An Untriggered Search for Optical Bursts

Abstract

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=(\frac{t}{t_{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 1.1\times 10^{-8} {\rm s}^{-1} {\rm deg}^{-2}. At $\sim$22 times the observed GRB rate from BATSE, this suggests a limit on \frac{θ_{opt}}{θ_γ}\lesssim 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.

8 pages, 6 figures, 1 table

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Cited by in corpus (8)

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• The ROTSE-III Robotic Telescope System
• Dirty Fireballs and Orphan Afterglows: A Tale of Two Transients
• A Search for Untriggered GRB Afterglows with ROTSE-III
• The Limited Contribution of Low- and High-Luminosity Gamma-Ray Bursts to Ultra-High Energy Cosmic Rays
• Tile or Stare? Cadence and Sky Monitoring Observing Strategies that Maximize the Number of Discovered Transients
• Orphan optical flare as SOSS emission afterglow, localization in time
• The CFHTLS Real Time Analysis System "Optically Selected GRB Afterglows"