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

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. = 11 h 18 m 10 s .85 , Decl. = +48 ◦ 02 ′ 12 ′′ .9. The outbursting object is identified with a 18.8 mag star in USNO catalog. Our prediscovery 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 ± 0.00004d 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.