We report on millisecond variability from the X-ray transient XTE J1739Ϫ285. We detected six X-ray type I bursts and found evidence for oscillations at Hz in the brightest X-ray burst. Taking into consideration 1122 ע 0.3 the power in the oscillations and the number of trials in the search, the detection is significant at the 99.96% confidence level. If the oscillations are confirmed, the oscillation frequency would suggest that XTE J1739Ϫ285 contains the fastest rotating neutron star yet found. We also found millisecond quasi-periodic oscillations in the persistent emission with frequencies ranging from 757 to 862 Hz. Using the brightest burst, we derive an upper limit on the source distance of about 10.6 kpc.
We report on the characterization of four Teledyne Imaging Systems HAWAII Hybrid Si CMOS detectors designed for X-ray detection. Three H1RG detectors were studied along with a specially configured H2RG. Read noise measurements were performed, with the lowest result being 7.1 e -RMS. Interpixel capacitive crosstalk (IPC) was measured for the three H1RGs and for the H2RG. The H1RGs had IPC upper limits of 4.0 -5.5 % (up & down pixels) and 8.7 -9.7 % (left & right pixels), indicating a clear asymmetry. Energy resolution is reported for two X-ray lines, 1.5 & 5.9 keV, at multiple temperatures between 150 -210 K. The best resolution measured at 5.9 keV was 250 eV (4.2 %) at 150 K, with IPC contributing significantly to this measured energy distribution. The H2RG, with a unique configuration designed to decrease the capacitive coupling between ROIC pixels, had an IPC of 1.8 ± 1.0 % indicating a dramatic improvement in IPC with no measurable asymmetry. We also measured dark current as a function of temperature for each detector. For the detector with the lowest dark current, at 150 K, we measured a dark current of 0.020 ± 0.001 (e -sec -1 pix -1 ). There is also a consistent break in the fit to the dark current data for each detector. Above 180 K, all the data can be fit by the product of a power law in temperature and an exponential. Below 180 K the dark current decreases more slowly; a shallow power law or constant must be added to each fit, indicating a different form of dark current is dominant in this temperature regime. Dark current figures of merit at 293 K are estimated from the fit for each detector.
We study the fluence distributions of over 3040 bursts from SGR 1806−20 and over 1963 bursts from SGR 1900+14 using the complete set of observations available from the Rossi X-Ray Timing Explorer/Proportional Counter Array through 2011 March. Cumulative event distributions are presented for both sources and are fitted with single and broken power laws as well as an exponential cutoff. The distributions are best fitted by a broken power law with exponential cutoff; however the statistical significance of the cutoff is not high and the upper portion of the broken power law can be explained as the expected number of false bursts due to random noise fluctuations. Event distributions are also examined in high and low burst rate regimes and power-law indices are found to be consistent, independent of the burst rate. The contribution function of the event fluence is calculated. This distribution shows that the energy released in the soft gamma repeater (SGR) bursts is dominated by the most powerful events for both sources. The power-law nature of these distributions combined with the dominant energy dissipation of the system occurring in the large, less frequent bursts is indicative of a self-organized critical system, as suggested by Goǧus et al. in 1999.
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