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Detections of extended Ly$\alpha$ halos (LAHs) around Ly$\alpha$ emitters (LAEs) have lately been reported on a regular basis, but their origin is still under investigation. Simulation studies predict that the outer regions of the extended…
Gamma-ray bursts are believed to be some catastrophic event in which material is ejected at a relativistic velocity, and internal collisions within this ejecta produce the observed $\gamma$-ray flash. The angular size of a causally…
Pulsar Wind Nebulae, Blazars, Gamma Ray Bursts and Magnetars all contain regions where the electromagnetic energy density greatly exceeds the plasma energy density. These sources exhibit dramatic flaring activity where the electromagnetic…
Following the eruption of a filament from a flaring active region, sunward-flowing voids are often seen above developing post-eruption arcades. First discovered using the soft X-ray telescope aboard Yohkoh, these supra-arcade downflows…
We briefly review the theory of gamma ray production in solar flares and present the highlights of the observations and their implications. Specifically: (i) the gamma ray data show that a large fraction of the released flare energy is in…
Nonlinear electron emission processes induced by surface plasmon oscillations have been studied both experimentally and theoretically. The measured above-threshold electron spectra extend up to energies whose appearance cannot be explained…
The recent years witnessed a dramatic improvement in our knowledge of the phenomenology and physics of Gamma Ray Bursts (GRBs). However, our "pillars of knowledge" remain a few, while many aspects remain obscure and not understood. There is…
In the external shock model, gamma-ray burst (GRB) emissions are produced by the energization and deceleration of a thin relativistic blast wave due to its interactions with the circumburst medium (CBM). We study the physical properties of…
White-light flares are the flares with emissions visible in the optical continuum. They are thought to be rare and pose the most stringent requirements in energy transport and heating in the lower atmosphere. Here we present a nearly…
We present broad band R and narrow band H_alpha emission line images of a sample of optically selected starburst galaxies from the Markarian lists. The emission line morphology is studied and global properties like luminosities, equivalent…
In this paper we shed light on late time (i.e. with peak time t_{pk} \gtrsim 1000 s) flaring activity. We address the morphology and energetic of flares in the window \sim 10^3-10^6 s to put constraints on the temporal evolution of the…
Blazars are a subclass of active galactic nuclei with relativistic jets pointing toward the observer. They are notable for their flux variability at all observed wavelengths and timescales. Together with simultaneous measurements at lower…
Relativistic flows resulting from sudden explosive events upscatter ambient interstellar photons of local radiation fields. For Lorentz factor $ > 100$ and dense optical - UV radiation fields the emergent signal is a typical gamma ray…
Swift-XRT observations of the X-ray emission from gamma ray bursts (GRBs) and during the GRB afterglow have led to many new results during the past two years. One of these exciting results is that approximately 1/3-1/2 of GRBs contain…
The radiation we observe from blazars is most likely the product of the transformation of bulk kinetic energy into random energy. This process must have a relatively small efficiency (e.g. 10%) if jets are to power the extended…
We show that a helical shear flow of a magnetized plasma may serve as an efficient amplifier of Alfven waves. We find that even when the flow is purely ejectional (i.e., when no rotation is present) Alfven waves are amplified through the…
Star quakes and internal phase transitions within anomalous x-ray pulsars (AXPs) and soft $\gamma$-ray repeaters (SGRs) can produce mini contractions and pulsar glitches. Shocks break out from their surface following such contractions…
Fast Radio Bursts (FRBs) are a class of short-duration transients at radio wavelengths with inferred astrophysical origin. The prototypical FRB is a broadband signal that occurs over the extent of the receiver frequency range, is narrow in…
We study the formation of a partially ionized zone behind the usual H II zone in photo-ionized gas for gas density intermediate between ordinary H II regions and AGN BLRs and without X-rays. While Lyman continuum photons are depleted before…
Recent Fermi results have focused attention on gamma-ray burst's (GRB) prompt emission phase, which is rich in phenomenology and poorly understood. The broad band spectra observed by Fermi does not fit into any of the frameworks of existing…