1995A&A...296..459B

We report on the ROSAT/PSPC All Sky Survey observations of the ultra soft X-ray transient TrA X-1. By chance, the source belongs to a sky region which was surveyed twice by ROSAT. Furthermore, the first scan occurred only a few days before a SIGMA observation which had revealed that the source was bright at hard X-ray energies, and was therefore probably undergoing an outburst phase. As expected, the source was clearly detected by ROSAT. The PSPC spectrum is equally well fitted by a power law of photon index in the range of 1 to 5, or a cool blackbody of temperature 0.3-0.7keV. Depending on the assumed spectral model the (0.1-2.4keV) unabsorbed X-ray luminosity of TrA X-1 varies from ≃4.5x10³⁶erg/s down to ≃8.0x10³⁵erg/s (d=4.4kpc). Using these data, we are unable to prove nor rule out the presence of an ultra soft component in the spectrum of TrA X-1 during this outburst. The second scan took place about six months later. No X-ray emission was detected, indicating that TrA X-1 had rapidly returned into quiescence. An upper limit of ≃2-10x10³³erg/s has been derived for its quiescent X-ray luminosity. The relatively low soft X-ray luminosity measured by ROSAT, together with the non-detection by X-ray all sky monitors (GINGA and WATCH) strengthens the idea that in 1990 TrA X-1 underwent a non classical outburst phase, characterized by a low maximum intensity. Such "mini-outbursts" are expected in the disk thermal instability model.