RXTE detection of a thermonuclear burst from IGR J17498-2921: distance estimate and burst oscillations

ATel #3568; M. Linares (MIT), D. Altamirano, A. Watts (Amsterdam), T. Strohmayer (NASA/GSFC), D. Chakrabarty (MIT), A. Patruno, M. van der Klis, R. Wijnands (Amsterdam), P. Casella (Southampton), M. Armas-Padilla, Y. Cavecchi, N. Degenaar, M. Kalamkar, R. Kaur, Y. Yang (Amsterdam), N. Rea (CSIC-IEEC)
on 18 Aug 2011; 00:40 UT
Credential Certification: Manuel Linares (linares@mit.edu)

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We report the detection of a type I X-ray burst with burst oscillations during an RXTE-PCA observation of the recently discovered accreting millisecond pulsar (AMP) IGR J17498-2921 (ATels #3551, #3555, #3556, #3558, #3559, #3560, #3561, #3562, #3563). The burst intensity peaked on 2011-08-16 at 15:21:45 UTC. We find a rise time of ~0.4s (time span for burst intensity to go from 25% to 90% of net peak value) and an approximate total duration in the RXTE-PCA bandpass of 30s.

The X-ray (2.5-15 keV) spectrum is well described throughout the burst by an absorbed blackbody model (nH fixed at 3e22 cm^{-2}; ATel #3555), showing a clear cooling tail typical of thermonuclear (type I) X-ray bursts (kT decreasing from ~2.6 to ~1.4 keV along the burst decay). This confirms the source as a thermonuclear burster (ATel #3560), and makes IGR J17498-2921 the 7th out of the 14 currently known AMPs to show thermonuclear bursts.

The burst light curve shows two peaks: a local intensity maximum followed by the global intensity maximum about 2s later. We find evidence for a moderate photospheric radius expansion (PRE) phase during the burst rise and peaks, followed by a temperature increase (from ~1.8 to 2.6 keV) simultaneous with a (factor of 3) radius decrease. After "touchdown" the apparent emitting area remains roughly constant. We measure a maximum burst bolometric net flux of 5.6e-8 erg/cm2/s. Assuming that the maximum luminosity was equal to the value found for PRE bursts (3.8e38 erg/s; Kuulkers et al. 2003, A&A, 399, 663), we estimate a distance to IGR J17498-2921 of 7.6 kpc. The total radiated burst energy at 7.6kpc was ~3.2e39 erg.

Oscillations at a frequency consistent with the persistent pulsar frequency (~401 Hz; ATel #3556) are detected during a 2-3 second interval in the tail of the burst after the PRE episode. The average pulsed amplitude (measured as the half-amplitude) during this interval corresponds to approximately 240 counts/sec, which is significantly larger than the 9 counts/sec half-amplitude of the persistent pulsations before the burst. This confirms that the burst flux must be modulated at the spin frequency.