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Astron. Astrophys. 346, 453-458 (1999)


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A BeppoSAX observation of the supersoft source 1E 0035.4-7230

P. Kahabka 1, A.N. Parmar 2 and H.W. Hartmann 3

1 Astronomical Institute and Center for High Energy Astrophysics, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
2 Astrophysics Division, Space Science Department of ESA, ESTEC, P.O. Box 299, 2200 AG Noordwijk, The Netherlands
3 SRON Laboratory for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands

Received 17 February 1999 / Accepted 25 March 1999

Abstract

Results from a 37 ks BeppoSAX Low-Energy Concentrator Spectrometer (LECS) observation of the supersoft source SMC 13 (=1E 0035.4-7230) in the Small Magellanic Cloud are reported. This source has probably the softest spectrum observed so far with BeppoSAX, with no detected counts [FORMULA]0.5 keV. The BeppoSAX spectrum is fitted either with a blackbody spectrum with an effective temperature kT = 26-58 eV, an LTE white dwarf atmosphere spectrum with kT = 35-50 eV, or a non-LTE white dwarf atmosphere spectrum with kT = 25-32 eV. The bolometric luminosity is not very well constrained, it is [FORMULA] and [FORMULA] for the LTE and the non-LTE spectrum (90% confidence).

We also applied a spectral fit to combined spectra obtained with BeppoSAX LECS and with ROSAT PSPC. We find that a blackbody spectrum with an effective temperature kT=(39-47) eV and a bolometric luminosity of [FORMULA] fits the data. The data are also fitted with a blackbody with a kT of (50-81) eV, an average C-edge at (0.38-0.47) keV with an optical depth [FORMULA], and a bolometric luminosity of [FORMULA] (90% confidence). We also applied LTE and non-LTE white dwarf atmosphere spectra. The kT derived for the LTE spectrum is (45-49) eV, the bolometric luminosity is [FORMULA], The kT derived for the non-LTE spectrum is (27-29) eV, the bolometric luminosity is [FORMULA]. We can exclude any spectrally hard component with a luminosity of more than [FORMULA] (for a bremmsstrahlung with a temperature of 0.5 keV) at a distance of 60 kpc. The LTE temperature is therefore in the range [FORMULA] K and the non-LTE temperature in the range [FORMULA] K.

Assuming the source is on the stability line for atmospheric nuclear burning, we constrain the white dwarf mass from the LTE and the non-LTE fit to [FORMULA]1.1 [FORMULA] and [FORMULA]0.9 [FORMULA] respectively. However, the temperature and luminosity derived with the non-LTE model for 1E 0035.4-7230 is consistent with a lower mass ([FORMULA]) white dwarf as predicted by Sion & Starrfield (1994). At the moment, neither of these two alternatives for the white dwarf mass can be excluded.

Key words: accretion, accretion disks – stars: binaries: close – stars: individual: 1E 0035.4-7230 – stars: white dwarfs – X-rays: stars

Send offprint requests to: ptk@astro.uva.nl

© European Southern Observatory (ESO) 1999

Online publication: May 21, 1999

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