GRB 130427A: A Very Bright, Nearby, Laboratory
for Synchrotron Shocks
Rob Preece
UAH Dept. of Space Science(with contributions by J. Michael Burgess, Nicola Omodei, Chuck Dermer,
Andreas von Kienlin and a host of others on the Fermi GBM and LAT teams!)

GRB 130427A• Highest gamma-ray fluence andhighest-energy (95 GeV) photon
ever seen in a GRB• Longest-lived gamma-ray emission
Fermi LAT(20 hours) from a GRB

Localization
Plots:• Observed simultaneously fromradio to gamma-ray energies inexceptional detail• Extremely bright afterglow (opticalflash is second-brightest onrecord, reached mag. 7)• Nearby – redshift 0.34, light travel
time 3.6 billion years• Supernova SN 2013cq detected

So bright that GBM NaI
detectors saturated
during the peak!

Spectral Analysis• Because the GBM data aresaturated throughout the main
emission episode, we looked
instead at the cleaner, firstpulse.
• The Band GRB function is fitted
to 0.1 s intervals, using threedatasets: GBM NaI, GBM BGO& LAT LLE.
• Strikingly, the peak energy
seems to show a very simple
behavior.

A –1 Power-law as a Function of Time!

The Case for a Photosphere• Ryde et al., 2006: fitted photosphericfunction + PL to archival BATSE spectra• Why does this work?
• GRB spectra with suﬃcient statistics
for spectral analysis (45 σ) typicallyallow only 4 free parameters forfitting, as with Band GRB function• BB + PL also has 4 free parameters• Line of death problem replaced byphysical values. Band α values arereplaced by sum of BB +1 index andfitted PL index• HE PL index change is also accounted for• Motivated by Baring & Braby(2004), Burgess et al. (2011) beganfitting spectra with numericallyintegrated synchrotron emission
from parametrized electron
distributions⌫F⌫ (ergs2 s 1 cm 2 keV 1 )Next Steps

10141015101610

1710181019101

GRB081224A Time Evolution

102103

104

Energy (keV)• Fitting is done to the electron
distribution, convolved withsynchrotron emissivity kernel - toonumerically intensive until recently

100ne (cm 3 )• Electron distributions are ‘life-like’:
Fermi shock accelerated PL froma thermal (rel. Maxwellian)
reservoir or fast cooling broken PL101102=3.2

th10min

3100101

Burgess et al. 2013

102103

Advantages• kT better determined

0.000030

FE (ergs s 1 cm 2 )• Photospheric component evolution
more tightly constrained• Need a really bright burst to test this

0.000020
0.000015
0.000010
0.000005

05

10

1520

Time (s)GRB100707A (synchrotron)

0.000035

FE (ergs s 1 cm 2 )• Can use to model the interplay
between physical emission
mechanisms

0.000025

0.000000• Probe independence (orcorrelation) of each component• Same number of parameters asBand GRB function

GRB100707A (band)

0.000030
0.000025
0.000020
0.000015
0.000010
0.000005
0.000000

0510Time (s)Burgess et al. 2013

1520

Peak Energy Evolution• Fitted Band function Epeak
values (blue) for the first
2.5 s of 130427A in 0.1 stime bins. Fitted with asingle power law (slope of–0. 96 +/– 0.02). Time has
been oﬀset by 0.1 s,• The Burgess et al.
Synchrotron + BB
characteristic energy
values are in red. A broken
power-law fit is indicatedby the dashed line (earlytime decay index is -0.37+/- 0.23, with a break at
0.38 +/- 0.08 s., breaking
to an index of -1.173 +/0.045).

PRELIMINARY

Preece et al., Science 2013

Luminosity-Epeak Correlation• The decay phase L- Epeak
correlation is fit with a powerlaw index of 1.43 +/- 0.04.
Analysis of high-latitude
curvature radiation from
relativistic, spherical blast
waves produced in shellcollisions show that theapparent isotropic luminosity
L ∝ Epeak3 during the decay
phase of a pulse [Dermer2004]

PRELIMINARY• We can obtain the 3/2 PLindex by assuming magnetic
flux freezing.
Preece et al., Science 2013

Derived Photosphere Parameters• Plot showing trends in thederived bulk Lorentz factor(red – left axis) andphotospheric radius (blue –right axis).

PRELIMINARY• We obtain both values fromthe instantaneous ratio of
the observed blackbody
component flux to the total
flux, following Eq. 4 & 5 inPe’er et al. 2007
• Really do not expect thebulk Lorentz factor todecrease in shock scenario.
Suggests magnetic model.
Preece et al., Science 2013

Lag Analysis• Composite light curves for the three Fermi
detector types (green: GBM NaI #6; blue: GBM
BGO #1; red: LAT LLE during the first 3 s ofGRB 130427A. Each curve has beennormalized so that their peak intensities
match. LAT photons > 100 MeV are indicatedby circles• (Inset Figure:) Time lag τ (filled symbols) as
determined by the CCF analysis between theLLE lightcurve and selected energy bands of
the NaI (green) and BGO (blue) lightcurves.

PRELIMINARY• Also displayed are fitted pulse widths as afunction of energy W(E) ~ Ep (hollow symbols).
The two dashed lines represent:1) the best fit power law model for W(E),2) the expected dependence of the time lag
τ as a function of energy, assuming thesame power law index as in 1).

Preece et al., Science 2013

LAT + GBMAfterglow• LAT Lightcurve belongs to the afterglow:• Except for 3 photons right at thetrigger, nearly all the LAT photons
come after 10 s.
• LAT Photon flux is a broken power law• GBM late-time activity is also due toafterglow:• Swift XRT and GBM late pulse @ 100s peaks are nearly coincident in time
and temporal decays are consistent• GBM late-time data is at the limit ofdetectability - would not be seen if
burst were less brightAckermann et al., Science 2013

Conclusions• The isolated initial pulse of GRB 130427A is apparently unmodified by preceding engine activity
or nascent external shock emission.
• Our analysis shows:• There is good agreement between the pulse width as a function of energy and the expectedlag,• The characteristic energy has roughly a –1 power-law decay with time during the decaying
phase,• The temperature of the blackbody component implies a photospheric radius that isincompatible with the internal shock radius,• The apparent isotropic luminosity is related to the characteristic energy to the 3/2 power.
• It is a challenge to explain all these behaviors simultaneously.