1999A&A...346..453K

Result. from a 37ks 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 >0.5keV. The BeppoSAX spectrum is fitted either with a blackbody spectrum with an effective temperature kT=26-58eV, an LTE white dwarf atmosphere spectrum with kT=35-50eV, or a non-LTE white dwarf atmosphere spectrum with kT=25-32eV. The bolometric luminosity is not very well constrained, it is <8x10³⁷erg/s and <3x10³⁷erg/s 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 (0.3-5)x10³⁷erg/s 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 τ>1.1, and a bolometric luminosity of (3-8)x10³⁶erg/s (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 (3-7)x10³⁶erg/s, The kT derived for the non-LTE spectrum is (27-29)eV, the bolometric luminosity is (1.1-1.2)x10³⁷erg/s. We can exclude any spectrally hard component with a luminosity of more than 2x10³⁵erg/s (for a bremmsstrahlung with a temperature of 0.5keV) at a distance of 60kpc. The LTE temperature is therefore in the range 5.5±0.2x10⁵K and the non-LTE temperature in the range 3.25±0.16x10⁵K. 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 ∼1.1M_☉ and ∼0.9M_☉ respectively. However, the temperature and luminosity derived with the non-LTE model for 1E 0035.4-7230 is consistent with a lower mass (M_WD∼0.6-0.7M_☉) white dwarf as predicted by Sion & Starrfield (1994ApJ...421..261S). At the moment, neither of these two alternatives for the white dwarf mass can be excluded.