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ABSTRACT: This paper presents a study on the spectral evolution of gamma-ray burst
(GRB) prompt emissions observed with the Suzaku Wide-band All-sky Monitor
(WAM). By making use of the WAM data archive, 6 bright GRBs exhibiting 7
well-separated fast-rise-exponential-decay (FRED) shaped light curves are
presented and the evaluated exponential decay time constants of the
energy-resolved light curves from these FRED peak light curves are shown to
indicate significant spectral evolution. The energy dependence of the time
constants is well described with a power-law function tau(E) ~ E^gamma, where
gamma ~ -(0.34 +/- 0.12) in average, although 5 FRED peaks show consistent
value of gamma = -1/2 which is expected in synchrotron or inverse-Compton
cooling models. In particular, 2 of the GRBs were located with accuracy
sufficient to evaluate the time-resolved spectra with precise energy response
matrices. Their behavior in spectral evolution suggests two different origins
of emissions. In the case of GRB081224, the derived 1-s time-resolved spectra
are well described by a blackbody radiation model with a power-law component.
The derived behavior of cooling is consistent with that expected from radiative
cooling or expansion of the emission region. On the other hand, the other 1-s
time-resolved spectra from GRB100707A is well described by a Band GRB model as
well as with the thermal model. Although relative poor statistics prevent us to
conclude, the energy dependence in decaying light curve is consistent with that
expected in the former emission mechanism model.
10/2011;
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Yuji Urata,
Makoto S. Tashiro,
Toru Tamagawa,
Fumihiko Usui,
Makoto Kuwahara,
Hungmiao Lin,
Shoichi Kageyama,
Wataru Iwakiri,
Takako Sugasahara,
Kazuki Takahara,
Natsuki Kodaka,
Keiichi Abe,
Keisuke Masuno, Kaori Onda
[show abstract]
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ABSTRACT: The WIDeField telescope for Gamma-ray burst Early Timing (WIDGET) is used for a fully automated, ultra-wide-field survey aimed at detecting the prompt optical emission associated with Gamma-ray Bursts (GRBs). WIDGET surveys the HETE-2 and Swift/BAT pointing directions covering a total field of view of 62 degree x 62 degree every 10 secounds using an unfiltered system. This monitoring survey allows exploration of the optical emission before the gamma-ray trigger. The unfiltered magnitude is well converted to the SDSS r' system at a 0.1 mag level. Since 2004, WIDGET has made a total of ten simultaneous and one pre-trigger GRB observations. The efficiency of synchronized observation with HETE-2 is four times better than that of Swift. There has been no bright optical emission similar to that from GRB 080319B. The statistical analysis implies that GRB080319B is a rare event. This paper summarizes the design and operation of the WIDGET system and the simultaneous GRB observations obtained with this instrument. Comment: 19 pages, 11 figures, Accepted to appear in PASJ
10/2010;
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Kaori Onda,
Makoto S. Tashiro,
Yujin E. Nakagawa,
Kazutaka Yamaoka,
Yukikatsu Terada,
Masanori Ohno,
Satoshi Sugita,
Takanori Sakamoto,
Kenji Toma,
Poshak Gandhi,
Natsuki Kodaka,
Wataru Iwakiri,
Yuji Urata,
Makoto Yamauchi,
Yasushi Fukazawa
[show abstract]
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ABSTRACT: Time-resolved spectral analyses of GRB 070125 with significant photon
detections over the MeV regime using Suzaku/WAM data are reported. The
prompt emission interval was divided into 10 time regions, and their
spectra were reproduced by a Band function or a cutoff power-law model.
The spectra show a complex evolution; a soft-hard-soft trend is seen,
rather than a typical hard-to-soft monotonic variation. In addition, a
flat spectral slope in the low-energy band exceeding the slope expected
from a simple synchrotron emission model is suggested in one of the time
regions. These results imply that the internal shocks in the
relativistic jet of GRB 070125 have different physical conditions. There
is a possible excess at ˜ 2.2 MeV in the spectrum of 33 seconds
after the trigger at a significant level of ≍ 98.5%, where the
continuum spectrum is not reproduced by the Band function. These
properties are only shown during the short prompt time interval of the
prompt emission, and emphasize the utility of the Suzaku/WAM for the
study of gamma-ray bursts.
