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Astron. Astrophys. 334, L65-L68 (1998)

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4. Discussion

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 [FORMULA] - 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 [FORMULA] 4.2 [FORMULA] 1035 erg s-1 (1991 October 8-9) and [FORMULA] 8.5 [FORMULA] 1034 erg s-1 (1996 April 26 - June 10). Moreover R XTE detected 2E 0053.2-7242 at a luminosity level of [FORMULA] 3 [FORMULA] 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 [FORMULA] 2.5 [FORMULA] 1036 erg s-1 is derived, implying a pronounced long-term variability of 2E 0053.2-7242 (a factor of [FORMULA] 30). This indicates that the source is probably a transient X-ray pulsar in a high-mass binary containing a Be star.

A [FORMULA] 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 [FORMULA] 8.5 [FORMULA] 1034 erg s-1). The ROSAT HRI error circle of contains only three stars in the ESO plates with [FORMULA] [FORMULA] 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; [FORMULA] = 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.

[FIGURE] Fig. 3. ESO plate including the position of 2E 0053.2-7242. The X-ray error circles obtained from different instruments and satellites are shown

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© European Southern Observatory (ESO) 1998

Online publication: May 15, 1998