Estimation of the detectability of optical orphan afterglows

Free access article

Issue		A&A Volume 461, Number 1, January I 2007


Page(s)		115 - 119
Section		Extragalactic astronomy
DOI		http://dx.doi.org/10.1051/0004-6361:20064853

A&A 461, 115-119 (2007)
DOI: 10.1051/0004-6361:20064853

Estimation of the detectability of optical orphan afterglows

Y. C. Zou¹, X. F. Wu^{2, 3}, and Z. G. Dai¹

¹ Department of Astronomy, Nanjing University, Nanjing 210093, PR China
e-mail: [zouyc;dzg]@nju.edu.cn
² Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, PR China
e-mail: xfwu@pmo.ac.cn
³ Joint Center for Particle, Nuclear Physics and Cosmology (J-CPNPC) between Nanjing University and Purple Mountain Observatory, Nanjing 210008, PR China

(Received 13 January 2006 / Accepted 22 August 2006)

Abstract
Aims.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.
Methods.This estimation is carried out by calculating the durations of off-axis optical afterglows whose flux density exceeds a certain observational limit.
Results.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 $\theta_{\rm j}=0.1$ . We also considered the effects of other parameters, and find that the effects of the ejecta energy $E_{\rm j}$ and post-jet-break temporal index $-\alpha_2$ are important but that the effects of the electron-energy distribution index p, electron energy equipartition factor $\epsilon_{\rm e}$ , and environment density n are insignificant. If $E_{\rm j}$ and $\alpha_2$ 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 $\times$ $10^{-2}~{\rm deg}^{-2}~{\rm yr}^{-1}$ , if the observed limiting magnitude is 20 in R-band.

Key words: gamma rays: bursts

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