Vladimir M. Loznikov

Extended emission from short gamma-ray bursts detected with SPI-ACS/INTEGRAL

The short duration (T90 < 2 s) gamma-ray bursts (GRBs) detected in the SPI-ACS experiment onboard the INTEGRAL observatory are investigated. Averaged light curves have been constructed for various groups of events, including short GRBs and unidentified short events. Extended emission has been found in the averaged light curves of both short GRBs and unidentified short events. It is shown that the fraction of the short GRBs in the total number of SPI-ACS GRBs can range from 30 to 45%, which is considerably larger than has been thought previously.

GRB 051008: a long, spectrally hard dust-obscured GRB in a Lyman-break galaxy at z ≈ 2.8

We present observations of the dark gamma-ray burst GRB 051008 provided by Swift/BAT, Swift/XRT, Konus-WIND, INTEGRAL/SPI-ACS in the high-energy domain and the Shajn, Swift/UVOT, Tautenburg, NOT, Gemini and Keck I telescopes in the optical and near-infrared bands. The burst was detected only in gamma- and X-rays and neither a prompt optical nor a radio afterglow was detected down to deep limits. We identified the host galaxy of the burst, which is a typical Lyman-break galaxy (LBG) with R-magnitude of 24.06 ± 0.10 mag. A redshift of the galaxy of z=2.77^(+0.15)_(−0.20) is measured photometrically due to the presence of a clear, strong Lyman-break feature. The host galaxy is a small starburst galaxy with moderate intrinsic extinction (A_V = 0.3) and has a star formation rate of ∼60 M_⊙ yr^(−1) typical for LBGs. It is one of the few cases where a GRB host has been found to be a classical LBG. Using the redshift we estimate the isotropic-equivalent radiated energy of the burst to be E_(iso) = (1.15 ± 0.20) × 10^(54) erg. We also provide evidence in favour of the hypothesis that the darkness of GRB 051008 is due to local absorption resulting from a dense circumburst medium.

Short gamma-ray bursts in the SPI-ACS INTEGRAL experiment

We analyze short-duration gamma-ray bursts (T90<2 s) recorded in the SPI-ACS experiment of the INTEGRAL observatory. We found an extended emission in the averaged light curve of both short-duration gamma-ray bursts and unidentified short-duration events. We show that the fraction of short-duration gamma-ray bursts among all the gamma-ray bursts recorded in the SPI-ACS experiment may be as high as 30 to 45%. We find the fraction of short gamma-ray bursts to augment while increasing the lower energy threshold. We report evidence for the absence of the class of very short gamma-ray bursts.

Searching for signature of extended emission in short GRBs registered by the SPI‐ACS of INTEGRAL Observatory

We investigate short duration gamma‐ray bursts registered by SPI‐ACS of the INTEGRAL observatory in 2002–2007. Average time profile of short GRB reveals an extended emission up to 25 s after the main peak in both confirmed by other experiments and unconfirmed short duration GRB. Unconfirmed group may consist of up to 70% of real short duration GRB. This is also one of the first evidence of the extended emission of short bursts in the energy range 80 keV–10 MeV.

Aperiodic properties of Gamma-Ray Bursts

Aperiodic properties of Gamma-Ray Burst (GRB) time profiles were investigated on the basis of BATSE catalog. Power density spectra (PDS) obtained with the FFT procedure for each GRB were averaged for all GRBs. We analyze the continuum and high-frequency part of the spectrum. The spectra averaged for different brightness groups of GRBs are found to be different. The hardness of PDS monotonously changes with brightness of bursts, which may point to the absence of “standard candle” model of Gamma-Ray Bursts.