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Data from four Fermi-detected gamma-ray bursts (GRBs) is used to set limits on spectral dispersion of electromagnetic radiation across the universe. The analysis focuses on photons recorded above 1 GeV for Fermi detected GRB 080916C, GRB…
Ultra-high energy (UHE) photons play an important role as an independent probe of the photo-pion production mechanism by UHE cosmic rays. Their observation, or non-observation, may constrain astrophysical scenarios for the origin of UHECRs…
The variable flux of TeV gamma-rays detected from Mkn 421 and Mkn 501 requires the presence of high energy electrons, which could in principle produce large numbers of electron/positron pairs, leading to an electromagnetic cascade. We point…
The diffuse meta-galactic radiation field at ultraviolet to infrared wavelengths - commonly labeled extragalactic background light (EBL) - contains the integrated emission history of the universe. Difficult to access via direct…
Ultra-high-energy photons with energies exceeding $10^{17}$ eV offer a wealth of connections to different aspects of cosmic-ray astrophysics as well as to gamma-ray and neutrino astronomy. The recent observations of photons with energies in…
Recently, the KM3NeT Collaboration announced the detection of a 220 PeV neutrino from the celestial coordinates RA=94.3\degree~ and Dec=-7.8\degree~ on 13 February 2023 at 01:16:47 UTC \cite{KM3NeT:2025npi}. The source for this…
We have calculated the opacities and secondary production mechanisms of high energy photons arising in gamma-ray burst internal shocks, using exact cross-sections for the relevant processes. We find that for reasonable choices of…
Light dark photons are subject to various plasma effects, such as Debye screening and resonant oscillations, which can lead to a more complex cosmological evolution than is experienced by conventional cold dark matter candidates.…
We calculate the exact width for a dark photon decaying to three photons at one loop order for dark photon masses m' below the e+e- production threshold of 2m_e. We find substantial deviations from previous results derived from the lowest…
The dark photon, an new hypothetical light spin 1 field, constitutes a well-motivated dark matter candidate. It manifests as an oscillating electric field with a fixed direction, which can be observed in magnetometric records. In this…
Gamma Ray Burst sources are apparently evolving around us in a harder and brighter samples at far and far redshift. The average output may range from a near Supernova (nearest events) output to a billion time that power for most distant…
For gamma-ray bursts (GRBs) born in a stellar wind, as the reverse shock crosses the ejecta, usually the shocked regions are still precipitated by the prompt MeV \gamma-ray emission. Because of the tight overlapping of the MeV photon flow…
Very high energy (VHE, energy $E \gtrsim 100$\,GeV) \gamma-rays from cosmological sources are attenuated due to the interaction with photons of the extragalactic background light (EBL) in the ultraviolet to infrared wavelength band. The…
The extragalactic magnetic field (EGMF) could be probed with $\gamma$-ray observations of distant sources. Primary very high energy (VHE) $\gamma$-rays from these sources absorb on extragalactic background light photons, and secondary…
Recent observations of blazars at high energy (HE, 0.1-100 GeV) and very high energy (VHE, >0.1 TeV) have provided important constraints on the intensity and spectrum of the diffuse Extragalactic Background Light (EBL), shedding light on…
Dark photons are a well-motivated candidate for dark matter, but their detection becomes challenging for ultralight masses with both experimental and astrophysical probes. In this work, we propose a new approach to explore this regime…
Dark photons, as a minimal extension of the Standard Model through an additional Abelian gauge group, may propagate relativistically across the galaxy, originating from dark matter decay or annihilation, thereby contributing to a galactic…
Dark photons, hypothesized to be sufficiently light and/or weakly interacting, offer a compelling candidate for dark matter. Their decay into three photons, referred to as the "dark photon trident" process, becomes the dominant channel when…
Recent work has suggested that an additional $\lesssim 6.9 \rm{\, eV}$ per baryon of heating in the intergalactic medium is needed to reconcile hydrodynamical simulations with Lyman-$\alpha$ forest absorption line widths at redshift…
Detections of very-high-energy (VHE; > 100 GeV) photons from Gamma-ray bursts (GRBs) can provide fundamental information on the involved radiative processes, physical composition of the ejecta and acceleration processes. The High Altitude…