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ATEL # 949; T. A. Jones, A. M. Levine, E. H. Morgan, and S. Rappaport (MIT Kavli nstitute for Astrophysics and Space Research)
on 17 Nov 2006; 18:37 UT
Password Certification: Alan M. Levine (email@example.com)
Subjects: X-ray, Cosmic Rays, Solar System Objects
Chang et al. (Nature, 442, 7103, 660, 2006) found millisecond duration dips in the X-ray intensity of Sco X-1 and attributed them to occultations of the source by small trans-Neptunian objects (TNOs). We have found evidence that these dips are in fact not astronomical in origin, but rather the result of high-energy charged particle events in the RXTE PCA detectors. Subsequent to the report by Chang et al., we identified ˜200 dips of the type they describe in some 500 ksec of RXTE/PCA observations of Sco X-1. In an attempt to detect the small counting rate increases that one might expect from the diffraction sidelobes if these are indeed occultation events, we have formed an average dip profile by superposing the PCA light curves that include the dips. This was done after carefully shifting and stretching each dip to a common centroid time and width. We find no evidence for diffraction sidelobes. Furthermore, the summed dip profile is distinctly asymmetric in shape (as Chang et al. suggested for many of the individual dips), and the distribution of dip widths is narrower than what one would expect from occultations by bodies with a power-law size distribution of index -4. These findings prompted us to further explore alternative explanations for the dips. Although there is no information available on the non-X-ray background with millisecond time resolution, we found highly significant enhancements in the superposed very large event (VLE) and propane-layer event rates at the times corresponding to the dip times within the 1/8 sec time resolution of the VLE and propane data. These enhancements are consistent with the interruption of normal event processing for 1-2 milliseconds in all of the PCUs roughly once per hour due to the collection of very large amounts of charge. These energetic events may be the consequence of particle showers produced in the RXTE spacecraft by cosmic rays. While our results cast doubt on whether any true occultation events have been detected, one cannot yet conclude that no such events have been detected. We intend to conduct further investigations of the dip phenomenon and its possible causes, and we will work to obtain a new measurement of, or upper limit on, the rate of occurrence of occultations of Sco X-1.