Cackett, E. M., Miller, J. M., Raymond, J., Homan, J., van der Klis, M., Mendez, M. M., Steeghs, D. (Danny), 1972- and Wijnands, R.. (2008) Investigating the nature of absorption lines in the Chandra X-ray spectra of the neutron star binary 4U 1820−30. The Astrophysical Journal, Vol.677 (No.2). pp. 1233-1240. ISSN 0004-637X

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Abstract

We use four Chandra gratings spectra of the neutron star low-mass X-ray binary 4U 1820–30 to better understand the nature of certain X-ray absorption lines in X-ray binaries, including the Ne II, Ne III, Ne IX, O VII, and O VIII lines. The equivalent widths of the lines are generally consistent between the observations, as expected if these lines originate in the hot interstellar medium. No evidence was found that the lines were blueshifted, again supporting the interstellar medium origin, although this may be due to poor statistics. There is apparent variability in the O VIII Lyα line equivalent width providing some evidence that at least some of the O VIII absorption arises within the system. However, the significance is marginal (2.4 σ), and the lack of variation in the other lines casts some doubt on the reality of the variability. From calculating the equivalent hydrogen column densities for a range of Doppler parameters, we find they are consistent with the interstellar origin of the lines. In addition, we fit the spectra with photoionization models for locally absorbing material, and find that they can reproduce the spectrum well, but only when there is an extremely low filling factor. We conclude that both the ISM and local absorption remain possible for the origin of the lines, but that more sensitive observations are needed to search for low-level variability.

Item Type:	Journal Article
Subjects:	Q Science > QB Astronomy
Divisions:	Faculty of Science > Physics
Library of Congress Subject Headings (LCSH):	X-ray binaries, Neutron stars
Journal or Publication Title:	The Astrophysical Journal
Publisher:	Institute of Physics Publishing Ltd.
ISSN:	0004-637X
Date:	2008
Volume:	Vol.677
Number:	No.2
Page Range:	pp. 1233-1240
Identification Number:	10.1086/529483
Status:	Peer Reviewed
Publication Status:	Published
Access rights to Published version:	Restricted or Subscription Access
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URI:	http://wrap.warwick.ac.uk/id/eprint/48672

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