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Title:
Discovery of a Second Transient Low-Mass X-ray Binary in the Globular Cluster NGC 6440
Authors:
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.; Grindlay, J. E.
Publication:
eprint arXiv:0911.0444
Publication Date:
11/2009
Origin:
ARXIV
Keywords:
Astrophysics - High Energy Astrophysical Phenomena, Astrophysics - Galaxy Astrophysics
Comment:
21 pages, 8 (color) figures, submitted to ApJ
Bibliographic Code:
2009arXiv0911.0444H

Abstract

We have identified a new transient luminous 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 (July 28-31, 2009) peaked at L_X~1.5*10^36 ergs/s, and lasted for <4 days above L_X=10^35 ergs/s. Three other outbursts (May 29-June 4, Aug. 29-Sept. 1, and Oct. 1-3, 2009) have been observed with RXTE/PCA (identifying millisecond pulsations, Altamirano et al. 2009a) and Swift/XRT (confirming a positional association with NGC 6440 X-2). Optical and infrared imaging did not detect a clear counterpart, with best limits of V>21, B>22 in quiescence from archival HST imaging, g'>22 during the third outburst from Gemini-South GMOS imaging, and J>~18.5$ and K>~17 during the second outburst from CTIO 4-m ISPI imaging. Archival Chandra X-ray images of the core do not detect the quiescent counterpart, and place a bolometric luminosity limit of L_{NS}< 5.6*10^31 ergs/s (one of the lowest measured) for a hydrogen atmosphere neutron star. A followup Chandra observation finds marginal evidence of enhanced quiescent emission at L_X (0.5-10 keV)~6*10^31 ergs/s 10 days into quiescence. NGC 6440 X-2 currently shows the shortest recurrence time (32 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.
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