Detection of pulsations and identification of SAX J1748.9-2021 as the X-ray transient in NGC 6440.

ATel #2407; Patruno, A.; Altamirano, D.; Watts, A.; Armas Padilla, M.; Cavecchi, Y.; Degenaar, N.; Kalamkar, M.; Kaur, R.; Soleri, P.; Yang, Y. J.; van der Klis, M.; Wijnands, R. (University of Amsterdam); Casella, P. (Southampton); Linares, M. (MIT); Rea, N. (IEEC-CSIC); Heinke, C. O. (U. Alberta); Pooley, D. (Eureka-Scientific)
on 27 Jan 2010; 18:18 UT
Credential Certification: Diego Altamirano (diego@science.uva.nl)

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On the 28th of December, MAXI/GSC detected an X-ray outburst from the globular cluster NGC6440 (Atel #2360), which has continued to the present. This globular cluster is known to have at least 24 X-ray sources (Pooley et al. 2002, ApJ 573, 184) of which two have been identified as accreting-millisecond X-ray pulsars: SAX J1748.9-2021 (Altamirano et al. 2008, 674, 45A) and NGC 6440 X-2 (Altamirano et al. 2009, arXiv:0911.0435v1). The new X-ray outburst was tentatively identified with the intermittent accreting millisecond X-ray pulsar SAX J1748.9-2021 (Atel #2360).

Due to visibility constraints, RXTE observations were only possible starting on January 18th, 2010. Since then, 11 observations were performed, with the source detected in the 2-16 keV range with an intensity between 130 and 180 mCrab. From the power spectral shape and color-color diagrams, it is evident that the source is in the soft (banana) state. No X-ray pulsations were detected in the first 10 RXTE observations, while significant coherent 442 Hz pulsations were observed in the 11th RXTE pointing performed on January 26th at 12:49:36 UT. The presence of coherent pulsations at this frequency confirms that the X-ray transient is indeed SAX J1748.9-2021.

We have derived a preliminary pulse timing solution for this new outburst by phase connecting the pulsations observed during 1.8 ks of observations in the interval MJD 55222.53-55222.55. The timing solution is entirely consistent with that previously reported for the 2001 outburst (Patruno et al. 2009, ApJ 690, 1856). The pulsations have a fractional amplitude of 0.7-1.9% rms (2-16 keV) and appear at a barycentered pulse frequency of 442.362(4) Hz, consistently with what was observed in the 2001 outburst. The pulsations sometimes disappear (for approximately 800 s out of 1.8 ks of data) with an upper limit on the fractional amplitude of 0.6% rms at 95% confidence level.

Additional RXTE observations are planned.