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Register a new userThe $gamma$-ray sky seen at X-ray energies I. Searching for the connection between X-rays and $gamma$-rays in Fermi BL Lac objects
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BL Lac objects are an extreme type of active galactic nuclei (AGNs) that belong to the largest population of $gamma$-ray sources: blazars. This class of AGNs shows a double-bumped spectral energy distribution that is commonly described in terms of a synchrotron self-Compton (SSC) emission process, whereas the low-energy component that dominates their emission between the infrared and the X-ray band is tightly connected to the high-energy component that peaks in the $gamma$-rays. Two strong connections that link radio and mid-infrared emission of blazars to the emission in the $gamma$-ray band are well established. They constitute the basis for associating $gamma$-ray sources with their low-energy counterparts. We searched for a possible link between X-ray and $gamma$-ray emissions for the subclass of BL Lacs using all archival Swift/XRT observations combined with Fermi data for a selected sample of 351 sources. Analyzing $sim$2400 ks of Swift/XRT observations that were carried out until December 2018, we discovered that above the $gamma$-ray flux threshold $F_{gamma}approx3times10^{-12},rm{erg},rm{cm}^{-2},rm{s}^{-1}$, 96% of all emph{Fermi} BL Lacs have an X-ray counterpart that is detected with signal-to-noise ratio higher than 3. We did not find any correlation or clear trend between X-ray and $gamma$-ray fluxes and/or spectral shapes, but we discovered a correlation between the X-ray flux and the mid-infrared color. Finally, we discuss on a possible interpretation of our results in the SSC framework.
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We present the spectral signatures of the Bethe-Heitler pair production ($pe$) process on the spectral energy distribution (SED) of blazars, in scenarios where the hard $gamma$-ray emission is of photohadronic origin. If relativistic protons interact with the synchrotron blazar photons producing $gamma$ rays through photopion processes, we show that, besides the $2-20$ PeV neutrino emission, the typical blazar SED should have an emission feature due to the synchrotron emission of $pe$ secondaries that bridges the gap betweeen the low-and high-energy humps of the SED, namely in the energy range 40 keV-40 MeV. We first present analytical expressions for the photopion and $pe$ loss rates in terms of observable quantities of blazar emission. For the $pe$ loss rate in particular, we derive a new approximate analytical expression for the case of a power-law photon distribution, which has an excellent accuracy with the numerically calculated exact one, especially at energies above the threshold for pair production. We show that for typical blazar parameters, the photopair synchrotron emission emerges in the hard X-ray/soft $gamma$-ray energy range with a characteristic spectral shape and non negligible flux, which may be even comparable to the hard $gamma$-ray flux produced through photopion processes. We argue that the expected $pe$ bumps are a natural consequence of leptohadronic models, and as such, they may indicate that blazars with a three-hump SED are possible emitters of high-energy neutrinos.
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.
The complex interplay of processes at the Galactic Center is at the heart of numerous past, present, and (likely) future mysteries. We aim at a more complete understanding of how spectra extending to >10 TeV result. We first construct a simplified model to account for the peculiar energy and angular dependence of the intense central parsec photon field. This allows for calculating anisotropic inverse Compton scattering and mapping gamma-ray extinction due to gamma gamma -> e^+ e^- attenuation. Coupling these with a method for evolving electron spectra, we examine several clear and present excesses, including the diffuse hard X-rays seen by NuSTAR and GeV gamma rays by Fermi. We address further applications to cosmic rays, dark matter, neutrinos, and gamma rays from the Center and beyond.
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In this paper, we compile the very-high-energy and high-energy spectral indices of 43 BL Lac objects from the literature. Based on a simple math model, $DeltaGamma_{obs}=alpha {rm{z}}+beta $, we present evidence for the origin of an observed spectral break that is denoted by the difference between the observed very-high-energy and high-energy spectral indices, $DeltaGamma_{obs}$. We find by linear regression analysis that $alpha e 0$ and $beta e 0$. These results suggest that the extragalactic background light attenuation and the intrinsic curvature dominate on the GeV-TeV $gamma$-ray energy spectral break of BL Lac objects. We argue that the extragalactic background light attenuation is an exclusive explanation for the redshift evolution of the observed spectral break.
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