Further X-ray observations of EXO 0748-676 in quiescence: evidence for a cooling neutron star crust

Authors

N. Degenaar, Astronomical Institute 'Anton Pannekoek', University of Amsterdam
M. T. Wolff, Naval Research Laboratory
P. S. Ray, Naval Research Laboratory
K. S. Wood, Naval Research Laboratory
J. Homan, MIT Kavli Institute for Astrophysics and Space Research
W. H. G. Lewin, MIT Kavli Institute for Astrophysics and Space Research
P. G. Jonker, SRON, Netherlands Institute for Space Research
E. M. Cackett, University of MichiganFollow
J. M. Miller, University of Michigan
E. F. Brown, Michigan State University
R. Wijnands, Astronomical Institute 'Anton Pannekoek,' University of Amsterdam

Document Type

Article

Abstract

In late 2008, the quasi-persistent neutron star X-ray transient and eclipsing binary EXO 0748-676 started a transition from outburst to quiescence, after it actively accreted for more than 24 yr. In a previous work, we discussed Chandra and Swift observations obtained during the first 5 months of this transition. Here, we report on further X-ray observations of EXO 0748-676, extending the quiescent monitoring to 1.6 yr. Chandra and XMM-Newton data reveal quiescent X-ray spectra composed of a soft, thermal component that is well fitted by a neutron star atmosphere model. An additional hard power-law tail is detected that changes non-monotonically over time, contributing between 4 and 20 per cent to the total unabsorbed 0.5-10 keV flux. The combined set of Chandra, XMM-Newton and Swift data reveals that the thermal bolometric luminosity fades from ˜ 1 × 10³⁴ to 6 × 10³³ (D/7.4 kpc)² erg s ^-1, whereas the inferred neutron star effective temperature decreases from ˜124 to 109 eV. We interpret the observed decay as cooling of the neutron star crust and show that the fractional quiescent temperature change of EXO 0748-676 is markedly smaller than observed for three other neutron star X-ray binaries that underwent prolonged accretion outbursts.

Disciplines

Cosmology, Relativity, and Gravity | Stars, Interstellar Medium and the Galaxy

Comments

NOTICE IN COMPLIANCE WITH PUBLISHER POLICY: This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©2011 The Authors. Published by Oxford University Press on behalf of The Royal Astronomical Society. All rights reserved. Available at doi:10.1111/j.1365-2966.2010.17562.x

Recommended Citation

Degenaar, N., Wolff, M. T., Ray, P. S., Wood, K. S., Homan, J., Lewin, W. H. G., Jonker, P. G., Cackett, E. M., Miller, J. M., Brown, E. F., Wijnands, R. 2011. Further X-ray observations of EXO 0748-676 in quiescence: evidence for a cooling neutron star crust. MNRAS 412(3), 1409-1418.