X射线暂现源Aql X-1的2个历元VLBI探探测

Two New VLBI Detections Verified of the X-ray Transient Aql X-1

天鹰座X-1 (Aquila X-1, Aql X-1)是一个低质量中子星X射线双星,在X射线和射电波段表现为暂现源。对该源的甚长基线干涉(very long baseline interferometry, VLBⅠ)观测不仅为中子星X射线双星的射电-X射线(或喷流-吸积盘)相关性研究提供了观测依据,而且VLBⅠ亚毫角秒级的定位精度还有助于实现对该源的天体测量(包括位置、视差和自行)。由于Aql X-1常伴随着光球层膨胀(photospheric radius expansion, PRE)特征的Ⅰ型X射线暴(X-ray burst),因此,独立于物理模型的视差测量可以用来估算这类X射线暴的本征光度,并有助于对该类X射线暴的理论研究。Aql X-1已在2009年11月和2013年6月的2个历元的VLBⅠ观测中被探测到,这使得它在其他历元的VLBⅠ观测中的搜寻范围显著变小,并大大降低了其他历元的VLBⅠ观测的探测阈值。处理并分析了观测时间为2010年1月和2010年7月的两组VLBA (Very Long Baseline Array)的历史数据,并证认了Aql X-1的2个新的亚毫秒级精度的VLBⅠ探测,其置信度分别达到99.998 6%和99.998 4%。此外,2010年7月Aql X-1出现在其2010年1月位置的西南面,这与之前发布的Aql X-1的自行方向大体一致,意味着Aql X-1在2个历元间的位移主要来源于自行。最后发现,这2个VLBⅠ观测的历元正对于X射线的软硬态转换窗口,这符合之前对该源的射电-X射线的相关性研究结果。

Aquila X-1(Aql X-1)is a neutron-star(NS)low-mass X-ray binary(LMXB),and a transient in radio and X-ray observations.VLBI(very long baseline interferometry)observations of Aql X-1play an essential role in the jet-disk coupling study of NS-LMXBs.They also offer locations of Aql X-1with sub-mas precision,which can be used in astrometric purpose.Since Aql X-1exhibits X-ray bursts with evident photospheric radius expansion(PRE burst),the model-independent astrometry(here parallax)can help estimate the luminosity of the PRE bursts,and facilitate the theoretical modeling of PRE bursts.The two reported(in2009November and2013June)VLBI detections of Aql X-1reduce the search area of Aql X-1in other VLBI observations,and effectively lower the S/N threshold of new detections.We devised a procedure to a)determine the search region of the target source with its prior positional information,b)estimate the confidence level of the potential detection using the search region and the statistical information from the relevant VLBI dirty map.This procedure is especially suitable for radio-weak sources with prior positional information.Following this procedure we re-analysed two sets of VLBA(Very Long Baseline Array)data obtained in the 2010 January and 2010 July observations,and find two new detections with confidence level of99.9986%and99.9984%respectively.The detections,suitable for astrometric usage,correspond to inconsistent locations showing a displacement towards southwest,which agrees with the previously reported proper motion of Aql X-1.Additionally,the two new radio detections are associated with state transitions of X-ray emission,supporting the widely accepted belief that radio flux of an atoll source is mainly generated at the transitions between hard and soft X-ray state during X-ray outbursts.