit at somewhat lowervelocity blurring than the inner disc effects. If the inner disc isocculted, the cloud reflection effects may dominate. The cloudswill also produce opaque partial covering, which can easily leadto an apparent reflection efficiency greater than 1. Merloni et al.

(2006) have considered inhomogeneous X-ray reflection models,using dense clouds with no central accretion disc, but embeddedin a very hot plasma. Their clouds are of similar column to thosediscussed here, but at higher density, and closer in. They find thatsoft excess and hard X-ray humps naturally go together.

(ii) Our clouds are closer and thicker than those deduced byRisaliti et al. (2007) and related papers to explain rapid absorptionchanges in many AGN. They may be more closely related to thematerial producing Compton-thick partial covering phenomena re-vealed by BeppoSAX and Suzaku at E >10k eV (Dadina & Cappi2004; Reeves et al. 2009; Turner et al. 2009), although it has beenclaimed that such objects may also be explained with disc reflection(Fabian et al. 2005). From a study of 165 AGN with INTEGRALdata, Ricci et al. (2011) have proposed that the large reflection com-ponent seen in apparently mildly absorbed Type 2 AGN may beexplained by Compton-thick clumps partially covering the source.At middling covering factors such opaque partial covering producesa striking spectral signature. At large, but still not complete, cov-ering factors, the leaked-through spectrum may look quite normal,and if there is additional thinner absorbing material at larger radii,the spectrum may easily be mistaken for being Compton-thin whenin fact it is largely Compton-thick. A consequence is that the ob-served X-ray luminosity may be systematically wrong. The effecton the luminosity function for obscured and unobscured AGN couldproduce an artificial dependence of obscuration on luminosity, asnoted by Lawrence & Elvis (2010) and explored in more detail byMayo and Lawrence (in preparation).

(iii) The clouds we have discussed may also be related to thehigh velocity absorbers seen in highly ionized Fe (Chartas, Brandt &Gallagher 2003; Pounds et al. 2003; Reeves et al. 2009). These seemto be very common (Tombesi et al. 2010), and have typical velocitiesof 0.1–0.2c. It is generally assumed that they represent outflows, butthere have also been claims of inflows at similar velocity (Dadinaet al. 2005; Tombesi et al. 2010). If material is in rotational orturbulent motion at such velocities, this is quite consistent withthe kind of blurring we have assumed in this paper. In at leastone case (Risaliti et al. 2005) the outflow velocity was seen tochange substantially from one observation to another, suggestiveof the kind of instability we have suggested for our UV-reflectingclouds. The columns deduced for the high velocity X-ray absorbersare large but somewhat less than we have been considering here,log NH = 23–24 as opposed to log NH > 25, but we note that partialcovering will dilute the strength of the absorption lines. Our UV-reflecting clouds may also be related to the fast moving materialinvolved in the proposed absorption explanation of the ubiquitoussoft excesses seen in AGN (Gierlinski & Done 2004; Middleton,Done & Gierlinski 2007).

(iv) In Section 4.3, I suggested that variations in covering fac-tor could explain the UV variability. Likewise, covering factorchanges could explain the observed X-ray variability, as argued byAbrassart & Czerny (2000). Several papers have argued that absorp-tion changes can explain the complex changes seen in the spectra ofindividual objects (e.g. Miller, Turner & Reeves 2009 and referencestherein; see also the review by Turner & Miller 2009). However,UV and X-ray variability cannot be explained by exactly the samereprocessing clouds. As discussed in Section 2.4, X-rays and UVtrack each other well over medium-time-scales, but X-rays alsoshow extra short time-scale variability. This is naturally explainedif UV reprocessing is dominated by thick clouds at ∼30RS, whereasX-rays are affected both by these clouds and by closer-in clouds aswell. However, it is not obvious why X-ray variations sometimesseem to lag the UV variations.



Overall, the clouds required to explain NH changes, X-ray vari-ability and spectral changes, thick partial covering, soft excessesand high velocity outflows are all likely to be somewhat related toour proposed UV-reflection clouds, but are not the same. A distribu-tion of cloud properties is required, and it is not yet clear if a singlemodel can reproduce everything consistently.

