Estimation of the detectability of optical orphan afterglows

Y. C. Zou; X. F. Wu; Z. G. Dai

doi:10.1051/0004-6361:20064853

paper

Estimation of the detectability of optical orphan afterglows

arXiv:astro-ph/0601292 · doi:10.1051/0004-6361:20064853

Abstract

By neglecting sideways expansion of gamma-ray burst (GRB) jets and assuming their half-opening angle distribution, we estimate the detectability of orphan optical afterglows. This estimation is carried out by calculating the durations of off-axis optical afterglows whose flux density exceeds a certain observational limit. We show that the former assumption leads to more detectable orphans, while the latter suppresses the detectability strongly compared with the model with half-opening angle $θ_{j} = 0.1$ . We also considered the effects of other parameters, and find that the effects of the ejecta energy $E_{j}$ and post-jet-break temporal index $- α_{2}$ are important but that the effects of the electron-energy distribution index $p$ , electron energy equipartition factor $ε_{e}$ , and environment density $n$ are insignificant. If $E_{j}$ and $α_{2}$ are determined by other methods, one can constrain the half-opening angle distribution of jets by observing orphan afterglows. Adopting a set of "standard" parameters, the detectable rate of orphan afterglows is about $1.3 \times 1 0^{- 2} d e g^{- 2} y r^{- 1}$ , if the observed limiting magnitude is 20 in R-band.

arXiv comment6 pages, 5 figures. Accepted by A&A

Estimation of the detectability of optical orphan afterglows

Abstract

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