Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userComptonization of photons near the photosphere of relativistic outflows
202
0
0.0
(
0
)
Added by
Gregory V. Vereshchagin
Publication date
2013
fields
Physics
and research's language is
English
Ask ChatGPT about the research
No Arabic abstract
We consider the formation of photon spectrum at the photosphere of ultrarelativistically expanding outflow. We use the Fokker-Planck approximation to the Boltzmann equation, and obtain the generalized Kompaneets equation which takes into account anisotropic distribution of photons developed near the photosphere. This equation is solved numerically for relativistic steady wind and the observed spectrum is found in agreement with previous studies. We also study the photospheric emission for different temperature dependences on radius in such outflows. In particular, we found that for $Tpropto r^{-2}$ the Band low energy photon index of the observed spectrum is $alphasimeq -1$, as typically observed in Gamma Ray Bursts.
rate research
Read More
We present five simultaneous UV/X-ray observations of IC4329A by AstroSat, performed over {a five-month} period. We utilize the excellent spatial resolution of the Ultra-Violet Imaging Telescope (UVIT) onboard AstroSat to reliably separate the intrinsic AGN flux from the host galaxy emission and to correct for the Galactic and internal reddening, as well as the contribution from the narrow and broad-line regions. We detect large-amplitude UV variability, which is unusual for a large black hole mass AGN, like IC4329A, over such a small period. In fact, the fractional variability amplitude is larger in the UV band than in the X-ray band. This demonstrates that the observed UV variability is intrinsic to the disk, and is not due to X-ray illumination. The joint X-ray spectral analyses of five SXT and LAXPC spectral data reveal a soft-X-ray excess component, a narrow iron-line (with no indication of a significant Compton hump), and a steepening power-law ($DeltaGammasim 0.21$) with increasing X-ray flux. The soft excess component could arise due to thermal Comptonization of the inner disk photons in a warm corona with $kT_esim 0.26$ keV. The UV emission we detect acts as the primary seed photons for the hot corona, which produces the broadband X-ray continuum. The X-ray spectral variability is well described by the cooling of this corona from $kT_esim42$ keV to $sim 32$ keV with increasing UV flux, while the optical depth remains constant at $tausim 2.3$.
In a core-collapse supernova, after the explosion is launched, neutrino heating above the protoneutron star creates an outflow of matter. This outflow has been extensively investigated as a nucleosynthesis site. Here, we revisit this problem motivated by the modeling of neutrino flavor transformations. In this case, it is crucial to understand whether the outflow has a termination shock: its existence observably alters neutrino oscillations a few seconds into the explosion. We derive physical criteria for the formation of this shock, in terms of neutrino luminosity, average energy, protoneutron star radius and mass, and the postshock density. For realistic physical conditions, the system is found to be on the edge of shock formation, thus reconciling seemingly disparate numerical results in the literature. Our findings imply that neutrino signatures of modulated matter effects are a sensitive probe of the inner workings of the supernova.
Outflows are observed in a variety of astrophysical sources. Remarkably, ultra-fast ($vgeq 0.1c$), outflows in the UV and X-ray bands are often seen in AGNs. Depending on their energy and mass outflow rate, respectively $dot{E}_{out}, dot{M}_{out}$, such outflows may play a key role in regulating the AGN-host galaxy co-evolution process through cosmic time. It is therefore crucial to provide accurate estimates of the wind properties. Here, we concentrate on special relativistic effects concerning the interaction of light with matter moving at relativistic speed relatively to the source of radiation. Our aim is to assess the impact of these effects on the observed properties of the outflows and implement a relativistic correction in the existing spectral modelling routines. We define a simple procedure to incorporate relativistic effects in radiative transfer codes. Following this procedure, we run a series of simulations to explore the impact of these effects on the simulated spectra, for different $v$ and column densities of the outflow. The observed optical depth is usually considered a proxy for the wind $N_H$, independently on its velocity. However, our simulations show that the observed optical depth of an outflow with a given column density $N_H$ decreases rapidly as the velocity of the wind approaches relativistic values. This, in turn, implies that when estimating $N_H$ from the optical depth, it is necessary to include a velocity-dependent correction, already for moderate velocities (e.g. $v geq 0.05c$). This correction linearly propagates to the derived $dot{M}_{out}, dot{E}_{out}$. As an example of these effects, we calculate the relativistically corrected values of $dot{M}_{out}$ and $dot{E}_{out}$ for a sample of $sim 30$ Ultra-Fast Outflows taken from the literature, and find correction factors of $20-120 %$ within the observed range of outflowing velocities.
We give an overview of the AGILE gamma-ray satellite scientific highlights. AGILE is an Italian Space Agency (ASI) mission devoted to observations in the 30 MeV - 50 GeV gamma-ray energy range, with simultaneous X-ray imaging in the 18-60 keV band. Launched in April 2007, the AGILE satellite has completed its tenth year of operations in orbit, and it is substantially contributing to improve our knowledge of the high-energy sky. Emission from cosmic sources at energies above 100 MeV is intrinsically non-thermal, and the study of the wide variety of observed Galactic and extragalactic gamma-ray sources provides a unique opportunity to test theories of particle acceleration and radiation processes in extreme conditions.
The afterglows to gamma-ray bursts (GRBs) are due to synchrotron emission from shocks generated as an ultra-relativistic outflow decelerates. A forward and a reverse shock will form, however, where emission from the forward shock is well studied as a potential counterpart to gravitational wave-detected neutron star mergers the reverse shock has been neglected. Here, we show how the reverse shock contributes to the afterglow from an off-axis and structured outflow. The off-axis reverse shock will appear as a brightening feature in the rising afterglow at radio frequencies. For bursts at $sim100$ Mpc, the system should be inclined $lesssim20^circ$ for the reverse shock to be observable at $sim0.1-10$ days post-merger. For structured outflows, enhancement of the reverse shock emission by a strong magnetic field within the outflow is required for the emission to dominate the afterglow at early times. Early radio photometry of the afterglow could reveal the presence of a strong magnetic field associated with the central engine.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
