Discovery and Photometric Observation of the Optical Counterpart in a Possible Galactic Halo X-ray Transient, XTE J1118+480

Abstract

We discovered the optical counterpart about 13 mag of a soft X-ray transient, XTE J1118+480 on 2000 March 30. We perform astrometry and provide the accurate position as R.A. = 11h18m10s.85, Decl. = +48o02'12".9. The outbursting object is identified with a 18.8 mag star in USNO catalog. Our pre-discovery data shows another outburst during 2000 January, again coinciding with an outburst detected in X-rays. Through the CCD time-series photometry, we found the presence of a periodic variation with the amplitude of 0.055 mag and the period of 0.17078(0.00004)d which we consider as promising candidate of orbital period. Because of the high galactic latitude and faint quiescence magnitude of 18.8, XTE J1118+480 is the possible first firmly identified black hole candidate (BHC) X-ray transient in the galactic halo.