Research Articles

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Long-duration gamma-ray bursts are associated with the collapse of massive stars to form black holes (1) or rapidly-spinning, highly-magnetized, neutron stars (2). This collapse is believed to eject collimat-ed relativistic jets that, through internal dissipation processes and colli-sions with the surroundings, generate luminous outbursts of electromagnetic radiation that have been detected at radio frequencies to very high (GeV) gamma-ray energies. Most of the outburst energy is emitted in the gamma-rays. But, starting with the first observations that established that GRBs occur at cosmological distances (3), correlative optical observations, in particular, have proven themselves as important tools for unraveling the nature of GRB explosions. Here we present ob-servations of the optical flash and early afterglow for a nearby burst that is bright enough in very high energy gamma-rays to allow a detailed comparison of the >100 MeV gamma-ray and optical light curves. These optical observations cover the critical early phases of the explosion from the time interval before the event onset, through the bright optical and prompt gamma-ray emitting period, and well into the early afterglow phase.

Starting at 27 April 2013 at 07:47:06.42 UTC (hereafter To), the Gamma Ray Burst Monitor (GBM) on the Fermi Satellite, the Burst Alert Telescope (BAT) on the Swift Satellite, and an armada of other space-based gamma-ray detectors detected the onset of a powerful gam-ma-ray burst (GRB) (4, 5). This GRB, called GRB 130427A, had the largest gamma-ray fluence (~2.7 × 10–3 erg/cm2 in the 20 keV-1200 keV band) measured in more than 18 years of operation by Konus-Wind (6) and set a record for duration of the >100 MeV gamma-ray emitting in-terval (5). Spectroscopy of the optical counterpart (7), coarsely localized by the Swift BAT and later refined by follow-up with optical telescopes, places the GRB at a redshift z = 0.34—a distance about five times closer

than the typical GRB localized by Swift. However, even accounting for its proximity, the intense gamma-ray flux-es observed imply an apparent isotropic energy release of nearly 1054 ergs and rank it among the more powerful GRBs ever detected (3). Subsequent optical monitoring discovered the emergence of a broad-line supernova at the GRB location (8).

This powerful GRB also generated an extremely bright flash of optical emission and a long-lived, bright, opti-cal afterglow. Three independent RAPTOR (RAPid Telescopes for Opti-cal Response) full sky monitoring tele-scopes (9), at locations in New Mexico and Hawaii, detected the emergence of a bright flash, temporally coincident with the onset of gamma-ray emission, at the location of GRB 130427A. The optical flash rapidly peaked at a magni-tude 7.03 ± 0.03 (unfiltered observa-tions calibrated to Sloan r’ band) in an exposure that covered the time interval To + 9.31 to To + 19.31 s. After the peak, the flash faded with a power-law flux decay with index α = –1.67 ± 0.07 (χ2 = 0.68/5 dof) and was detected for about 80 s until it faded below the ~10th magnitude sensitivity limit of the RAPTOR full sky monitors.

The taxonomy for optical emission detected during the prompt gamma-ray

emitting interval identifies two broad classes: prompt optical emission correlated with prompt gamma-ray emission (10–12) and early optical afterglow emission uncorrelated with the prompt gamma-ray emission (11, 13, 14). In context of the standard fireball model (15, 16), the prompt optical emission is attributed to internal shocks in an ultra-relativistic jet outflow generated by the central engine and the afterglow emission to external shocks generated by interaction with the surround-ing medium. The prompt optical emission therefore reflects the impul-sive energy injection into the jet and the early afterglow emission measures the response of the jet/environment system to the energy injec-tion. Bright optical flashes from reverse shocks were predicted on theo-retical grounds (16, 17) before observational evidence was seen in GRB 990123 (13). The optical flash light-curve for GRB 130427A shows a single peak delayed with respect to the keV-MeV prompt gamma-ray peak (Fig. 1) and a steep power-law flux that is consistent with the pre-dictions of models for optical flashes from reverse shocks (17). Based on the above taxonomy, the brightness of the flash, and the rapid power law flux decay, it makes sense to associate the optical flash with reverse shock emission.

