A Link between Prompt Optical and Prompt Gamma-Ray
Emission in Gamma-Ray Bursts
W.T. Vestrand1, P.R. Wozniak1, J.A. Wren1, E. E. Fenimore1, T. Sakamoto2, R. R. White1, D.
Casperson1, H. Davis1, S. Evans1, M. Galassi1, K. E. McGowan1, J.A. Schier3, J. W. Asa3, S.
D. Barthelmy2, J. R. Cummings2, N. Gehrels2, D. Hullinger2, H.A. Krimm2, C. B.
Markwardt2, K. McLean1, D. Palmer1, A. Parsons 2 & J. Tueller2
1Los Alamos National Laboratory, Space Science and Applications Group, ISR-1, MS-D466,
Los Alamos, New Mexico 87545, USA.
2 NASA Goddard Space Flight Center, Code 661, Greenbelt, MD 20771 USA.
3 The Pilot Group, 128 W. Walnut Ave., Unit C, Monrovia, CA 91016, USA.
NOTE: This paper has been accepted for publication in Nature, but is embargoed for
discussion in the popular press until formal publication in Nature.
The prompt optical emission that arrives with the γ γ-rays from a cosmic γ γ-ray burst
(GRB) is a signature of the engine powering the burst, the properties of the ultra-
relativistic ejecta of the explosion, and the ejecta’s interactions with the surroundings1–
5. Until now, only GRB 990123 had been detected6 at optical wavelengths during the
burst phase. Its prompt optical emission was variable and uncorrelated with the
prompt γ γ-ray emission, suggesting that the optical emission was generated by a reverse
shock arising from the ejecta’s collision with surrounding material. Here we report
prompt optical emission from GRB 041219a. It is variable and correlated with the
prompt γ γ-rays, indicating a common origin for the optical light and the γ γ-rays. Within
the context of the standard fireball model of GRBs, we attribute this new optical
component to internal shocks driven into the burst ejecta by variations of the inner
engine. The correlated optical emission is a direct probe of the jet isolated from the
medium. The timing of the uncorrelated optical emission is strongly dependent on the
nature of the medium.
Starting on 19 December 2004 at 01:42:18 UT, high-energy emission from a bright and
very long duration gamma-ray burst, named GRB 041219a, was measured by both the IBIS
(Imager on Board the INTEGRAL Satellite) detector of the INTEGRAL satellite7 and the
Burst Alert Telescope (BAT) of the Swift satellite8. The 15-350 keV fluence measured by the
Swift BAT was approximately 1.55 x 10–4 ergs/cm2, placing it among the top few percent of
the 1637 GRB events listed in the comprehensive fourth BATSE (Burst and Transient Source
Experiment) catalog9. The duration of gamma ray emission from GRB 041219a was
approximately 520 seconds, making it one of the longest ever measured.
One of our RAPTOR (RAPid Telescopes for Optical Response) telescopes10 began
optical imaging of the GRB 041219a region at 01:44:13 UT, just 8 seconds after receipt of
the INTEGRAL alert. The long duration of the burst allowed RAPTOR-S to measure the
optical emission in a series of 30-second images for an unprecedented 6.4 minutes while
prompt gamma rays were being emitted. At the location of an IR transient identified11 in
subsequent images (starting at 01:49:18 UT) taken by the PAIRITEL telescope, our images
show an earlier flash of optical emission (see Figure 1) temporally coincident with the main
gamma-ray pulses. At its peak, the optical flash reached a measured magnitude of
Rc=18.6±0.1 magnitude. However, the location of the event placed it in the galactic plane and
in a direction with high optical extinction (galactic longitude and latitude: l=120o, b=+0.1o).
Using standard extinction maps12, we estimate an R-band extinction of ~4.9 magnitudes, but
the true extinction may be larger13. Correcting for the nominal extinction, the peak flux we
measured corresponds to a peak optical magnitude of Rc~ 13.7. (Error analysis and our
transformation of unfiltered instrumental magnitudes to standard Rc-band magnitudes
employing standard stars14 are discussed in a supplementary information file that
accompanies this paper on www.nature.com/nature.)
Light curves for prompt optical emission and prompt gamma-ray emission from
GRB041219a are shown in the top panel of figure 2. The optical light curve shows: the onset
of an optical flash as the dominant first gamma-ray pulse begins, peak brightness during the
first gamma-ray pulse, continued optical emission during the secondary gamma-ray peak, and
a decay of the optical emission to below our detection threshold during the tertiary gamma-
Optical emission has been detected during the interval of prompt gamma-ray emission
only once before6, for GRB 990123. Except for an overall temporal scaling factor—GRB
041219a was about 6 times longer—the temporal morphology of the two gamma-ray light
curves is remarkably similar (see figure 2). Like GRB 041219a, the gamma ray light-curve
for GRB 990123 had a precursor followed by a much larger primary pulse, a secondary pulse,
and a smaller amplitude tertiary flux enhancement composed of minor pulses. But in contrast
to GRB 041219a, the optical light-curve from GRB 990123 was low during the primary
gamma-ray pulse and, though more sparsely sampled, reached peak brightness after the
second major pulse. This anti-correlation suggests prompt optical emission from GRB
990123 was generated by a different process than the prompt gamma rays. The consensus
interpretation is that the delayed optical peak is generated by a reverse shock 1,3,15, an
interpretation supported by detections of the predicted rise to a peak radio flux about one day
after the burst 16.
