The Space multi-band Variable Object Monitor(SVOM) is a proposed Chinese astronomical satellite,dedicated to the detection,localization and measurement of gamma-ray bursts(GRBs) on a cosmological scale.An efficient algorithm is developed for the purpose of onboard star extraction from the CCD images obtained with the Visible Telescope(VT) onboard the SVOM.The CCD pixel coordinates of the reference stars will be used to refine the astronomical position of the satellite,which will facilitate triggering rapid ground-based follow-up observations of the GRBs.In this algorithm,the image is divided into a number of grid cells and the "global" pixel-value maximum within each cell is taken as the first-guess position of a "bright" star.The correct center position of a star is then computed using a simple iterative method.Applying two additional strategies,i.e.,scanning the image only by even(or odd) lines or in a black-white chess board mode,we propose to further reduce the time to extract the stars.To examine the efficiency of the above algorithms,we applied them to the experimental images obtained with a ground-based telescope.We find that the accuracy of the astronomical positioning achieved by our method is comparable to that derived by using the conventional star-extraction method,while the former needs about 25 times less CPU time than the latter.This will significantly improve the performance of the SVOM VT mission.
WANG TianZhou1,2,QIU YuLei1,CAI HongBo1 & DENG JingSong1 1 National Astronomical Observatories,Chinese Academy of Sciences,Beijing 100012,China
A gamma-ray burst （GRB） optical photometric follow-up system at the Xinglong Observatory of National Astronomical Observatories of China （NAOC） has been constructed. It uses the 0.8-m Tsinghua-NAOC Telescope （TNT） and the 1-m EST telescope, and can au- tomatically respond to GRB Coordinates Network （GCN） alerts. Both telescopes slew relatively fast, being able to point to a new target field within - 1 min upon a request. Whenever available, the 2.16-m NAOC telescope is also used. In 2006 the system responded to 15 GRBs and detected seven early afterglows. In 2007 six GRBs have been detected among 18 follow- up observations. TNT observations of the second most distant GRB 060927 （z = 5.5） are shown, which started as early as 91 s after the GRB trigger. The afterglow was detected in the combined image of the first 19 × 20 s unfiltered exposures. This GRB follow-up system has joined the East-Asia GRB Follow-up Observation Network （EAFON）.
Wei-Kang ZhengJin-Song DengMeng ZhaiLi-Ping XinYu-Lei QiuJing WangXiao-Meng LuJian-Yan WeiJing-Yao Hu
