Astronomy & Astrophysics

Free access article

Issue		A&A Volume 399, Number 3, March I 2003
Page(s)		1109 - 1114
Section		Stellar atmospheres
DOI		10.1051/0004-6361:20021747

A&A 399, 1109-1114 (2003)
DOI: 10.1051/0004-6361:20021747

The thermal radiation of the isolated neutron star RX J1856.5-3754 observed with Chandra and XMM-Newton

V. Burwitz, F. Haberl, R. Neuhäuser, P. Predehl, J. Trümper and V. E. Zavlin

Max-Planck-Institut für extraterrestrische Physik, PO Box 1312, 85741 Garching, Germany
(Received 24 October 2002 / Accepted 21 November 2002)

Abstract
We present results of the analysis of data collected in 57-ks XMM-Newton and 505-ks Chandra observations of the nearby ( $\simeq$ 120 pc) isolated neutron star RX J1856.5-3754. We confirm most of the statements made by Burwitz et al. (2001) who discussed the original 55-ks Chandra data. Detailed spectral analysis of the combined X-ray and optical data rules out the currently available nonmagnetic light and heavy element neutron star atmosphere (LTE) models with hydrogen, helium, iron and solar compositions. We find that strongly magnetized atmosphere models also are unable to represent the data. The X-ray and optical data show no spectral features and are best fitted with a two-component blackbody model with $kT_{\rm bb,X}^\infty \simeq 63.5$ eV and $R_{\rm bb,X}^\infty\simeq\,4.4\,(d/120\,\rm pc)$ km for the hot X-ray emitting region, and $kT_{\rm bb,opt}^\infty < 33$ eV and $R_{\rm bb,opt}^\infty >$ 17 ( d/120 pc) km for the rest of the neutron star surface responsible for the optical flux. The large number of counts collected with XMM-Newton allows us to reduce the upper limit on periodic variation in the X-ray range down to 1.3% (at a 2 $\sigma$ confidence level) in the 10^-3-50 Hz frequency range. In an attempt to explain this small variability, we discuss an one-component model with $kT_{\rm bb}^{\infty}\simeq 63$ eV and $R_{\rm bb}^{\infty}\simeq 12.3\,(d/120\,{\rm pc})$ km. This model requires a low radiative efficiency in the X-ray domain, which may be expected if the neutron star has a condensed matter surface.

Key words: stars: atmospheres -- stars: individual: RX J1856.5-3754 -- stars: neutron -- X-rays: stars

Offprint request: V. Burwitz, burwitz@mpe.mpg.de

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