We compare the properties of optically dark GRBs, defined by the optical-to-X- ray spectral indexβox 〈 0.5, and normal ones discovered by the Swift satellite before the year 2008 in a statistical way, using data collected from the literature and online databases. Our sample includes 200 long bursts, 19 short bursts, and 10 with measured high redshifts （z ≥ 4）. The ratio of dark bursts is found to be -10% - 20%, and is similar among long bursts, short ones, and the high-z sub-sample. The result for long bursts is consistent with both the pre-Swift sample and studies by other authors on smaller Swift samples. The existence of dark short GRBs is pointed out for the first time. The X-ray derived hydrogen column densities of dark GRBs clearly prefer large values compared with those of normal bursts. This supports the dust extinction scenario as the main cause of dark GRBs. Other possibilities like very high redshifts and non-standard emission mechanisms are less likely, although not fully excluded.
We present a brief overview of follow-up observations of GRB afterglows made by the 0.8-m TNT and other telescopes at the Xinglong observatory during the last three years.Our system,dedicated to the measurement of early-time optical afterglow emission,responded to 50 GRBs from Jan.2006 to Apr.2009.Among them,about 50% have been successfully detected.The fastest response time is 76 sec(GRB 061110A and GRB 090426) after the space-borne GRB detector was triggered.The redshift distribution spans the range from z = 0.033(GRB 060218) to z = 5.6(GRB 060927).We also report the optical photometric follow-up of GRB 080330,which is an X-ray flash,as an example of our observations.
XIN LiPing1,2,DENG JingSong1,ZHENG WeiKang1,2,WEI JianYan1,WANG Jing1,QIU YuLei1 & HU JingYao1 1 National Astronomical Observatories,Chinese Academy of Sciences,Beijing 100012,China
A two-component jet model is proposed to explain the unusual afterglow of GRB 070419A.Regarding the optical light curve,a wide "jet" with an opening angle of >30-40 degrees is assumed to produce the late shallow decay,while the three early power-law segments must be caused by a narrow jet with an opening angle of-2-4 degrees.Additional energy injections to both components are required.Late X-ray emission may come from either the wide jet or the narrow one.If the latter is correct,the jets may run into an ISM environment and the temporal index of the late energy injection may be q-0.65.
ZHENG WeiKang1,2 & DENG JinSong1 1 National Astronomical Observatories,Chinese Academy of Sciences,Beijing 100012,China
