Swift and Suzaku observations of the X-ray afterglow from the GRB 060105

[著者版]Results are presented of early X-ray afterglow observations of GRB 060105 by Swift and Suzaku. The bright, long gamma-ray burst GRB; 060105 triggered the Swift Burst Alert Telescope at 06:49:28 on 2006 January 5 (T-0). The Suzaku team commenced a pre-planned target of opportunity observation at 19 ks (5.3 hr) after the Swift trigger. The X-ray flux faded during observations from 6.8 x 10(-9) ergs(-1) cm(-2) (at T-0 + 87 s with the Swift/XRT) to 1.5 x 10(-13) erg s(-1) cm(-2) [at T-0 + 94-101 ks with the Suzaku X-ray Imaging Spectrometer (XIS)] in the 2-10 keV energy band. Following prompt emission and successive very steep decay, a shallow decay was observed from To + 187 s to T-0 + 1287 s. After an observation gap during T-0 + (1.5-3) ks, an extremely early steep decay was observed in T-0 + (4-30) ks. The lightcurve flattened again at T-0 + 30ks, and another steep decay followed from T-0 + 50ks to the end of observations. Both steep decays exhibited decay indices of 2.3-2.4. This very early break, if it is a jet break, is the earliest case among X-ray afterglow observations, suggesting a very narrow jet whose opening angle is well below 1 degrees. The unique Suzaku/XIS data allow us to set very tight upper limits on line emission or absorption in this GRB. For the reported pseudo-redshift of z = 4.0 +/- 1.3 the upper limit on the iron line equivalent width is 50 eV