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ATEL # 1106; N. Degenaar(UvA), S. Campana (OA Brera), D. Galloway (U. Melbourne), W. Lewin, J. Homan, D. Chakrabarty, E. Morgan (MIT), P.Jonker (SRON/CfA), E. Cackett, J. Miller (University of Michigan), R. Wijnands (UvA)
on 13 Jun 2007; 22:23 UT
Password Certification: Rudy Wijnands (firstname.lastname@example.org)
Subjects: X-ray, Binaries, Neutron Stars, Pulsars, Transients
Following the non-detection of HETE J1900.1-2455 on June 6 (ATel #1098), we find this system considerably brighter in a ~5 ksec Swift/XRT observation performed on June 10, 2007 (2:25-12:14 UT). The X-ray spectrum can be well described by an absorbed powerlaw plus blackbody model, with Nh~0.19, kT~0.16 keV and a photon index of ~2.24. The inferred 2.0-10.0 keV flux is ~2.8E-10 erg/cm2/s, which corresponds to an X-ray luminosity ~8.4E35 (d/5kpc) erg/s. The increased flux detected with Swift/XRT is confirmed by RXTE/PCA observations performed on June 12, 2007 (14:14-15:51 UT), where HETE J1900.1-2455 is found at a 2.0-10.0 keV flux of ~5.5E-10 erg/cm2/s.
The X-ray behavior of HETE J1900.1-2455 is reminiscent of that of another accreting millisecond pulsar, SAX J1808.4-3658. This system displayed outbursts where the initial gradual flux decline accelerated after ~2 weeks. Following this rapid decay, the flux suddenly increased and the source entered a period of rapid (hours to days) luminosity fluctuations, before fully returning into quiescence several weeks later (Wijnands et al. 2001, ApJ 560). The current flux of HETE J1900.1-2455 is roughly consistent with the average flux displayed during its ~2 year outburst. HETE J1900.1-2455 may experience a complete recovery to full outburst, but it is also possible that the system has entered a state of variable X-ray flux, similar to what has been observed for SAX J1808.4-3658. HETE J1900.1-2455 will be further monitored with RXTE the coming weeks.
We would like to thank both the Swift team and Jean Swank for the prompt scheduling of the XRT and PCA observations.