Optical and X-Ray Observations of GRB 060526: A Complex Afterglow Consistent with An Achromatic Jet Break

Abstract

We obtained 98 R-band and 18 B, r', i' images of the optical afterglow of GRB 060526 (z=3.21) with the MDM 1.3m, 2.4m, and the PROMPT telescopes in Cerro Tololo over the 5 nights following the burst trigger. Combining these data with other optical observations reported in GCN and the Swift-XRT observations, we compare the optical and X-ray afterglow light curves of GRB 060526. Both the optical and X-ray afterglow light curves show rich features, such as flares and breaks. The densely sampled optical observations provide very good coverage at T>1.e4 sec. We observed a break at 2.4e5 sec in the optical afterglow light curve. Compared with the X-ray afterglow light curve, the break is consistent with an achromatic break supporting the beaming models of GRBs. However, the pre-break and post-break temporal decay slopes are difficult to explain in simple afterglow models. We estimated a jet angle of θ_j ~ 7 degrees and a prompt emission size of R_{prompt} ~ 2e14 cm. In addition, we detected several optical flares with amplitudes of Δm ~ 0.2, 0.6, and 0.2 mag. The X-ray afterglows detected by Swift have shown complicated decay patterns. Recently, many well-sampled optical afterglows also show decays with flares and multiple breaks. GRB 060526 provides an additional case of such a complex, well observed optical afterglow. The accumulated well-sampled afterglows indicate that most of the optical afterglows are complex.