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New outburst of the accreting-millisecond X-ray pulsar NGC 6440 X-2

ATEL # 2500; D. Altamirano (UvA), A. Patruno (Uva), C. Heinke (U Alberta), M. Linares (MIT), C. Markwardt (U. Maryland & NASA/GSFC) & T. Strohmayer (NASA/GSFC).
on 22 Mar 2010; 22:02 UT
Distributed as an Instant Email Notice (Transients)
Password Certification: Diego Altamirano (diego@science.uva.nl)

Subjects: X-ray, Neutron Stars, Pulsars, Transients

On Friday 19th, 2010 (19h 13m 26s UT), the RXTE galactic bulge scans detected a flux excess in the direction of the globular cluster NGC 6440. This globular cluster is known to harbor 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 (AMXPs): NGC 6440 X-2 (Altamirano et al. 2010, ApJ, 712, 58) and SAX J1748.9-2021 (Altamirano et al. 2008, ApJ, 674, 45A).

To identify the X-ray transient, a ~1 ksec Swift observation was performed in PC mode on March 20, at 18h 16m 32s (UT). The spectrum is well fitted with an absorbed (Nh=0.6 10^22 / cm^2) power law with index ~1.9. The 0.5-10 keV luminosity is ~1.6+-0.2 10^36 ergs/sec (assuming a distance of 8.5 kpc). The source position is consistent with the AMXP NGC 6440 X-2 (Heinke et al. 2010, astro-ph:0911.0444) and not with any other identified X-ray source in NGC 6440.

RXTE observed the globular cluster NGC 6440 in three occasions on March 21st. Each observation lasted for about 2 ksec. At 05h19m25s, 16h13m33s and 23:57:17 (UTC) the source was at 7.4, 6.7 and 2.7 x 10^35 erg/s, respectively (2.5-25 keV, assuming a distance of 8.5 kpc - the spectra were all well fitted with the same model used for the Swift data). We found pulsations in all three observations at a barycentric frequency of 205.8921 Hz and with fractional rms amplitudes between ~9 and ~12% (2-16 keV range). These values are consistent with those previously measured for NGC 6440 X-2.

Short duration outbursts (3-5 days) of this source were observed between August-November 2009. Heinke et al. 2010 estimated that the outbursts were occurring quasi-periodically every ~30 days. Due to visibility constraints and the fact that SAX J1748.9-2021 was in outburst until recently (ATELs #2360 and #2407), we do not know of the occurrence of any other outburst of NGC 6440 between November 2009 and now. This new detection confirms that the source is still undergoing short and faint outbursts. However, its occurrence is out of phase by ~10 days, which shows that the outburst recurrence rate has changed. The detection of future outbursts will allow us to understand if the recurrence rate has increased or decreased.

We thank both RXTE and Swift teams for rapidly scheduling the observations.


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