The M31 population of supersoft sources

The 1991 ROSAT PSPC M31 X-ray point source catalog has been screened in order to set up a sample of candidate supersoft sources in this galaxy, additional to the 16 supersoft sources of Supper et al. (1997). 26 candidates have been found. Absorbing hydrogen column densities, effective temperatures and white dwarf masses (assuming the sources are on the stability line of surface nuclear burning) are derived. An observed white dwarf mass distribution is derived which indicates that the masses are constrained to values above 0.90 solar masses. The entire population of supersoft sources in M31 is estimated taking a theoretical white dwarf mass distribution into account, assuming that the observationally derived sample is restricted to white dwarf masses above 0.90 solar, and taking into account that the gas and the source population have a different scale height. A total number of at least 200-500 and at most 6,000-15,000 sources is deduced depending on the used galaxy N_H model, making use of the population synthesis calculation of Yungelson (1996). The spatial distribution favors a disk (or spiral-arm) dominated young stellar population with a ratio of 1/(4-7) of bulge/disk systems. Assuming that all supersoft sources with masses in excess of 0.5 solar masses are progenitors of supernovae of type Ia, a SN Ia rate of (0.8-7) 10^-3 yr^-1 is derived for M31 based on these progenitors. Supersoft sources might account for 20-100% of the total SN Ia rate in a galaxy like M31

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

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.Comment: 6 pages, accepted by A&A March 30th 199

A ROSAT PSPC X-Ray Survey of the Small Magellanic Cloud

We present the results of a systematic search for point-like and moderately extended soft (0.1-2.4 keV) X-ray sources in a raster of nine pointings covering a field of 8.95 deg^2 and performed with the ROSAT PSPC between October 1991 and October 1993 in the direction of the Small Magellanic Cloud. We detect 248 objects which we include in the first version of our SMC catalogue of soft X-ray sources. We set up seven source classes defined by selections in the count rate, hardness ratio and source extent. We find five high luminosity super-soft sources (1E 0035.4-7230, 1E 0056.8-7146, RX J0048.4-7332, RX J0058.6-7146 and RX J0103-7254), one low-luminosity super-soft source RX J0059.6-7138 correlating with the planetary nebula L357, 51 candidate hard X-ray binaries including eight bright hard X-ray binary candidates, 19 supernova remnants, 19 candidate foreground stars and 53 candidate background active galactic nuclei (and quasars). We give a likely classification for ~60% of the catalogued sources. The total count rate of the detected point-like and moderately extended sources in our catalogue is 6.9+/-0.3 s^-1, comparable to the background subtracted total rate from the integrated field of ~6.1+/-0.1 s^-1.Comment: Accepted by A&AS, 13 pages, 2 Postscript figure

X-Ray Binary Systems in the Small Magellanic Cloud

We present the result of a systematic search for spectrally hard and soft X-ray binary systems in the Small Magellanic Cloud (SMC). This search has been applied to ROSAT PSPC data (0.1-2.4 keV) collected during 9 pointed observations towards this galaxy covering a time span of 2 years from October 91 till October 93. Selection criteria have been defined in order to confine the sample of candidates. Finally 7 spectrally hard and 4 spectrally soft sources were selected from the list as candidates for binaries in the SMC. The sample is luminosity limited (>3.10**35 erg/s). SMC X-1 has been observed during a full binary orbit starting with a low-state covering an X-ray eclipse and emerging into a bright long-duration flare with two short-duration flares separated by 10 hours. The Be type transient SMC X-2 has been redetected with ROSAT. Variability has been found in the sources RX J0051.8-7231 and RX J0052.1-731 already discovered with Einstein. RX J0101.0-7206 has been discovered at the north-eastern boundary of the giant SMC HII region N66 during an X-ray outburst and half a year later during a quiescent phase. A variable source, RX J0049.1-7250, located north-east of the SMC supernova remnant N19 and which may either be an X-ray binary or an AGN turns out to be strongly absorbed. It may be located behind the SMC. If it is an X-ray binary then it radiates at the Eddington limit in the X-ray bright state. Another variable and hard X-ray source RX J0032.9-7348 has been discovered at the south-eastern border of the body of the SMC. A high mass X-ray binary nature is favored for this source. We searched for CAL87 like systems in the SMC catalog and found none. A new candidate supersoft source RX J0103.8-7254 has been detected. We cannot exclude that it is a foreground object.Comment: 22 pages, Late

The recent pulse period evolution of SMC X-1

We report observations of SMC X-1 in three new high-intensity states with ROSAT HRI in December 1995, May 1997, and March 1998 in which pulsations with a period of 0.70769+/-0.00006 sec, 0.70706+/-0.00001 sec, and 0.706707+/-0.00001 sec respectively were detected. Combining the pulse periods from observations with ROSAT PSPC in high-intensity states in October 1991 and March 1998, respectively, we obtain the spin-up rate of the pulsar in recent 6.5 years, -dP/dt = 1.18+/-0.06 10^-11 s s^-1, consistent with the average spin-up rate -dP/dt = 1.2 10^-11 s s^-1 determined from previous measurements indicating that the stable spin-up has continued. Pulsations with a period of 0.709103+/-0.000003 sec were also detected ~2 weeks after an X-ray turn-off during an X-ray low-intensity state in October 1991, and the period derivative derived within ~10 days is -dP/dt ~ (1.1+/-0.7) 10^-11 s s^-1. This is consistent with a constant accretion torque in sign and magnitude. The magnitude of the magnetic moment of the pulsar is discussed based on different description of the apparent spin-up behavior

Luminous supersoft X-ray emission from the recurrent nova U Scorpii

BeppoSAX detected luminous 0.2-2.0 keV supersoft X-ray emission from the recurrent nova U Sco ~19-20 days after the peak of the optical outburst in February 1999. U Sco is the first recurrent nova to be observed during a luminous supersoft X-ray phase. Non-LTE white dwarf atmosphere spectral models (together with a ~0.5 keV optically thin thermal component) were fitted to the BeppoSAX spectrum. We find that the fit is acceptable assuming enriched He and an enhanced N/C ratio. This implies that the CNO cycle was active during the outburst, in agreement with a thermonuclear runaway scenario. The best-fit temperature is ~9 10^5 K and the bolometric luminosity those predicted for steady nuclear burning on a WD close to the Chandrasekhar mass. The fact that U~Sco was detected as a supersoft X-ray source is consistent with steady nuclear burning continuing for at least one month after the outburst. This means that only a fraction of the previously accreted H and He was ejected during the outburst and that the WD can grow in mass, ultimately reaching the Chandrasekhar limit. This makes U~Sco a candidate type Ia supernova progenitor.Comment: 4 pages, accepted by A&A Letters 15 June 199