Perley, DA, Cenko, SB, Corsi, A, Tanvir, NR, Levan, AJ, Kann, DA, Sonbas, E,
Wiersema, K, Zheng, W, Zhao, XH, Bai, JM, Bremer, M, Castro-Tirado, AJ,
Chang, L, Clubb, KI, Frail, D, Fruchter, A, Gö üş, E, Greiner, J, Güver, T, ǧ
Horesh, A, Filippenko, AV, Klose, S, Mao, J, Morgan, AN, Pozanenko, AS,
Schmidl, S, Stecklum, B, Tanga, M, Volnova, AA, Volvach, AE, Wang, JG,
Winters, JM and Xin, YX
The afterglow of GRB 130427A from 1 to 10<sup>16</sup> GHz
http://researchonline.ljmu.ac.uk/id/eprint/6446/
Article
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Citation (please note it is advisable to refer to the publisher’s version if you
intend to cite from this work)
Perley, DA, Cenko, SB, Corsi, A, Tanvir, NR, Levan, AJ, Kann, DA, Sonbas, E,
Wiersema, K, Zheng, W, Zhao, XH, Bai, JM, Bremer, M, Castro-Tirado, AJ,
Chang, L, Clubb, KI, Frail, D, Fruchter, A, Gö üş, E, Greiner, J, Güver, T, ǧ
Horesh, A, Filippenko, AV, Klose, S, Mao, J, Morgan, AN, Pozanenko, AS,
LJMU Research Online
The Astrophysical Journal, 781:37 (21pp), 2014 January 20 doi:10.1088/0004-637X/781/1/37
C
2014. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
THE AFTERGLOW OF GRB 130427A FROM 1 TO 10
16
GHz
D. A. Perley
1,23
,S.B.Cenko
2,3,4
, A. Corsi
5
,N.R.Tanvir
6
, A. J. Levan
7
,D.A.Kann
8,9
, E. Sonbas
10
, K. Wiersema
6
,
W. Zheng
3
, X.-H. Zhao
11,12
,J.-M.Bai
11,12
, M. Bremer
13
, A. J. Castro-Tirado
14,15
, L. Chang
11,12
,K.I.Clubb
3
,
D. Frail
16
, A. Fruchter
17
,E.G
¨
o
ˇ
g
¨
u¸s
18
, J. Greiner
8
,T.G
¨
uver
19
, A. Horesh
1
, A. V. Filippenko
3
, S. Klose
9
,
J. Mao
11,20
, A. N. Morgan
3
, A. S. Pozanenko
21
, S. Schmidl
9
, B. Stecklum
9
, M. Tanga
8
, A. A. Volnova
21
,
A. E. Volvach
22
,J.-G.Wang
11,12
, J.-M. Winters
13
,andY.-X.Xin
11,12
1
Department of Astronomy, California Institute of Technology, MC 249-17, 1200 East California Blvd, Pasadena, CA 91125, USA; dperley@astro.caltech.edu
2
Astrophysics Science Division, NASA Goddard Space Flight Center, Mail Code 661, Greenbelt, MD 20771, USA
3
Department of Astronomy, University of California, Berkeley, CA 94720-3411, USA
4
Joint Space Science Institute, University of Maryland, College Park, MD 20742, USA
5
Physics Department, George Washington University, 725 21st St, NW Washington, DC 20052, USA
6
Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK
7
Department of Physics, University of Warwick, Coventry CV4 7AL, UK
8
Max-Planck-Institut f
¨
ur extraterrestrische Physik, Giessenbachstraße, D-85748 Garching, Germany
9
Th
¨
uringer Landessternwarte Tautenburg, Sternwarte 5, D-07778 Tautenburg, Germany
10
Department of Physics, University of Adiyaman, 02040 Adiyaman, Turkey
11
Yunnan Observatories, Chinese Academy of Sciences, P.O. Box 110, 650011 Kunming, China
12
Key Laboratory for the Structure and Evolution of Celestial Bodies, Chinese Academy of Sciences, P.O. Box 110, 650011 Kunming, China
13
Institute de Radioastronomie Millim
`
etrique (IRAM), 300 rue de la Piscine, F-38406 Saint Martin d’H
`
eres, France
14
Instituto de Astrof
´
ısica de Andaluc
´
ıa (IAA-CSIC), Glorieta de la Astronom
´
ıa s/n, E-18008 Granada, Spain
15
Unidad Asociada Departamento de Ingenier
´
ıa de Sistemas y Autom
´
atica, E.T.S. de Ingenieros Industriales, Universidad de M
´
alaga, Spain
16
National Radio Astronomy Observatory, P.O. Box O, Socorro, NM 87801, USA
17
Space Telescope Science Institute, Baltimore, MD 21218, USA
18
Sabancı University, Orhanlı- Tuzla,
˙
Istanbul 34956, Turkey
19
