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

Abstract

Results from a 37,000 s BeppoSAX Low-Energy Concentrator Spectrometer (LECS) observation of the supersoft source SMC 13 (=1E 0035.4-7230) in the Small Magellanic Cloud are reported. 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 < 8 10^37 erg s-1 and < 3 10^37 erg s^-1 for the LTE and the non-LTE spectrum. We also applied a spectral fit to combined spectra obtained with BeppoSAX LECS and with ROSAT PSPC. The kT derived for the non-LTE spectrum is 27-29 eV, the bolometric luminosity is 1.1-1.2 10^37 erg s^-1. We can exclude any spectrally hard component with a luminosity > 2 10^35 erg s^-1 (for a bremmstrahlung with a temperature of 0.5 keV) at a distance of 60 kpc. The LTE temperature is therefore in the range 5.5+/-0.2 10^5 K and the non-LTE temperature in the range 3.25+/-0.16 10^5 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 ~1.1 M-solar and ~0.9 M-solar respectively. However, the temperature and luminosity derived with the non-LTE model for 1E 0035.4-7230 is consistent with a lower mass M~0.6-0.7 M-solar white dwarf as predicted by Sion and Starrfield (1994). At the moment, neither of these two alternatives for the white dwarf mass can be excluded.