Swift and Suzaku Observations of the X-Ray Afterglow from the GRB 060105

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 (T₀). The Suzaku team commenced a pre-planned target of opportunity observation at 19ks (5.3hr) after the Swift trigger. The X-ray flux faded during observations from 6.8 × 10^-9 erg s^-1 cm^-2 (at T₀ + 87s with the Swift/XRT) to 1.5 × 10^-13 erg s^-1 cm^-2 [at T₀ + 94-101 ks with the Suzaku X-ray Imaging Spectrometer (XIS)] in the 2-10keV energy band. Following prompt emission and successive very steep decay, a shallow decay was observed from T₀ + 187s to T₀ + 1287s. After an observation gap during T₀ + (1.5-3) ks, an extremely early steep decay was observed in T₀ + (4-30) ks. The lightcurve flattened again at T₀ + 30 ks, and another steep decay followed from T₀ + 50 ks 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°. 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 50eV.