arXiv subjects

Gor Oganesyan

Publications and source records attributed to Gor Oganesyan.

39 records · Page 3Linked to original sources

arXiv2018

THESEUS and Gamma-Ray Bursts: a valuable contribution to the understanding of prompt emission

Recent advances in fitting prompt emission spectra in gamma-ray bursts (GRBs) are boosting our understanding of the still elusive origin of this radiation. These progresses have been possible thanks to a more detailed analysis of the low-energy part ($<$\,100\,keV) of the prompt spectrum, where the spectral shape is sometimes found to deviate from a simple power-law shape. This deviation is well described by a spectral break or, alternatively by the addition of a thermal component. Spectral data extending down to less than 1\,keV are extremely relevant for these studies, but presently they are available only for a small subsample of {\it Swift} GRBs observed by XRT (the X-ray telescope, 0.3-10\,keV) during the prompt emission. The space mission þ will allow a systematic study of prompt spectra from 0.3\,keV to several MeV. We show that observations performed by þ will allow us to discriminate between different models presently considered for GRB prompt studies, solving the long-standing open issue about the nature of the prompt radiation, with relevant consequences on the location of the emitting region, magnetic field strength and presence of thermal components.

astro-ph.HE↗

arXiv2017

Detection of Low-energy Breaks in Gamma-Ray Burst Prompt Emission Spectra

The radiative process responsible for gamma-Ray Burst (GRB) prompt emission has not been identified yet. If dominated by fast-cooling synchrotron radiation, the part of the spectrum immediately below the $νF_ν$ peak energy should display a power-law behavior with slope $α_2=-3/2$, which breaks to a higher value $α_1=-2/3$ (i.e. to a harder spectral shape) at lower energies. Prompt emission spectral data (usually available down to $\sim10-20\,$keV) are consistent with one single power-law behavior below the peak, with typical slope $\langleα\rangle=-1$, higher than (and then inconsistent with) the expected value $α_2=-3/2$. To better characterize the spectral shape at low energy, we analyzed 14 GRBs for which the Swift X-ray Telescope started observations during the prompt. When available, Fermi-GBM observations have been included in the analysis. For 67% of the spectra, models that usually give a satisfactory description of the prompt (e.g., the Band model) fail in reproducing the $0.5-1000\,$keV spectra: low-energy data outline the presence of a spectral break around a few keV.We then introduce an empirical fitting function that includes a low-energy power law $α_1$, a break energy $E_{\rm break}$, a second power law $α_2$, and a peak energy $E_{\rm peak}$. We find $\langleα_1\rangle=-0.66$ ($ \rm σ=0.35$), $\langle \log (E_{\rm break}/\rm keV)\rangle=0.63$ ($ \rm σ=0.20$), $\langleα_2\rangle=-1.46$ ($\rm σ=0.31$), and $\langle \log (E_{\rm peak}/\rm keV)\rangle=2.1$ ($ \rm σ=0.56$).The values $\langleα_1\rangle$ and $\langleα_2\rangle$ are very close to expectations from synchrotron radiation. In this context, $E_{\rm break}$ corresponds to the cooling break frequency.

astro-ph.HE↗

arXiv2015

Statistical Analysis of the Parameters of Gamma-Ray Bursts with Known Redshifts and Peaked Optical Light Curves

We present the statistical analysis of the properties of gamma-ray bursts with measured host galaxy redshifts and peaked optical light curves in proper frames of reference. The optical transients are classified by comparing the time lag of the optical peak relative to the GRB trigger with the duration of the gamma-ray emission itself. The results of the correlation analysis of all possible pairs of energy, spectral, and temporal characteristics of both gamma-ray and optical emissions are given. We specify the pairs of the parameters with correlation coefficients greater than 50 % at significance levels better than 1 %. The following empirical relations, obtained for the first time, are specifically discussed: a correlation between the peak optical afterglow $R$ band luminosity and redshift $L_{R} \propto (z+1)^{5.39 \pm 0.74}$ and a correlation between the peak luminosity of the prompt optical emissions and the time of the peak $L_{R} \propto T_{\rm peak}^{-3.85 \pm 1.22}$. We also analyze the similarity of the relationships between the peak optical luminosity and the isotropic equivalent of the total energy of gamma-ray bursts for afterglows ($L_{R} \propto E_{\rm iso}^{1.06 \pm 0.22}$) and for prompt optical emissions ($L_{R} \propto E_{\rm iso}^{1.59 \pm 0.21}$).

astro-ph.HE↗