To explore the evolution of the broad band GRB spectrum during the optical flash, we constructed spectral energy distributions (SED) using simultaneous measurements taken with the Fermi GBM and the Fermi Large Area Telescope (LAT). Each snapshot of the time evolving SED was formed by integrating the GRB flux over the same time interval as the optical exposure. We found that the broad-band SEDs (Fig. 2) varied rapidly during the first 40 s and the optical measurements fell far from the values expected from extrapolation of the keV-MeV SED. However as the intensity of the outburst declined during the next 40 s interval, the SED shape stabilized and the optical measurements started to converge

The Bright Optical Flash and Afterglow from the Gamma-Ray Burst GRB 130427A W. T. Vestrand,1* J. A. Wren,1 A. Panaitescu,1 P. R. Wozniak,1 H. Davis,1 D. M. Palmer,1 G. Vianello,2 N. Omodei,2 S. Xiong,3 M. S. Briggs,3 M. Elphick,4 W. Paciesas,5 W. Rosing4 1Los Alamos National Laboratory, P.O. Box 1663, Los Alamos, NM 87545, USA. 2W.W. Hansen Experimental Physics Laboratory, Kavli Institute for Particle Astrophysics and Cosmology, Department of Physics, and SLAC National Accelerator Laboratory, Stanford University, Stanford, CA 94305, USA. 3Center for Space Plasma and Aeronomic Research, University of Alabama in Huntsville, 320 Sparkman Dr., Huntsville, AL 35899, USA. 4Las Cumbres Observatory Global Telescope Network, Inc., 6740 Cortona Drive, Suite 102, Santa Barbara, CA 93117, USA. 5Universities Space Research Association, 320 Sparkman Dr., Huntsville, AL 35899, USA.

*Corresponding author. E-mail: vestrand@lanl.gov

The optical light generated simultaneously with x-rays and gamma-rays during a gamma-ray burst (GRB) provides clues about the nature of the explosions that occur as massive stars collapse. We report on the bright optical flash and fading afterglow from powerful burst GRB 130427A. The optical and >100 MeV gamma-ray flux show a close correlation during the first 7000 s, best explained by reverse shock emission co-generated in the relativistic burst ejecta as it collides with surrounding material. At later times, optical observations show the emergence of emission generated by a forward shock traversing the circumburst environment. The link between optical afterglow and >100 MeV emission suggests that nearby early peaked afterglows will be the best candidates for studying particle acceleration at GeV/TeV energies.

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on the values predicted by a straightforward linear extrapolation of the keV-MeV SED. By the end of the optical flash, the optical to 10 MeV spectrum is consistent with a single power law with index β = –0.64.

In response to the Swift BAT localization alert at 127.8 s after the GBM trigger, our RAPTOR response telescopes began unfiltered and simultaneous multicolor (g’, r’, i’, z’) optical observations at To + 132.9 s that continued until To + 7585.9 s. This photometry begins near the peak of a prominent flare in keV-MeV x-ray/gamma-rays that lasts until ~To +400 s. The optical light curves show a smooth monotonic decline but no indication of the steep decline nor the break to a slower power-law decay at ~400 s measured at x-ray energies (4). Instead, the structure of the optical light-curve shows a steepening at about To + 270 s. This steepening is essentially achromatic and the color of the optical emission is consistent with a ν–0.70 ± 0.05 spectrum and constant until it starts to become bluer (ν–0.59 ± 0.05) at ~3000 s after the GBM trigger (see bottom panel of Fig. 3).

In marked contrast with the keV-MeV emission, the optical light curves after To + 100 s show a striking similarity with the >100 MeV photon flux light curve measured by the Fermi LAT (5). The LAT light curve has a break at about 300 s, just like the optical afterglow. Straight-forward scaling of the RAPTOR optical light curve by a factor of ~10−6 provides a reasonable description of the LAT observations out to ~To + 7000 s. This close correspondence argues for a common origin of both components in external shocks.