Publications- Astronomical Society of Japan 05/2010; 62:547. · 2.44 Impact Factor
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Kazutaka Yamaoka,
Satoshi Sugita,
Makoto Tashiro,
Yukikatsu Terada,
Yuji Urata,
Akira Endo, Kaori Onda,
Kouichi Morigami,
Natsuki Kodaka,
Takako Sugasawara, [......],
Kazuhiro Nakazawa,
Teruaki Enoto,
Hiroyasu Tajima,
Tadayuki Takahashi,
Motohide Kokubun,
Masanori Ohno,
Yujin E. Nakagawa,
Tohru Tamagawa,
Toshio Murakami,
Kevin Hurley
[show abstract]
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ABSTRACT: The Suzaku Wide‐band All‐sky Monitor (WAM) consists of 20 BGO anti‐coincidence scintillators for the Hard X‐ray Detector (HXD). The WAM has a wide field of view (FOV), about half of the whole sky, a large collecting area, 800 cm2, and broad‐band energy coverage from 50 to 5000 keV. Thus it has been designed to work as a gamma‐ray burst detector. For the three years since Suzaku launch in July 2005, the WAM has been working very well. About 500 GRBs have been detected through the end of 2008, corresponding to a detection rate of ∼ 140 GRBs per year. The current status of the WAM is presented in this paper.
AIP Conference Proceedings. 05/2009; 1133(1):91-93.
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Kazutaka Yamaoka,
Akira Endo,
Teruaki Enoto,
Yasushi Fukazawa,
Ryuji Hara,
Yoshitaka Hanabata,
Soojing Hong,
Tsuneyoshi Kamae,
Chie Kira,
Natsuki Kodaka, [......],
Makoto S. Tashiro,
Yukikatsu Terada,
Takeshi Uehara,
Yuji Urata,
Makoto Yamauchi,
Atsumasa Yoshida,
Kevin Hurley,
Valentin Pal'Shin,
Takanori Sakamoto,
Jay Cummings
[show abstract]
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ABSTRACT: The Suzaku Wide-band All-sky Monitor (WAM) consists of thick BGO
anti-coincidence shields of the Hard X-ray Detectors (HXD). It views
about half of the sky and has a geometrical area of 800cm2
per side and an effective area of 400cm2, even at 1MeV.
Hence, the WAM can provide unique opportunities to detect high-energy
emission from GRBs and solar flares in the sub-MeV toMeV range. The WAM
has detected more than 400GRBs and 100 solar flares since its launch.
This paper describes the in-flight performance of the HXD/WAM during the
initial two years of operations, including the in-flight energy
response, spectral and timing capabilities, and in-orbit background.
Publications- Astronomical Society of Japan 12/2008; 61:35. · 2.44 Impact Factor
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Eric C. Bellm,
Kevin Hurley,
Valentin Pal'shin,
Kazutaka Yamaoka,
Mark S. Bandstra,
Steven E. Boggs,
Soojing Hong,
Natsuki Kodaka,
A. S. Kozyrev,
M. L. Litvak,
I. G. Mitrofanov,
Yujin E. Nakagawa,
Masanori Ohno, Kaori Onda,
A. B. Sanin,
Satoshi Sugita,
Makoto Tashiro,
V. I. Tretyakov,
Yuji Urata,
and Claudia Wigger
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ABSTRACT: The long, bright gamma-ray burst GRB 070125 was localized by the Interplanetary Network. We present light curves of the prompt gamma-ray emission as observed by Konus-Wind, RHESSI, Suzaku WAM, and Swift BAT. We detail the results of joint spectral fits with Konus and RHESSI data. The burst shows moderate hard-to-soft evolution in its multipeaked emission over a period of about 1 minute. The total burst fluence as observed by Konus is 1.79 × 10−4 ergs cm−2 (20 keV-10 MeV). Using the spectroscopic redshift z = 1.548, we find that the burst is consistent with the "Amati" Epeak,i − Eiso correlation. Assuming a jet opening angle derived from broadband modeling of the burst afterglow, GRB 070125 is a significant outlier to the "Ghirlanda" Epeak,i − Eγ correlation. Its collimation-corrected energy release, Eγ = 2.5 × 1052 ergs, is the largest yet observed.
