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The Quiescent X-Ray Spectrum of the Neutron Star in Centaurus X-4 Observed with Chandra/ACIS-S

Rutledge, Robert E. and Bildsten, Lars and Brown, Edward F. and Pavlov, G. G. and Zavlin, Vyacheslav E. (2001) The Quiescent X-Ray Spectrum of the Neutron Star in Centaurus X-4 Observed with Chandra/ACIS-S. Astrophysical Journal, 551 (2). pp. 921-928. ISSN 0004-637X. doi:10.1086/320247.

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We report on spectral and intensity variability analysis from a Chandra/ACIS-S observation of the transient, type I X-ray bursting low-mass X-ray binary Cen X-4. The quiescent X-ray spectrum during this observation is statistically identical to one observed previously with BeppoSAX and close, but not identical, to one observed previously with ASCA. The X-ray spectrum is best described as a pure hydrogen atmosphere thermal spectrum plus a power-law component that dominates the spectrum above 2 keV. The best-fit radius of the neutron star is r = 12.9 ± 2.6 (d/1.2 kpc) km if the interstellar absorption is fixed at the value implied by the optical reddening. Allowing the interstellar absorption to be a free parameter yields r = 19 ^(+45)_(-10) (d/1.2 kpc) km (90% confidence). The thermal spectrum from the neutron star surface is inconsistent with a solar metallicity. We find a 3 σ upper limit of rms variability ≤18% (0.2-2.0 keV; 0.0001-1 Hz) during the observation. On the other hand, the 0.5-10.0 keV luminosity decreased by 40 ± 8% in the 4.9 yr between the ASCA and Chandra observations. This variability can be attributed to the power-law component. Moreover, we limit the variation in thermal temperature to ≾10% over these 4.9 yr. The stability of the thermal temperature and emission-area radius supports the interpretation that the quiescent thermal emission is caused by the hot neutron star core.

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Additional Information:© 2001 American Astronomical Society. Received 2000 December 5; accepted 2000 December 18. The authors are grateful to the Chandra X-Ray Observatory team for producing this exquisite observatory. R. R. thanks Andy Fabian for a useful conversation regarding historical measurements of photospheric absorption lines. The authors thank Dany Page and Andrew Cumming for comments on the text prior to submission. This research was partially supported by the National Science Foundation under grant PHY99-07949 and by NASA through grant NAG 5-8658, NAG 5-7017, and the Chandra Guest Observer program through grant NAS 8-39073. L. B. is a Cottrell Scholar of the Research Corporation. E. F. B. acknowledges support from an Enrico Fermi Fellowship.
Group:Space Radiation Laboratory
Funding AgencyGrant Number
NASANAG 5-8658
NASANAG 5-7017
NASANAS 8-39073
Cottrell Scholar of Research CorporationUNSPECIFIED
Enrico Fermi FellowshipUNSPECIFIED
Subject Keywords:stars: neutron; stars: individual (Centaurus X-4); X-rays: binaries; X-rays: individual (Centaurus X-4); X-rays: stars
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Other Numbering System NameOther Numbering System ID
Space Radiation Laboratory2000-09
Issue or Number:2
Record Number:CaltechAUTHORS:20150219-092718794
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:54997
Deposited By: Deborah Miles
Deposited On:24 Feb 2015 01:05
Last Modified:10 Nov 2021 20:40

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