1 Introduction

The origin and nature of gamma-ray bursts (GRBs) remains an important unresolved problem in astrophysics. Much of the difficulty in studying gamma-ray bursts is due to the poor directional precision of GRB detectors ($1 \sim 10^\circ 1\sigma$ statistical error) and their short duration ($1 \sim 100$ s). Recently an Italian-Dutch satellite (BeppoSAX) provided GRB coordinates with arcmin accuracy leading to the observation of fading X-ray, optical and radio afterglows (Costa 1997; Heise 1997; Djorgovski 1997; van Paradijs 1997; Frail 1997). These events provided information on their distance scale (Metzger 1997) and external environment. However, these observations were made many hours after the GRB event and the signals may be due to processes different from the GRB production mechanism. An observation of optical activity occurring at the same time as the gamma-ray emission may provide clues to understanding GRB physics (Sari 1998).

To search for simultaneous optical counterparts of GRBs, we are operating an automated wide field-of-view telescope at Lawrence Livermore National Laboratory (LLNL) to rapidly image GRB coordinate error boxes distributed by the Gamma-ray burst Coordinate distribution Network (GCN) (Barthelmy 1998). This system is called LOTIS for Livermore Optical Transient Imaging System.

Currently, the real-time trigger derived from BATSE telemetry in $\sim 5$ s has large angular error ($1\sigma$ error of $2 \sim 10^\circ$). This requires wide field-of-view optics to obtain significant coverage of an event. LOTIS utilizes commercially available Canon f/1.8 telephoto lenses of 200 mm focal length and 110 mm diameter effective aperture. The electronic focal plane sensors are $2048 \times 2048$ pixel Loral 442A CCDs with $15 \times 15\mu$m pixels driven by custom read-out electronics. Each lens/camera assembly has a field-of-view of $8.8\times 8.8^\circ$ with a pixel scale of 15 arcsec. Four cameras are arranged in a $2\times 2$ array to cover a total field-of-view of $17.4\times 17.4^\circ$ overlapping $0.2^\circ$ in each dimension. We are running these cameras without filters to enhance our detection sensitivity to dim objects. While we are waiting for GRB triggers which occur $\sim$once per 20 days, we systematically monitor the entire available night sky 4 times a night.

LOTIS started operating in October 1996 at LLNL's remote test facility, 25 miles east of Livermore, California. We recorded $\sim40$ GCN triggers as of Nov. 1998 including GRB970223 (Park 1997), GRB 971227 and GRB 980703. In this paper we report on 2 events: GRB 971227 and GRB 980703. Many follow-up observations were made for these events because BeppoSAX detected fading hard X-ray counterparts and was able to localize the GRB coordinate to $\sim 1$ arcmin. The ealiest observations were made by LOTIS responding to the BATSE triggers.