The Suzaku-swift joint observation of the early X-ray afterglow of GRB060105

Results are presented of a Suzaku and Swift joint observation for an early X-ray afterglow of GRB 060105. The bright, long gamma-ray burst GRB 060105 triggered the Swift Burst Alert Telescope (BAT) at 06:49:28 on 5 January 2006 (T0). The Suzaku team commenced a pre-planned target of opportunity observation at 12:10 and continued till 12:00 on January 6. The X-ray flux faded during the observations from 6.8x10-9 erg s-1cm-2 to 1.5x10-13 erg s-1 cm-2 in the 2-10 keV energy band. Following the prompt emission and successive 'very steep decay', a 'shallow decay' was observed from T0 + 187 s to T0 + 1287 s. After an observation gap during T0 + (1.5-3) ks, an extremely early steep decay was observed in T0 + (4-30) ks. The steep decay 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 1deg. (author)