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A Method for Distinguishing Between Transiently Accreting Neutron Stars and Black Holes in Quiescence

Rutledge, Robert E. and Bildsten, Lars and Brown, Edward F. and Pavlov, George G. and Zavlin, Vyacheslav E. (2000) A Method for Distinguishing Between Transiently Accreting Neutron Stars and Black Holes in Quiescence. Astrophysical Journal, 529 (2). pp. 985-996. ISSN 0004-637X. doi:10.1086/308303.

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Neutron stars and black holes often reside in binaries where the accretion rate onto the compact object varies by orders of magnitude. These "X-ray transients" are observed both in outburst (when the high accretion rate makes them X-ray bright) and quiescence (when the accretion rate is very low, or potentially zero). In a previous paper, we showed that the quiescent X-ray emission from three neutron star transients (Aql X-1, Cen X-4, and 4U 1608-522) were well represented by thermal emission from the neutron star's hydrogen atmosphere and that the emitting area was consistent with the whole surface. Previous blackbody spectral fits (which are not accurate representations of the thermal spectrum) severely underestimated the true emitting area. In this paper, we fit hydrogen atmosphere models to the X-ray data for four neutron stars (the three from the previous paper, plus 4U 2129+47) and six black hole candidates (A0620-00, GS 2000+25, GS 1124-68, GS 2023+33, GRO J1655-40, and GRO J0422+32) with masses ≳ 3 M_☉. While the neutron stars are similar in their intrinsic X-ray spectra (that is, similar effective temperatures and emission area radii ~10 km), the spectra of two black hole candidates are significantly different, and the spectra of the remaining four are consistent with a very large parameter space that includes the neutron stars. The spectral differences between the neutron stars and black hole candidates favors the interpretation that the quiescent neutron star emission is predominantly thermal emission from the neutron star surface. Higher quality data from Chandra, XMM, and ASTRO-E will yield a much better contrast. There are many transients which do not have clear neutron star characteristics (such as type I X-ray bursts or coherent pulsations) and where the mass of the compact object is not constrained. In these cases, it is ambiguous as to whether the compact object is a neutron star or black hole. Our work suggests that an X-ray spectral comparison in quiescence provides an additional means for distinguishing between neutron stars and black holes. The faint X-ray sources in globular clusters—thought to be either cataclysmic variables or quiescent neutron stars—are a class of objects which can be investigated in this manner.

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Additional Information:© 2000. The American Astronomical Society. Received 1999 May 20; accepted 1999 September 22. This research was supported by NASA via grant NAGW-4517 and through a Hellman Family Faculty Fund Award (UC-Berkeley) to L. B., who is also a Cottrell Scholar of the Research Corporation. E. F. B. is supported by NASA GSRP Graduate Fellowship under grant NGT 5-50052. G. G. P. acknowledges support from NASA grants NAG 5-6907 and NAG 5-7017. This research was supported in part by the National Science Foundation under grant PHY 94-07194. We acknowledge use of data obtained through the High Energy Astrophysics Science Archive Research Center Online Service, provided by the NASA/Goddard Space Flight Center. We gratefully acknowledge useful conversations with D. Fox and A. Prestwich regarding ROSAT/HRI response issues. We gratefully acknowledge helpful comments from J. McClintock and from the referee, J. Grindlay.
Group:Space Radiation Laboratory
Funding AgencyGrant Number
Hellman Family Faculty FundUNSPECIFIED
Cottrell Scholar of Research CorporationUNSPECIFIED
NASA Graduate FellowshipNGT 5-50052
NASANAG 5-6907
NASANAG 5-7017
NSFPHY 94-07194
Subject Keywords:accretion, accretion disks; black hole physics; stars: atmospheres; stars: neutron
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Other Numbering System NameOther Numbering System ID
Space Radiation Laboratory2000-05
Issue or Number:2
Record Number:CaltechAUTHORS:20150219-085446936
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:54989
Deposited By: Deborah Miles
Deposited On:24 Feb 2015 18:02
Last Modified:10 Nov 2021 20:40

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