Unidentified EGRET sources and the extragalactic gamma-ray background

  • Vasiliki Pavlidou
  • Jennifer M. Siegal-Gaskins
  • Carolyn Brown
  • Brian D. Fields
  • Angela V. Olinto
Conference paper


The large majority of EGRET point sources remain to this day without an identified low-energy counterpart. Whatever the nature of the EGRET unidentified sources, faint unresolved objects of the same class must have a contribution to the diffuse gamma-ray background: if most unidentified objects are extragalactic, faint unresolved sources of the same class contribute to the background, as a distinct extragalactic population; on the other hand, if most unidentified sources are Galactic, their counterparts in external galaxies will contribute to the unresolved emission from these systems. Understanding this component of the gamma-ray background, along with other guaranteed contributions from known sources, is essential in any attempt to use gamma-ray observations to constrain exotic high-energy physics. Here, we follow an empirical approach to estimate whether a potential contribution of unidentified sources to the extragalactic gamma-ray background is likely to be important, and we find that it is. Additionally, we comment on how the anticipated GLAST measurement of the diffuse gamma-ray background will change, depending on the nature of the majority of these sources.


Gamma rays: observations Gamma-ray sources: astronomical Radiation sources: unidentified 


95.85.Pw 98.70.Rz 98.70.-f 


Unable to display preview. Download preview PDF.

Unable to display preview. Download preview PDF.


  1. [Dar and De Rújula, 2001]
    Dar, A., De Rújula, A.: Is the diffuse gamma background radiation generated by galactic cosmic rays?. Mon. Not. Roy. Astron. Soc. 323 391–401 (2001) ADSCrossRefGoogle Scholar
  2. [Harding and Stecker, 1981]
    Harding, A.K., Stecker, F.W.: Pulsar and diffuse contributions to observed galactic gamma radiation. Nature 290, 316–318 (1981) ADSCrossRefGoogle Scholar
  3. [Hartman et al., 1999]
    Hartman, R.C., et al.: The third EGRET catalog of high-energy gamma-ray sources. Astrophys. J. Suppl. Ser. 123, 79–202 (1999) ADSCrossRefGoogle Scholar
  4. [Hunter et al., 1997]
    Hunter, S.D., et al.: EGRET observations of the diffuse gamma-ray emission from the galactic plane. Astrophys. J. 482, 205–240 (1997) ADSCrossRefGoogle Scholar
  5. [Keshet et al., 2004]
    Keshet, U., Waxman, E., Loeb, A.: The case for a low extragalactic gamma-ray background. J. Cosmol. Astropart. Phys. 04 006 (2004) ADSCrossRefGoogle Scholar
  6. [Lichti et al., 1978]
    Lichti, G.G., Bignami, G.F., Paul, J.A.: The gamma-ray luminosity of spiral galaxies—its evolution and its contribution to the diffuse background above 100 MeV. Astrophys. Space Sci. 56, 403–414 (1978) ADSCrossRefGoogle Scholar
  7. [Mücke and Pohl, 2000]
    Mücke, A., Pohl, M.: The contribution of unresolved radio-loud AGN to the extragalactic diffuse gamma-ray background. Mon. Not. Roy. Astron. Soc. 312 177–193 (2000) ADSCrossRefGoogle Scholar
  8. [Mukherjee and Chiang, 1999]
    Mukherjee, R., Chiang, J.: EGRET gamma-ray blazars: luminosity function and contribution to the extragalactic gamma-ray background. Astropart. Phys. 11, 213–215 (1999) ADSCrossRefGoogle Scholar
  9. [Narumoto and Totani, 2006]
    Narumoto, T., Totani, T.: Gamma-ray luminosity function of blazars and the cosmic gamma-ray background: evidence for the luminosity-dependent density evolution. Astrophys. J. 643, 81–91 (2006) ADSCrossRefGoogle Scholar
  10. [Pavlidou and Fields, 2002]
    Pavlidou, V., Fields, B.D.: The guaranteed gamma-ray background. Astrophys. J. 575, L5–L8 (2002) ADSCrossRefGoogle Scholar
  11. [Pohl et al., 1997]
    Pohl, M., Kanbach, G., Hunter, S.D., Jones, B.B.: The pulsar contribution to the diffuse galactic gamma-ray emission. Astrophys. J. 491, 159 (1997) ADSCrossRefGoogle Scholar
  12. [Reimer, 2005]
    Reimer, O.: High energy gamma-ray astronomy. AIP Conf. Proc. 745, 184 (2005) ADSCrossRefGoogle Scholar
  13. [Siegal-Gaskins et al., 2007]
    Siegal-Gaskins, J.M., Pavlidou, V., Olinto, A.V., Brown, C., Fields, B.D.: Population studies of the unidentified EGRET sources. Astrophys. Space Sci., doi: 10.1007/s10509-007-9468-x (2007), astro-ph/0611273
  14. [Sreekumar et al., 1998]
    Sreekumar, P., et al.: EGRET observations of the extragalactic gamma-ray emission. Astrophys. J. 494 523–534 (1998) ADSCrossRefGoogle Scholar
  15. [Stecker and Salamon, 1996]
    Stecker, F.W., Salamon, M.H.: The gamma-ray background from blazars: a new look. Astrophys. J. 464, 600–605 (1996) ADSCrossRefGoogle Scholar
  16. [Strong, 2007]
    Strong, A.W.: Source population synthesis and the galactic diffuse gamma-ray emission. Astrophys. Space Sci., doi: 10.1007/s10509-007-9480-1 (2007), astro-ph/0609359
  17. [Strong et al., 2004]
    Strong, A.W., Moskalenko, I.V., Reimer, O.: A new determination of the extragalactic diffuse gamma-ray background from EGRET data. Astrophys. J. 613 956–961 (2004) ADSCrossRefGoogle Scholar
  18. [Venters and Pavlidou, 2007]
    Venters, T.M., Pavlidou, V.: The spectral index distribution of EGRET blazars. Astrophys. J. (2007, submitted) Google Scholar
  19. [Zhang and Cheng, 1998]
    Zhang, L., Cheng, K.S.: The contribution to the galactic diffuse gamma-ray spectrum from unresolved rotation-powered pulsars. Mon. Not. Roy. Astron. Soc. 301, 841–848 (1998) ADSCrossRefGoogle Scholar

Copyright information

© Springer 2007

Authors and Affiliations

  • Vasiliki Pavlidou
    • 1
  • Jennifer M. Siegal-Gaskins
    • 1
  • Carolyn Brown
    • 1
  • Brian D. Fields
    • 2
  • Angela V. Olinto
    • 1
  1. 1.The University of ChicagoChicagoUSA
  2. 2.University of Illinois at Urbana-ChampaignUrbanaUSA

Personalised recommendations