Istanbul University Science Faculty, Department of Astronomy and Space Sciences, 34119, University-Istanbul, Turkey
20
Astrophysical Big Bang Laboratory, RIKEN, Saitama 351-0198, Japan
21
Space Research Institute, 117997, 84/32 Profsoyuznaya, Moscow, Russia
22
Radio Astronomy Laboratory of the Crimean Astrophysical Observatory, Katsiveli, Yalta 98688, Ukraine
Received 2013 July 16; accepted 2013 November 18; published 2014 January 6
ABSTRACT
We present multiwavelength observations of the afterglow of GRB 130427A, the brightest (in total fluence) gamma-
ray burst (GRB) of the past 29 yr. Optical spectroscopy from Gemini-North reveals the redshift of the GRB to
be z = 0.340, indicating that its unprecedented brightness is primarily the result of its relatively close proximity
to Earth; the intrinsic luminosities of both the GRB and its afterglow are not extreme in comparison to other
bright GRBs. We present a large suite of multiwavelength observations spanning from 300 s to 130 days after
the burst and demonstrate that the afterglow shows relatively simple, smooth evolution at all frequencies, with no
significant late-time flaring or rebrightening activity. The entire data set from 1 GHz to 10 GeV can be modeled
as synchrotron emission from a combination of reverse and forward shocks in good agreement with the standard
afterglow model, providing strong support to the applicability of the underlying theory and clarifying the nature
of the GeV emission observed to last for minutes to hours following other very bright GRBs. A tenuous, wind-
stratified circumburst density profile is required by the observations, suggesting a massive-star progenitor with a
low mass-loss rate, perhaps due to low metallicity. GRBs similar in nature to GRB 130427A, inhabiting low-density
media and exhibiting strong reverse shocks, are probably not uncommon but may have been difficult to recognize
in the past owing to their relatively faint late-time radio emission; more such events should be found in abundance
by the new generation of sensitive radio and millimeter instruments.
Key words: gamma-ray burst: individual (GRB 130427A) – radiation mechanisms: non-thermal
Online-only material: color figures, machine-readable tables
1. INTRODUCTION
The majority of long-duration gamma-ray bursts (GRBs)
detected by orbiting satellites are extremely energetic events
originating from the distant universe: the mean redshift among
Swift GRBs is z ≈ 2.0, and ∼80% of Swift events originate
from z>1 (e.g., Jakobsson et al. 2006, 2012; Fynbo et al.
2009). While this makes GRBs excellent potential probes of
early phasesof cosmic history, it also implies that nearby analogs
of these high-redshift events must be relatively rare: the ratio
23
Hubble Fellow.
of observable comoving volume within the range 0 <z<0.4
compared to 1 <z<3, for example, is approximately a factor
24
of 60. Because GRBs are associated with star formation, the
sharp decline in the cosmic star formation rate since z ≈ 1(bya
factor of 5–10; e.g., Madau et al. 1998) further serves to reduce
the relative fraction of GRBs observed from the nearby universe.
The probable sensitivity of the GRB rate to metallicity (e.g., Le
Floc’h et al. 2006; Modjaz et al. 2008; Kistler et al. 2008; Butler
et al. 2010; Levesque et al. 2010; Graham & Fruchter 2013;
24
Here and elsewhere we assume a standard ΛCDM cosmological model with
Ω
Λ
= 0.7, Ω
m
= 0.3, h = 0.7.
1
The Astrophysical Journal, 781:37 (21pp), 2014 January 20 Perley et al.