4.5 Apparent UV source size

Following equation (2) of Morgan et al. (2010), the characteristicUV size of a thin accretion disc around a black hole with M =109 M� accreting at the Eddington limit with efficiency η = 0.1should be ∼10RS. However we have found that reprocessing cloudscould be producing a false UV continuum over a considerable radialscale, 10–100RS – energy produced in the central region emerges ona larger scale. Potentially therefore reprocessing clouds can solvethe size problem implied by gravitational microlensing studies (seeSection 2.5). Whether this works quantitatively is not yet clear– the effective size will depend on the density and covering fac-tor of the clouds with radius, which obviously requires a physicalmodel. Such a model of the physical distribution of the clouds isalso needed to predict the chromatic behaviour of microlensingfluctuations.

4.6 Next steps

In this paper I have concentrated on the properties of clouds thatcould plausibly explain the worrying observed features of the UVbump and its variability, and demonstrating the plausibility of suchclouds as an explanation. What is needed is a more detailed specificmodel. This is rather a challenge; to go beyond the plausibilitydemonstrated here one would need to specify the distribution ofclouds in density, column and radius, and to specify their velocityfield. This is too large a volume of parameter space to blindlyexplore as a grid. It really needs a prediction from a physical model.However, there are some general features that could be exploredfirst.

(i) There should be some testable dependence on black hole massand accretion rate, as these properties together predict the hardnessof the intrinsic continuum. As noted in Section 3.3.2, the relativestrengths of Lyβ and He II Lyα, and hence the 1100 Å bump andthe EUV peak, will depend on the temperature of the intrinsiccontinuum. This may produce measurable effects on the UV-FUVSED, and on the observed BLR line ratios.

(ii) Another possible mass dependence comes from cloud size.The expected cloud size is ∼1013 cm, but with a likely order ofmagnitude range. For quasar-size black holes, M ∼ 109 M�, thisis a small fraction of the source size, so that any one cloud hasa small covering factor, and any changes are global ones pertain-ing to the ensemble of clouds. For lower-luminosity Seyferts, withM ∼ 106 −7 M�, the movement and physical changes of individualclouds may be significant, producing qualitatively different changes.

(iii) Although I have argued that physically motivated cloud mod-els are needed next, an obvious step is models with a small numberof cloud zones, to see whether UV-reflection effects, X-ray reflec-tion effects, X-ray absorption effects and microlensing chromaticeffects can all be explained together without the various cloud pop-ulations destructively interfering as it were. This could also lead toinsights which would help the design of simultaneous UV/X-raymonitoring campaigns.

5 C O N C L U S I O N S

The problems with the accretion disc interpretation of the BBB– the universal knee and FUV falloff, the ionization paradox, thetime-scale issue, the co-ordination and amplitude-colour problems,and the microlensing size discrepancy – can all potentially be ex-plained if EUV emission from the inner disc is reprocessed by denseclouds at a few tens times RS. A variety of clouds may be present,but the ones that produce the desired effect have densities in therange 1011 < n < 1013 cm−3, and column densities NH > 4 × 1024.Such clouds are opaque and reflect most of the incident continuum,but recycle a substantial fraction of the EUV energy as emissionlines, dominated by He II Lyα and H I Lyβ. Blurring this emissionproduces a localized false peak at 1100 Å, followed by a turn backupwards in the FUV. Producing such clouds requires only a smallfraction of the mass available in the accretion disc, and does notrequire the accretion disc to be completely disrupted, and may bemost likely to happen at a few tens of RS. Such clouds will feel avery strong outward radiation force, but they may be formed and de-stroyed on a faster time-scale than they are ejected, so that there willnot necessarily be a continuous wind, but rather an unstably variablecloudy atmosphere. Variations in covering factor may explain theobserved UV variability.

Such clouds may be related to those deduced from various X-rayobservations, but are not the same. There is likely to be a large rangeof cloud properties at various different radii in an AGN, with thosein different parts of parameter space producing different effects. Itremains to be seen whether a physically realistic model can explainall the observations simultaneously and consistently.

In the 1980s, there was confused argument over whether the ‘bluebump’ was really due to an accretion disc as proposed by Malkan& Sargent (1982) – some researchers claimed that the turn-up seenin optical spectra was actually due to Balmer continuum and Fe II

emission lines on top of an underlying power law (Grandi 1982;Wills, Netzer & Wills 1985). The solution was to take a wider (IR-optical-UV-X-ray) perspective and see that both phenomena werepresent. Elvis & Lawrence (1985) first used the term ‘little bluebump’ to distinguish the atomic feature from the ‘BBB’. If it iscorrect that the 1100 Å peak is due to blurred atomic features, wemay have to rename this feature the ‘medium blue bump’, with thetrue ‘BBB’ residing at 300 Å.

