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ATEL # 2004; Watts, A.; Altamirano, D.; Casella, P.; Cavecchi, Y.; Degenaar, N.; Linares, M.; Patruno, A.; Rea, N.; Soleri, P.; van der Klis, M.; Wijnands, R (University of Amsterdam)
on 2 Apr 2009; 18:07 UT
Password Certification: Manuel Linares (email@example.com)
Subjects: X-ray, Binaries, Neutron Stars, Pulsars, Transients
RXTE observations of the intermittent accreting millisecond X-ray pulsar HETE J1900.1-2455 (Kaaret et al. 2006 ApJ 638, 963; Galloway et al. 2007 ApJ 654, L73) showed a Type I X-ray burst on April 2 2009 (08:57 UTC). We detect strong burst oscillations during the peak and initial decay of the burst, drifting upwards from 375.5 Hz to 376.5 Hz. Maximum Leahy power is in the range 25-45 in 4 independent consecutive 2s bins (using all photons in the 2-60 keV band), putting the detection beyond doubt.
The burst oscillation frequency is just below the known spin frequency of 377.3 Hz (the satellite orbit file is not available yet so we could not barycenter the data at this stage, but shifts due to barycentering and orbital effects will not exceed 0.1 Hz). Several bursts from this source have been detected with RXTE (Galloway et al. 2008 ApJS, 179, 360), but this is the first to show burst oscillations. HETE J1900.1-2455 is the second of the three known intermittent pulsars, all of which burst, to show burst oscillations (Casella et al. 2008 ApJ, 674, L41; Altamirano et al. 2008 ApJ, 674, L45). This new result supports the general picture that burst oscillation frequency lies within a few Hz of spin frequency (see Watts 2008, AIP Conference Proceedings, 1068,199 for a review).