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Takuro Narumoto (Department of Astronomy, Kyoto Univ.) Tomonori Totani

3rd Workshop on the Nature of Unidentified High Energy Sources@Barcelona 4 th July 2006. Gamma-Ray Luminosity Function of Blazars and the Cosmic Gamma-Ray Background: Evidence for the Luminosity-Dependent Density Evolution. Takuro Narumoto (Department of Astronomy, Kyoto Univ.)

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Takuro Narumoto (Department of Astronomy, Kyoto Univ.) Tomonori Totani

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  1. 3rd Workshop on the Nature of Unidentified High Energy Sources@Barcelona 4th July 2006 Gamma-Ray Luminosity Function of Blazars and the Cosmic Gamma-Ray Background: Evidence for the Luminosity-Dependent Density Evolution Takuro Narumoto (Department of Astronomy, Kyoto Univ.) Tomonori Totani (Department of Astronomy, Kyoto Univ.) T. Narumoto & T. Totani, 2006, ApJ, 643, 81

  2. 1. INTRODUCTION

  3. Extragalactic Gamma-Ray Background (EGRB) EGRET confirmed the presence of the extragalactic gamma-ray background (EGRB) EGRET Strong et al. (2004) However, the origin of the EGRB is still an open problem blazars? (e.g., Stecker & Salamon 1996; Chiang & Mukherjee 1998) galaxy clusters? (e.g., Loeb & Waxman 2000; Totani & Kitayama 2000) dark matter annihilation?(e.g., Oda, Totani, & Nagashima 2005)

  4. Gamma-Ray Luminosity Function of Blazar and the EGRB • Most of the identified extragalactic EGRET sources are blazars • blazars are the most likely candidate for the origin of the EGRB however, the gamma-ray luminosity function (GLF) of blazars and its cosmological evolution are poorly understood the estimate of the blazar contribution is highly uncertain SS96 (Stecker & Salamon 1996) blazar contribution to the EGRB is~ 100% problem : overpredict the number of low-redshift blazars CM98 (Chiang & Mukherjee 1998) blazar contribution to the EGRB is only ~ 25% Earlier studies treated the cosmological evolution of the blazar GLF as a Pure Luminosity Evolution (PLE)

  5. Cosmological Evolution of the AGN X-Ray Luminosity Function (XLF) On the other hand, the cosmological evolution of the luminosity function of AGNs has been investigated in various wavelengths low luminosity Peak redshift of the density evolution increases with AGN luminosity (e.g., Ueda et al. 2003; Hasinger et al. 2005) Number density high luminosity Ueda et al. (2003) Redshift Evolutionary nature of the AGN XLF is best described by the Luminosity-Dependent Density Evolution (LDDE)

  6. Radio Detectability for Blazar Identification • Most of the EGRET blazars are identified by finding radio counterparts, and they would remain unidentified if their radio counterparts are under the flux limit of radio surveys identification probability must be included in the analysis this probability was included in CM98, but it was calculated by assuming no correlation between gamma-ray and radio luminosities of blazars we assume the gamma-ray and radio luminosity correlation based on the observations

  7. In this study… • For the first time, in addition to the PLE, we introduce the LDDE into the blazar GLF and perform the likelihood analysis for the redshift and luminosity distribution of the EGRET blazars • In the likelihood analysis, we introduce the gamma-ray and radio luminosity correlation with a modest dispersion which is consistent with observations to calculate the radio detectability • Then, we examine the blazar contribution to the EGRB and address the prospects for the GLAST mission

  8. 2. BLAZAR GLF MODELS

  9. PLE Model • We derive the blazar GLF from the flat-spectrum radio-loud quasar (FSRQ) radio luminosity function (RLF) by assuming a linear relation between the gamma-ray and radio luminosities of blazars • Since the faint-end slope index is poorly constrained, we take it as a free parameter • and are constrained by likelihood analysis normalization factor

  10. LDDE Model • We construct the blazar GLF based on the AGN XLF by assuming a linear relation between the blazar gamma-ray luminosity and the AGN X-ray luminosity • Since the faint-end slope index is poorly constrained, we take it as a free parameter • and are constrained by likelihood analysis X-ray luminosity of normal AGNs (not blazars) normalization factor « 1

  11. 3. RESULTS

  12. Constraints from Likelihood Analysis PLE model Best-fit parameters is quite similar to the value obtained directly from the EGRET blazars ( ) is smaller (i.e., flatter faint-end slope) than that of the FSRQ RLF ( )

  13. Constraints from Likelihood Analysis LDDE model Best-fit parameters is a little larger (i.e., steeper faint-end slope) than the value inferred from the AGN XLF ( ), but the AGN XLF value is within the 95% CL contour faint-end slope of the AGN XLF

  14. Redshift and Luminosity Distribution of the EGRET Blazars KS probability (luminosity) LDDE model : 99.3% PLE model : 27.0% The LDDE model can explain the redshift and luminosity distribution of the EGRET blazars better than the PLE model KS probability (redshift) LDDE model : 67.8% PLE model : 3.1%

  15. Blazar Contribution to the EGRB (PLE Model) Best-fit PLE model can explain only 50~55% of the EGRB Steep faint-end slope (within the 68% CL contour) can explain 100% of the EGRB However, since the PLE model poorly fits the observed data, it is not appropriate to derive any conclusion

  16. Blazar Contribution to the EGRB (LDDE Model) Best-fit LDDE model can explain only 25~50% of the EGRB Steep faint-end slope (within the 68% CL contour) can explain 100% of the EGRB faint-end slope of the AGN XLF However, such a steep faint-end slope is not favored from the AGN XLF

  17. 4. PREDICTIONS FOR THE GLAST MISSION

  18. Expected Number of GLAST Blazars The Number of blazars detectable by GLAST is ~ 3000 : best-fit LDDE model ~ 5250 : best-fit PLE model ~ 10000 : SS96 model strongly dependent on the blazar GLF models The LDDE model predicts three times fewer blazars than the previous estimate Blazar GLF and its evolution can be constrained from the number count of GLAST blazars

  19. Contribution of GLAST Blazarsto the EGRB There are two peaks of the contribution to the EGRB as a function of flux, and major contribution comes from blazars under the GLAST detection limit The contribution to the EGRB decreses with decreasing flux just below the EGRET sensitivity limit thick lines : best-fit models thin lines : models that can explain 100% of the EGRB (with steeper faint-end slope) The resolvable fraction of the EGRB by GLAST blazars is ~ 20% (best-fit LDDE) ~ 26% (LDDE with steeper faint-end slope) ~ 33% (best-fit PLE) ~ 42% (PLE with steeper faint-end slope)

  20. SUMMARY • The LDDE model can explain the redshift and luminosity distribution of the EGRET blazars better than the PLE model • Only 25%~50% of the EGRB can be explained by the best-fit LDDE model • 100% of the EGRB can be explained by the LDDE model with steeper faint-end slope, but such a model is not favored from the AGN XLF • The LDDE model predicts considerably fewer (by a factor of more than 3) blazars down to the GLAST sensitivity limit, compared with the previous estimate • Based on the LDDE model, the contribution to the EGRB will decrease with decreasing flux just below the EGRET sensitivity limit

  21. Redshift and Luminosity Distribution of the GLAST Blazars Distributions of the LDDE model are wider than those of the other models

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