The Astrophysical Journal 12/2008; 688(1):491. · 6.02 Impact Factor
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Masanori Ohno,
Yasushi Fukazawa,
Takuya Takahashi,
Kazutaka Yamaoka,
Satoshi Sugita,
Valentin Pal'shin,
Takanori Sakamoto,
Goro Sato,
Kevin Hurley,
Dmitry Frederiks,
Philipp Oleynik,
Mikhail Ulanov,
Makoto Tashiro,
Yuji Urata, Kaori Onda,
Toru Tamagawa,
Yukikatsu Terada,
Motoko Suzuki,
Hong Soojing
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ABSTRACT: We have performed a joint analysis of prompt emission from four bright short gamma-ray bursts (GRBs) with the Suzaku-WAM and the Konus-Wind experiments. This joint analysis allows us to investigate the spectral properties of short-duration bursts over a wider energy band with a higher accuracy. We find that these bursts have a high E$_{\rm peak}$, around 1 MeV and have a harder power-law component than that of long GRBs. However, we can not determine whether these spectra follow the cut-off power-law model or the Band model. We also investigated the spectral lag, hardness ratio, inferred isotropic radiation energy and existence of a soft emission hump, in order to classify them into short or long GRBs using several criteria, in addition to the burst duration. We find that all criteria, except for the existence of the soft hump, support the fact that our four GRB samples are correctly classified as belonging to the short class. In addition, our broad-band analysis revealed that there is no evidence of GRBs with a very large hardness ratio, as seen in the BATSE short GRB sample, and that the spectral lag of our four short GRBs is consistent with zero, even in the MeV energy band, unlike long GRBs. Although our short GRB samples are still limited, these results suggest that the spectral hardness of short GRBs might not differ significantly from that of long GRBs, and also that the spectral lag at high energies could be a strong criterion for burst classification.
02/2008;
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Daisuke Yonetoku,
Sachiko Tanabe,
Toshio Murakami,
Naomi Emura,
Yuka Aoyama,
Takashi Kidamura,
Hironobu Kodaira,
Yoshiki Kodama,
Ryota Kozaka,
Takuro Nashimoto, [......],
Shouta Maeno,
Eri Sonoda,
Makoto Yamauchi,
Makoto Kuwahara,
Toru Tamagawa,
Daisuke Matsuura,
Motoko Suzuki,
Scott Barthelmy,
Neil Gehrels,
John Nousek
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ABSTRACT: We observed an X-ray afterglow of GRB 060904A with the Swift and Suzaku satellites. We found rapid spectral softening during both the prompt tail phase and the decline phase of an X-ray flare in the BAT and XRT data. The observed spectra were fit by power-law photon indices which rapidly changed from $\Gamma = 1.51^{+0.04}_{-0.03}$ to $\Gamma = 5.30^{+0.69}_{-0.59}$ within a few hundred seconds in the prompt tail. This is one of the steepest X-ray spectra ever observed, making it quite difficult to explain by simple electron acceleration and synchrotron radiation. Then, we applied an alternative spectral fitting using a broken power-law with exponential cutoff (BPEC) model. It is valid to consider the situation that the cutoff energy is equivalent to the synchrotron frequency of the maximum energy electrons in their energy distribution. Since the spectral cutoff appears in the soft X-ray band, we conclude the electron acceleration has been inefficient in the internal shocks of GRB 060904A. These cutoff spectra suddenly disappeared at the transition time from the prompt tail phase to the shallow decay one. After that, typical afterglow spectra with the photon indices of 2.0 are continuously and preciously monitored by both XRT and Suzaku/XIS up to 1 day since the burst trigger time. We could successfully trace the temporal history of two characteristic break energies (peak energy and cutoff energy) and they show the time dependence of $\propto t^{-3} \sim t^{-4}$ while the following afterglow spectra are quite stable. This fact indicates that the emitting material of prompt tail is due to completely different dynamics from the shallow decay component. Therefore we conclude the emission sites of two distinct phenomena obviously differ from each other.
