The new transient X-ray pulsar XTE J0111.2–7317 was observed with Advanced Satellite for Cosmology and Astrophysics ( ASCA ) on 1998 November 18 , a few days after its discovery with the Proportional Counter Array onboard the Rossi X-ray Timing Explorer .
The source was detected at a flux level of 3.6 \times 10 ^ { -10 } erg cm ^ { -2 } s ^ { -1 } in the 0.7–10.0 keV band , which corresponds to the X-ray luminosity of 1.8 \times 10 ^ { 38 } erg s ^ { -1 } , if a distance of 65 kpc for this pulsar in the Small Magellanic Cloud is assumed .
Nearly sinusoidal pulsations with a period of 30.9497 \pm 0.0004 s were unambiguously detected during the ASCA observation .
The pulsed fraction is low and slightly energy dependent with average value of \sim 27 % .
The energy spectrum shows a large soft excess below \sim 2 keV when fitted to a simple power-law type model .
The soft excess is eliminated if the spectrum is fitted to an “ inversely broken power-law ” model , in which photon indices below and above a break energy of 1.5 keV are 2.3 and 0.8 , respectively .
The soft excess can also be described by a blackbody or a thermal bremsstrahlung when the spectrum above \sim 2 keV is modeled by a power-law .
In these models , however , the thermal soft component requires a very large emission zone , and hence it is difficult to explain the observed pulsations at energies below 2 keV .
A bright state of the source enables us to identify a weak iron line feature at 6.4 keV with an equivalent width of 50 \pm 14 eV .
Pulse phase resolved spectroscopy revealed a slight hardening of the spectrum and marginal indication of an increase in the iron line strength during the pulse maximum .