For GRB 041219a, we find the observed optical light curve is well fit by assuming that
the generation of prompt optical emission is correlated with the generation of prompt gamma-
ray emission. By integrating the observed 15-350 keV flux measured by Swift during the
optical exposure intervals and multiplying by a derived constant optical to high-energy flux
ratio, we predicted the optical light curve expected if the optical emission and gamma ray
emission were perfectly correlated. As shown by the green circles in Figure 3, this simple
constant flux ratio assumption predicts both the fast rise of the prompt optical emission
observed at the start of the primary gamma-ray pulse and the rapid decline observed after that
dominant pulse. Our derived Rc-band optical to gamma-ray flux logarithmic color ratio for
GRB 041219a is (Rc-γ)= –2.5 log(Fopt/Fγ) =17.2 or, after correcting for an R-band extinction
of 4.9 magnitudes, (Rc-γ)=12.3.
The fast rise of optical emission simultaneous with the dominant gamma-ray pulse, and
a general correlation with the prompt gamma-ray emission would naturally arise if emission
in both energy bands were generated by a common mechanism. The broadband spectra
measured during the optical observation intervals are shown in Figure 4. Modeling of the
observed spectra is beyond the scope of this paper, but it can distinguish between emission
mechanisms and provide important constraints on physical conditions in the emitting region.
A particularly attractive possibility, within the standard internal-external model for GRB
fireballs, is that the prompt optical emission observed in GRB 041219a is a low energy tail of
the synchrotron emission generated by internal shocks in the GRB outflow2,30. In that model,
a nearly constant optical to gamma-ray flux ratio requires cooling times short compared to the
expansion time, and therefore magnetic fields near equipartition in the ejecta. However,
possibilities exist for the emission mechanism, including, for example, saturated
Comptonization, which can generate correlated optical and gamma ray emission17.
Internal shock models2 typically predict fainter prompt optical emission than reverse
shock models. Using the gamma-ray fluxes measured for GRB 990123 and scaling by
1.2x10–5 (from the (Rc-γ) color derived for GRB 041219a) one predicts significantly lower
optical fluxes than measured in GRB 990123—except for the first point in the optical light
curve. That first optical measurement, which occurred during the dominant gamma-ray pulse,
is consistent, within the prediction uncertainty, with the value predicted for an internal shock
using the (Rc-γ) color for GRB 041219a. But after the first measurement, any optical
emission generated by internal shocks in GRB 990123 was outshined by bright optical
emission from the external reverse shock. To generate the correlated optical and gamma-ray
variations measured throughout the full interval of gamma-ray emission in GRB 041219a
with internal (forward) shocks, reverse shock emission must be suppressed and/or delayed.
The timing and strength of the reverse shock component depends strongly on the physical
properties of the relativistic ejecta and the surrounding medium. In fact, the PAIRITEL near
IR observations of GRB 041219a show the emergence of a weaker component after the end
of the prompt gamma-ray emission that can be interpreted as delayed reverse shock
With the addition of the new optical properties displayed by GRB 041219a to the set of
known properties for optical emission from GRBs, we can construct a taxonomy of GRB
optical emission with three classes: (1) prompt optical emission varying simultaneously with
the prompt gamma-rays; (2) early afterglow emission that may start during the prompt
gamma-ray emission, but persists for ten minutes or more after the prompt gamma-rays have
faded 6,19-21; and (3) late afterglow emission that can last for many hours to days 22-24. Within
the context of the standard fireball model, it makes sense to attribute the prompt emission to
internal shocks in the ultra-relativistic ejecta driven by the GRB engine2, the early afterglow
to a reverse shock driven into the ejecta by interaction with the surrounding medium 1-5,15,
and the late afterglow to forward external shocks driven into the surrounding medium
generated by interaction with the ejecta 25-26. This theoretical framework, in turn, allows
predictions about the timing, spectra, and relative strength of the optical components that
hinge on the properties of the inner engine, the ejecta, and the surrounding medium. The
ability of the Swift satellite to provide precise real-time positions and make panchromatic
observations of GRBs27, supplemented by a new generation of sensitive ground-based rapid
response telescopes, therefore brings us into a new era in the study of the critical first few
minutes during and after GRBs—one that will allow us to probe deeply the physics of these
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Acknowledgements: The RAPTOR project is supported by the Laboratory Directed Research and Development
program at Los Alamos National Laboratory.
