DECIPHERING CONTRIBUTIONS TO THE EXTRAGALACTIC GAMMA-RAY BACKGROUND FROM 2 GeV TO 2 TeV

Abstract

Astrophysical sources outside the Milky Way, such as active galactic nuclei and star-forming galaxies, leave their imprint on the gamma-ray sky as nearly isotropic emission referred to as the extragalactic gamma-ray background (EGB). While the brightest of these sources may be individually resolved, their fainter counterparts contribute diffusely. In this work, we use a recently developed analysis method, called the Non-Poissonian Template Fit, on up to 93 months of publicly available data from the Fermi Large Area Telescope to determine the properties of the point sources (PSs) that comprise the EGB. This analysis takes advantage of photon-count statistics to probe the aggregate properties of these source populations below the sensitivity threshold of published catalogs. We measure the source-count distributions and PS intensities, as a function of energy, from ∼2 GeV to 2TeV. We find that the EGB is dominated by PSs, likely blazars, in all seven energy sub-bins considered. These results have implications for the interpretation of IceCube's PeV neutrinos, which may originate from sources that contribute to the nonblazar component of the EGB. Additionally, we comment on implications for future TeV observatories such as the Cherenkov Telescope Array. We provide sky maps showing locations most likely to contain these new sources at both low (50 GeV) and high (50 GeV) energies for use in future observations and cross-correlation studies.

Probing the Extragalactic Cosmic-Ray Origin with Gamma-Ray and Neutrino Backgrounds

“…It is compatible, with even more room for UHECRs, with the estimates of Lisanti et al (2016) for the PS contributions (∼54% and 68% of the EGB Model A around 2 GeV and above 50 GeV, respectively). Only very strong evolutions are excluded by the current observations.…”

Evidence against Star-forming Galaxies as the Dominant Source of Icecube Neutrinos

“…Taking uncertainties into account, the upper bound for the non-blazar fraction of the EGB is 28% (6.6 × 10 −10 ph cm −2 s −1 sr −1 ). Lisanti et al (2016) performed a similar NonPoissonian Template Fit (NPTF) of LAT data in the > 50 GeV energy range and found that point sources account for at least 68 +9 −8 % (±10% systematic uncertainty) of the total EGB intensity. The NPTF method loses sensitivity to sources below the single-photon limit, corresponding to a flux of ∼ 4 × 10 −12 ph cm −2 s −1 in the Lisanti et al (2016) analysis, and therefore represents a lower bound on the point-source contribution, as a realistic source population would include contributions from members of the same population significantly below that flux threshold.…”

“…Lisanti et al (2016) performed a similar NonPoissonian Template Fit (NPTF) of LAT data in the > 50 GeV energy range and found that point sources account for at least 68 +9 −8 % (±10% systematic uncertainty) of the total EGB intensity. The NPTF method loses sensitivity to sources below the single-photon limit, corresponding to a flux of ∼ 4 × 10 −12 ph cm −2 s −1 in the Lisanti et al (2016) analysis, and therefore represents a lower bound on the point-source contribution, as a realistic source population would include contributions from members of the same population significantly below that flux threshold. We note that the contribution of subthreshold sources inferred from the photon fluctuation analysis of Ackermann et al (2016b) is consistent with expectations based on blazar luminosity functions Di Mauro et al 2014b;.…”

Tomographic Imaging of the Fermi-LAT γ-Ray Sky through Cross-correlations: A Wider and Deeper Look

“…Among these we can list anisotropy analyses (Ando & Komatsu 2006Ando 2009;Ackermann et al 2012c;Cuoco et al 2012;Harding & Abazajian 2012;Fornasa et al 2013Fornasa et al , 2016Di Mauro et al 2014b;, pixel statistic analyses (Dodelson et al 2009;Malyshev & Hogg 2011;Feyereisen et al 2015;Lisanti et al 2016;Zechlin et al 2016aZechlin et al , 2016b, and cross-correlations with tracers of the large-scale structure of the universe (Ando 2014;Ando et al 2014;Fornengo & Regis 2014;Shirasaki et al 2014;Camera et al 2015;Cuoco et al 2015;Fornengo et al 2015;Regis et al 2015;Xia et al 2015;Feng et al 2017;Shirasaki et al 2016;Tröster et al 2017), which we will investigate in the following.…”