Kavli Affiliate: Stuart Marshall
| Summary:
We present an untriggered search for optical bursts with the ROTSE-I telephoto array. Observations were taken which monitor an effective 256 square degree field continuously over 125 hours to m_ROTSE=15.7. The uniquely large field, moderate limiting magnitude and fast cadence of $sim$10 minutes permits 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=(fractt_0) ^α_t, 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 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 1.1times 10^-8 rm s^-1 rm deg^-2. At $sim$22 times the observed GRB rate from BATSE, this suggests a limit on fracθ_optθ_γlesssim 5 where θ_opt and θ_γ 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 magnitudes should offer a more definitive probe.
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