Search for X-ray occultation faster photometry smaller Fresnel scale shorter events smaller bodies * The background X-ray source needs to be bright enough. * Sco X-1 is the brightest in the Sky. * RXTE/PCA has the largest effective area. * The typical PCA count rate of Sco X-1 is 10 5 cps. * Detection of msec time-scale occultation is possible.
In the first attempt…… RXTE/PCA data of Sco X-1, 1996-2002, 564 ksec (Chang et al. 2006, Nature 442, 660; Chang et al. 2007, MNRAS 378, 1287; Jones et al. 2008, ApJ 677, 1241) * 107 significant dips were found. * There are signatures indicating that most, if not all, of them could be due to some possible instrumental effects, which are previously unknown. * Those signatures allow 10% of the dip events to be non-instrumental.
Light curves of the 12 possible non-instrumental events (no anomaly in ‘remaining’ and ‘VLE’ counts)
In the second attempt …… RXTE/PCA data of Sco X-1, 2007/2008/2009, 240 ksec (Liu et al. 2008, MNRAS 388, L44; Chang et al. 2011, MNRAS 411, 427) * New event modes for recording the so-called ‘very large events’ (VLEs) were designed, in the hope to identify clearly the instrumental effect associated with dip events. * The arrival time of each VLE is recorded with 125- μs resolution. The identification of anodes which detect the recorded VLE is also known. RXTE PCA Very Large Events (VLEs) are those events which deposit more than 100 keV in any one of the 6 active xenon layers or the propane layer.
Association of dips with RXTE/PCA VLEs RXTE PCA Very Large Events (VLEs) are those events which deposit more than 100 keV in any one of the 6 active xenon layers or the propane layer. This number is consistent with random fluctuation, but the others are not. See Chang et al. (2011) for details. 39 ‘significant’ dips and 253 ‘less significant’ dips were found in the 240-ks data (June 2007 – Oct 2009). No indication of any instrumental effect for this event; -7.1σ, r.p.<7.5x10 -5 (0.1) (2.4) (3.2) (0.7) (7.4)
The fitting result suggests, assuming a circular orbit and a minimum inclination, the occulting body is at r = 3.4 +24 -2.4 AU, with D = 38 +26 -37 m. It might be (1) a TNO of 150-m size, but with a rare retrograde orbit, or, (2) an MBA of 40-m size, but the associated detection rate is incredibly high, or, (3) a very nearby object of metre size moving at a relative speed of a few km s −1.
Summary * Serendipitous occultation search can explore properties of small TNOs in the size range for which direct imaging is impossible in the foreseeable future. * Such search in optical bands can study TNOs of kilometer size, while in the X- ray band, it can be hectometer or even down to decameter size. * It is possible to estimate the distance of the occulting body with good enough statistics. * It is also possible to study the size distribution of decameter MBAs, which may have further impact on the study of NEAs. * Upper limit to the size distribution of TNO at small size end has been obtained, which is not attainable by any other means known to date. In the future ASTROSAT/LAXPC observation of Sco X-1 ??? AXTAR, Athena, LOFT ???