Discovery of 11 Hz burst oscillations from the 11 Hz Eclipsing Pulsar in Terzan 5

ATel #2932; D. Altamirano (Amsterdam), A. Watts (Amsterdam), M. Kalamkar (Amsterdam), J. Homan (MIT), Y. Yang (Amsterdam), P. Casella (Southampton), M. Linares (MIT), A. Patruno (Amsterdam), M. Armas-Padilla (Amsterdam), Y. Cavecchi (Amsterdam), N. Degenaar (Amsterdam), R. Kaur (Amsterdam), M. van der Klis (Amsterdam), N. Rea (CSIC-IEEC), R. Wijnands (Amsterdam)
on 13 Oct 2010; 16:36 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: Diego Altamirano (diego@science.uva.nl)

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An RXTE pointed observation of the ongoing outburst of of the transient source in the globular cluster Terzan 5 (ATEL #2919, #2920, #2922, #2924, #2929) began at approximately 2010-10-13 at 00:13 UTC, for an exposure of 3.2 ksec. This observation revealed strong pulsations at a barycentric frequency of 11.0452(2) Hz as well as one Type I X-ray burst (Atel #2929).

In a detailed analysis of the X-ray burst we find strong burst oscillations at the non-barycentered frequency of ~11.05 Hz, consistent with the spin frequency of this neutron star. Maximum Leahy power exceeds 50 in at least 15 independent 4s bins during the burst (using all photons in the 2-60 keV band). Even accounting for red noise at low frequencies, this puts the detection beyond doubt. Burst oscillation amplitudes reach up to 20-30% rms in this energy band. Note that we have verified that we are not simply detecting the accretion-powered pulsations during the burst: the amplitudes are such, and the countrates in the peak of the burst sufficiently high compared to the persistent emission, that the burst emission itself must be pulsed.

This is the most slowly rotating neutron star for which burst oscillations have been detected, indicating that rapid rotation is not essential to the oscillation generation mechanism.

We also reanalyzed the Swift data (ObsID: 00031841002) reported by Bozzo et al (Atel #2922) and found a weak signal also at ~11 Hz, confirming that the pulsations come from the source detected in Terzan 5 and not other one in the 1^o FoV of XTE.

Observations at other wavelengths are strongly encouraged.