The optical light curve until ~To + 3000 s is best modeled by syn-chrotron emission from a reverse shock in a wind density profile. Most optical afterglows have been modeled with forward shocks in a homoge-neous medium. But the peak brightness (~6 Jy) and steep decay of the optical flash suggest origin in a reverse shock. The relative faintness of the radio afterglow peak (~1 mJy) also argues for generation by a reverse shock in a wind-like medium (18). To explain the optical flash by re-verse shock emission in a wind (R–2) requires either a long-lived electron energy injection up to ~40 s or a shorter (~20 s) followed by adiabatic cooling. Figure 4 shows the best fit to the optical flash with the short interval of injection (on at To + 4 s and off at To + 20 s) and, for self-consistency, the same dynamical parameters that we infer from fits to the later afterglow forward shock emission discussed below. This model employs an electron distribution with power-law energy index p = 1.88 and corresponds to an injected energy of 8.0 × 1053 ergs. The slower optical fading after the flash interval requires a second episode of energy injection to sustain the optical afterglow or a continuous outflow with a variable Lorentz factor (19). This sustained reverse shock model repro-duces the closely tracking variability observed by the RAPTOR tele-scopes and the Fermi LAT and suggests that the optical and most of the >100 MeV emission is generated by synchrotron emitting electrons that are accelerated by the reverse shocks.

This reverse shock model cannot, by itself, explain the properties of the prompt keV-MeV emission nor some of the properties of the late time afterglows. The evolution to a bluer color after ~3000 s observed by RAPTOR and the slowing of the optical brightness decay suggests the emergence of a forward shock component. This transition to forward shock dominance at late times would also naturally explain the late time x-ray light-curve and the sustained >100 MeV emission after 10,000 s. Emergence of a bluer optical component at late time is similar to the afterglow evolution of GRB 080319B—another burst with a bright opti-cal flash. For GRB 080319B, the color change was also interpreted as marking the transition from reverse shock emission dominance to for-ward shock dominance (12, 20, 21).

During most of the interval before To + 400 s, the keV-MeV x-ray/gamma-ray emission is consistent with the standard assumption that the prompt emission is generated by internal shocks in the relativistic jet ejecta. Our predicted >100 MeV flux from the reverse shock that gener-ates the optical flash is slightly less (~ factor of 2) than the peak meas-

ured by the Fermi LAT (Fig. 4). But the keV-MeV flux is significantly underpredicted by at least a factor of 10. So the reverse shock in a wind model requires prompt emission to explain the keV-MeV emission and might require additional >100 MeV emission to explain the LAT light-curve peak. In this picture, the keV-MeV light-curve is a proxy that trac-es the injection of internal jet energy by the central engine. The keV-MeV emission therefore indicates two periods of significant energy in-jection into the jet: the initial 20 s and a period from ~120 to 300 s. The interesting potential exception to prompt emission dominance in the keV-MeV range is the period just before onset of the flare at To + 120 s. During the interval To + 79 s to To + 89 s, the optical afterglow flux measured by RAPTOR falls right on the extrapolation of the power-law (index = ~–0.6) measured in the 10 keV-20 MeV energy band by the GBM. This gamma-ray spectral slope is also similar to spectral slope that we measure for optical afterglow emission at later times. The “notch” in the keV-MeV light curve before the flare at To + 120 may be providing a rare glimpse, similar to that seen in GRB 980923 (22), of afterglow emission at MeV energies between prompt emission intervals.

The exceptional optical properties observed for the optical flash and afterglow from GRB 130427A are mostly a result of burst proximity. The flash peak luminosity for GRB 130427A is among the most power-ful events, but its value is consistent with the anti-correlation between peak time and peak luminosity (Fig. 5) found for optical afterglows (23). If optical afterglows and >100 MeV gamma-ray afterglows have a com-mon origin, then the peaked optical afterglows that peak early should be the best candidates for detection at GeV/TeV gamma-ray energies.

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Acknowledgments: This gamma-ray burst research was supported by NASA and the Laboratory Directed Research and Development program at Los Alamos National Laboratory. The optical measurements reported in this paper are available online in the supplementary materials.

