Thermal Emission from Isolated Neutron Stars:Thermal Emission from Isolated Neutron Stars:Spectral Features and Featureless SpectraSpectral Features and Featureless Spectra

Silvia Zane, MSSL, UCL, UKCongresso Nazionale Oggetti Compatti |||

Osservatorio Astronomico di Roma, 9-11 Dicembre 2003

Over the last few years, intense observational resources have been devoted to study the faint thermal emission from neutron stars and to search for features in their spectrum.

Isolated neutron stars play a key role in compact objects astrophysics: these are the only sources in which we can see directly the surface of the compact star.

we can measure physical parameters as star mass, radius, probing our understanding of the EOS.

we can measure the surface temperature and reconstruct the cooling history of the source. we can detect/undetect spectral features, constraining chemical composition and/or magnetic field strength in the atmosphere.

THIS MEANS THAT, AS SINGLE OBJECTSTHEY ARE INTERESTING BECAUSE:

X-ray Dim Isolated Neutron Star (INS) X-ray Dim Isolated Neutron Star (INS)

RINSs are the largest class of thermally emitting Neutron Stars (Treves et al, 2000)

Thermal emission detected in more than 20 NSs (SGRs, AXPs,PSRs, Radio-quiet NSs)

Soft X-ray sources in ROSAT survey BB-like X-ray spectra, no non thermal hard emission Low absorption, nearby (NH ~1019-1020 cm-2) Constant X-ray flux on time scales of years Some are X-ray pulsars (3.45-11.37 s) No radio emission ? No obvious association with SNR Optically faint

As a class, they are interesting As a class, they are interesting because:because:

They imply the existence of They imply the existence of a fair number a fair number of neutron stars different from standard of neutron stars different from standard radio pulsars and X-ray binaries radio pulsars and X-ray binaries

Accreting from ISM?Accreting from ISM? Unlikely: high Unlikely: high proper motionproper motion

Cooling NS or descendant from AXP, Cooling NS or descendant from AXP, SGRs SGRs (old magnetars?)(old magnetars?)

Standard Standard radio pulsars beamed awayradio pulsars beamed away from the Earth? (however: they are from the Earth? (however: they are relatively numerous and all close-by)relatively numerous and all close-by)

Genuinely radio-quiet? Genuinely radio-quiet? (as Geminga, (as Geminga, SGRs, AXPs)? Population synthesis SGRs, AXPs)? Population synthesis modelsmodels

The The strikingstriking case of RX J1856.5- case of RX J1856.5-37543754

500 ks DDT Chandra exposure 500 ks DDT Chandra exposure

(i) RX J1856.5-3754 has a featureless X-ray continuum(ii) better fit with a simple bb than with more sophisticated atmospheric models (Burwitz et al 2001, Drake et al 2002, Burwitz et al, 2002)

XMM-Newton and Chandra spectra of RXJ1856 together with the best single blackbody fit to each instrument (see table).

Mission Instrument NH kT ∞ R ∞BB χ2 /D.O.F.

1022 cm-2 eV km (d/120 pc)

Rosat PSPC 1.46 ±0.20 56.7±1.0 7.5 ± 0.5 0.9/16 Chandra LETGS 0.95±0.02 62.5±0.2 4.4± 0.1 1.2/114 XMM EPIC-PN 0.12±0.02 62.3±0.2 4.2± 0.1 2.2/122 XMM EPIC-MOS2 0.60 ±0.02 62.6±0.4 4.4± 0.1 6.1/41 XMM RGS1+RGS2 0.87 ±0.08 62.4±0.3 4.0± 0.2 1.1/717

The The strikingstriking case of RX J1856.5- case of RX J1856.5-37543754

Optical excess of ~6 over the Rayleigh-Jeans tail of the X-ray best fitting bb (Walter & Lattimer, 2002)

No X-ray pulsations: upper limit on the pulsed fraction 1% (Burwitz et al., 2003)

previous d ~120-140 pc (Kaplan et al, 2001; Walter & Lattimer, 2002)

revised revised d ~175 pc (Kaplan et al., 2003, Korea meeting) radiation radius of only 7-8 km!

• Two-T model: x-ray = caps; optical = star surface (Pons et al. 2002; Walter & Lattimer, 2002)

• Is it the first quark/strange star discovered? (Drake et al, 2002; Xu, 2002)

• Phase transition to a solid surface (B>few x1013 G) (Turolla et al. 2003)

Pulsating neutron stars: 4 so Pulsating neutron stars: 4 so far!far!

RX J0420: previous pulsation 22.7s , 1 in ROSAT HRI (Haberl et al. 1999).

1 RXS 1308: previous pulsation 5.157 s (Hambaryan et al. 2002)

Haberl et al. 2003: double peaked light curve, P=10.31 s

Haberl et al., 2004 in prep.: spurious. Instead, P= 3.45 s (4 XMM PN and 4 XMM MOS observations in 2003)

RX J0806.4-4123Epic-PN (0.12-1.2) keV

RX J072.4-3125Epic-PN (0.12-1.2) keV

RBS 1223Epic-PN (0.12-1.2) keV

RX J0420.0-5022Epic-PN (0.12-0.7) keV

Spectral variations with pulse Spectral variations with pulse phasephase

Hardness ratio is max at the pulse maximum: counter-intuitive! Same observed in RX J0420 and RXJ0806 (Haberl et al., 2004, in prep.)

Beaming effects ? (Cropper et al. 2001) Phase-dependent cyclotron absorption? (Haberl et al., 2003)

dP/dt measured in 1 case: the brightest pulsating source RXJ0720.

dP/dt = 1.4 ± 0.6 x10-13 s/s B (2.8-4.2) x 1013 G ; Ecp 0.2-0.3 keV (Cropper et al. 2004 in prep.)

