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Astron. Astrophys. 323, L33-L36 (1997)

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Letter to the Editor

A BeppoSAX observation of the super-soft source CAL 87

A.N. Parmar 1, P. Kahabka 2, H.W. Hartmann 3, J. Heise 3, D.D.E. Martin 1, M. Bavdaz 1 and T. Mineo 4

1 Astrophysics Division, Space Science Department of ESA, ESTEC, P.O. Box 299, 2200 AG Noordwijk, The Netherlands
2 Astronomical Institute and Center for High Energy Astrophysics, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
3 SRON Laboratory for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
4 IFCAI/CNR, via U. Malfa 163, I-90146 Palermo, Italy

Received 15 April 1997 / Accepted 6 May 1997

Abstract

We report on a BeppoSAX Concentrator Spectrometer observation of the super-soft source (SSS) CAL 87. The X-ray emission in SSS is believed to arise from nuclear burning of accreted material on the surface of a white dwarf (WD). An absorbed blackbody spectral model gives a [FORMULA] of 1.18 and a temperature of [FORMULA]  eV. However, the derived luminosity and radius are greater than the Eddington limit and radius of a WD. Including an O VIII edge at 0.871 keV gives a significantly better fit (at [FORMULA] 95% confidence) and results in more realistic values of the source luminosity and radius. We also fit WD atmosphere models to the CAL 87 spectrum. These also give reasonable bolometric luminosities and radii in the ranges 2.7-4.8 [FORMULA]  erg s-1 and 8-20 [FORMULA]  cm, respectively. These results support the view that the X-ray emission from CAL 87 results from nuclear burning in the atmosphere of a WD.

Key words: X-rays: stars - accretion - binaries:close - stars:individual (CAL 87) - white dwarfs

Send offprint requests to: A.N. Parmar: aparmar@astro.estec .esa.nl

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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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