tev blazars and their distance e. prandini, padova university & infn g. bonnoli, l. maraschi, m....
TRANSCRIPT
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TeV blazars and their distance
E. Prandini, Padova University & INFN
G. Bonnoli, L. Maraschi, M. Mariotti and F. Tavecchio
Cosmic Radiation Fields - Sources in the early UniverseHamburg 9-12 November 2010
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E. Prandini TeV blazars and their distance CRF2010, Hamburg2
Outline
TeV blazars and EBL
Combined GeV+TeV spectra:
TEST the max slope hypothesis
Use it to set u.l. on the distance of unknown z sources
Propose an empirical method to estimate blazars z
Update: new data
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E. Prandini TeV blazars and their distance CRF2010, Hamburg3
TeV Blazars
Blazars: “radio loud” AGNs with the jet almost aligned to the line of sight of the observer
log(E)
log(
ener
gy d
ensi
ty)
eV keV MeV GeV TeV
Simplified SED model of emitted spectrum of TeV blazars
Fermi TeV
The blazar emission is non thermal and covers the entire e.m. spectrum
It is composed by two bumps:
Syncrotron emission High energy emission
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E. Prandini TeV blazars and their distance CRF2010, Hamburg4
The effect of EBL on blazars spectra
The blazar EMITTED spectrum is partially absorbed and deformed (at TeV instruments energy range)
The absorption is related to the DISTANCE of the source
BUT: some blazars have unknown distance!
log(E)
log(
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gy d
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ty)
eV keV MeV GeV TeV
SED
EBL: the EMITTED spectrum is deformed
Hauser & Dwek (2001)
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E. Prandini TeV blazars and their distance CRF2010, Hamburg5
Constraints from the absorption
log(E)
log(
ener
gy d
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eV keV MeV GeV TeV
SED
EBL model
Hypotheses on the intrinsic spectrum
Blazar distance
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E. Prandini TeV blazars and their distance CRF2010, Hamburg6
Constraint on the distance
With Fermi + TeV spectral points the second bump is resolved!
Use the Fermi slope as limiting slope for the TeV de-absorbed spectrum in order to set a limit on the source distance: z*
Observed spectrum De-absorbed spectrum
Abdo et al. (2010)
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E. Prandini TeV blazars and their distance CRF2010, Hamburg7
The sample
Fermi TeV sources (from Abdo et al. 2009) + TeV spectra from last generation of Cherenkov Telescopes (Magic, Veritas, H.E.S.S.)
14 sources with well known redshift
2 sources of uncertain redshift (S5 0716+714 and 3C 66A)
Spectral break
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E. Prandini TeV blazars and their distance CRF2010, Hamburg8
Results: z* VS z[true]
All the limits (z*) are above the bisector
Open points: uncertain redshift: the only values below the bisector!
THIS TEST CONFIRMS THAT THE SLOPE MEASURED BY FERMI CAN BE USED AS A LIMIT ON THE VHE SLOPE FOR
CONSTRAINING THE REDSHIFT
bisector
Uncertain redshift
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E. Prandini TeV blazars and their distance CRF2010, Hamburg9
A step further: from limit to estimate
Is there any relation among z* and z[true]?
Following Stecker & Scully, 2010 (1): linear expression for
the steepening of the observed TeV slope
z* is also related to the steepening: LINEAR RELATION
Linear fit:
z* = A + B z[true]Ref (1): Stecker and
Scully, 2010 ApJ 709 L124
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E. Prandini TeV blazars and their distance CRF2010, Hamburg10
Reconstructed redshift
We can use the fit to ESTIMATE THE REDSHIFT of a source (and not only a limit)
Method: De-absorb TeV data according to the Fermi
spectral measure Estimate z* (and obtain a LIMIT on the distance) Invert the linear relation and estimate z[rec]
z[rec] = (z* - A)/B
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Test on known distance blazars
E. Prandini TeV blazars and their distance CRF2010, Hamburg
Residuals distribution z[true]-z[rec]
Sigma of the Gaussian fit: 0.05
Uncertain redshift sources are well outside the distribution...
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Comparison between different EBL Models
Similar results with extreme EBL models: Low EBL: Kneinske & Dole 2010
Mean EBL: Franceschini et al. 2008
High EBL: Stecker et al. 2006
E. Prandini TeV blazars and their distance CRF2010, Hamburg
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E. Prandini TeV blazars and their distance CRF2010, Hamburg13\
Conclusions of the Preliminary study
Combining Fermi and TeV spectra: the max hardness hypothesis is successfully tested on known redshift sources LIMIT on z
Moreover: we found a linear relation between the z limit (z*) and the real redshift (z[true]), that can be inverted and used for the estimate of the unknown distance of blazars, z[rec]. The result is EBL model independent!
According to our results, the “uncertain” z attributed to 3C66A and S5 0716+714 is largely overestimated or these sources are peculiar
More details in: E. Prandini et al. “Constraining blazar distances with combined Fermi and TeV data: an empirical approach”, MNRAS 405, L76-L80 (2010)
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E. Prandini TeV blazars and their distance CRF2010, Hamburg14\
Update
Since last year:
New Fermi catalogue (1 y)
New TeV sources
New EBL model (Dominguez et al. 2010)
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E. Prandini TeV blazars and their distance CRF2010, Hamburg15\
The updated sample
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E. Prandini TeV blazars and their distance CRF2010, Hamburg16\
Results: Franceschini & Dominguez models
Confirm previous results
The two EBL models give very similar results
Prandini et al. , in prep.
Prandini et al. , in prep.
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Application: the distance of PKS 1424+240
E. Prandini TeV blazars and their distance CRF2010, Hamburg
estimate on the distance of unknown redshift sources observed at both TeV and GeV ranges: PKS 1424+240
Deabsorbed spectrum of PKS 1424+240 (Fermi 1y catal., Franceschini et al. EBL model)
z* = 0.45 ± 0.15
z[rec] = 0.26 ± 0.06
Prandini et al. , in prep.
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Conclusions… The updated work basically
confirms the results
Fermi slope as limiting slope for TeV blazars
Empirical law allows an estimate on blazars distance
Increase statistics (new TeV sources)
Use simultaneous Fermi - TeV data
Extend the z range (CTA?)
… & Outlook
Newly detected TeV extragalactic objects:
E. Prandini TeV blazars and their distance CRF2010, Hamburg
Danke!
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E. Prandini TeV blazars and their distance CRF2010, Hamburg19\
Backup slides
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E. Prandini TeV blazars and their distance CRF2010, Hamburg20
The TeV extragalactic sky
45 sources (November 2010)
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E. Prandini TeV blazars and their distance CRF2010, Hamburg21
Assumptions
Not simultaneous GeV-TeV observations
But slopes seem somehow less variable than the corresponding fluxes (es. 1ES 1218+304 Veritas Coll.)
Different TeV instruments, sensitivities and energy thresholds (systematics?)
IC peak position: could be different
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E. Prandini TeV blazars and their distance CRF2010, Hamburg22
Results: z* VS z[true] in LINEAR SCALE
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E. Prandini TeV blazars and their distance CRF2010, Hamburg23
Application: the distance of PKS 1424+240
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E. Prandini TeV blazars and their distance CRF2010, Hamburg24
Steepening of blazars spectra
Stecker & Scully, 2010
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E. Prandini TeV blazars and their distance CRF2010, Hamburg25\
Results: Linear Relation
Linear releation still fits well!
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E. Prandini TeV blazars and their distance CRF2010, Hamburg26\
z*: updated values
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E. Prandini TeV blazars and their distance CRF2010, Hamburg27\
z[rec]