Infrared emission from dust and the spectral features of extragalactic gamma-ray sources

Abstract

In this work, we investigate the role of emission by dust at infrared wavelengths in the absorption of gamma radiation from distant extragalactic sources, especially blazars. We use an existing EBL model based on direct starlight emission at UV/visible and secondary radiation due to dust (PAHs (polycyclic aromatic hydrocarbons), small and large grains) at IR due to partial absorption of the stellar component. The relative contribution of each grain type to the total EBL energy density was determined from a combined fit to the Markarian 501 (z∼ 0.034) SED in flare state, where both the parameters of the intrinsic source spectrum (with or without curvature) and the dust fractions were allowed to vary. By separating the attenuation due to each EBL component, the importance of individual grain types to the opacity of the extragalactic medium for the TeV emission of a blazar like Markarian 501 could be better understood. Using a nested log-likelihood ratio test, we compared null hypotheses represented by effective 1- and 2-grain models against a 3-grain alternative scenario. When the temperatures of the grains are fixed a priori, the 1-grain scenario with only PAHs can be excluded at more than 5σ (p=2.9× 10−8), irrespective of the curvature in the intrinsic spectrum. The effective 3-grain EBL model with the tuned fractions was finally used to fit the SEDs of a sample of extragalactic gamma-ray sources (dominated by blazars). Such a sample is still dominated by starlight attenuation, therefore, no statistically significant improvement in the quality of fits was observed when the tuned fractions are used to account for the EBL attenuation and the intrinsic spectrum parameters are allowed to vary during the fit. The potential of this kind of analysis when the next generation of IACTs, represented by the Cherenkov Telescope Array (CTA), starts observations is enormous. The newly discovered AGNs at a broad range of redshifts should break many of the degeneracies currently observed.

Lorentz Invariance Violation Tests in Astroparticle Physics

“…The current systematics are discussed in [41]. New EBL measurements are foreseen using the propagation of TeV photons [79]. The key experimental future is the extension of the detectable energy range to the highest possible energies.…”

Modeling the Extragalactic Background Light and the Cosmic Star Formation History

“…The results constrained by only the γ-rays (Model B) demonstrate that the existing γ-ray data do not strongly constrain the dust parameters, although the results of this model are completely consistent with Model A. A similar analysis was performed by Pimentel & Moura-Santos (2019), who used the VHE γ-ray spectrum of Mrk 501 from HEGRA to attempt to constrain these dust parameters as well. Our Model B results are consistent with theirs.…”

Simultaneously unveiling the EBL and intrinsic spectral parameters of gamma-ray sources with Hamiltonian Monte Carlo

“…When working with the simulated sources, we use the EBL grid (i.e., the optical depth in the E γ × z parameter space) as computed by [33]. However, for analyzing real IACT flux data, we have recomputed the optical depth using a larger range in redshift.…”

“…Description of the EBL optical depth grids, based on the F10 model. We use the grid computed by [33] to analyse the synthetic sources (section 3), while having an updated grid used for the IACT data inference (section 4).…”

“…In fact, data from IACTs, reaching a few tens of TeV, are essential for constraining the mid-IR (3-25 µm), as the cross-section of pair-production peaks at an EBL wavelength λ(µm) ∼ 1.24E γ (TeV) for a gamma-ray photon detected at energy E γ [13]. Yet, current IACT data, as identified by [33] using a sample cataloged in TeVCat, are heavily dominated by spectra whose attenuation originates from the stellar component of the EBL (mainly UV, optical light and near-IR). Consequently, further observations reaching higher energies or performed with improved precision, such as expected with the CTA, will refine the determination of the infrared EBL spectrum and the mid-IR portion in particular, where direct measurements are heavily affected by foreground contamination [34].…”