Orphan Afterglows of Collimated Gamma-ray Bursts: Rate Predictions and Prospects for Detection

We make a quantitative prediction for the detection rate of orphan gamma-ray burst (GRB) afterglows as a function of flux sensitivity in X-ray, optical, and radio wavebands, based on a recent model of collimated GRB afterglows. We find that the orphan afterglow rate strongly depends on the opening angle of the jet (roughly ∝ θ−2 jet ), as expected from simple geometrical consideration, if the total jet energy is kept constant as suggested by recent studies. The relative beaming factor brel, i.e., the ratio of all afterglow rate including orphans to those associated with observable prompt GRBs, could be as high as brel & 100 for searches deeper than R ∼ 24, depending on afterglow parameters. To make the most plausible predictions, we average the model emission for ten sets of afterglow parameters obtained through fits to ten well-observed, collimated GRB jets, weighted by the sky coverage of each jet. Our model expectations are consistent with the results (or constraints) obtained by all past searches. We estimate the number of orphan afterglows in the first 1500deg2 field of the Sloan Digital Sky Survey (SDSS) to be about 0.2. The relative beaming factor brel is rapidly increasing with the search sensitivity: brel ∼ 3 for the SDSS sensitivity to transient objects in the northern sky (R ∼ 19), ∼14 for the past high-z supernova searches (R ∼ 23), and ∼50 for the sensitivity of the Subaru Suprime-Cam (R ∼ 26). Predictions are made for the current facilities and future projects in X-ray, optical, and radio bands. Among them, the southern-sky observation of the SDSS (sensitive to transients down to R ∼ 23) could detect ∼40 orphan afterglows during the five-year operation. Allen Telescope Array would find about 200 afterglows in a radio band at ∼0.1–1mJy with brel ∼ 15. Subject headings: gamma rays: bursts — ISM: jets and outflows — surveys