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[en] Various interpretations are discussed of the apparent absence of radio pulses at 151 MHz in time coincidence with the Vela satellite γ-ray bursts. While the negative results are consistent with either a galactic or an extragalactic origin for the γ-ray bursts, the constraints are in most cases less stringent for models based on a local or galactic origin. The absence of VHF radio pulses suggests, however, that the γ-ray bursts do not arise from stellar superflares, on the restrictive assumption that they are directly scaled-up events of solar type
[en] Gamma-ray bursts (GRBs) are the most energetic sources in the universe and among the farthest known astrophysical sources. These features make them appealing candidates as standard candles for cosmological applications such that studying the physical mechanisms for the origin of the emission and correlations among their observable properties is an interesting task. We consider here the luminosity L*X and break time T*a (hereafter LT) correlation and investigate whether there are systematics induced by selection effects or redshift-dependent calibration. We perform this analysis both for the full sample of 77 Swift GRBs with known redshift and for the subsample of GRBs having canonical X-ray light curves, hereafter called the U0095 sample. We do not find any systematic bias, thus confirming the existence of physical GRB subclasses revealed by tight correlations of their afterglow properties. Furthermore, we study the possibility of applying the LT correlation as a redshift estimator both for the full distribution and for the canonical light curves. The large uncertainties and the non-negligible intrinsic scatter make the results not so encouraging, but there are nevertheless some hints motivating a further analysis with an increased U0095 sample.
[en] Some criticisms are offered on the paper by Carter and others in Nature; 262:370 (1976). These authors derived an upper limit to the intensity of small γ-bursts at approximately 100 below the extrapolated known size spectrum, and concludes that γ-ray bursts are of galactic origin. The present authors give reasons for claiming, however, that Carter's results are entirely consistent with the -1.5 index power law extrapolation, and therefore no conclusions can be drawn regarding the nature or origin of γ-ray bursts from their measurements. A reply by Carter is appended. (U.K.)
[en] We consider the possible gravitational wave radiation from a rotating black hole and a torus at the center of a Gamma Ray Burst(GRB). We propose gravitational waves generated by the precession of the torus. Using typical values for the black hole mass and for the mass and the radius of the torus, one can expect gravitational-wave production with frequencies roughly in the range of 102 up to 103 Hz. In the small-wobble-angle approximation, the amplitude of the gravitational wave associated with a GRB at a cosmic distance of z ∼ 1 is estimated. The amplitude is found to be not smaller than the recent estimate for a precessing neutron star. The effect of the back reaction of gravitational radiation is estimated to be not so significant during a GRB. We also point out for this particular configuration that the magnetic alignment process can be effective such that the angular momentum and the rotational energy of the central, rotating black hole are transferred to its surrounding torus.
[en] The design and implementation of main control system of polarimeter (POLAR) which is to measure the polarization of gamma-ray burst were described. As the important part of POLAR, this system consists of control unit and low power supply unit, and it is responsible for POLAR's power supply, data management and system control. Based on the isostructural and redundant design, the compactness and reliability of the main control system were enhanced. The test results show that this system can meet the design requirements and works normally and stably. (authors)
[en] In this work, we present the first results of our imaging campaign at Keck Observatory to identify the host galaxies of 'dark' gamma-ray bursts (GRBs), events with no detected optical afterglow or with detected optical flux significantly fainter than expected from the observed X-ray afterglow. We find that out of a uniform sample of 29 Swift bursts rapidly observed by the Palomar 60 inch telescope through 2008 March (14 of which we classify as dark), all events have either a detected optical afterglow, a probable optical host-galaxy detection, or both. Our results constrain the fraction of Swift GRBs coming from very high redshift (z>7), such as the recent GRB 090423, to between 0.2% and 7% at 80% confidence. In contrast, a significant fraction of the sample requires large extinction columns (host-frame AV ∼> 1 mag, with several events showing AV > 2-6 mag), identifying dust extinction as the dominant cause of the dark GRB phenomenon. We infer that a significant fraction of GRBs (and, by association, of high-mass star formation) occurs in highly obscured regions. However, the host galaxies of dark GRBs seem to have normal optical colors, suggesting that the source of obscuring dust is local to the vicinity of the GRB progenitor or highly unevenly distributed within the host galaxy.