Contribution from unresolved discrete sources to the extragalactic gamma-ray background (EGRB).pdf

Research in Astron．As~ophys．2009 Vo1．9 No．11，1205-1214 http：／／www．raa-journa1．org htp：／／www．iop．org／journals／raa Contribution from unresolv discrete sources to gamma-ray background(EGRB) Researchin Astronomy and Astrophysics the extragalactic Debbijoy Bhatacharya ．Parameswaran Sreekumar and Reshmi Mukherjee3 1 Department of Physics， Indian Institute of Science，Bangalore 560012，India； debbijoy@physics．isc．ernet．in 2 Space Astronomy Instrumentation Division，ISRO Satellite Centre，Bangalore 560017，India 3 Bamard College，Columbia University，USA Received 2009 May 9：accepted 2009 June 1 2 Abstract The origin of the extragalactic gamma．ray background(EGRB1 is still an open question，even nearly forty years after its discovery．The emission could originate either from truly diffuse processes or from unresolved point sources．A1though the maiority of the 27 1 point sources detected by EGRET(Energetic Gamma Ray Experiment Telescope) are unidentified，of the identified sources，blazars are the dominant candidates．Therefore， unresolved blazars may be considered the main contributor to the EGRB．and many stud ies have been carried out to understand their distribution，evolution and contribution to the EGRB．Considering that-y—ray emission comes mostly from iets of blazars and that the iet emission decreases rapidly with increasing jet to line—of-sight angle，it is not surprising tl1at EGRET was not able to detect many 1arge inclination angle active galactic nuclei (AGNs)．Though Fermi could only detect a few large inclination angle AGNs during the first three months of its survey，it is expected to detect many such sources in the near fu． ture．Since non．blazar AGNs are expected to have higher density as compared to blazars． these could also contribute significantly to the EGRB．In this Paper，we discuss contri— butions from unresolved discrete sources including normal galaxies，starburst galaxies， blazars and off-axis AGNs to the EGRB． Key words：galaxies：active—galaxies：jets— gamma-ray