Constraining blazars distance with gamma-rays

Blazar distance indications from Fermi and TeV data: an empirical approachE. Prandini, Padova University & INFN G. Bonnoli, L. Maraschi, M. Mariotti and F. Tavecchio

SciNeGHE, Trieste, September 2010

OutlineBlazars physics and Spectral Energy DistributionExtragalactic Background LightCombining GeV and TeV spectral information and EBL: a limit on blazar distanceTEST on known distance blazarsNew empirical method to set blazars z2E. Prandini Blazars distance constraint2TeV BlazarsBlazar: radio loud AGNs with the jet almost aligned to the line of sight of the observer

3E. Prandini Blazars distance constraintlog(E)log(energy density)eV keV MeV GeV TeVSimplified SED model of emitted spectrumFermiTeVThe blazar emission is non thermal and covers the entire e.m. spectrum It is composed by two bumps:Syncrotron emissionHigh energy emission3The Extragalactic Background LightThe blazar EMITTED spectrum is partially absorbed and deformed (at TeV instruments energy range)The absorption is related to the DISTANCE of the sourceMany blazars have unknown distance!

4E. Prandini Blazars distance constraint

log(E)log(energy density)eV keV MeV GeV TeVSEDeblEBL: light that fills the Universe emitted by stars and reprocessed by dust (Hauser & Dwek 2001)

4

Blazars observed spectrumIngredientsE. Prandini Blazars distance constraint

5EBL Model

Blazars emitted spectrumBlazars distanceBlazars distance

Limit on the distance: the ideaWith Fermi + TeV spectral points the second bump is resolved!

6E. Prandini Blazars distance constraint

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 spectrumDe-absorbed spectrum6The sample Fermi TeV sources (1) + 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)

7E. Prandini Blazars distance constraintRef: (1) Abdo A. A. et al., 2009, ApJ, 707, 1310

Spectral break7Results: z* VS z[true]All the limits (z*) are above the bisector: method confirmed

Open points: uncertain redshift. This method suggests that these sources are peculiar or the z estimated are too large8E. Prandini Blazars distance constraint

THIS TEST CONFIRMS THAT THE SLOPE MEASURED BY FERMI CAN BE USED AS A LIMIT ON THE VHE SLOPE FOR CONSTRAINING THE REDSHIFT z*: at this redshift the INDEX of the power law fit of the DEABSORBED SPECTRUM (Franceschini et al. 2008 EBL model) equals the index measured by Fermi/LAT (5.5 months)

8A step further: from limit to estimateIs there any relation among z* and z[true]?

Following Stecker & Scully, 2010 (1):linear expression for the steepening of the observed TeV slopez* is also related to the steepening: LINEAR RELATION

9E. Prandini Blazars distance constraint

Linear fit are drawn in the figure (log-log scale) :

z* = A + B z[true]Ref (1): Stecker and Scully, 2010 ApJ 709 L1249Reconstructed redshiftWe can use the fit to estimate the redshift of the source (and not only a limit)Method:De-absorb TeV data according to the Fermi spectral measureEstimate z* (and obtain a LIMIT on the distance)Invert the linear relation and estimate z[rec]

10E. Prandini Blazars distance constraint

z[rec] = (z* - A)/B10Test on known distance blazarsResiduals distribution z[true]-z[rec] (each source is excluded from the fit)Sigma of the Gaussian fit: 0.05Uncertain redshift sources are well outside the distribution...

11E. Prandini Blazars distance constraint11Application: the distance of PKS 1424+240The natural application of this work is to give an estimate on the distance of unknown redshift sources observed at both TeV and GeV ranges: es. PKS 1424+240z*: 0.4 0.1

E. Prandini Blazars distance constraint12z[rec] = 0.24 0.05Comparison between different EBL ModelsSimilar results with extreme EBL models:Low EBL: Kneinske & Dole 2010Mean EBL: Franceschini et al. 2008High EBL: Stecker et al. 2006

13E. Prandini Blazars distance constraint

13ConclusionsCombining 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 PKS 1424+240 --> z[rec] = 0.24 0.05 More details in: E. Prandini et al. Constraining blazar distances with combined Fermi and TeV data: an empirical approach, MNRAS 405, L76-L80 (2010)14E. Prandini Blazars distance constraint14OutlookIncrease statistics (new TeV sources)Use new EBL modelsUse simultaneous Fermi-TeV dataExtend the z range (CTA?)E. Prandini Blazars distance constraint15THANKS!

BackupE. Prandini Blazars distance constraint16The TeV extragalactic sky17E. Prandini Blazars distance constraint

42 sources (September 2010)17Results: z* VS z[true] in LINEAR SCALE18E. Prandini Blazars distance constraint

18Application: the distance of PKS 1424+240E. Prandini Blazars distance constraint19

Assumptions Not simultaneous GeV-TeV observationsBut 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

E. Prandini Blazars distance constraint20Steepening of blazars spectra

E. Prandini Blazars distance constraint21Stecker & Scully, 2010

Log Energy [GeV]210

]-1 T

eV-1

s-2

dN/d

E[ph

ot c

m

-1210

-1110

-1010

Deabsorbed spectrum source PKS 1424+240 VERITAS 2009 at redshift 0.382

Fermi Slope (0.2-300GeV)= 1.85 +/- 0.05Real Redshift unknown