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ATEL # 1086; Duncan Galloway (U. Melbourne), Ed Morgan (MIT), Deepto Chakrabarty (MIT) and Phil Kaaret (U. Iowa)
on 28 May 2007; 5:18 UT
Distributed as an Instant Email Notice (Request for Observations)
Password Certification: Duncan K. Galloway (firstname.lastname@example.org)
Subjects: X-ray, Request for Observations, Binaries, Neutron Stars, Pulsars, Transients
We report ongoing Rossi X-ray Timing Explorer (RXTE) monitoring observations of the accretion-powered millisecond pulsar HETE J1900.1-2455 (Kaaret et al. 2006, ApJ 638, 963). This source has been X-ray active continuously since it first went into outburst in June 2005 (see e.g. ATel #657), although the 377.3 Hz pulsations became undetectable in August 2005 and have not been detected since.
The last two RXTE observations have found HETE J1900.1-2455 at the faintest flux level yet seen. On 21 May the flux was 3.7e-11 erg/cm^2/s (2.5-25 keV), a factor of approximately 10 fainter than in the previous observation (14 May). On 27 May the source was even fainter, at 1.3e-11 erg/cm^2/s. Twice before in 2007 (on 3 February and 11 March) the flux decreased below the lowest level seen during 2005-6, reaching 3.1 and 3.6e-10 erg/cm^2/s respectively. However, the flux subsequently recovered each time, within a week. That the source has been consistently faint and fading for at least a week strongly suggests that HETE J1900.1-2455 is returning to quiescence.
The source was the brightest yet seen just a month earlier, on 15 April, at 1.34e-9 erg/cm^2/s; more typically the flux was within a factor of two of the outburst-averaged mean value, 7.5e-10 erg/cm^2/s. Since the source became visible again to RXTE in February 2007, the X-ray flux has been significantly more variable, by a factor of ~4 on time-scales of ~50 d. During 2005 and 2006 the flux varied quasi-periodically on a time-scale of ~120 d with rms amplitude of ~20%. Accompanying the increased variability in 2007 were changes in the X-ray color evolution, with the source spectrum switching frequently to low hard color (i.e. a softer spectrum) characteristic of a flare observed early in the outburst, in 2005.
We will continue our RXTE observations until the source becomes undetectable. More sensitive X-ray observations, as well as observations at other wavelengths, are strongly encouraged.