2E 0053.2-7242 was detected three times between 1991 and 1996 in the ROSAT archival data. Highly significant pulsations, at a period of 59.072 s were detected on 1991 October 8-9. These findings, together with the Beppo SAX results, yield a mean period derivative of - 0.016 s yr-1 between 1991 and 1998.
In one case a spectral analysis could be performed. The spectrum was found to be consistent with a relatively flat low absorbed power-law model that is typical of accreting X-ray pulsars in this energy range.
The 0.1-2.4 keV luminosity of 2E 0053.2-7242 as observed with ROSAT ranges between 4.2 1035 erg s-1 (1991 October 8-9) and 8.5 1034 erg s-1 (1996 April 26 - June 10). Moreover R XTE detected 2E 0053.2-7242 at a luminosity level of 3 1037 erg s-1 in the 2-10 keV energy band. Extrapolating to the ROSAT energy range the luminosity measured by R XTE on 1998 January 20, a 0.1-2.4 keV luminosity of 2.5 1036 erg s-1 is derived, implying a pronounced long-term variability of 2E 0053.2-7242 (a factor of 30). This indicates that the source is probably a transient X-ray pulsar in a high-mass binary containing a Be star.
A accurate position was obtained thanks to a
ROSAT HRI observation during which the source was detected
(1996 April; 0.1-2.4 keV luminosity of 8.5
1034 erg s-1). The
ROSAT HRI error circle of contains only three stars in the ESO
plates with 15.5, the
likely optical counterpart of
2E 0053.2-7242 (see Fig. 3). Assuming a B-V = -0.2 and a distance modulus of 19 mag, these optical counterpart candidates are consistent with main sequence A9 - B2 stars. We note that a similar spectral-type star (B1.5Ve; = 16) is the companion of the nearby X-ray source SMC X-2. Future optical follow-up observations of these candidates should determine the counterpart of 2E 0053.2-7242 and its probable Be star X-ray transient nature. The optical and/or infrared activity brightening of the counterpart will allow further X-ray triggers and studies.
© European Southern Observatory (ESO) 1998
Online publication: May 15, 1998