DISCOVERY OF A SECOND TRANSIENT LOW-MASS X-RAY BINARY IN THE GLOBULAR CLUSTER NGC 6440

Journal Article · Sat May 01 04:00:00 EDT 2010 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/714/1/894· OSTI ID:21448866

Heinke, C O; Budac, S A ^[1]; Altamirano, D; Linares, M; Wijnands, R ^[2]; Cohn, H N; Lugger, P M ^[3]; Servillat, M; Grindlay, J E ^[4]; Strohmayer, T E; Markwardt, C B; Swank, J H ^[5]; Knigge, C ^[6]; Bailyn, C ^[7]

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1. Department of Physics, University of Alberta, Room 238 CEB, Edmonton, AB T6G 2G7 (Canada)
2. Astronomical Institute 'Anton Pannekoek', University of Amsterdam, Science Park 904, 1098 XH, Amsterdam (Netherlands)
3. Department of Astronomy, Indiana University, 727 East Third St., Bloomington, IN 47405 (United States)
4. Harvard College Observatory, 60 Garden Street, Cambridge, MA 02138 (United States)
5. Astrophysics Science Division, NASA/GSFC, Greenbelt, MD 20771 (United States)
6. School of Physics and Astronomy, University of Southampton, Hampshire SO17 1BJ (United Kingdom)
7. Department of Astronomy, Yale University, P.O. Box 208101, New Haven, CT 06520 (United States)

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{sub X} {approx} 1.5 x 10{sup 36} erg s{sup -1} and lasted for <4 days above L{sub X} = 10{sup 35} erg s{sup -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 {approx_gt} 18.5 and K {approx_gt} 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{sub X} < (1-2) x 10{sup 31} erg s{sup -1}) and place a bolometric luminosity limit of L{sub NS} < 6 x 10{sup 31} erg s{sup -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{sub X} {approx} 6 x 10{sup 31} erg s{sup -1}. NGC 6440 X-2 currently shows the shortest recurrence time ({approx}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.

• OSTI ID: 21448866
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 1 Vol. 714; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ATMOSPHERES
BARYONS
BOLOMETERS
BOSONS
COSMIC RADIO SOURCES
ELECTROMAGNETIC RADIATION
ELEMENTARY PARTICLES
ELEMENTS
FERMIONS
HADRONS
HYDROGEN
IONIZING RADIATIONS
LUMINOSITY
MASS
MASSLESS PARTICLES
MEASURING INSTRUMENTS
NEUTRON STARS
NEUTRONS
NONMETALS
NUCLEONS
OPTICAL PROPERTIES
PHOTONS
PHYSICAL PROPERTIES
PULSARS
RADIATIONS
STARS
STELLAR ATMOSPHERES
TELESCOPES
X RADIATION