RXJ 0720 RBS 1223

Phase

No

rmal

ised

Flu

xH

ard

nes

s ra

tio

Phase

Har

dn

ess

0.5

-1.0

ke

VN

orm

. In

ten

s.

0.1

2-0

.5 k

eV

No

rm.

Inte

ns

.

Thermal Spectra: blackbody fitsThermal Spectra: blackbody fits

The situation changed only this year….

Energy (keV)

Co

un

ts/s

/keV

Co

un

ts/s

/keV

SCRIVO

Energy (keV)

RX J1605:kT = 96 eVNH = 2.7x1019 cm -2

RX J0420:kT = 44 eVNH = 1.3 x1020 cm -2

Scrivo

RX J0720:kT = 86 eVNH = 1.3 x1020 cm -2

Scri

RBS 1223:kT = 95 eVNH = 7.1 x1020 cm -2

Absorption features: RBS 1223Absorption features: RBS 1223 (Haberl et al., (Haberl et al., 2003)2003)

Eline 0.3 keV ; 100 eV , EW 150 eV B 5(1+z) x 1013 GP = 10.3 s; cooling age 5 x 105 yrsdP/dt P/2t 3 x 10-13 s/s Bdip 6 x1013 G

B consistent with what is required for a proton cyclotron line Line parameters (EW, sigma) consistent with models (Zane et al. 2001)

Energy (keV)

Co

un

ts/s

/keV

Scri

Absorption features: RX Absorption features: RX J1605.3+3249 (van J1605.3+3249 (van Kerkwijk et al., 2003)Kerkwijk et al., 2003)

Two gaussians:Eline 0.45 keV + a narrower marginally significant one at 0.55 keV

B 7(1+z) x 1013 GNo detected pulsations to a limit of 3% impossible to verify the B-field strength from timing measures

RGS spectrum of RX J1605.3+3249. Overdrawn is the best fit model: a slightly extincted blackbody with two Gaussian absorption features.

(Ǻ)

n (

ks-1

cm

-2 Ǻ

-1) RX J1605.3+3249

Absorption features and magnetic Absorption features and magnetic fields:fields:

SummarySummary

RX J1605:

The cyclotron line needs to be weaker at the pulse max to explain the observed correlation between hardness ratio/pulse maxVacuum polarization effects?

• Eline 0.3 keV B 5(1+z) x 1013 G• hardness ratio shifted in phase wrt pulse max

RX J0720:

• no P, Eline 0.45 keV B 7(1+z) x 1013 G

RBS 1223:

• no line yet, dP/dt = 1.4± 0.6 x 10-13 s/s B (2.8-4.2)x1013 G • hardness ratio shifted in phase wrt pulse max

RX J0420:• no dP/dt, no line yet • hardness ratio shifted in phase wrt pulse max

An hotter isolated neutron An hotter isolated neutron star:star:

1E1207: still radio-silent, but hottest and associated with a SNR

2 Multiple absorption features at ~0.7 and ~1.4 keV in Chandra and XMM data + 1 marginal feature at ~2 keV

1) Sanwal et al. 2002:

1E1207.4-52091E1207.4-5209

no cyclotron, no H atmosphere He atmosphere with B=1.5x1014 G

2) Mereghetti et al. 2002:

Fe or other high Z atmosphere with B1012 G

3) Hailey and Mori 2002:

He-like oxygen or neon with B1012 G

(the longest EPIC observation of a galactic (the longest EPIC observation of a galactic

source)source)

1E1207.4-5209: 257,303 s with XMM-Newton1E1207.4-5209: 257,303 s with XMM-Newton

• Data and best fitting continuum spectral model

3 Multiple absorption features: i. 0.72 ± 0.02 keVii. 1.37 ± 0.02 keViii. 2.11 ± 0.03 keV iv. less significant at 2.85 ± 0.06 keV

(Bignami et al., 2003, Nature)

(two bb at kT=0.211± 0.0001 keV and kT=0.40 ± 0.02 keV; NH = 1.0 ± 0.1 cm -2)

• Residuals in unit of standard deviations from the best-fitting continuum

MOS

PN

• P =0.424 s • dP/dt = 1.4 ± 0.3 10-14 s/s

Evidence of cyclotron absorption

• Proton cyclotron B 1.6 x 1014 G:

• Electron Cyclotron B 8 x 1010 G

Better agreement if:

TOO HIGH!

Additional breaking mechanisms (debris disk..);Cyclotron scattering at R ~3-4 stellar radii….

(but also t ~4.8 x 105 yrs, incompatible with that of the SNR < 104 yrs)

B (2-3) x 1012 G

1E1207.4-5209:1E1207.4-5209:

1E1207.4-5209:1E1207.4-5209: Lines vary in phaseLines vary in phase Comparison of 4 PN spectra at different phase intervals.

Residuals of the phase-dependent spectra from the two- blackbody continuum fit.

The peak of the total light curve corresponds to the phase-interval where absorption lines are at their minimum; Lines are more important at the light curve trough.

Phase

No

rm. I

nte

nsi

ty

Pea

k

Dec

line

Tro

ugh

Ris

e

Energy (keV)

Cou

nts/

s/ke

V

Pulsed phase spectroscopy Pulsed phase spectroscopy of proton cyclotron lines: of proton cyclotron lines: theory theory

1) Computing atmospheric models at different magnetic co-latitudes

2) Assuming surface temperature profile and B-field topology

3) Ray-tracking in the strong gravitational field.

+ +

=

4) Predicting spin variation of the line parameters!

GOAL: probe the surface properties of the neutron star via pulse-phase spectroscopy of cyclotron absorption lines

= 0˚ = 40˚ = 80˚

Zane, Turolla, Perna, Llyod, 2004 in prep.