AC K N OW L E D G M E N T S

In worrying about the problems with the BBB over a number ofyears, I have talked to many people, but particularly influential con-versations were had with Andrew King, Martin Elvis, Julian Kro-lik, Christine Done, Omer Blaes and Ski Antonucci. Several veryuseful suggestions were also made by an anonymous referee, par-ticular concerning microlensing and covering factor issues. When Idescribed how any dense clumps formed in the accretion disc wouldfeel a very strong radiation force, Ski, colourful as ever, exclaimedthat they would jump out of the disc ‘like popcorn out of a pan’.

R E F E R E N C E S

Abrassart A., Czerny B., 2000, A&A, 356, 475Alloin D., Pelat D., Phillips M., Whittle M., 1985, ApJ, 288, 205Antonucci R., 2002, in Trujillo-Bueno J., Moreno-Insertis F., Sanchez

F., eds, Astrophysical Spectropolarimetry. Proceedings of the XII Ca-nary Islands Winter School of Astrophysics. Cambridge Univ. Press,Cambridge


12 A. Lawrence

Barvainis R., 1993, ApJ, 412, 513Binette L., Magris C. G., Krongold Y., Morisset C., HaroCorzo S., de Diego

J. A., Mutschke H., Andersen A. C., 2005, ApJ, 631, 661Binette L., Haro-Corzo S., Krongold Y., Andersen A. C., 2008, Rev. Mex.

Astron. Astrofis., 32, 115Blaes O., Hubeny I., Agol E., Krolik J. H., 2001, ApJ, 563, 560Breedt E. et al., 2009, MNRAS, 394, 427Brotherton M. S., 1996, ApJS, 102, 1Celotti A., Rees M. J., 1999, MNRAS, 305, L41Celotti A., Fabian A. C., Rees M. J., 1992, R. Astron. Soc., 255, 419Chartas G., Brandt W. N., Gallagher S. C., 2003, ApJ, 595, 85Chartas G., Kochanek C. S., Dai X., Morgan., Agol E., Poindexter.,

Blackburne J., Kozlowski S., 2010, American Astron. Soc., 41Chen B., Dai X., Kochanek C. S., Chartas G., Blackburne J. A., Szymon K.,

2011, ApJ, 740, 34Chiang J., Blaes O., 2003, ApJ, 586, 97Chiang J., Reynolds C. S., Blaes O. M., Nowak M. A., Murray N., Madejski

G., Marshall H. L., Magdziarz P., 2000, ApJ, 528, 292Clavel J. et al., 1991, ApJ, 366, 64Collier S., Peterson B. M., 2001, ApJ, 555, 775Collier S. J. et al., 1998, ApJ, 500, 162Collier S. et al., 2001, ApJ, 561, 146Collin-Souffrin S., 1986, A&A, 166, 115Collin-Souffrin S., Czerny B., Dumont A.-M., Zycki P. T., 1996, A&A, 314,

393Crenshaw D. M. et al., 1996, ApJ, 470, 322Cutri R. M., Wisniewski W. Z., Rieke G. H., Lebofsky M. J., 1985, ApJ,

296, 423Czerny B., Dumont A.-M., 1998, A&A, 338, 386Dadina M., Cappi M., 2004, A&A, 413, 921Dadina M., Cappi M., Malaguti G., Ponti G., De Rosa A., 2005, A&A, 442,

461Dai X., Kochanek C. S., Chartas G., KozÅowski S., Morgan C. W., Garmire

G., Agol E., 2010, ApJ, 709, 278de Kool M., Begelman M. C., 1995, ApJ, 455, 448Dexter J., Agol E., 2011, ApJ, 727, L24Doroshenko V. T., Sergeev S. G., Merkulova N. I., Sergeeva E. A., Golubin-

sky Y. V., 2005, A&A, 437, 87Dumont A.-M., Collin-Souffrin S., Nazarova L., 1998, A&A, 331, 11Edelson R. A. et al., 1996, ApJ, 470, 364Edelson R. et al., 2000, ApJ, 534, 180Eigenbrod A., Courbin F., Meylan G., Agol E., Anguita T., Schmidt R. W.,

Wambsganss J., 2008, A&A, 490, 933Elvis M., Lawrence A., 1985, in Miller J. S., ed., Astrophysics of Ac-

tive Galaxies and Quasi-stellar Objects. University Science Books, MillValley, p. 289

Elvis M. et al., 1994, ApJS, 95, 1Elvis M., 2000, ApJ, 545, 63Everett J. E., 2005, ApJ, 631, 689Fabian A. C., Miniutti G., Iwasawa K., Ross R. R., 2005, MNRAS, 361, 795Ferland G. J., Rees M. J., 1988, ApJ, 332, 141Ferland G. J., Korista K. T., Peterson B. M., 1990, ApJ, 363, L21Ferland G., Korista K., Verner D., Ferguson J., Kingdon J., Verner E., 1998,

PASP, 110, 761Frank J., King A., Raine D. J., 2002, Accretion Power in Astrophysics.

