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Author

G. Sato

Bio: G. Sato is an academic researcher. The author has contributed to research in topics: Gamma-ray burst. The author has an hindex of 1, co-authored 4 publications receiving 6 citations.

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Journal ArticleDOI
TL;DR: In this article, a large sample of Gamma-ray Burst (GRB) afterglow light curves was used to investigate the possibility of achromatic steepening in GRB light curves, known as jet breaks.
Abstract: Gamma-ray Burst (GRB) collimation has been inferred with the observations of achromatic steepening in GRB light curves, known as jet breaks. Identifying a jet break from a GRB afterglow light curve allows a measurement of the jet opening angle and true energetics of GRBs. In this paper, we re-investigate this problem using a large sample of GRBs that have an optical jet break that is consistent with being achromatic in the X-ray band. Our sample includes 99 GRBs from 1997 February to 2015 March that have optical and, for Swift GRBs, X-ray light curves that are consistent with the jet break interpretation. Out of the 99 GRBs we have studied, 55 GRBs are found to have temporal and spectral behaviors both before and after the break, consistent with the theoretical predictions of the jet break models, respectively. These include 53 long/soft (Type II) and 2 short/hard (Type I) GRBs. Only 1 GRB is classified as the candidate of a jet break with energy injection. Another 41 and 3 GRBs are classified as the candidates with the lower and upper limits of the jet break time, respectively. Most jet breaks occur at 90 ks, with a typical opening angle θ j = (2.5 ± 1.0)°. This gives a typical beaming correction factor for Type II GRBs, suggesting an even higher total GRB event rate density in the universe. Both isotropic and jet-corrected energies have a wide span in their distributions: log(E γ,iso/erg) = 53.11 with σ = 0.84; log(E K,iso/erg) = 54.82 with σ = 0.56; log(E γ /erg) = 49.54 with σ = 1.29; and log(E K/erg) = 51.33 with σ = 0.58. We also investigate several empirical correlations (Amati, Frail, Ghirlanda, and Liang–Zhang) previously discussed in the literature. We find that in general most of these relations are less tight than before. The existence of early jet breaks and hence small opening angle jets, which were detected in the Swfit era, is most likely the source of scatter. If one limits the sample to jet breaks later than 104 s, the Liang–Zhang relation remains tight and the Ghirlanda relation still exists. These relations are derived from Type II GRBs, and Type I GRBs usually deviate from them.

71 citations

Journal ArticleDOI
TL;DR: In this article, the authors performed a comprehensive multi-wavelength comparative study between duration-defined long GRBs and short GRBs as well as the so-called consensus GRBs, which are believed to be more closely related to the two types of progenitor systems.
Abstract: Gamma-ray bursts (GRBs) are classified into long and short categories based on their durations. Broadband studies suggest that these two categories of objects roughly correspond to two different classes of progenitor systems, i.e., compact star mergers (Type I) versus massive star core collapse (Type II). However, the duration criterion sometimes leads to mis-identification of the progenitor systems. We perform a comprehensive multi-wavelength comparative study between duration-defined long GRBs and short GRBs as well as the so-called "consensus" long GRBs and short GRBs (which are believed to be more closely related to the two types of progenitor systems). The parameters we study include two parts: the prompt emission properties including duration (T90), spectral peak energy (), low energy photon index (α), isotropic γ-ray energy (), isotropic peak luminosity (), and the amplitude parameters (f and ); and the host galaxy properties including stellar mass (), star formation rate, metallicity ([X/H]), half light radius (R50), angular and physical () offset of the afterglow from the center of the host galaxy, the normalized offset (), and the brightness fraction . For most parameters, we find interesting overlapping properties between the two populations in both one-dimensional (1D) and 2D distribution plots. The three best parameters for the purpose of classification are T90, , and . However, no single parameter alone is good enough to place a particular burst into the right physical category, suggesting the need for multiple criteria for physical classification.

67 citations

Journal ArticleDOI
TL;DR: A catalog of the redshifts for most long-duration gamma-ray bursts (GRBs) by Swift from 2004 December 20 to 2008 July 23 (258 bursts in total) is presented in this article.
Abstract: We present a catalog of the redshifts for most long-duration gamma-ray bursts (GRBs) by Swift from 2004 December 20 to 2008 July 23 (258 bursts in total) All available information is collected, including spectroscopic redshifts, photometric redshift limits, and redshifts calculated from various luminosity relations Error bars for the redshifts derived from the luminosity relations are asymmetric, with tails extended to the high-redshift end, and this effect is evaluated by looking at the 30% of Swift bursts with spectroscopic redshifts A simulation is performed to eliminate this asymmetric effect, and the resultant redshift distribution is deconvolved We test and confirm this simulation on the sample of bursts with known spectroscopic redshifts and then apply it to the 70% of Swift bursts that do not have spectroscopic measures A final intrinsic redshift distribution is then made for almost all Swift bursts, and the efficiency of the spectroscopic detections is evaluated The efficiency of spectroscopic redshifts varies from near unity at low redshift to 05 at z = 1, to near 03 at z = 4, and to 01 at z = 6 We also find that the fraction of GRBs with z>5 is ~10%, and this fraction is compared with simulations from a cosmological model

26 citations

Dissertation
01 Jan 2009
TL;DR: In this article, a multivariate data analysis technique, Principal Component Analysis (PCA), is applied to the data in order to determine parameters such as seasonal and diurnal changes which affect the variation of these signals.
Abstract: Very Low Frequency (VLF) radio waves propagate within the Earth-ionosphere waveguide with very little attenuation. Modifications of the waveguide geometry affect the propagation conditions, and hence, the amplitude and phase of VLF signals. Changes in the ionosphere, such as the presence of the D-region during the day, or the precipitation of energetic particles, are the main causes of this modification. Using narrowband receivers monitoring remote VLF transmitters, the amplitude and phase of these signals are recorded. A multivariate data analysis technique, Principal Component Analysis (PCA), is applied to the data in order to determine parameters such as seasonal and diurnal changes which affect the variation of these signals. Data was then analysed for effects from extragalactic gamma ray bursts, terrestrial gamma ray flashes and solar flares. Only X-rays from solar flares were shown to have an appreciable affect on ionospheric propagation.

1 citations