Type-II bursts from the new Terzan 5 transient: a GRO J1744-28 analogue?

ATel #3000; D. K. Galloway (Monash University), J. J. M. in 't Zand (SRON)
on 2 Nov 2010; 12:41 UT
Credential Certification: Duncan K. Galloway (duncan@space.mit.edu)

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We report on a further analysis of Rossi X-ray Timing Explorer observations of the new bursting transient in the globular cluster Terzan 5 (ATels #2919, #2920, #2922, #2924, #2929, #2932, #2933, #2935, #2937, #2939, #2946, #2952, #2958), identified with CXOG1b J174804.8-244648 (ATel #2974), aka IGR J17480-2446 (ATel #2940). We identified a total of 202 X-ray bursts from this source between 2010 Oct 13 and 26; the true number of bursts in some of the observations is uncertain, due to their faintness and (at times) high persistent variability. We performed time-resolved spectroscopy of the net burst spectra, subtracting the pre-burst emission as background, following the conventional approach; we fit an absorbed blackbody to each spectrum, with the column density for the neutral absorption component fixed at 3.8e22 cm^{-2} (Kuulkers et al. 2003, A&A 399, 663). The typical integration time was 2s, and 99% of the spectra (above a flux limit of 0.05 of the maximum reached in each burst) had reduced chi^2 values for the fit of <1.84, indicating that the blackbody fit is statistically acceptable in the entire sample.

The first burst, on 2010 October 13 (see also ATel #2929), reached a peak flux of (9.9+/-0.5)e-9 erg/cm^2/s (corresponding to 9e37 erg/s at 8.7 kpc, the approximate distance to Terzan 5) and showed a clear cooling trend from a maximum of approximately 2.2 keV down to 1.6 keV after 60s. However, we did not measure significant (>2 sigma confidence) cooling in any of the subsequent bursts, which were fainter, peaking at between 1 and 5e9 erg/cm^2/s. For selected observations, we also combined spectra from subsequent bursts to improve the precision of the spectral parameters, but still found no significant evidence for cooling.

We conclude that the faint events detected after October 13 are likely type-II bursts, for which the usual explanation is episodic accretion. Further support for this conclusion comes from the burst recurrence times; separations as short as 5 min have been observed previously for type-I (thermonuclear) bursts, but for groups of no more than 4 consecutive events (Keek et al. 2010, ApJ 718, 292), and never for trains of 10 or more, as seen from the new transient. We note that the relatively wide orbit, slow pulsations, and type-II bursts are characteristics shared by the bursting pulsar, GRO J1744-28, discovered in 1996 (e.g. Kouveliotou et al. 1996, Nature 379, 799; Finger et al. 1996, Nature 381, 291); we suggest that the new Terzan 5 transient is only the second such source ever discovered, and the first to also show a type-I (thermonuclear) burst. In that case, the mHz QPOs (ATel #2958) are likely also related to the accretion dynamics, rather than quasi-stable nuclear burning.

This analysis was carried out for the Multi-INstrument Burst ARchive (MINBAR) project; see http://users.monash.edu.au/~dgallow/minbar