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The bright optical afterglow of the nearby gamma-ray burst of 29 March 2003

Many past studies of cosmological gamma-ray bursts (GRBs) have been limited because of the large distance to typical GRBs, resulting in faint afterglows. There has long been a recognition that a nearby GRB would shed light on the origin of these mysterious cosmic explosions, as well as the physics of their fireballs. However, GRBs nearer than z=0.2 are extremely rare, with an estimated rate of localisation of one every decade. Here, we report the discovery of bright optical afterglow emission from GRB 030329. Our prompt dissemination and the brilliance of the afterglow resulted in extensive followup (more than 65 telescopes) from radio through X-ray bands, as well as measurement of the redshift, z=0.169. The gamma-ray and afterglow properties of GRB 030329 are similar to those of cosmological GRBs (after accounting for the small distance), making this the nearest known cosmological GRB. Observations have already securely identified the progenitor as a massive star that exploded as a supernova, and we anticipate futher revelations of the GRB phenomenon from studies of this source.

Many past studies of cosmological γ-ray bursts (GRBs) have been limited because of the large distance to typical GRBs, resulting in faint afterglows.
There has long been a recognition that a nearby GRB would shed light on the origin of these mysterious cosmic explosions, as well as the physics of their fireballs. However, GRBs nearer than z = 0.2 are extremely rare, with an estimated rate of localisation of one every decade. 1 Here, we report the discovery of bright optical afterglow emission from GRB 030329. 2 Our prompt dissemination 3 and the brilliance of the afterglow resulted in extensive followup (more than 65 telescopes) from radio through X-ray bands, as well as measurement of the redshift, z = 0.169. 4 The γ-ray and afterglow properties of GRB 030329 are similar to those of cosmological GRBs (after accounting for the small distance), making this the nearest known cosmological GRB. Observations have already securely identified the progenitor as a massive star that exploded as a supernova, 5  error-circle with the Wide Field Imager (WFI) on the 40-inch telescope at Siding Spring Observatory under inclement conditions (nearby thunderstorms). Nevertheless, we were able to identify a bright source not present on the Digitised Sky Survey ( Figure 1) and rapidly communicated the discovery to the community. 3 The same source was independently detected by the RIKEN automated telescope. 6 With a magnitude of 12.6 in the R band at 1.5 hours, the optical afterglow of GRB 030329 is unusually bright. At the same epoch, the well-studied GRB 021004 was R ∼ 16 mag, 7 and the famous GRB 990123 was R ∼ 17 mag. 8 The brightness of this afterglow triggered observations by over 65 optical telescopes around the world, ranging from sub-metre apertures to the Keck I 10-metre telescope. Unprecedented bright emission at radio, 9 millimetre, 10 sub-millimetre, 11 and X-ray 12 was also reported ( from those of cosmological GRBs. In particular, the extrapolated isotropic X-ray luminosity at t = 10 hr is L X,iso ∼ 6.4 × 10 45 erg s −1 , not distinctly different from that of other X-ray afterglows (e.g. ref. 15).
Nonetheless, two peculiarities about the afterglow of GRB 030329 are worth noting.
First, the optical emission steepens from f (t) ∝ t −α with α 1 = 0.873 ± 0.025 to α 2 = 1.97 ± 0.12 at epoch t * = 0.481 ± 0.033 d (Figure 4; see also ref. 16). This change in α is too large to be due to the passage of a cooling break (∆α = 1/4; ref. 17) through the optical bands. On the other hand, α ∼ 2 is typically seen in afterglows following the so-called "jet-break" epoch (t j ). Before this epoch, the explosion can be regarded as isotropic, and following this epoch the true collimated geometry is manifested. Such an early jet break would imply a substantially-lower energy release than E γ,iso .
If t * ∼ t j then using the formalism and adopting the density and γ-ray efficiency normalisations of Frail et al. 14 we estimate the true γ-ray energy release to be E γ ∼ 3×10 49 erg. This estimate is 4σ lower than the "standard energy" of 5×10 50 erg found by Frail et al. 14 The geometric-corrected x-ray luminosity 15 would also be the lowest of all xray afterglows. If the above interpretation is correct then GRB 030329 may be the missing link between cosmological GRBs and the peculiar GRB 980425 18 (E γ,iso ∼ 10 48 erg) which has been associated with SN 1998bw at z = 0.0085.
Second, the decay of the optical afterglow is marked by bumps and wiggles (e.g. ref. 19). These features could be due to inhomogeneities in the circumburst medium or additional shells of ejecta from the central engine. In either case, the bumps and wiggles complicate the simple jet interpretation offered above. We note that if the GRB had been more distant, and hence the afterglow fainter, then the break in the light curve would likely have been interpreted as the jet break without question.
The proximity of GRB 030329 offers us several new opportunities to understand the origin of GRBs. Red bumps in the light curve have been seen in several more-distant (z ∼ 0.3 to 1) GRB afterglows (e.g. refs 20,21) and interpreted as underlying SNe that caused the GRBs. While these red bumps appeared to be consistent with a SN light curve, prior to GRB 030329, it had not yet been unambiguously demonstrated that they were indeed SNe. As this paper was being written, a clear spectroscopic signature for an underlying SN has been identified in the optical afterglow of GRB 030329. 5 Our own spectroscopy at Palomar and Keck confirm the presence of these SN features. This demonstrates once and for all that the progenitors of at least some GRBs are massive stars that explode as SNe.
However, there remain a number of issues still to be resolved, relating to the physics of GRB afterglows and the environment around the progenitor. The "fireball" model of GRB afterglows predicts a broad-band spectrum from centimetre wavelengths to x-rays that evolves as the GRB ejecta expand and sweep up the surrounding medium. 17 Testing this model in detail has in the past been difficult, primarily due to both low signal-to-noise and interstellar scintillation at the longer wavelengths, from which come the majority of spectral and temporal coverage of the afterglow evolution (e.g. refs. 22,23). GRB 030329, with bright emission at all wavelengths (Figure 2), and limited scintillation (due to the larger apparent size) will allow astronomers to test the predictions of the fireball model with unprecedented precision through the time evolution of the broad-band spectrum, the angular size of the fireball, and its proper motion (if any). It has been long predicted that if the progenitors of GRBs are massive stars, the circumburst medium should be rich and inhomogenous, 24 but it has been difficult to find evidence for this. However, for GRB 030329, it should be possible to trace the distribution of circumburst material and determine the environment of the progenitor.
Even in the Swift (launch December 2003) era, we expect only one such nearby (z < 0.2) GRB every decade (scaling from ref. 1). Thus GRB 030329, the nearest of the cosmological GRBs to date, has given astronomers a rare opportunity to be up close and personal with a GRB and its afterglow. We eagerly await reports of the many experiments that have been and will be conducted to shed new light on the GRB phenomenon.     lines are caused by gas in the disk of the galaxy. In addition, we identify Ca II at z ≈ 0, presumably due to clouds in our own Galaxy.