The brightest optical flash from a gamma-ray burst (GRB) was, briefly, a naked-eye object. Several other GRBs have produced optical transients only slightly fainter. We argue that, based upon the recently accumulated data from hundreds of GRB transients, many such optical events should have been visible to the unaided eye in the course of human history. The most likely repositories of such observations are historical records from the Orient, and we have located and discuss a number of candidates. We also consider the value of such observations, should any very likely ones be uncovered, to modern astrophysics.
Optical transients (OTs) associated with GRBs were first observed some 13 years ago (Van Paradijs et al. 1997), and were the key to understanding the nature of these events, or at least a large fraction of them (Van Paradijs et al. 2000). After attaining their peak brightness, which may be reached in some tens of seconds, most transients decay rapidly. A quick response by optical telescopes to a GRB detection is therefore essential, but that requires an equally rapid position determination by the γ-ray instruments, followed by communication of the information to optical observatories. The necessary logistics have been available, in several forms, for the past decade, but it is especially the advent of dedicated GRB observatories like Swift (Gehrels et al. 2004), designed to respond rapidly (and with its own X-ray and UV-optical telescopes), which has led to the detection of hundreds of OTs. In the 13 years since the first (pre-Swift) optical detection -of GRB 970228 -well over 300 OTs have been observed; they have originated in some 40% of the GRBs discovered over the period. The annual rate of OT discoveries to the end of 2010 is shown in Fig. 1, where the influence of Swift -which began operating near the end of 2004 -is obvious.
Optically, the brightest γ-ray burst yet observed was the OT produced by GRB 080319B which peaked at V = 5.3 mag (Racusin et al. 2008). With a redshift of 0.937, this remarkable event could have been observed with a modest-sized near-IR telescope at z ≃ 17 (Bloom et al. 2009). In our corner of the universe it was, for half a minute, visible to the unaided eye. It reached its peak brightness just 18 s after the GRB trigger, but by the time the Swift-UVOT slewed on source, it had faded to V = 7.6 mag. The GRB 080319B OT onset and peak had been fortuitously captured by several wide-field telescopes which were still monitoring an earlier outburst (GRB 080319A) just 10 • away. Although the next brightest OT, associated with GRB 990123, was over 3 mag fainter, there is a handful of GRB transients which were not observed until minutes after the trigger and which could have been of similar brightness to GRB 990123. Most of the GRB transients known have been discovered in less than a decade (see Fig. 1); it seems inescapable, as we argue below, that there have been significant numbers of naked-eye OTs in the course of human history.
Transient nighttime events -in particular, (super)novae, comets, meteors -have been recorded in a variety of written documents for over 2 millennia. While the bright supernova (SN) of 1006 is mentioned in the annals of a monastery in St. Gallen, Switzerland (Clark & Stephenson 1977;Stephenson & Green 2002), and the 1066 appearance of Halley’s Comet was even embroidered on the Bayeux tapestry (Mardon & Mardon 2002), no European or other records so far uncovered are the equal of those from East Asia, particularly China. For example, from 240 BC until 1910, every passage of Halley bar one (that of 164 BC) has been described in Chinese historical records (Eddy et al. 1989). In this paper we consider what an ancient observer might have seen on the basis of GRB OTs recorded until now, and what description would mark such an event as different from other transient phenomena. We give some examples of records which refer to transients that might have been GRB associated. We hope this preliminary investigation will alert scholars who peruse ancient chronicles to the possible existence of further OT records. We also discuss what the value of such observations, if any can be located, might be to astrophysics.
There are several GRB catalogues on the internet keeping track of bursts which have produced an optical counterpart. We have made use of the Berkeley list1 maintained by D. Perley, which included OTs up to March 2010. To get some idea of the OT brightnesses, Fig. 2 is a histogram of the minimum observed magnitude (in bins of ∆m = 1) for 330 GRBs in the Berkeley list. It must be emphasized that this is a very heterogeneous collection of OT brightness for several reasons. First, the observations were made in a number of color bands (including unfiltered “white”), so individual objects could be relatively shifted by 1-2 magnitudes (and certain “dark” GRBs by even more). There has been little uniformity in the choice of band, and some GRBs were, for a variety of reasons, only observed (or detected) in a single band, making correction to a uniform color problematic. Second, integration times also differ, which will affect the observed brightness of a quickly varying object. The third significant factor is that observation of many GRBs only begins after the light curve has begun to decline. This problem will be discussed further below. Despite these shortcomings, the histogram (Fig. 2) provides useful information. The distribution is slightly asymmetric, with a peak in the m = 19 -20 mag bin, and a steep decline for m > 20 mag. Although that aspect does not concern us h
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