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ATEL # 1945; C. B. Markwardt (U. Maryland/NASA/GSFC) N. Shaposhnikov (U. Maryland/NASA/GSFC) J. H. Swank (NASA/GSFC) D. Altamirano (U. Amsterdam) K. Yamaoka (Aoyama Gakuin U.) Y. Ueda (Kyoto U.)
on 27 Feb 2009; 21:39 UT
Password Certification: Craig B. Markwardt (firstname.lastname@example.org)
Subjects: X-ray, Binaries, Black Holes, Neutron Stars, Transients
Recently, an X-ray burst was reported fom SAX J1747.0-2853 by the INTEGRAL Galactic Bulge team (Chenevez et al, ATEL #1944). RXTE PCA scans of the galactic center region have been underway since early February. The most recent PCA scanning observation on 2009-02-26 detected a small increase in the persistent flux of the source of about 15 +/- 7 mCrab (2-10 keV). The uncertainty is large because of source confusion near the galactic center. In the past several years, SAX J1747.0-2853 has exhibited approximately 1 outburst every two years, with a peak flux of about 100-150 mCrab.
On 2009-02-26, PCA scans also detected an increase in the flux of GX 339-4, to about 11 mCrab (2-10 keV). The most recent time a comparable flux was reached was summer 2008 (ATEL #1588, ATEL #1586). Over the past five years, the source has been regularly active more than 60% of the time (brighter than 3 mCrab). If the light curve can be divided into five "outbursts," then two have been brighter than 100 mCrab at their peak, and three have been fainter than 100 mCrab.
A dedicated PCA pointing observation 2009-02-27 (MJD 54889.120) showed a similar count rate. The PCA spectrum had a power law shape with photon index 1.45 +/- 0.02, modified by smeared edge at 7.1 keV. The flux was 1.52e-10 ergs/cm2/s (2-10 keV) and 3.68e-10 erg/cm2/s (2-30 keV). The power spectrum is a pure power law with a index 1.10 +/- 0.02 and the root-mean-square variability of about 50 per cent. These properties are consistent with the source being in the extreme low-hard state. Although GX 339-4 exhibited hard "failed" outbursts before, the timing of previous strong outbursts may indicate that this event can develop in a full-scale outburst with the transition to the high-soft state. The RXTE team has scheduled additional short pointing observations. Simultaneous coverage in other wavelengths is recommended.