ROSAT archival observations of 1SAX J0054.9-7226 = 2E 0053.2-7242, a newly discovered X-ray pulsar in the SMC

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We analysed 13 archival ROSAT PSPC and HRI observations which included the position of a newly discovered 59s X-ray pulsar in the Small Magellanic Cloud, 1SAX J0054.9-7226 = 2E 0053.2-7242. The source was detected three times between 1991 and 1996 at a luminosity level ranging from about 8 x 10³⁴ - 4 x 10³⁵ erg s⁻¹ (0.1-2.4 keV). Highly significant pulsations at 59.072s were detected during the 1991 October 8-9 observation. The ROSAT period, together with those measured by RXTE and BeppoSAX yields a period derivative of Pdot = - 0.016 s yr⁻¹. A much more accurate source position (10 arcsec uncertainty) was obtained through the ROSAT HRI detection on 1996 April restricting to three mᵥ > 15.5 stars the likely counterpart of 1SAX J0054.9-7226 = 2E 0053.2-7242.