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 0 , i 0 images of the optical afterglow of GRB 060526 (z ¼ 3:21) with the MDM 1.3 m, 2.4 m, and the PROMPT telescopes at CTIO over the five 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 > 10 4 s. We observed a break at 2:4 ; 10 5 s 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 prebreak and postbreak temporal decay slopes are difficult to explain in simple afterglow models. We estimated a jet angle of j $ 7 and a prompt emission size of R prompt $ 2 ; 10 14 cm. In addition, we detected several optical flares with amplitudes of Ám $ 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.

Photometry and spectroscopy of GRB 060526: a detailed study of the afterglow and host galaxy of az = 3.2 gamma-ray burst

“…= 3.12 with 302 d.o.f. This is due to the strong variability in the light curve which was first found by Halpern et al (2006a) and is also discussed in Dai et al (2007). This fit, along with the residuals showing the strong variability, is shown in Fig.…”

“…Still, the significant improvement shows that the light curve is basically a double smoothly-broken power law, and the steep late decay indicates that this break is a jet break, as first noted by Thöne et al (2006) and also found by Dai et al (2007). The rest frame jet break time of 0.52 days is typical for the optical afterglows of the pre-Swift GRB sample .…”

“…3. The parameters we find for this fit (α plateau = 0.288±0.026, α 1 = 0.971±0.008, α 2 = 2.524±0.052, t b1 = 0.090 ±0.005 days, t b = 2.216 ±0.049 days) are concurrent with those of Dai et al (2007) (who find α 1 ≈ 1.0, α 2 ≈ 2.9 and t b ≈ 2.55 days).…”

“…The light curve was followed up sparsely every night over nearly a week with DFOSC (Danish Faint Object Spectrograph Dai et al (2007). The early light curve especially shows strong variability, with two flares seen in the UVOT v band data.…”

“…Jet breaks, a common feature in well-monitored pre-Swift optical afterglows (e.g., Zeh et al 2006), have not been found in many Swift afterglows 2 , especially in the X-rays (e.g., Mangano et al 2007;Grupe et al 2007;Sato et al 2007;Racusin et al 2009), and if there are breaks, then the comparison between the optical and X-ray light curves show them to often be chromatic (Panaitescu et al 2006;Oates et al 2007). Analysing both the optical and X-ray data of GRB 060526, Dai et al (2007) suggest that the (jet) break is achromatic.…”