0 1 2 3 4 5
Redshift
Isotropic energy release E
γ,iso
10
48
10
49
10
50
10
51
10
52
10
53
10
54
130427A
030329
080319B
990123
1
109
1
8
A
1
1
12
0
9A
090618
06
0
2
1
8
98
0
4
2
5
090323
080916C
130505A
130702A
031203
Fermi-GBM
Konus
pre-Swift
Swift
10
-7
erg cm
-2
10
-6
erg cm
-2
10
-5
erg cm
-2
1
0
-4
erg cm
-2
Figure 1. Total (bolometric) isotropic-equivalent gamma-ray energy release of pre-Swift, Swift,andFermi-GBM long-duration GRBs versus redshift. “High-luminosity”
(E
γ,iso
10
52
erg) GRBs dominate the observed population and represent the only type of GRB visible at z>2. However, such events have an extremely low intrinsic
rate and are rarely observed in the nearby universe due to simple volumetric considerations. Studies of low-redshift GRBs have instead been forced to target more
intrinsically common populations of low-luminosity GRBs that may not serve as good analogs of the energetic, high-z population. GRB 130427A, the subject of this
paper, is the closest example by far of a highly luminous GRB. Dotted curves are lines of constant fluence. The bottom-right portion of the diagram is empty owing
to the undetectability of low-luminosity bursts beyond very low redshifts. (E
γ,iso
values are taken from Amati 2006, Goldstein et al. 2012, Paciesas et al. 2012,and
Butler et al. 2007, and from Konus GCN Circulars: Golenetskii et al. 2005a, 2005b, 2005c, 2006a, 2006b, 2006c, 2006d, 2006e, 2006f, 2007a, 2007b, 2008a, 2008b,
2008c, 2008d, 2008e, 2008f, 2008g, 2008h, 2008i, 2009a, 2009b, 2009c, 2009d, 2009e, 2009f, 2009g, 2010a, 2010b, 2010c, 2010d, 2011a, 2011b, 2011c, 2011d,
2011e, 2011f, 2012a, 2012b, 2012c, 2012d, 2012e, 2013a, 2013b, 2013d
, 2013e; Pal’shin et al. 2009a, 2009b, 2013; Pal’shin 2011; Sakamoto et al. 2009, 2011.) Some
GRBs of particular interest (very luminous and nearby events) are circled and labeled.
(A color version of this figure is available in the online journal.)
Robertson & Ellis 2012; cf. Savaglio et al. 2009; Mannucci
et al. 2011; Elliott et al. 2012) also decreases the local rate.
Indeed, a simple scaling of the observed z ≈ 1–2 GRB rates
would naively suggest that “nearby” events (those at z<0.4)
should be extraordinarily uncommon, perhaps one per decade
within Swift’s field of view. Fortunately, however, GRBs span a
wide range of luminosities (Butler et al. 2010; Cao et al. 2011),
and a population ofless luminous but intrinsically more common
events that cannot be detected at higher redshifts becomes
visible in the nearby universe (Cobb et al. 2006; Guetta & Della
Valle 2007; Soderberg et al. 2006), raising the observed rate
of nearby GRBs to a more respectable (but still relatively low)
∼1yr
−1
during the Swift era. The existence of this population
has been critical in tying GRBs conclusively to massive stellar
death, since at z<0.4 optical observations are capable of
unambiguously recognizing an accompanying supernova (SN)
signature and classifying it spectroscopically (see Woosley &
Bloom 2006 for a review), whereas at high redshifts this task is
very challenging or impossible.
However, the differences in intrinsic rate and luminosity
between these “nearby” events and the high-redshift population
are quite large. The typical GRB selected by Swift or other major
satellites has an isotropic-equivalent energy scale of E
γ,iso
≈
10
52
–10
53
erg, which is about the energy scale necessary for
detection at z ≈ 1 (Figure 1). In contrast, the two nearest Swift
GRBs (060218 at z = 0.033 and 100316D at z = 0.059) and
the two nearest pre-Swift GRBs (980425 at z = 0.0085 and
031203 at z = 0.105) produced only E
γ,iso
≈ 10
48
–10
49
erg,
a difference of four orders of magnitude. These nearby events
couple very little or no energy to the highly relativistic emission
normally responsible for producing a GRB (Kaneko et al.
