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Thermonuclear burst oscillations from the 401 Hz pulsar IGR J17498-2921

ATel #3643; Manoneeta Chakraborty (TIFR), Sudip Bhattacharyya (TIFR)
on 10 Sep 2011; 19:17 UT
Credential Certification: Sudip Bhattacharyya (sudip@tifr.res.in)

Subjects: X-ray, Binary, Neutron Star, Transient, Pulsar

A thermonuclear burst has been reported from the RXTE data of the transient neutron star low-mass X-ray binary IGR J17498-2921 (Atel #3568), which is also a 401 Hz pulsar and went into an outburst in August 2011 (Atels #3551, #3555, #3556, #3558, #3559, #3560, #3561, #3562, #3563, #3601, #3606, #3622, #3638). We report the detection of nine additional bursts from the RXTE data up to September 7, 2011. One of these bursts (2011-08-20; observation starts at 13:16:32) was clearly a photospheric radius expansion (PRE) burst. Its bolometric touchdown flux was roughly 4.8e-8 ergs/cm2/s, which would give a similar distance as reported in Atel #3568. The peak count rates of all other bursts are at least an order of magnitude smaller than those of this burst and the first RXTE burst. Linares et al. (Atel #3568) have reported burst oscillations from the first RXTE burst. Searching within 3 Hz (on both sides) of the known pulsar frequency and considering 10 bursts, we have found an approximately 4 sigma significance of these oscillations. This suggests that a further and stronger detection would be required for confirmation of burst oscillation from this source. From the above mentioned PRE burst of 2011-08-20, we have detected burst oscillations during a few seconds of the burst tail. We have calculated power spectrum from each of 20 one second segments of the burst (from rise to 10% of the peak flux), and from the average of these 20 power spectra we have found a peak power of ~7.4 at ~401 Hz. With a number of trials of 60 (6 from the searched frequency range, 10 from the number of bursts), the significance of the burst oscillations is roughly 7 sigma. The amplitude of these oscillations cannot be explained from the persistent pulsations. These confirm burst oscillations from IGR J17498-2921.