DISCOVERY OF A 205.89 Hz ACCRETING MILLISECOND X-RAY PULSAR IN THE GLOBULAR CLUSTER NGC 6440

Journal Article · Sat Mar 20 04:00:00 EDT 2010 · Astrophysical Journal Letters

DOI:https://doi.org/10.1088/2041-8205/712/1/L58; COUNTRY OF INPUT: INTERNATIONAL ATOMIC ENERGY AGENCY (IAEA)· OSTI ID:21305089

Altamirano, D; Patruno, A; Linares, M; Wijnands, R; Van der Klis, M ^[1]; Heinke, C O ^[2]; Markwardt, C; Strohmayer, T E

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1. Astronomical Institute, 'Anton Pannekoek', University of Amsterdam, Science Park 904, 1098XH, Amsterdam (Netherlands)
2. Department of Physics, University of Alberta, Room 238 CEB, 11322-89 Avenue, Edmonton, AB T6G 2G7 (Canada)

We report on the discovery of the second accreting millisecond X-ray pulsar (AMXP) in the globular cluster NGC 6440. Pulsations with a frequency of 205.89 Hz were detected with RXTE on 2009 August 30, October 1 and October 28, during the decays of {approx}<4 day outbursts of a newly X-ray transient source in NGC 6440. By studying the Doppler shift of the pulsation frequency, we find that the system is an ultra-compact binary with an orbital period of 57.3 minutes and a projected semimajor axis of 6.22 lt-ms. Based on the mass function, we estimate a lower limit to the mass of the companion to be 0.0067 M {sub sun} (assuming a 1.4 M {sub sun} neutron star). This new pulsar shows the shortest outburst recurrence time among AMXPs ({approx}1 month). If this behavior does not cease, this AMXP has the potential to be one of the best sources in which to study how the binary system and the neutron star spin evolve. Furthermore, the characteristics of this new source indicate that there might exist a population of AMXPs undergoing weak outbursts which are undetected by current all-sky X-ray monitors. NGC 6440 is the only globular cluster to host two known AMXPs, while no AMXPs have been detected in any other globular cluster.

• OSTI ID: 21305089
• Journal Information: Astrophysical Journal Letters, Journal Name: Astrophysical Journal Letters Journal Issue: 1 Vol. 712; ISSN 2041-8205
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
99 GENERAL AND MISCELLANEOUS
DOPPLER EFFECT
MASS
NEUTRON STARS
PULSARS
PULSATIONS
SPIN
X RADIATION