DISCOVERY OF A SECOND TRANSIENT LOW-MASS X-RAY BINARY IN THE GLOBULAR CLUSTER NGC 6440
Author(s)Heinke, C. O.; Altamirano, D.; Cohn, H. N.; Lugger, P. M.; Budac, S. A.; Servillat, M.; Linares, Manuel Alegret; Strohmayer, T. E.; Markwardt, Craig B.; Wijnands, R.; Swank, J. H.; Knigge, C.; Bailyn, Charles D.; Grindlay, Jonathan E.; ... Show more Show less
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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 × 10[superscript 36] erg s[superscript –1] and lasted for <4 days above LX = 10[superscript 35] erg s[superscript –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 gsim 18.5 and K gsim 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) × 10[superscript 31] erg s[superscript –1]) and place a bolometric luminosity limit of L NS < 6 × 1031 erg s[superscript –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 LX ~ 6 × 10[superscript 31] erg s[superscript –1]. 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.
DepartmentMIT Kavli Institute for Astrophysics and Space Research
Institute of Physics/American Astronomical Society
Heinke, C. O., D. Altamirano, H. N. Cohn, P. M. Lugger, S. A. Budac, M. Servillat, M. Linares, et al. “DISCOVERY OF A SECOND TRANSIENT LOW-MASS X-RAY BINARY IN THE GLOBULAR CLUSTER NGC 6440.” The Astrophysical Journal 714, no. 1 (April 15, 2010): 894–903. © 2010 American Astronomical Society.
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