Cambridge Univ. Press, CambridgeGierlinski M., Done C., 2004, MNRAS, 349, L7Giveon U., Maoz D., Kaspi S., Netzer H., Smith P. S., 1999, MNRAS, 306,

637Grandi S. A., 1982, ApJ, 255, 25Guilbert P. W., Rees M. J., 1988, MNRAS, 233, 475HaroCorzo S. A. R., Binette L., Krongold Y., Benitez E.,

Humphrey A., Nicastro F., Rodriguez-Martinez M., 2007, ApJ, 662,145

Hawkins M. R. S., 2003, MNRAS, 344, 492Hawkins M. R. S., 2007, A&A, 462, 581Hubeny I., Agol E., Blaes O., Krolik J. H., 2000, ApJ, 533, 710Jarvis M. J., McLure R. J., 2006, MNRAS, 369, 182

Jimenez-Vicente J., Mediavilla E., Munoz J. A., Kochanek C. S., 2012,preprint (arXiv:1201.3187)

Karas V., Czerny B., Abrassart A., Abramowicz M. A., 2000, MNRAS, 318,547

Kaspi S., Smith P. S., Netzer H., Maoz D., Jannuzi B. T., Giveon U., 2000,ApJ, 533, 631

Kazanas D., Nayakshin S., 2001, ApJ, 550, 655Kishimoto M., Antonucci R., Blaes O., 2003, MNRAS, 345, 253Kishimoto M., Antonucci R., Blaes O., Lawrence A., Boisson C., Albrecht

M., Leipski C., 2008, Nat, 454, 492Konigl A., Kartje J. F., 1994, ApJ, 434, 446Korista K. T., Goad M. R., 2001, ApJ, 553, 695Korista K. T. et al., 1995, ApJS, 97, 285Korista K., Baldwin J., Ferland G., Verner D., 1997, ApJS, 108, 401Korista K., Ferland G., Baldwin J., 1997, ApJ, 487, 555Kriss G. A., Davidsen A. F., Zheng W., Lee G., 1999, ApJ, 527, 683Kriss G. A., Peterson B. M., Crenshaw D. M., Zheng W., 2000, ApJ, 535,

58Krolik J., 1998, ApJ, 498, L13Krolik J. H., Horne K., Kallman T. R., Malkan M. A., Edelson R. A., Kriss

G. A., 1991, ApJ, 371, 541Kuncic Z., Blackman E. G., Rees M. J., 1996, MNRAS, 283, 1322Kuncic Z., Celotti A., Rees M. J., 1997, MNRAS, 284, 717Lawrence A., 2005, MNRAS, 363, 57Lawrence A., Elvis M., 2010, ApJ, 714, 561Lewis G. F., Irwin M. J., Hewett P. C., Foltz C. B., 1998, MNRAS, 295, 573Lightman A. P., White T. R., 1988, ApJ, 335, 57Malkan M. A., Sargent W. L. W., 1982, ApJ, 254, 22Malzac J., Celotti A., 2002, MNRAS, 335, 23Marshall H. L. et al., 1997, ApJ, 479, 222Merloni A., Malzac J., Fabian A. C., Ross R. R., 2006, MNRAS, 370,

1699Middleton M., Done C., Gierlinski M., 2007, MNRAS, 381, 1426Miller L., Turner T. J., Reeves J. N., 2009, MNRAS, 399, L69Morgan C. W., Kochanek C. S., Morgan N. D., Falco E. E., 2010, ApJ, 712,

1129Mosquera A. M., Munoz J. A., Mediavilla E., Kochanek C. S., 2011, ApJ,

728, 145Murray N., Chiang J., Grossman S. A., Voit G. M., 1995, ApJ, 451, 498Netzer H., 1985, ApJ, 289, 451Oknyanskij V. L., Horne K., Lyuty V. M., Sadakane K., Honda S., Tanabe