Supplementary Materials www.sciencemag.org/cgi/content/full/science.1242316/DC1 Supplementary Text Table S1 References (25–31)

24 June 2013; accepted 21 October 2013 Published online 21 November 2013 10.1126/science.1242316

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Fig. 1. A comparison of the relative flux variations measured for GRB 130427A by the Fermi LAT, RAPTOR, and the Swift BAT during the first 90 s after the gamma-ray burst trigger. The >100 MeV emission and the 50-150 keV emission have been integrated over the same time intervals as the optical exposures and multiplied by a scaling factor to allow comparison with the optical light curve. Both the optical and >100 MeV light curves rise to a peak in the second interval, 50-150 keV emission peaks in the first interval.

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Fig. 2. The spectral energy distributions measured for GRB 130427A during the early phases of the burst development. The points and solid lines represent actual measurements. The dotted straight lines indicate the extrapolation the keV-MeV measurements for comparison to the optical measurements. The dashed lines indicate the connection between energy bands and are not actual measurements. The measurements were obtained by RAPTOR, the Fermi GBM, and the Fermi LAT. The errors bars on the GBM measurements have been increased by 25% to allow for systematics, such as pulse pile-up (PPU), background subtraction during and following the repointing of Fermi, and the rapid spectral evolution during the exposure intervals.

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Fig. 3. Simultaneous multi-color measurements of the early optical afterglow from GRB 130427A. The top panel shows the light-curves obtained using standard Sloan g’, r’, i’, z’ filters (24). The light-curves show more structure than is captured by power law fits. But the r’ band light-curve can be characterized as a power-law decay with index ~0.7 before 270 s, ~1.1 between 270 and 3000 s, and 0.88 between 3000 and 7500 s. Also plotted for comparison, in blue, is the photon flux light curve for >100 MeV emission measured by the Fermi LAT (4). The bottom panel shows the g’-r’ color evolution of GRB 130427A. The red line indicates the mean g’-r’ value (0.286 ± 0.018) measured before T0 + 1000 s. After about T0 + 3000 s a bluer component emerges as the flux decay slows.

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Fig. 4. Best-fit with a reverse-forward shock model to the optical light-curve of GRB afterglow 130427A. Three episodes of ejecta and energy injection are needed to explain the optical flash, the early optical emission (up to few ks), and the late optical emission (after a few ks). Here each “injection episode” represents a change in the dynamical and micro-physical parameters of the reverse shock in an otherwise continuous ejecta injection into the reverse-shock. For the first two episodes, the optical emission arises from the reverse shock, and the kinetic energy of the incoming ejecta (6.1052 erg/sr and 4.1053 erg/sr, respectively) is less than that of the leading shock (1054 erg/sr). During the last injection episode, the optical afterglow emission arises from the forward-shock, with the incoming ejecta carrying 3.1054 erg/sr, which is more than that already existing in the forward-shock (thus its deceleration is mitigated by the energy injection). Other parameters are: 1) first injection episode, flash reverse-shock (fRS)–onset at 4 s, end at 15 s, incoming ejecta Lorentz factor 730, magnetic field parameter 0.008, electron energy parameter 0.006, index of electron power-law distribution with energy 1.9 2) second injection episode, reverse-shock (RS, parameters in same order as above)–15 s, 3 ks, 1800, 0.0010, 0.012, 2.0. 3) third injection episode, forward-shock (FS)–3 ks, > 2 Ms, > 100, 3.10−4, 0.14, 2.3, energy injection law E ~ t0.3. The micro-physical parameters for the forward shock are kept constant throughout the entire course of the event. We also require that the ambient medium have a wind-like density stratification (n ~ r–2) corresponding to a GRB progenitor with a mass-loss rate–to–wind speed ratio of 4.10−11 (M_sun/yr)/(km/s). Dot-dash lines show the model fits for the flash phase emission, the solid lines show the reverse shock contributions during the early afterglow phase, and the dashed lines shows the contributions of the forward shock emission to the late optical afterglow and all phases of the x-ray afterglow. The reverse-shock emission during the third injection episode is not shown and would be dimmer than that of the forward-shock, if the reverse-shock has the same micro-physical parameters as the forward-shock.

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Fig. 5. A comparison of the light-curve from GRB 130427A (in green) with those measured for other prominent GRBs with peaked optical afterglows. All of the light-curves have been transformed to show how they would appear if they had occurred at redshift z = 2.