Observations of the optical counterpart to XTE J1118+480 during outburst by the Robotic Optical Transient Search Experiment I telescope

The X-ray nova XTE J1118 + 480 exhibited two outbursts in the early part of 2000. As detected by the Rossi X-Ray Timing Explorer (RXTE), the first out burst began in early January and the second began in early March. Routine i maging of the northern sky by the Robotic Optical Transient Search Experime nt (ROTSE) shows the optical counterpart to XTE J1118 + 480 during both out bursts. These data include over 60 epochs from January to June 2000. A sear ch of the ROTSE data archives reveal no previous optical outbursts of this source in selected data between 1998 April and 2000 January. While the X-ra y-to-optical flux ratio of XTE J1118 + 480 was low during both outbursts, w e suggest that they were full X-ray novae and not minioutbursts based on co mparison with similar sources. The ROTSE measurements taken during the 2000 March outburst also indicate a rapid rise in the optical flux that precede d the X-ray emission measured by the RXTE by approximately 10 days. Using t hese results, we estimate a preoutburst accretion disk inner truncation rad ius of similar to1.2 x 10(4) Schwarzschild radii.