Session Z10: Gamma Ray Astronomy: Extragalactic

Live

The high-energy Diffuse Gamma-Ray Background (DGRB) is expected to be produced by unresolved extra-galactic objects such as active galactic nuclei, isotropic Galactic $\gamma$-rays, and possibly emission from dark matter annihilations or decays in Galactic or extra-galactic structures. The DGRB has only been observed below 1 TeV and above this energy, upper-limits have been reported. Observations or stringent limits on the DGRB above this energy could have strong multi-messenger consequences, such as constraining the origin of TeV-PeV astrophysical neutrinos detected by IceCube. The High Altitude Water Cherenkov (HAWC) Observatory, located in central Mexico at 4100 m above sea level, is sensitive to gamma rays from 300 GeV to $>$100 TeV and continuously observes a wide field-of-view ($\sim$2 sr). With its high energy reach and large area coverage, HAWC is well-suited to significantly improve searches for the DGRB at TeV energies. In this work, strict cuts have been applied to the HAWC dataset to better isolate gamma-ray air showers from background hadronic showers. The sensitivity to the DGRB was then verified using 535 days of Crab data and Monte Carlo simulations, leading to a new limit on the DGRB with HAWC as well as its implications for multi-messenger and dark matter studies.   

Live

The extragalactic background light (EBL) is comprised of all the radiation emitted by nuclear and accretion processes since the epoch of recombination. Direct measurements of the EBL in the near-IR to mid-IR waveband are extremely difficult due, mainly, to the zodiacal light foreground. Instead, gamma-ray astronomy offers the possibility to indirectly set limits to the EBL by studying the effects of gamma-ray absorption in the spectra of distant blazars in the very high energy range (VHE:>100 GeV). HAWC is a water Cherenkov observatory that can detect extragalactic gamma rays significantly up to ~10 TeV, making it one of the few instruments sensitive to gamma rays in the multi-TeV range. This offers the opportunity to probe the EBL in the near IR region: $\lambda$ = 5 $\mu$m - 15 $\mu$m. In this study, we assume an intrinsic spectrum as the extrapolation of the Fermi-LAT GeV spectrum and derive a large number of absorbed spectra for different randomly generated EBL model shapes. We then calculate confidence bands in the EBL intensity space by comparing and testing the agreement between the absorbed spectra and HAWC data.   

Live

Proper measurements of the Extragalactic Background Light (EBL), the radiation field of all infrared to light emitted in the Universe since reionization, are key to understanding the cosmic makeup and evolution of the universe. However, its direct measurement is difficult due to bright foreground emissions. An alternative method is to indirectly probe the EBL from its interaction with gamma rays emitted by blazars. The Fermi-LAT and H.E.S.S collaborations proposed using a scaling factor alpha to normalize EBL density based on a previously existing model. However, numerous “problematic” sources that deviate more than a discrepancy of 3 sigma from an EBL model were present in Fermi-LAT’s 4FGL-DR2 catalog, which contained 10 years of data. We performed a new gamma ray analysis on 12 years of Fermi-LAT observations, focusing on “problematic” and bright sources. The changes on the scaling factor alpha derived from our analysis resolve the issue for most of the “problematic sources”, while creating a new outlier from our “bright sources” sample. By estimating the factor alpha for a large number of blazars observed by the Fermi Large Area Telescope (Fermi-LAT), this study will contribute to the creation of a map of the density of the EBL.   

Live

The mystery of the extragalactic gamma-ray background (EGB) has been investigated since its first detection. To unveil its origin and composition, it is necessary to resolve the different gamma-ray emitting populations. Relying on 8 years of Fermi-Large Area Telescope data, we obtained the most sensitive source count distribution of blazars >100 MeV to date. This allowed us to derive the contribution of blazars to the EGB, highlighting that this population cannot reproduce the entire EGB and that, indeed, another source class is required to explain the residual emission. In this talk, I will present the latest results of our analysis in light of blazars' evolutionary models and discuss alternatives for the origin of the missing EGB component.   

Live

High-mass microquasars (HMMQs) are powerful particle accelerators, but the mechanism of their high-energy emission is poorly understood. A few of these particle engines have been observed to emit gamma-ray photons and are potential very-high-energy (VHE) gamma-ray emitters. In this work, we study four HMMQs (LS 5039, Cyg X-1, Cyg X-3, and SS 433) using 1,523 days of data from the High Altitude Water Cherenkov (HAWC) observatory. We report the most stringent limit to date on the gamma-ray emission by each source above 10 TeV. By stacking the likelihoods of all HMMQs that are accessible to HAWC, we constrain the fraction of the jet luminosity in emitting VHE gamma rays and high-energy neutrinos. We also show that the non-detection of VHE gamma-rays implies a significant magnetic field, which challenges synchrotron radiation as the dominant mechanism of the microquasar emission between 10 keV and 10 MeV.   

Live

We started from a well-defined and representative sample of High-Energy peaked BL Lac (HBL) selected by cross-matching radio and X-ray data. We searched for very high energy gamma-ray emission using the five-year data set from the HAWC Observatory. We will present the results of our search in the multi-TeV energy range assuming an extrapolated power law for each source.   

Live

Many star-forming galaxies and those hosting active galactic nuclei (AGN) show evidence of massive outflows of material in a variety of phases including ionized, neutral atomic, and molecular outflows. Molecular outflows in particular have been the focus of recent interest as they may be responsible for removing gas from the galaxy, thereby suppressing star formation. As the material is ejected from the core of the galaxies, interactions of the outflowing material with the interstellar medium can produce high energy gamma rays. However, the gamma-ray emission from these individual objects is expected to be below the threshold for LAT detection and has yet to be directly observed. In order to search for this faint gamma-ray signal we conduct a stacked analysis of a sample of molecular outflows in the nearby universe using roughly 11 years of Fermi-LAT data. Evidence of gamma-ray emission from these sources can have significant implications for our understanding of AGN feedback mechanisms and the extragalactic gamma-ray background.   

Live

On 22 Sept 2017, neutrinos likely from the blazar TXS 0506+056 were detected in concert with flaring activity in the gamma-rays. Since then, multiwavelength spectra have been analyzed alongside the neutrino signature to constrain and narrow down models applicable to the plausible physical scenarios giving rise to this observed event, but no consensus has been found. We analyze the multiwavelength spectrum and neutrino detection during the 2014-2015 and 2017 neutrino activity periods, using a self-consistent particle transport model in order to track the effects of acceleration and emission on the protons, electrons, and subsequent neutrinos. From this careful consideration of the particle spectra, we then produce a model of the observable spectrum, including the MeV, which is underpinned with the historical TXS 0506+056 data, and make projections for AMEGO/AMEGO-X in detecting similar events.