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Register a new userEffect of Dust Extinction on Gamma-ray Burst Afterglows
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Gu-Jing Lv
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In order to study the effect of dust extinction on the afterglow of gamma-ray bursts (GRBs), we carry out numerical calculations with high precision based on rigorous Mie theory and latest optical properties of interstellar dust grains, and analyze the different extinction curves produced by dust grains with different physical parameters. Our results indicate that the absolute extinction quantity is substantially determined by the medium density and metallicity. However, the shape of the extinction curve is mainly determined by the size distribution of the dust grains. If the dust grains aggregate to form larger ones, they will cause a flatter or grayer extinction curve with lower extinction quantity. On the contrary, if the dust grains are disassociated to smaller ones due to some uncertain processes, they will cause a steeper extinction curve with larger amount of extinction. These results might provide an important insight into understanding the origin of the optically dark GRBs.
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Aims: Drawing an analogy with Active Galactic Nuclei, we investigate the one-zone SSC model of Gamma Ray Bursts afterglows in the presence of electron injection and cooling both by synchrotron and SSC losses. Methods: We solve the spatially averaged kinetic equations which describe the simultaneous evolution of particles and photons, obtaining the multi-wavelength spectrum as a function of time. We back up our numerical calculations with analytical solutions of the equations using various profiles of the magnetic field evolution under certain simplifying assumptions. Results: We apply the model to the afterglow evolution of GRBs in a uniform density environment and examine the impact various parameters have on the multiwavelength spectra. We find that in cases where the electron injection and/or the ambient density is high, the losses are dominated by SSC and the solutions depart significantly from the ones derived in the synchrotron standard cases.
Extra-galactic X-ray absorption and optical extinction are often found in gamma-ray burst (GRB) afterglows and they could be tracers of both circumburst and host galaxy environments. By performing spectral analyses for spectral energy distribution of 9 short GRB (SGRB) afterglows with known redshift, we investigated a ratio of the equivalent hydrogen column density to the dust extinction, N^{rest}_{H}/A^{rest}_{V}, in the rest frame of each SGRB. We found that the distribution of N^{rest}_{H}/A^{rest}_{V} is systematically smaller than the one for long GRBs, and is roughly consistent with the gas-to-dust ratio in the Milky Way. This result means that the measured gas-to-dust ratio of SGRBs would originate from the interstellar medium in each host galaxy. This scenario supports the prediction that SGRBs occur in non star-forming regions in the host galaxies.
We study thermal emission from circumstellar structures heated by gamma-ray burst (GRB) radiation and ejecta and calculate its contribution to GRB optical and X-ray afterglows using the modified radiation hydro-code small STELLA. It is shown that thermal emission originating in heated dense shells around the GRB progenitor star can reproduce X-ray plateaus (like observed in GRB 050904, 070110) as well as deviations from a power law fading observed in optical afterglows of some GRBs (e.g. 020124, 030328, 030429X, 050904). Thermal radiation pressure in the heated circumburst shell dominates the gas pressure, producing rapid expansion of matter similar to supenova-like explosions close to opacity or radiation flux density jumps in the circumburst medium. This phenomenon can be responsible for so-called supernova bumps in optical afterglows of several GRBs. Such a `quasi-supernova suggests interpretation of the GRB-SN connection which does not directly involve the explosion of the GRB progenitor star.
The afterglow emission from gamma-ray bursts (GRBs) is believed to originate from a relativistic blast wave driven into the circumburst medium. Although the afterglow emission from radio up to X-ray frequencies is thought to originate from synchrotron radiation emitted by relativistic, non-thermal electrons accelerated by the blast wave, the origin of the emission at high energies (HE; $gtrsim$~GeV) remains uncertain. The recent detection of sub-TeV emission from GRB~190114C by MAGIC raises further debate on what powers the very high-energy (VHE; $gtrsim 300$GeV) emission. Here, we explore the inverse Compton scenario as a candidate for the HE and VHE emissions, considering two sources of seed photons for scattering: synchrotron photons from the blast wave (synchrotron self-Compton or SSC) and isotropic photon fields external to the blast wave (external Compton). For each case, we compute the multi-wavelength afterglow spectra and light curves. We find that SSC will dominate particle cooling and the GeV emission, unless a dense ambient infrared photon field, typical of star-forming regions, is present. Additionally, considering the extragalactic background light attenuation, we discuss the detectability of VHE afterglows by existing and future gamma-ray instruments for a wide range of model parameters. Studying GRB~190114C, we find that its afterglow emission in the fermi-LAT band is synchrotron-dominated.The late-time fermi-LAT measurement (i.e., $tsim 10^4$~s), and the MAGIC observation also set an upper limit on the energy density of a putative external infrared photon field (i.e. $lesssim 3times 10^{-9},{rm erg,cm^{-3}}$), making the inverse Compton dominant in the sub-TeV energies.
While all but one Gamma-Ray Bursts observed in the X-ray band showed an X-ray afterglow, about 60 per cent of them have not been detected in the optical band. We demonstrate that in many cases this is not due to adverse observing conditions, or delay in performing the observations. We also show that the optically non-detected afterglows are not affected by particularly large Galactic absorbing columns, since its distribution is similar for both the detected and non-detected burst subclasses. We then investigate the hypothesis that the failure of detecting the optical afterglow is due to absorption at the source location. We find that this is a marginally viable interpretation, but only if the X-ray burst and afterglow emission and the possible optical/UV flash do not destroy the dust responsible for absorption in the optical band. If dust is efficiently destroyed, we are led to conclude that bursts with no detected optical afterglow are intrinsically different. Prompt infrared observations are the key to solve this issue.
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