ASCAObservation of the New Transient X‐Ray Pulsar XTE J0111.2−7317 in the Small Magellanic Cloud

The new transient X-ray pulsar XTE J0111.2-7317 was observed with AdvancedSatellite for Cosmology and Astrophysics (ASCA) on 1998 November 18, a few daysafter its discovery with the Proportional Counter Array onboard the Rossi X-rayTiming Explorer. The source was detected at a flux level of 3.6x10^-10 ergcm^-2 s^-1 in the 0.7--10.0 keV band, which corresponds to the X-ray luminosityof 1.8x10^38 erg s^-1, if a distance of 65 kpc for this pulsar in the SmallMagellanic Cloud is assumed. Nearly sinusoidal pulsations with a period of30.9497 +/- 0.0004 s were unambiguously detected during the ASCA observation.The pulsed fraction is low and slightly energy dependent with average value of\~27%. The energy spectrum shows a large soft excess below ~2 keV when fittedto a simple power-law type model. The soft excess is eliminated if the spectrumis fitted to an ``inversely broken power-law'' model, in which photon indicesbelow and above a break energy of 1.5 keV are 2.3 and 0.8, respectively. Thesoft excess can also be described by a blackbody or a thermal bremsstrahlungwhen the spectrum above ~2 keV is modeled by a power-law. In these models,however, the thermal soft component requires a very large emission zone, andhence it is difficult to explain the observed pulsations at energies below 2keV. A bright state of the source enables us to identify a weak iron linefeature at 6.4 keV with an equivalent width of 50 +/- 14 eV. Pulse phaseresolved spectroscopy revealed a slight hardening of the spectrum and marginalindication of an increase in the iron line strength during the pulse maximum.