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Detection of coherent millisecond pulsations at ~518 Hz in the LMXB Swift J1749.4-2807 (GRB060602B)

ATEL # 2565; D. Altamirano, R. Wijnands, M. van der Klis, A. Patruno, A. Watts , M. Armas Padilla, Y. Cavecchi, N. Degenaar, M. Kalamkar, R. Kaur, Y.J. Yang (University of Amsterdam), P. Casella (Southampton), M. Linares (MIT), P. Soleri (Groningen) and N. Rea (IEEC-CSIC)
on 15 Apr 2010; 14:18 UT
Distributed as an Instant Email Notice (Request for Observations)
Password Certification: Diego Altamirano (diego@science.uva.nl)

Subjects: X-ray, Request for Observations, Binaries, Neutron Stars, Pulsars, Transients
Referred to by ATEL #: 2568, 2576

On April 10th, it was discovered (ATel #2548) using INTEGRAL that the neutron-star X-ray binary Swift J1749.4-2807 (Wijnands et al. 2009, MNRAS, 393, 126) was again in outburst. Using follow-up Swift and INTEGRAL observations the source activity was confirmed (ATel #2548 and #2561).

On April 14th, 2010 (20:58:00 UT), RXTE performed a pointed observation of Swift J1749.4-2807. The observation lasted for about 1.6 ksec, at a background-corrected count rate of ~30 cts/sec/PCU (2 PCUs were active during this observation) in the 2-16 keV range. This corresponds to an intensity of about ~17 mCrab. In these RXTE data we discovered highly coherent signals at an unbarycentered frequency of ~518 Hz and its first overtone at ~1036 Hz. Using 128 seconds data segments, the frequency was seen to be drifting by 0.12 Hz, suggestive of Doppler drifts due to a combination of the satellite orbit and a possible binary orbit. Given the short observation and the lack of RXTE orbital information it is difficult to estimate the orbital period, but the present data suggests it should be larger than 2500 seconds. Assuming that both signals are present during the whole observation, we estimate an average fractional rms amplitude of ~5.5% and ~6.8% for the signals at ~518 Hz and ~1036 Hz, respectively.

Further RXTE observations have been requested to study the pulsations in detail and to constrain the binary orbital parameters. Observations at other wavelengths are strongly encouraged.


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