S., 2003, in Collin S., Combes F., Shlosam I., Active Galactic Nuclei:from Central Engine to Host Galaxy. ASP Conf. Ser. Vol. 290, Astron.Soc. Pac., San Francisco

Paltani S., Turler M., 2005, A&A, 435, 811Peterson B. M., Wanders I., Horne K., Collier S., Alexander T., Kaspi S.,

Maoz D., 1998, PASP, 110, 660Peterson B. M. et al., 2004, ApJ, 613, 682Pooley D., Blackburne J. A., Rappaport S., Schechter P. L., 2007, ApJ, 661,

19Pounds K. A., Nandra K., Stewart G. C., George I. M., Fabian A. C., 1990,

Nat, 344, 132Pounds K. A., Reeves J. N., King A. R., Page K. L., O’Brien P. T., Turner

M. J. L., 2003, MNRAS, 345, 705Proga D., 2005, ApJ, 630, L9Proga D., 2007, in Ho L. C., Wang J. M., eds, The Central Engine of

Active Galactic Nuclei. ASP Conf. Ser. Vol. 373, Astron. Soc. Pac., SanFrancisco

Proga D., Stone J. M., Kallman T. R., 2000, ApJ, 543, 686Rauch K. P., Blandford R. D., 1991, ApJ, 381, L39Reeves J. N. et al., 2009, ApJ, 701, 493Ricci C., Walter R., Courvoisier T. J.-L., Paltani S., 2011, A&A, 532, A102Risaliti G., Bianchi S., Matt G., Baldi A., Elvis M., Fabbiano G., Zezas A.,

2005, ApJ, 630, L129Risaliti G., Elvis M., Fabbiano G., Baldi A., Zezas A., Salvati M., 2007,

ApJ, 659, L111Rokaki E., Lawrence A., Economou F., Mastichiadis A., 2003, MNRAS,

340, 1298



Sanders D. B., Phinney E. S., Neugebauer G., Soifer B. T., Matthews K.,1989, ApJ, 347, 29

Scott J. E., Kriss G. A., Brotherton M., Green R. F., Hutchings J., ShullJ. M., Zheng W., 2004, ApJ, 615, 135

Sergeev S. G., Doroshenko V. T., Golubinskiy Y. V., Merkulova N. I.,Sergeeva E. A., 2005, ApJ, 622, 129

Shang Z. et al., 2005, ApJ, 619, 41Shields G. A., 1978, Nat, 272, 706Shlosman I., Vitello P. A., Shaviv G., 1985, ApJ, 294, 96Sivron R., Tsuruta S., 1993, ApJ, 402, 420Strateva I. V., Brandt W. N., Schneider D. P., Vanden Berk D. G., Vignali

C., 2005, AJ, 130, 387Telfer R. C., Zheng W., Kriss G. A., Davidsen A. F., 2002, ApJ, 565, 773Tombesi F., Cappi M., Reeves J. N., Palumbo G. G. C., Yaqoob T., Braito

V., Dadina M., 2010, A&A, 521, A57Trevese D., Vagnetti F., 2002, ApJ, 564, 624Trevese D., Kron R. G., Bunone A., 2001, ApJ, 551, 103

Turler M. et al., 1999, A&AS, 134, 89Turner T. J., Miller L., 2009, A&AR, 17, 47Turner T. J., Miller L., Kraemer S. B., Reeves J. N., Pounds K. A., 2009,

ApJ, 698, 99Wambsganss J., 2006, in Meylan G., Jetzer P., North P., eds, Saas-Fee

Advanced Courses, Vol. 33, Gravitational Lensing: Strong, Weak andMicro. Springer, Berlin, Heidelberg, p. 93

Wambsganss J., Schneider P., Paczynski B., 1990, ApJ, 358, L33Wanders I. et al., 1997, ApJS, 113, 69Wilhite B. C., Vanden Berk D. E., Kron R. G., Schneider D. P., Pereyra N.,

Brunner R. J., Richards G. T., Brinkmann J. V., 2005, ApJ, 633, 638Wills B. J., Netzer H., Wills D., 1985, ApJ, 288, 94Zheng W., Kriss G. A., Telfer R. C., Grimes J. P., Davidsen A. F., 1997,

ApJ, 475, 469

This paper has been typeset from a TEX/LATEX file prepared by the author.


the uv peak in active galactic nuclei: a false continuum from blurred reflection?

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