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Register a new userDiffuse $gamma$-ray emission from misaligned active galactic nuclei
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Active galactic nuclei (AGN) with jets seen at small viewing angles are the most luminous and abundant objects in the $gamma$-ray sky. AGN with jets misaligned along the line-of-sight appear fainter in the sky, but are more numerous than the brighter blazars. We calculate the diffuse $gamma$-ray emission due to the population of misaligned AGN (MAGN) unresolved by the Large Area Telescope (LAT) on the {it Fermi} Gamma-ray Space Telescope ({it Fermi}). A correlation between the $gamma$-ray luminosity and the radio-core luminosity is established and demonstrated to be physical by statistical tests, as well as compatible with upper limits based on {it Fermi}-LAT data for a large sample of radio-loud MAGN. We constrain the derived $gamma$-ray luminosity function by means of the source count distribution of the radio galaxies (RGs) detected by the {it Fermi}-LAT. We finally calculate the diffuse $gamma$-ray flux due to the whole MAGN population. Our results demonstrate that the MAGN can contribute from 10% up to nearly the entire measured Isotropic Gamma-Ray Background (IGRB). We evaluate a theoretical uncertainty on the flux of almost an order of magnitude.
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We calculate the diffuse $gamma$-ray emission due to the population of misaligned AGN (MAGN) unresolved by the Large Area Telescope (LAT) on the {it Fermi} Gamma-ray Space Telescope ({it Fermi}). A correlation between the $gamma$-ray luminosity and the radio-core luminosity is established and demonstrated to be physical by statistical tests, as well as compatible with upper limits based on {it Fermi}-LAT data for a large sample of radio-loud MAGN. We constrain the derived $gamma$-ray luminosity function by means of the source count distribution of the MAGN detected by the {it Fermi}-LAT. We finally estimate the diffuse $gamma$-ray flux due to the whole MAGN population which ranges from 10% up to nearly the entire measured Isotropic Gamma-Ray Background (IGRB). We evaluate also the room left to galactic DM at high latitudes ($>10^circ$), by taking into account the results on the MAGN together with the other significant galactic and extragalactic $gamma$-rays emitting sources.
Millisecond Pulsars are second most abundant source population discovered by the Fermi-LAT. They might contribute non-negligibly to the diffuse emission measured at high latitudes by Fermi-LAT, the IDGRB. Gamma-ray sources also contribute to the anisotropy of the IDGRB measured on small scales by Fermi-LAT. We aim to assess the contribution of the unresolved counterpart of the detected MSPs population to the IDGRB and the maximal fraction of the measured anisotropy produced by this source class. We model the MSPs spatial distribution in the Galaxy and the gamma-ray emission parameters by considering radio and gamma-ray observational constraints. By simulating a large number of MSPs populations, we compute the average diffuse emission and the anisotropy 1-sigma upper limit. The emission from unresolved MSPs at 2 GeV, where the peak of the spectrum is located, is at most 0.9% of the measured IDGRB above 10 degrees in latitude. The 1-sigma upper limit on the angular power for unresolved MSP sources turns out to be about a factor of 60 smaller than Fermi-LAT measurements above 30 degrees. Our results indicate that this galactic source class represents a negligible contributor to the high-latitude gamma-ray sky and confirm that most of the intensity and geometrical properties of the measured diffuse emission are imputable to other extragalactic source classes. Nevertheless, given the MSP distribution, we expect them to contribute significantly to the gamma-ray diffuse emission at low latitudes. Since, along the galactic disk, the population of young Pulsars overcomes in number the one of MSPs, we compute the gamma-ray emission from the whole population of unresolved Pulsars in two low-latitude regions: the inner Galaxy and the galactic center.
Blazars represent the most abundant class of high-energy extragalactic $gamma$-ray sources. The subset of blazars known as BL Lac objects is on average closer to Earth and characterized by harder spectra at high energy than the whole sample. The fraction of BL Lacs that is too dim to be detected and resolved by current $gamma$-ray telescopes is therefore expected to contribute to the high-energy isotropic diffuse $gamma$-ray background (IGRB). The IGRB has been recently measured over a wide energy range by the Large Area Telescope (LAT) on board the Gamma-ray Space Telescope ({it Fermi}). We present a new prediction of the diffuse $gamma$-ray flux due to the unresolved BL Lac blazar population. The model is built upon the spectral energy distribution and the luminosity function derived from the fraction of BL Lacs detected (and spectrally characterized) in the $gamma$-ray energy range. We focus our attention on the ${cal O}(100)$ GeV energy range, predicting the emission up to the TeV scale and taking into account the absorption on the extragalactic background light. In order to better shape the BL Lac spectral energy distribution, we combine the {it Fermi}-LAT data with Imaging Atmospheric Cerenkov Telescopes measurements of the most energetic sources. Our analysis is carried on separately for low- and intermediate-synchrotron-peaked BL Lacs on one hand, and high-synchrotron-peaked BL Lacs on the other one: we find in fact statistically different features for the two. The diffuse emission from the sum of both BL Lac classes increases from about 10$%$ of the measured IGRB at 100 MeV to $sim$100$%$ of the data level at 100 GeV. At energies greater than 100 GeV, our predictions naturally explain the IGRB data, accommodating their softening with increasing energy. Uncertainties are estimated to be within of a factor of two of the best-fit flux up to 500 GeV.
Diffuse $gamma$-ray emission is the most prominent observable signature of celestial cosmic-ray interactions at high energies. While already being investigated at GeV energies over several decades, assessments of diffuse $gamma$-ray emission at TeV energies remain sparse. After completion of the systematic survey of the inner Galaxy, the H.E.S.S. experiment is in a prime position to observe large-scale diffuse emission at TeV energies. Data of the H.E.S.S. Galactic Plane Survey are investigated in regions off known $gamma$-ray sources. Corresponding $gamma$-ray flux measurements were made over an extensive grid of celestial locations. Longitudinal and latitudinal profiles of the observed $gamma$-ray fluxes show characteristic excess emission not attributable to known $gamma$-ray sources. For the first time large-scale $gamma$-ray emission along the Galactic Plane using imaging atmospheric Cherenkov telescopes has been observed. While the background subtraction technique limits the ability to recover modest variation on the scale of the H.E.S.S. field of view or larger, which is characteristic of the inverse Compton scatter-induced Galactic diffuse emission, contributions of neutral pion decay as well as emission from unresolved $gamma$-ray sources can be recovered in the observed signal to a large fraction. Calculations show that the minimum $gamma$-ray emission from $pi^0$-decay represents a significant contribution to the total signal. This detection is interpreted as a mix of diffuse Galactic $gamma$-ray emission and unresolved sources.
Intergalactic magnetic fields (IGMF) can cause the appearance of halos around the gamma-ray images of distant objects because an electromagnetic cascade initiated by a high-energy gamma-ray interaction with the photon background is broadened by magnetic deflections. We report evidence of such gamma-ray halos in the stacked images of the 170 brightest active galactic nuclei (AGN) in the 11-month source catalog of the Fermi Gamma-Ray Space Telescope. Excess over point spread function in the surface brightness profile is statistically significant at 3.5sigma (99.95% confidence level), for the nearby, hard population of AGN. The halo size and brightness are consistent with IGMF, B_{IGMF} ~ 10^{-15} G. The knowledge of IGMF will facilitate the future gamma-ray and charged-particle astronomy. Furthermore, since IGMF are likely to originate from the primordial seed fields created shortly after the Big Bang, this potentially opens a new window on the origin of cosmological magnetic fields, inflation, and the phase transitions in the early Universe.
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