Competing interests statement. The authors declare that they have no competing financial interests.
Correspondence and requests for materials should be addressed to W.T.V. (e-mail: email@example.com).
Figure 1 The prompt optical emission detected from GRB 041219a. This finder chart shows
the location (right ascension 00 h 24 min 27.7 s, declination+62° 50′ 33.5″ (J2000)) of the
prompt optical flash that we detected, simultaneously with the prompt γ-ray emission
detected by the INTEGRAL7 and Swift8 satellites, during the time interval 01:45:41–
01:49:01 UT on 2004 December 19. The location of the optical transient (OT) is identical to
that found both for the subsequent infrared transient11 and the optical counterpart28 measured
later during the late afterglow phase. Our observations of the prompt optical emission were
obtained by RAPTOR-S, a 0.4-m, f/5, fully autonomous rapid response telescope owned by
Los Alamos National Laboratory and located at an altitude of 2,500 m in the Jemez
Mountains of New Mexico. The CCD camera employed for those observations has a
1,056×1,027 pixel format, back-illuminated, Marconi CCD47-10 chip with 13-µm pixels.
Table 1 Simultaneous RAPTOR and Swift measurements of GRB 041219a
Figure 2 Comparison of the prompt γ-ray and prompt optical light curves measured9 for both
GRB 041219a and GRB 990123. The black trace in Panel a shows the relative (normalized
to the peak value) γ-ray flux, Fγ , measured for GRB 041219a by the Swift satellite29 as a
function of elapsed time, t - ttrig , after recognition of the onset of a GRB at trigger time
ttrig=01:42:18 UT. The γ-ray lightcurve for GRB 041219a shows an outburst of precursor
emission, followed by a mostly quiet period of 250 s, a primary pulse peaking at about 280 s,
a secondary pulse centred at about 380 s, and finally a smaller-amplitude tertiary flux
enhancement composed of minor pulses starting at 420 s after the trigger. The black trace in
Panel b shows the γ-ray light curve for GRB 990123, which is remarkably similar, except for
a temporal scaling factor, to that measured for GRB 041219a. The relative optical fluxes,
Fopt, are indicated by red crosses on both panels with observing intervals denoted by
horizontal red lines and 1σ flux error bars represented by red vertical lines. Together the
panels show the relationship between optical emission and γ-ray emission was quite different
for the two events. Notice that the early optical flux from GRB 990123 was relatively low
during the most intense γ-ray peak and only peaked in the optical6 after the two primary γ-ray
peaks. For GRB 041219a, on the other hand, the prompt optical emission rises rapidly at the
start and reaches a maximum during the primary γ-ray pulse, declines but persists during the
secondary γ-ray pulse, and then fades below the detection threshold during the tertiary γ-ray
enhancement. At peak brightness, GRB 041219a reached Rc=18.6±0.1 mag, corresponding
to an estimated peak magnitude of Rc≈13.7 after correction for extinction by dust.
Figure 3 The measured optical light curve and that predicted for GRB 041219a assuming a
constant prompt optical to prompt γ-ray flux ratio. All the optical photometry measurements
are derived from stacks of two 30-s images separated by a eight-second readout time, except
during the dominant γ-ray peak where a single image yielded a >9σ detection. The γ-ray
fluxes used for this comparison are derived by integrating the 15–350-keV counting rate
measured by the Swift BAT, plotted as the grey trace, during the optical observation
intervals. The black crosses show the actual measurements, and the circles show the predicted
values. The error bars for detections are given as 1σ values, and non-detections are plotted as
2σ upper limits. The reduced χ2 for the best-fitting model for GRB 041219a, with
Fopt/Fγ=1.3×10−7 (1.2×10−5 after correcting for extinction), is χ2/d.f.=1.79 (4 degrees of
freedom, d.f.). In contrast, the best-fitting model employing a constant flux ratio to predict
the GRB 990123 optical light curve yields a reduced χ2 of χ2/d.f.=1,950.65 (2 d.f.).
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Figure 4 Broad-band spectra of GRB 041219a, here plotted in flux density Fν as a function
of observed frequency ν, measured during the period of simultaneous prompt optical and γ-
ray emission. The RAPTOR-S optical measurements, after correcting for a nominal 4.9
magnitudes of extinction, are shown as circles. Simultaneous high-energy measurements
from the BAT instrument on board the Swift satellite29 are shown as crosses. All the error
bars represent the 1σ statistical errors. We estimate that the systematic uncertainty for the
normalization of the optical fluxes is about a factor of three due to uncertainty in the intrinsic
colour of the optical transient and the true extinction along the line of sight. The four
integration time intervals, int1–int4, are measured in seconds from the Swift GRB trigger
time at 01:42:18.7 UT on 2004 December 19. Notice the optical and γ-ray fluxes vary in
concert so that the spectra never cross, and also that the highest-energy band seems to be a
slightly better predictor of the behaviour of the optical emission.