Astrophys. J., 714, 894-903 (2010/May-1)
Discovery of a second transient low-mass X-ray binary in the globular cluster NGC 6440.
HEINKE C.O., ALTAMIRANO D., COHN H.N., LUGGER P.M., BUDAC S.A., SERVILLAT M., LINARES M., STROHMAYER T.E., MARKWARDT C.B., WIJNANDS R., SWANK J.H., KNIGGE C., BAILYN C. and GRINDLAY J.E.
Abstract (from CDS):
We have discovered a new transient low-mass X-ray binary, NGC 6440 X-2, with Chandra/ACIS, RXTE/PCA, and Swift/XRT observations of the globular cluster NGC 6440. The discovery outburst (2009 July 28-31) peaked at LX∼ 1.5 x 1036 erg/s and lasted for <4 days above LX= 1035 erg/s. Four other outbursts (2009 May 29-June 4, August 29-September 1, October 1-3, and October 28-31) have been observed with RXTE/PCA (identifying millisecond pulsations) and Swift/XRT (confirming a positional association with NGC 6440 X-2), with similar peak luminosities and decay times. Optical and infrared imaging did not detect a clear counterpart, with best limits of V>21, B>22 in quiescence from archival Hubble Space Telescope imaging, g'>22 during the August outburst from Gemini-South GMOS imaging, and J ≳ 18.5 and K ≳ 17 during the July outburst from CTIO 4 m ISPI imaging. Archival Chandra X-ray images of the core do not detect the quiescent counterpart (LX< (1-2)x1031 erg/s) and place a bolometric luminosity limit of LNS< 6x1031 erg/s (one of the lowest measured) for a hydrogen atmosphere neutron star. A short Chandra observation 10 days into quiescence found two photons at NGC 6440 X-2's position, suggesting enhanced quiescent emission at LX∼ 6x1031 erg/s. NGC 6440 X-2 currently shows the shortest recurrence time (∼31 days) of any known X-ray transient, although regular outbursts were not visible in the bulge scans before early 2009. Fast, low-luminosity transients like NGC 6440 X-2 may be easily missed by current X-ray monitoring.
dense matter - pulsars: general - stars: neutron - X-rays: binaries
[HAC2010] NGC 6440 X-2 N=1 = CXOGlb JHHMMSS.s+DDMMSS N=1.
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