Exploring Broadband GRB Behavior during γ‐Ray Emission

Abstract

The robotic ROTSE-III telescope network detected prompt optical emission contemporaneous with the γ-ray emission of Swift events GRB 051109A and GRB 051111. Both datasets have continuous coverage at high signal-to-noise levels from the prompt phase onwards, thus the early observations are readily compared to the Swift XRT and BAT high energy detections. In both cases, the optical afterglow is established, declining steadily during the prompt emission. For GRB 051111, there is evidence of an excess optical component during the prompt emission. The component is consistent with the flux spectrally extrapolated from the γ-rays, using the γ-ray spectral index. A compilation of spectral information from previous prompt detections shows that such a component is unusual. The existence of two prompt optical components -one connected to the high-energy emission, the other to separate afterglow flux, as indicated in GRB 051111 -is not compatible with a simple "external-external" shock model for the GRB and its afterglow.-4telescopes, the GRB field now has several examples of optical lightcurves that begin during, or within seconds after, the γ-ray emission.Broadband prompt emission is one of the least-understood aspects of GRB phenomena. The first prompt optical detection, GRB 990123 (Akerlof et al. 1999), exhibited an optical flare that was interpreted as the signature of a reverse shock passing through the relativistic ejecta (however, for another interpretation, see Liang et al. 1999). Reverse shock emission was expected to be common (Sari & Piran 1999), so it has come as a surprise that nearly all rapidly-detected GRB afterglows show scant evidence of it. There are alternatives to the standard "internal shocks" formulation for prompt emission. Such models include, e.g., external shocks (Meszaros & Rees 1993) or magnetic reconnection (Meszaros et al. 1994;Thompson 1994;Usov 1994) as the mechanism to release energy as γ-rays. The nature of GRB prompt emission is best investigated in conjunction with prompt observations at lower frequencies, with ongoing measurements at the same frequency to connect to the longerlasting, and better understood, afterglow.The Swift XRT's early X-ray observations have revealed a nearly standard morphology seen in most bursts' X-ray afterglow O'Brien et al. 2006). The typical early X-ray afterglow includes two surprises: a stage of relatively slow decay preceding the faster decline, known from pre-Swift observations, hours to days post-burst, and flaring well after the cessation of γ-ray emission. To connect this interesting early behavior to the later afterglow evolution, it is essential to compare such high-energy emission to lower-energy evolution. Such comparisons elucidate which features also occur at low energies, indicating a process affecting the entire early afterglow rather than a separate high-energy component (see GRB 050801, Rykoff et al. 2006).There is a small but growing sample of events for which it is possible to compare the very early optical lightcurve with X-ray emission (o...

THE AFTERGLOWS OFSWIFT-ERA GAMMA-RAY BURSTS. I. COMPARING PRE-SWIFTANDSWIFT-ERA LONG/SOFT (TYPE II) GRB OPTICAL AFTERGLOWS

“…z Possible very early plateau or peak, as there is hardly any decay between the first two points (Yost et al 2005). aa The early decay is slightly steeper than later, probably due to a tail component associated with the prompt emission (Yost et al 2007a;Butler et al 2006). ab Followed by a small chromatic bump and a later, strong rebrightening (Perley et al 2008d).…”

“…An early reverse-shock component has been proposed for GRB 050525A (Klotz et al 2005b;Shao & Dai 2005); this is also an interpretation for the early steep decay of GRB 061126 (Perley et al 2008c) and GRB 060908 (Covino et al 2010). In the case of GRB 051111, an early steep decay is associated with the tail of the prompt emission (Yost et al 2007a;Butler et al 2006). Once again, there are counterexamples, e.g., for GRB 060418, early upper limits on the polarization of the optical afterglow point to a weak (or even negligible) reverse-shock component (Mundell et al 2007a;Jin & Fan 2007), in agreement with a dominating forward-shock at very early times.…”

“…GRB 051109A, z = 2.346. We constructed the light curve and the SED with data from the following sources: Yost et al (2007a), Holland et al (2005), Bloom et al (2005), Milne et al (2005b), Haislip et al (2005b), Huang et al (2005), Woźniak et al (2005a), Li (2005b), Misra et al (2005), and Kinugasa & Torii (2005), as well as our own data (one point from the 0.38 m K-380 telescope). While this GRB had one of the densest early multi-color follow-ups ever obtained, most of these data are as yet unpublished; therefore our analysis is preliminary.…”

Optical behavior of GRB 061121 around its X-Ray shallow decay phase

“…Panaitescu et al (2006) report that the early break after the shallow decay phase is chromatic on the basis of 6 afterglows. Yost et al (2007) additionally report that GRB 051109A also exhibits a clear chromatic break. Panaitescu et al (2006) propose that the observed light curves require the temporal evolution of microphysical parameters in the emitting region of early and late afterglows.…”

Gamma Ray Burst Prompt Emission Variability in Synchrotron and Synchrotron Self-Compton Lightcurves

“…Most often these detections were limited to a few observations in the entire duration of the burst. Nevertheless, in a few cases a temporal correlation could be established between the optical and the γ-ray lightcurves, indicative of their possible origin from the same dynamical process (Yost et al 2007;Page et al 2007). This improved spectral coverage has led to a better understanding of the prompt emission region (Shen & Zhang 2009).…”