Discovery of a Second Transient Low-mass X-ray Binary in the Globular Cluster Ngc 6440

Abstract

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 L-X similar to 1.5 x 10(36) erg s(-1) and lasted for < 4 days above L-X = 10(35) erg s(-1). 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 greater than or similar to 18.5 and K greater than or similar to 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 (L-X < (1-2) x 10(31) erg s(-1)) and place a bolometric luminosity limit of LNS < 6 x 10(31) erg s(-1) (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 L-X similar to 6 x 10(31) erg s(-1). NGC 6440 X-2 currently shows the shortest recurrence time (similar to 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.