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ATEL # 1599; H. A. Krimm (CRESST/GSFC/USRA), P. Soleri (Universiteit van Amsterdam), S. D. Barthelmy (GSFC), W. Baumgartner (CRESST/GSFC/UMBC), J. Cummings (CRESST/GSFC/UMBC), E. Fenimore (LANL), N. Gehrels (GSFC), C. B. Markwardt (CRESST/GSFC/UMD), D. Palmer (LANL), A. Parsons (GSFC), T. Sakamoto (CRESTT/GSFC/UMBC), G. Sato (GSFC/ISAS), G. Skinner (CRESST/GSFC/UMBC), M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC)
on 2 Jul 2008; 20:00 UT
Password Certification: Hans A. Krimm (Hans.Krimm@nasa.gov)
Subjects: X-ray, Request for Observations, Binaries, Black Holes, Transients
Referred to by ATEL #: 1608
The Swift/BAT hard X-ray transient monitor reports increased activity in the 15-50 keV band for the black hole candidate Swift J1753.5-0127. Starting on about June 19, 2008 (MJD 54636), the source has been increasing in intensity from an average rate of 0.015 cts/cm2/sec (65 mcrab) in the 15-50 keV energy band to a current daily average level of (0.024 ± 0.003) cts/cm2/sec (105 mcrab). This is the highest rate seen in the BAT monitor since the end of its very high state in September 2005. Since that time Swift J1753.5-0127 has remained in a low/hard state. In the BAT it has slowly risen in intensity from ~35 mcrab in September 2005 to ~65 mcrab in mid-June 2008. It has been detectable in the RXTE All-Sky Monitor for the past year as well.
The source was observed with the Swift satellite as a target of opportunity for a total of 1994 seconds beginning at UT 21:27:18 on June 30, 2008. The rate in the XRT (2-10 keV) was 21.89 ± 0.11 cts/sec. The spectrum was fitted with an absorbed power law with a photon index 1.711 ± 0.013 and nH = (0.141 ± 0.0024) X 1022 cm-2 (chi-squared/dof = 1022.4/865). The calculated unabsorbed flux values are:
0.3-10 keV (1.222 +/- 0.008) X 10^-9 erg/cm2/s
2-10 keV (7.14 +/- 0.038) X 10^-10 erg/cm2/s
Further Swift observations are planned for July 3, 6 and 10.
This increase in the BAT energy range may portend a state change from the low/hard state to the high/soft state within the next few weeks.
Swift/BAT light curve for SWIFT J1753.5-0127