An untriggered search for optical bursts

We present an untriggered search for optical bursts with the Robotic Optical Transient Search Experiment (ROTSE-I) telephoto array. Observations were taken that monitor an effective 256 deg(2) field continuously over 125 hr to m(ROTSE) = 15.7. The uniquely large field, moderate limiting magnitude, and fast cadence of 10 minutes permit transient searches in a new region of sensitivity. Our search reveals no candidate events. To quantify this result, we simulate potential optical bursts with peak magnitude m(p) at t = 10 s, which fade as f = (t/t(0))(alphat), where alpha(t) < 0. Simple estimates based on observational evidence indicate that a search of this sensitivity begins to probe the possible region occupied by gamma-ray burst ( GRB) orphan afterglows. Our observing protocol and image sensitivity result in a broad region of high detection efficiency for light curves to the bright and slowly varying side of a boundary running from [alpha(t), m(p)] = [-2.0, 6.0] to [-0.3, 13.2]. Within this region, the integrated rate of brief optical bursts is less than 1.1 x10(-8) s(-1) deg(-2). At similar to 22 times the observed GRB rate from BATSE, this suggests a limit on theta(opt)/theta(gamma)less than or similar to 5, where theta(opt) and theta(gamma) are the optical and gamma-ray collimation angles, respectively. Several effects might explain the absence of optical bursts, and a search of the kind described here but more sensitive by about 4 mag should offer a more definitive probe.