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Continuing flux decay of HETE J1900.1-2455 as observed with Swift/XRT

ATEL # 1091; N. Degenaar, R. Wijnands (UvA), D. Galloway (U. Melbourne), W. Lewin, J. Homan, D. Chakrabarty (MIT), S. Campana (OA Brera), J. Miller, E. Cackett (University of Michigan)
on 31 May 2007; 15:06 UT
Distributed as an Instant Email Notice (Transients)
Password Certification: Rudy Wijnands (

Subjects: X-ray, Binaries, Neutron Stars, Pulsars, Transients

Galloway et al. (2007; ATEL #1086) reported that the X-ray luminosity of the accreting millisecond pulsar HETE J1900.1-2455 exhibited a steady decay between 14 and 27 May, and suggested that the source may be finally returning to quiescence after an outburst of ~2 years. A similar declined was seen in the optical flux of the source (ATEL #1090). To follow the X-ray luminosity and spectral behavior of the source, we observed it with Swift/XRT during a ~1 ksec exposure performed on May 31, 2007 (00:02-00:18 UT). The source was clearly detected at an absorbed flux of ~2.7E-12 erg/s/cm^2 (2-10 keV; using an absorbed power-law model with Nh of ~1.4E21 cm^-2 and an power-law index of 2.5+-0.5). This extrapolates to a flux of ~3.0E-12 erg/s/cm^2 in the 2.5-25 keV band, which is a factor of ~4 fainter than during the RXTE observation on 21 May (ATEL #1086). The continued fading of the X-ray flux confirms that HETE J1900.1-2455 is returning to quiescence after an outburst episode of about 2 years. Swift/XRT observations will continue throughout June; a Chandra observation is currently planned for the last week of June.

It is a pleasure to thank the Swift team for making the XRT observations possible.

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R. E. Rutledge , Editor
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