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
Credential Certification: Diego Altamirano (diego@science.uva.nl)
Subjects: X-ray, Request for Observations, Binary, Neutron Star, Transient, Pulsar
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.