Optical behavior of GRB 061121 around its X-Ray shallow decay phase

T. Uehara, M. Uemura, A. Arai, R. Yamazaki, K. S. Kawabata, et al., “Optical behavior of GRB 061121 around its X-Ray shallow decay phase”, A&A, vol. 526, p. 92, 2011.

We report on a detailed study of the optical afterglow of GRB 061121 with our original time-series photometric data. Along with our optical observations and public X-ray data, we discuss the origin of its optical and X-ray afterglows. We observed the optical afterglow of Swift burst GRB 061121 with the Kanata 1.5-m telescope at Higashi-Hiroshima Observatory. Our observation covers a period just after an X-ray plateau phase. We also performed deep imaging with the Subaru telescope in 2010 in order to estimate the contamination by the host galaxy. In the light curve, we find that the optical afterglow also exhibited a break as in the X-ray afterglow. However, our observation suggests a possible hump structure or a flattening period before the optical break in the light curve. There is no sign of such a hump in the X-ray light curve. This implies that the optical emitting region was distinct from the X-ray one. The hump in the optical light curve was possibly caused by a passage with the typical frequency of synchrotron emission from another forward shock distinct from the early afterglow. The observed decay and spectral indices are inconsistent with the standard synchrotron-shock model. As a results, the observation requires a change in microphysical parameters in the shock region or prior activity by the central engine. Alternatively, the emission during the shallow decay phase may be a composition of two forward shock emissions, as indicated by the hump structure in the light curve.


Infrared/optical – X-ray simultaneous observations of X-ray flares in GRB 071112C and GRB 080506

T. Uehara, M. Uemura, K. S. Kawabata, Y. Fukazawa, R. Yamazaki, “Infrared/optical – X-ray simultaneous observations of X-ray flares in GRB 071112C and GRB 080506”, A&A, vol. 519, p. 56, 2010.

We investigate the origin of short X-ray flares which are occasionally observed in early stages of afterglows of gamma-ray bursts (GRBs). We observed two Swift events, GRB 071112C and GRB 080506, before the start of X-ray flares in the optical and near-infrared (NIR) bands with the 1.5-m Kanata telescope. In conjunction with published X-ray and optical data, we analyzed densely sampled light curves of the early afterglows and spectral energy distributions (SEDs) in the NIR-X-ray ranges. We found that the SEDs had a break between the optical and X-ray bands in the normal decay phases of both GRBs regardless of the model for the correction of the interstellar extinction in host galaxies of GRBs. In the X-ray flares, X-ray flux increased by 3 and 15 times in the case of GRB 071112C and 080506, respectively, and the X-ray spectra became harder than those in the normal decay phases. No significant variation in the optical-NIR range was detected together with the X-ray flares. These results suggest that the X-ray flares were associated with either late internal shocks or external shocks from two-component jets.


Structure in the early afterglow light curve of the γ-ray burst of 29 March 2003

M. Uemura, T. Kato, R. Ishioka, H. Yamaoka, B. Monard, et al., “Structure in the early afterglow light curve of the γ-ray burst of 29 March 2003,” Nature, vol. 423, p. 843, 2003.

Gamma-ray bursts (GRBs) are energetic explosions that for 0.01-100s are the brightest γ-ray sources in the sky. Observations of the early evolution of afterglows are expected to provide clues about the nature of the bursts, but their rapid fading has hampered such studies; some recent rapid localizations of bursts have improved the situation. Here we report an early detection of the very bright afterglow of the burst of 29 March 2003 (GRB030329). Our data show that, even early in the afterglow phase, the light curve shows unexpectedly complicated structures superimposed on the fading background.