2007), show no evidence for collimation (Kulkarni et al. 1998;
Soderberg et al. 2004, 2006), and early X-ray/UV/optical
observations reveal an expanding thermal component instead
of a classical optical/X-ray afterglow (Campana et al. 2006;
Starling et al. 2011).
25
This is probably because the fastest
ejecta in these events do not contain sufficient energy to produce
a bright relativistic shock wave in their surrounding media (as
universally seen in high-luminosity GRBs), so at most times and
frequencies the shock is dominated by other emission processes
such as shock breakout and the SN itself, precluding the use of
these events for studies of afterglow emission.
Until now, the best nearby analog of a traditional high-
luminosity GRB has been GRB 030329 at z = 0.169. With
L
iso
≈ 10
51
erg s
−1
, it would likely have been detected (by
Swift) as far away as z ≈ 2; this event had an extremely bright
and well-studied optical/millimeter/X-ray afterglow, as well as
a spectroscopically confirmed SN that emerged after a few days
(Price et al. 2003; Tiengo et al. 2003; Greiner et al. 2003; Sheth
et al. 2003; Hjorth et al. 2003; Stanek et al. 2003). However,
until 2013 this event has remained singular: no other comparably
luminous GRB has been found at z<0.4. Even GRB 030329
is at the low end of the overall GRB population in terms of
its gamma-ray energetics and was peculiar in many ways: in
particular, the optical light curve showed continued variability
and rebrightenings as late as ∼8 days post-trigger (Uemura et al.
2003; Lipkin et al. 2004), and its bright and long-lived radio
afterglow seemed to require a second, wide jet unassociated
with any gamma-ray emission (Berger et al. 2003).
25
This thermal component may exist in “standard,” high-luminosity GRBs as
well, but is subdominant relative to the afterglow (Sparre & Starling 2012).
2
The Astrophysical Journal, 781:37 (21pp), 2014 January 20 Perley et al.
In this paper, we present observations of Swift/Fermi
GRB 130427A, the closest high-luminosity (E
γ,iso
> 10
51
erg)
GRB since GRB 030329. With z = 0.34 and E
γ,iso
≈
8 × 10
53
erg, its combination of proximity and luminosity is
unprecedented in the history of the field, producing the highest
gamma-ray and X-ray fluence of any GRB or afterglow observed
during the past 29 yr. Furthermore, this burst occurred under a
series of favorable circumstances for observations. The GRB
position was located within the field of view of the Fermi Large
Area Telescope (LAT; Atwood et al. 2009), providing coverage
of the GeV-photon component at early times and continuing
for many hours after the GRB. It also occurred over the western
hemisphere near local midnight during a period of good weather
in the western United States, enabling a number of telescopes
to observe the GRB at optical wavelengths within minutes, or
in one case starting before the burst began (Wren et al. 2013).
For all these reasons, GRB 130427A is a keystone event
that is likely to represent a gold standard for comparisons with
other GRB afterglows for decades. In this paper, we present
a large suite of multiwavelength observations of the afterglow
of GRB 130427A stretching from the radio band to X-rays
and from three minutes to four months after the burst. Our
acquisition and reduction of the observational data are presented
in Section 2. Examination of the key features of the observations
as a function of wavelength, including detailed comparisons
to samples of past GRBs, is presented in Section 3.Having
identified the key observational features, in Section 4 we then
attempt to explain the data using a standard reverse+forward
shock synchrotron model. We find that this model provides
an excellent description of the entire data set from 400 s to
130 days and at frequencies ranging from the low-frequency
radio to the high-energy gamma-rays, providing support for the
standard afterglow model and explaining the origin of the long-
lived LAT emission seen in this and previous GRBs as a simple
extension of the forward shock. We summarize our conclusions
in Section 5 and examine the implications of our results for
modeling of more complex GRBs and for the GRB progenitor.
2. OBSERVATIONS
2.1. Swift BAT and XRT
GRB 130427A triggered the Burst Alert Telescope (BAT;
Barthelmy et al. 2005) on board the Swift satellite (Gehrels
et al. 2004) on 2013 April 27 at 07:47:57 (UT dates are used
throughout this paper). This trigger time actually corresponds
to a point near the end of burst activity, as the BAT was in the
middle of a preplanned slew when burst emission began and
could not trigger until the slew was complete. Consequently,
the BAT trigger time does not provide a useful reference time
for the burst; we instead employ the Fermi Gamma-Ray Burst
Monitor (GBM; Meegan et al. 2009) trigger time of 07:47:06.42
(von Kienlin 2013)ast
0
in all of our subsequent analyses, an
adjustment of 50.58 s for times referenced to the BAT trigger.
