Blazar distance indications from Fermi and TeV data: an empirical approach

1. Blazar distance indications from Fermi and TeV data: an empirical approach E. Prandini, Padova University & INFN G. Bonnoli, L. Maraschi, M. Mariotti and F. Tavecchio SciNeGHE, Trieste, September 2010

2. Outline • Blazarsphysics and Spectral Energy Distribution • Extragalactic Background Light • Combining GeV and TeV spectral information and EBL: a limit on blazar distance • TEST on known distance blazars • New empirical method to set blazarsz E. Prandini Blazars distance constraint

3. TeVBlazars Simplified SED model of emitted spectrum • The blazar emission isnon thermal and covers the entire e.m. spectrum • It is composed by two bumps: • Syncrotron emission • High energy emission log(energy density) eVkeVMeVGeVTeV log(E) Fermi TeV • Blazar:“radio loud” AGNs with the jet almost aligned to the line of sight of the observer E. Prandini Blazars distance constraint

4. The Extragalactic Background Light • EBL: light that fills the Universe emitted by stars and reprocessed by dust (Hauser & Dwek 2001) SED log(energy density) ebl eVkeVMeVGeVTeV log(E) • The blazar EMITTED spectrum is partially absorbed and deformed (at TeV instruments energy range) • The absorption is related to the DISTANCE of the source • Many blazars have unknown distance! E. Prandini Blazars distance constraint

5. Ingredients EBL Model Blazars emitted spectrum Blazars observed spectrum Blazars distance Blazars distance E. Prandini Blazars distance constraint

6. Limit on the distance: the idea Observed spectrum De-absorbed spectrum • Use the Fermi slope as limiting slope for the TeV de-absorbed spectrum in order to set alimit on the source distance:z* • With Fermi + TeV spectral points the second bump is resolved! E. Prandini Blazars distance constraint

7. Ref: (1) Abdo A. A. et al., 2009, ApJ, 707, 1310 The sample Spectral break 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) E. Prandini Blazars distance constraint

8. Results: z* VS z[true] 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) THIS TEST CONFIRMS THAT THE SLOPE MEASURED BY FERMI CAN BE USED AS A LIMIT ON THE VHE SLOPE FOR CONSTRAINING THE REDSHIFT • All the limits (z*) are above the bisector: method confirmed • Open points: uncertain redshift. This method suggests that these sources are peculiar or the zestimated are too large E. Prandini Blazars distance constraint

9. A step further: from limit to estimate • Linear fit are drawn in the figure (log-log scale) : • z* = A + B z[true] Ref (1): Stecker and Scully, 2010 ApJ709 L124 • Is there any relation among z* and z[true]? • FollowingStecker & Scully, 2010 (1): • linear expression for the steepening of the observed TeV slope • z* is also related to the steepening: LINEAR RELATION E. Prandini Blazars distance constraint

10. Reconstructedredshift • z[rec] = (z* - A)/B • We can use the fit to estimate the redshiftof the source (and not only a limit) • Method: • De-absorbTeV data according to the Fermi spectral measure • Estimate z* (and obtain a LIMIT on the distance) • Invert the linear relation and estimate z[rec] E. Prandini Blazars distance constraint

11. Test on known distance blazars • Residuals distribution z[true]-z[rec] (each source is excluded from the fit) • Sigma of the Gaussian fit: 0.05 • Uncertain redshiftsources are well outside the distribution... E. Prandini Blazars distance constraint

12. Application: the distance of PKS 1424+240 z[rec] = 0.24 ± 0.05 The 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+240 z*: 0.4 ± 0.1 E. Prandini Blazars distance constraint

13. 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 Blazars distance constraint

14. Conclusions • Combining Fermi and TeV spectra: themax hardness hypothesis is successfully tested on known redshiftsources LIMIT on z • Moreover: we found a linear relation between the zlimit (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) E. Prandini Blazars distance constraint

15. Outlook • Increase statistics (new TeV sources) • Use new EBL models • Use simultaneous Fermi-TeV data • Extend the z range (CTA?) THANKS! E. Prandini Blazars distance constraint

16. Backup E. Prandini Blazars distance constraint

17. The TeV extragalactic sky 42 sources (September 2010) E. Prandini Blazars distance constraint

18. Results: z* VS z[true] in LINEAR SCALE E. Prandini Blazars distance constraint

19. Application: the distance of PKS 1424+240 E. Prandini Blazars distance constraint

20. 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 E. Prandini Blazars distance constraint

21. Steepening of blazars spectra Stecker & Scully, 2010 E. Prandini Blazars distance constraint