Continuing activity of the 2.65 ms accretion-powered pulsar HETE J1900.1-2455

ATel #657; Duncan Galloway (U. Melbourne), Ed Morgan (MIT), Phil Kaaret (U. Iowa), Deepto Chakrabarty (MIT), Nat Butler (MIT) and Motoko Suzuki (RIKEN)
on 17 Nov 2005; 00:08 UT
Credential Certification: Duncan K. Galloway (duncan@space.mit.edu)

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We report on the ongoing monitoring of the 2.65 ms pulsar, HETE J1900.1-2455 (ATels 516, 523, 525, 526, 538 and others) with the Rossi X-ray Timing Explorer (RXTE). The source remains active at a flux of approximately 9e-10 ergs/cm²/s (2.5-25 keV), more than 150 d after the outburst commenced (around June 14). This is the longest outburst of any of the accretion-powered millisecond pulsars, by a factor of approximately three. The X-ray flux dropped as low as 4e-10 ergs/cm²/s on July 19, and reached a second (local) minimum of 6e-10 ergs/cm²/s on September 7. Since then the flux has been rising gradually.

We detected a single thermonuclear (type-I) X-ray burst with the Proportional Counter Array (PCA) on July 21 which exhibited photospheric radius expansion, and which reached an estimated peak (bolometric) flux of 1.1e-7 ergs/cm²/s. The inferred distance (assuming an He-rich burst) is 5 kpc, consistent with the estimate from the discovery burst (ATel 534). Two additional bursts were detected with the High Energy Transient Explorer (HETE-II) satellite, and a third fell within an observation by the All Sky Monitor aboard RXTE on September 24. The accretion rate, adopting a bolometric correction to the flux in the 2.5-25 keV band of 1.96, is around 5e-10 M_sun/yr or 3% of the Eddington rate.

The 377.3 Hz pulsations, initially present between 0.8 and 2.7% rms amplitude, became undetectable (< 0.5% rms) shortly after a flare to around 1.2e-9 ergs/cm²/s on July 8 (see also Kaaret et al. 2005, astro-ph/0510483). The pulsations returned at a slightly lower pulse fraction of 0.5-1.7% rms between July 19 and August 19. Since then the rms amplitude has never risen above 0.5% and is usually undetectable.

We encourage observations in other wavebands.