Following the end of its preplanned slew and trigger, Swift
slewed immediately to the BAT location and began observations
with the X-Ray Telescope (XRT; Burrows et al. 2005) beginning
at 07:50:17.7 (t − t
0
= 190.8 s). Observations continued until
t − t
0
= 1984 s, at which point Swift slewed to another location
owing to orbital visibility constraints. After a gap of about 20 ks
(0.23 days), Swift returned to the source for further observations;
regular additional observing epochs continued as long as the
position remained visible to Swift.
We downloaded the Swift BAT and XRT light curves and
spectral analysis from the Swift Burst Analyser (Evans et al.
2010)
26
and Swift XRT repository (Evans et al. 2007, 2009).
Specifically, we obtained the 15–50 keV BAT flux light curve
(in 64 ms, 1 s, 10 s, and signal-to-noise ratio (S/N) = 7
binning modes) and the 0.3–10 keV XRT flux light curve;
each light curve was appropriately rebinned by our own scripts
depending on the application. Where necessary, these bandpass-
integrated fluxes were converted to flux-density values in Jy
using a smoothed value of the photon index Γ for each bin and
a correction factor of 1.16 for X-ray absorption (taken from
the ratio of absorbed to unabsorbed fluxes on the XRT spectral
analysis page for this event
27
).
2.2. UVOT
Ultraviolet–Optical Telescope (UVOT) data were also ac-
quired by Swift in parallel with XRT follow-up observations
beginning at t − t
0
= 197 s. The GRB field is at high Galactic
latitude (b = +72
◦
), and there are relatively few bright stars
nearby; consequently, the spacecraft experienced difficulties
guiding and most of the initial exposures are trailed, though
this difficulty was corrected in subsequent epochs.
We reduced the UVOT data using standard procedures within
the HEASoft
28
environment (e.g., Brown et al. 2009). Flux from
the transient was extracted from a 3
radius aperture for images
with good star lock (a much larger aperture was used on trailed
exposures to include all of the trailed flux), with a correction
applied to put the photometry on the standard UVOT system
(Poole et al. 2008). For observations after t = 8 days the object
is not detected in individual epochs, so we stacked observations
in three blocks spanning the time periods t = 9–15 days,
16–30 days, and 30–60 days. The resulting measurements are
listed in Table 1.
2.3. Palomar 60 inch Telescope
The Palomar 60 inch telescope (P60; Cenko et al. 2006)
responded automatically to the BAT trigger and began imaging
the field starting at 07:52:21.7 (Figure 2), detecting a bright
source at the location reported by Elenin et al. (2013). This
initial set of observations consisted of a repeating cycle of 60 s
exposures in the r, i, and z filters. P60 temporarily slewed away
from thetarget after completing this sequence ∼90 minutes later,
but was manually instructed to return to the field and continue
observations, this time in a repeating cycle of 60 s exposures in
the g, r, and i filters. Observations continued until a telescope
limit was reached at airmass 4.2. P60 was not available for
observations the following night, but the GRB was monitored
the night after that (and for a majority of the next several nights)
in the g, r, i, and z filters for most of the window in which it was
observable, switching to 120 s and then 180 s exposures. As the
source faded and the Moon brightened, the z and g exposures
were dropped in favor of r and i; observations continued nightly
(except during periods of bad weather) until May 31, after which
a less regular cadence was used.
Basic reductions (bias subtraction, flat-fielding, and astrome-
try) are provided in real time by an automated IRAF
29
pipeline.
26
http://www.swift.ac.uk/burst_analyser/
27
http://www.swift.ac.uk/xrt_spectra/00554620/
28
http://heasarc.nasa.gov/lheasoft
29
IRAF is distributed by the National Optical Astronomy Observatory, which
is operated by the Association of Universities for Research in Astronomy
(AURA) under cooperative agreement with the National Science Foundation
(NSF).
3
