Terzan 5 transient IGR J17480-2446: return of thermonuclear bursts or were they always there?

ATel #3044; Manoneeta Chakraborty (TIFR), Sudip Bhattacharyya (TIFR)
on 19 Nov 2010; 21:05 UT
Credential Certification: Sudip Bhattacharyya (sudip@tifr.res.in)

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We report Rossi X-ray Timing Explorer data analysis of the Terzan 5 source IGR J17480-2446, which is currently in outburst (ATels #2919, #2920, #2922, #2924, #2929, #2932, #2933, #2935, #2937, #2939, #2946, #2952, #2958, #2974, #3000). Galloway and in 't Zand (ATel #3000) reported that after the 2010 October 13 thermonuclear burst, no other burst up to 2010 October 26 showed significant (>2 sigma confidence) cooling during burst decay. This led them to suggest that except the first burst, all other bursts (and hence the mHz QPOs) were powered by the gravitational potential energy. It has been noticed that as the source intensity increased during the rise of the outburst, the burst peak flux and the burst recurrence time roughly decreased. Similarly, during the decay of the source outburst, the burst peak flux and the burst recurrence time roughly increased. We have found significant (>2 sigma confidence) cooling during decay of the bursts observed on 2010 November 13, 14, 15, 18. For example, the temperature of the first 2010 November 14 burst approximately decreased from 2.83(-0.25,+0.31) keV to 1.89(-0.24,+0.27) keV in 41 seconds. This might indicate the return of thermonuclear bursts as the persistent intensity to burst peak intensity ratio decreased. For example, this ratio was approximately 104mCrab/365mCrab, 281mCrab/229mCrab, 268mCrab/232mCrab, 250mCrab/260mCrab, 271mCrab/292mCrab for our analyzed 2010 October 13, November 13, 14, 15, 18 bursts respectively. On the other hand this ratio was 416mCrab/90mCrab for a 2010 October 15 burst, for which no significant cooling was detected. However, it is not yet clear if the frequent faint bursts during the high intensity of the source were different from the 2010 October 13, November 13, 14, 15, 18 bursts, and were powered by the gravitational potential energy. The reasons are the following. (1) The difference of properties between these two kinds of bursts is usually gradual, and not abrupt (see also #2958). For example, the burst decay temperature profiles of even 2010 October 13, November 13, 14, 15, 18 bursts show large fluctuations, like those of the faint bursts.(2) Several (e.g., 18th burst of EXO 0748-676) thermonuclear bursts mentioned in Galloway et al., 2008, ApJS, 179, 360 showed apparent temperature increase during burst decay. So cooling trend is not always clear even for established thermonuclear bursts. (3) The estimated integrated persistent flux to integrated burst flux ratio (in 3-15 keV), which is approximately a measure of the gravitational potential energy release per nucleon to nuclear energy release per nucleon, is roughly close (within a factor of 3) to the canonical value 40 for the 2010 October 15 bursts, and hence these bursts might be powered by nuclear energy. Note that these bursts did not show any cooling trend. We also note that Lewin et al., 1996, ApJ, 462, L39 reported a ratio of 4 in 20-50 keV (which is an order of magnitude different from the canonical value 40) for the bursts of GRO J1744-28, and hence concluded that these bursts had been powered by the gravitational potential energy. If our estimated ratio is confirmed and is true for the entire data set of the Terzan 5 source, then it might be difficult to argue that this source is a GRO J1744-28 analogue (as mentioned in Atel #3000).