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

Abstract

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 on board the Rossi X-ray Timing Explorer. The source was detected at a flux level of 3.6 × 10-10 ergs cm-2 s-1 in the 0.7-10.0 keV band, which corresponds to the X-ray luminosity of 1.8 × 1038 ergs 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 ± 0.0004 s were unambiguously detected during the ASCA observation. The pulsed fraction is low and slightly energy dependent with an average value of ∼27%. The energy spectrum shows a large soft excess below ∼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 ∼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 ± 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.