A BeppoSAX observation of the supersoft source 1 E 0035 . 4 - 7230

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 ∼ >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 < 8 1037 erg s−1 and < 3 1037 erg s−1 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) 1037 erg s−1 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) 1036 erg s−1 (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) 1036 erg s−1, The kT derived for the non-LTE spectrum is (27–29) eV, the bolometric luminosity is (1.1–1.2) 1037 erg s−1. We can exclude any spectrally hard component with a luminosity of more than 2 1035 erg s−1 (for a bremmsstrahlung with a temperature of 0.5 keV) at a distance of 60 kpc. The LTE temperature is therefore in the range 5.5 ± 0.2 105 K and the non-LTE temperature in the range 3.25 ± 0.16 105 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 and ∼0.9 M respectively. However, the temperature and luminosity derived with the non-LTE model for 1E 0035.4-7230 is consistent with a lower mass (MWD ∼ 0.6–0.7M ) white dwarf as predicted by Sion & Starrfield (1994). At the moment, neither of these two alternatives for the white dwarf mass can be excluded. Send offprint requests to: [email protected]