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J.C Lochner, L.A. Whitlock, R.H.D. Corbet (USRA \& NASA/GSFC), F.E. Marshall (NASA/GSFC)
We report on observations by RXTE of the outbursts of four x-ray transient pulsar systems in the Small Magellanic Cloud made from 1997 November to 1998 July. Each of these four systems, AX J0051-722 (pulsation period P=91.0s), XTE J0054-720 (P=168.9s), 1SAX J0054.9-7226 (P=58.9s), and 1WGA J0053.8-7226 (P=46.6s), had outbursts lasting approximately 30 days, but at times more than one source was active. All but XTE J0054-720 have confirmed optical counterparts indicating that they are Be-neutron star systems. After initial detection in 1997 November, two subsequent outbursts of AX J0051-722 in 1998 March and 1998 July point to a possible orbital period for this system of 116 days. Similarly, reappearances of 1SAX J0054.9-7226 imply an orbital period of 65 days. In addition, XTE J0054-720 and 1SAX J0054.9-7226 show a significant decrease in their pulsation periods, with the former large enough to suggest a spin-up of the neutron star due to accreting material.
Spectra for these sources can be modeled as a power law with a high-energy cutoff. Because multiple sources were at times simultaneously active, discerning properties of individual sources is sometimes difficult. The observations show that the combined spectrum of 1WGA J0053.8-7226 and AX J0051-722 has a power law index of 0.8 and energy cutoffs between 10-16 keV. XTE J0054-720 has a power law index of 0.9, and a cutoff ranging between 10-15 keV. 1SAX J0054.9-7226 has power law index of 0.4 and cutoff of 7 keV. There is no significant evolution of these parameters through the course of the outbursts. Features in the folded light curves range from an eclipse-like dip in AX J0051-722 to sinusoidal-like variations in 1WGA J0053.8-7226. Further, in many of the observations of AX J0051-722 the hardness [(10-20 keV)/(6-10 kev)] shows an increase during the dip.
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