09/2007;
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Yuji Urata,
Kui-Yun Huang,
Ping-Hung Kuo,
Wing-Huen Ip,
Yulei Qiu,
Keisuke Masuno,
Makoto Tashiro,
Keichi Abe, Kaori Onda,
Natsuki Kodaka,
Makoto Kuwahara,
Toru Tamagawa,
Fumihiko Usui,
Kunihito Ioka,
Yi-Hsi Lee,
Jianyan Wei,
Jinsong Deng,
Weikang Zheng,
Kazuo Makishima
[show abstract]
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ABSTRACT: Observations were made of the optical afterglow of GRB 051028 with the Lulin observatory's 1.0 m telescope and the WIDGET robotic telescope system. R band photometric data points were obtained on 2005 October 28 (UT), or 0.095-0.180 days after the burst. There is a possible plateau in the optical light curve around 0.1 days after the burst; the light curve resembles optically bright afterglows (e.g. GRB 041006, GRB 050319, GRB060605) in shape of the light curve but not in brightness. The brightness of the GRB 051028 afterglow is 3 magnitudes fainter than that of one of the dark events, GRB 020124. Optically dark GRBs have been attributed to dust extinction within the host galaxy or high redshift. However, the spectrum analysis of the X-rays implies that there is no significant absorption by the host galaxy. Furthermore, according to the model theoretical calculation of the Ly$\alpha$ absorption to find the limit of GRB 051028's redshift, the expected $R$ band absorption is not high enough to explain the darkness of the afterglow. While the present results disfavor either the high-redshift hypothesis or the high extinction scenario for optically dark bursts, they are consistent with the possibility that the brightness of the optical afterglow, intrinsically dark.
07/2007;
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Kazutaka Yamaoka,
Satoshi Sugita,
Masanori Ohno,
Takuya Takahashi,
Yasushi Fukazawa,
Yukikatsu Terada,
Tohru Tamagawa,
Keiichi Abe,
Yasuhiko Endo, Kaori Onda, [......],
Makoto Tashiro,
Hong Soojing,
Goro Sato,
Kazuhiro Nakazawa,
Tadayuki Takahashi,
Ryohei Miyawaki,
Teruaki Enoto,
Motohide Kokubun,
Kazuo Makishima,
Toshio Murakami
[show abstract]
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ABSTRACT: The X-ray astronomical satellite Suzaku was successfully launched in July 2005. The onboard Wideband All-sky Monitor (WAM) is designed as the second function of the large, thick BGO anti-coincidence shields of the Hard X-ray Detectors (HXD). It views about half of the whole sky and has a geometrical area of 800 cm2 per side, with a large effective area of 400 cm2 even at 1 MeV. Hence, the WAM is expected to provide unique opportunities to detect high energy emission from GRBs and solar flares in the MeV range. In fact, the WAM has detected at least 47 GRBs, although the fine-tuning of the GRB functions is still in progress. The most impressive GRB result is the bright, hard spectrum GRB 051008, which was detected up to 1 MeV with the WAM. We will present here the in-flight performance of the HXD/WAM during the initial eight-months of operations. The in-flight energy response, spectral and timing capabilities, and in-orbit background are described in this paper.© (2006) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
06/2006;
