2002ApJ...577..845K

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 deg² 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_pat t=10 s, which fade as f=t/t₀^αt_^, where α_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 [α_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 1.1x10^–8s^–1.deg^–2. At ∼22 times the observed GRB rate from BATSE, this suggests a limit on θ_opt/θ_γ≲5, where θ_optand θ_γare 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.