ASCA Observation 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 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× 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 × 10 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 ± 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 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. Subject headings: binaries: general — Magellanic Clouds — pulsars: individual (XTE J0111.2–7317) — X-rays: stars 1Department of Physics, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan; [email protected] 2Institute of Space and Astronautical Science, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510, Japan; [email protected], [email protected], [email protected] 3On leave from the Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai, 400005, India 4Department of Physics and Center for Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA; [email protected] 5Laboratory for High Energy Astrophysics, NASA, Goddard Space Flight Center, Greenbelt, MD 20771, USA; [email protected] 6Universities Space Research Association, Seabrook, MD 20706, USA