Related papers: Self-Shielding of X-rays and Gamma-Rays in Compact…
A $\gamma$-ray source must have a limited optical depth to pair production. This simple condition, called compactness, implies that gamma-ray bursts (GRBs) must involve a highly relativistic motion ($\Gamma \gtrsim 100$) giving the first…
A BeppoSAX survey of bright Neutron Star (NS) systems in globular clusters, together with the results obtained on Galactic LMXRB, suggests that a two-component model is a reasonable description of the 0.1-200 keV spectra. At energies lower…
We suggest that ultraluminous X-ray sources (ULXs) and some of the variable low latitude EGRET gamma-ray sources may be two different manifestations of the same underlying phenomena: high-mass microquasars with relativistic jets forming a…
Simulations have indicated that most of the escaped Lyman continuum photons escape through a minority of solid angles with near complete transparency, with the remaining majority of the solid angles largely opaque, resulting in a very broad…
In examining a select sample of over 200 blazars of known jet kinetic power (L_kin) and well-characterized SEDs, we found (Meyer et al., 2011) that Intermediate synchrotron-peaking (ISP) blazars may have lower gamma-ray output than high…
We have constructed a new laser-Compton-scattering facility, called the LEPS2 beamline, at the 8-GeV electron storage ring, SPring-8. This facility provides a linearly polarized photon beam in a tagged energy range of 1.3--2.4 GeV. Thanks…
Recent works have shown that collective single photon spontaneous emission from an ensemble of $N$ resonant two-level atoms is a rich field of study. Superradiance describes emission from a completely symmetric state of $N$ atoms, with a…
If the large-scale galaxy distribution is filamentary, as suggested by some observations and recent hydrodynamical simulations, then lengthwise views of filaments will apparently produce compact groups (CGs) that are in reality stretched…
We have obtained a firm detection of Cyg X-1 during its hard and intermediate spectral states in the energy range of 40 MeV--60 GeV based on observations by the Fermi Large Area Telescope, confirming the independent results at $\geq$60MeV…
The accretion of matter onto intermediate polar White Dwarfs (IPWDs) seems to provide attractive conditions for acceleration of particles to high energies in a strongly magnetized turbulent region at the accretion disk inner radius. We…
To reionize the early universe, high-energy photons must escape the galaxies that produce them. It has been suggested that stellar feedback drives galactic outflows out of star-forming regions, creating low density channels through which…
We consider the search for gamma-rays produced by the annihilation or decay of low-mass dark matter which couples to quarks. In this scenario, most of the photons are produced from the decays of $\pi^0$ or $\eta$ mesons. These decays…
Colliding winds of massive binaries have long been considered as potential sites of non-thermal high-energy photon production. This is motivated by the detection of non-thermal spectra in the radio band, as well as by correlation studies of…
The conditions for the appearance of a sharp laser transition are formulated in terms of a scaling limit, involving vanishing cavity loss and light-matter coupling, $\kappa \to 0$, $g \to 0$, such that $g^2/\kappa$ stays finite. It is shown…
X-ray flares and other solar brightenings have been discarded as potential axion signatures. An axion X-ray signal must appear exclusively near the disk centre, and its spectrum must peak at ~4.2keV, contrary to observation. We argue here…
The surface of a low mass star inside a compact low mass X-ray binary system (LMXB) can be heated by the external X-ray source which may appear due to the accretion process onto a companion compact object (a neutron star or a black hole).…
The propagation of $\gamma$ rays over very large distances provides new insights on the intergalactic medium and on fundamental physics. On their path to the Earth, $\gamma$ rays can annihilate with diffuse infrared or optical photons of…
Becker et al. 2021 measured the mean free path of Lyman limit photons in the IGM at $z=6$. The short value suggests that absorptions may have played a prominent role in reionization. Here we study physical properties of ionizing photon…
Inverse Compton (IC) scattering is one of two viable mechanisms that can produce the prompt gamma-ray emission in Gamma-Ray Bursts. IC requires low energy seed photons and a population of relativistic electrons that upscatter them. The same…
We calculate the properties of gamma-ray burst spectral lines formed by resonant cyclotron scattering in a radiation-driven outflow. Most previous models of line formation in gamma-ray bursts are appropriate at the polar cap of a neutron…