Detection of a spectral break in the extra hard component of GRB 090926A

0.55 MB·English

by  Fermi-LAT

Accepted by ApJ, January 9th 2011
Detection of a spectral break in the extra hard component of
GRB 090926A
1
1
M. Ackermann2, M. Ajello2, K. Asano3, M. Axelsson4,5,6, L. Baldini7, J. Ballet8,
0
2 G. Barbiellini9,10, M. G. Baring11, D. Bastieri12,13, K. Bechtol2, R. Bellazzini7, B. Berenji2,

n P. N. Bhat14, E. Bissaldi15, R. D. Blandford2, E. Bonamente16,17, A. W. Borgland2,
a
J A. Bouvier2, J. Bregeon7,1, A. Brez7, M. S. Briggs14, M. Brigida18,19, P. Bruel20,

2 R. Buehler2, S. Buson12,13, G. A. Caliandro21, R. A. Cameron2, P. A. Caraveo22,
1
S. Carrigan13, J. M. Casandjian8, C. Cecchi16,17, O¨. C¸elik23,24,25, V. Chaplin14, E. Charles2,

E] A. Chekhtman26,27, J. Chiang2, S. Ciprini17, R. Claus2, J. Cohen-Tanugi28,
H V. Connaughton14, J. Conrad29,6,30, S. Cutini31, C. D. Dermer26, A. de Angelis32,
h. F. de Palma18,19, B. L. Dingus33, E. do Couto e Silva2, P. S. Drell2, R. Dubois2,
p C. Favuzzi18,19, S. J. Fegan20, E. C. Ferrara23, W. B. Focke2, M. Frailis32,34, Y. Fukazawa35,
-
o S. Funk2, P. Fusco18,19, F. Gargano19, D. Gasparrini31, N. Gehrels23, S. Germani16,17,
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t N. Giglietto18,19, F. Giordano18,19, M. Giroletti36, T. Glanzman2, G. Godfrey2,
s
a
A. Goldstein14,1, J. Granot37, J. Greiner15, I. A. Grenier8, J. E. Grove26, S. Guiriec14,
[

D. Hadasch21, Y. Hanabata35, A. K. Harding23, K. Hayashi35, M. Hayashida2, E. Hays23,
2
v D. Horan20, R. E. Hughes38, R. Itoh35, G. J´ohannesson2, A. S. Johnson2, W. N. Johnson26,
2
T. Kamae2, H. Katagiri35, J. Kataoka39, R. M. Kippen33, J. Kno¨dlseder40, D. Kocevski2,
8
0 C. Kouveliotou41, M. Kuss7, J. Lande2, L. Latronico7, S.-H. Lee2, M. Llena Garde29,6,
2
. F. Longo9,10, F. Loparco18,19, M. N. Lovellette26, P. Lubrano16,17, A. Makeev26,27,
1
0 M. N. Mazziotta19, S. McBreen15,42, J. E. McEnery23,43, S. McGlynn44,6, C. Meegan45,
1
J. Mehault28, P. M´esz´aros46, P. F. Michelson2, T. Mizuno35, C. Monte18,19, M. E. Monzani2,
1
: E. Moretti44,6, A. Morselli47, I. V. Moskalenko2, S. Murgia2, H. Nakajima48, T. Nakamori39,
v
i M. Naumann-Godo8, S. Nishino35, P. L. Nolan2, J. P. Norris49, E. Nuss28, M. Ohno50,
X
r T. Ohsugi51, A. Okumura50, N. Omodei2, E. Orlando15, J. F. Ormes49, M. Ozaki50,
a
W. S. Paciesas14, D. Paneque2, J. H. Panetta2, D. Parent26,27, V. Pelassa28, M. Pepe16,17,
M. Pesce-Rollins7, V. Petrosian2, F. Piron28, T. A. Porter2, R. Preece14,1, J. L. Racusin23,
S. Raino`18,19, R. Rando12,13, A. Rau15, M. Razzano7, S. Razzaque26,52, A. Reimer53,2,
O. Reimer53,2, T. Reposeur54, L. C. Reyes55, J. Ripken29,6, S. Ritz56, M. Roth57,
F. Ryde44,6, H. F.-W. Sadrozinski56, A. Sander38, J. D. Scargle58, T. L. Schalk56, C. Sgro`7,
E. J. Siskind59, P. D. Smith38, G. Spandre7, P. Spinelli18,19, M. Stamatikos23,38,
F. W. Stecker23, M. S. Strickman26, D. J. Suson60, H. Tajima2, H. Takahashi51,1,
T. Tanaka2, Y. Tanaka50, J. B. Thayer2, J. G. Thayer2, L. Tibaldo12,13,8,61, D. Tierney42,
K. Toma46,1, D. F. Torres21,62, G. Tosti16,17, A. Tramacere2,63,64, Y. Uchiyama2,
T. Uehara35,1, T. L. Usher2, J. Vandenbroucke2, A. J. van der Horst41,65, V. Vasileiou24,25,
– 2 –
N. Vilchez40, V. Vitale47,66, A. von Kienlin15, A. P. Waite2, P. Wang2, C. Wilson-Hodge41,
B. L. Winer38, K. S. Wood26, X. F. Wu46,67,68, R. Yamazaki69, Z. Yang29,6, T. Ylinen44,70,6,
M. Ziegler56
– 3 –
1Corresponding authors: J. Bregeon, [email protected]; A. Goldstein, [email protected];
R. Preece, [email protected]; H. Takahashi, [email protected]; K. Toma,
[email protected]; T. Uehara, [email protected].
2W. W. Hansen ExperimentalPhysicsLaboratory,KavliInstitute for ParticleAstrophysics andCosmol-
ogy, Department of Physics and SLAC National Accelerator Laboratory,Stanford University, Stanford, CA
94305,USA
3Interactive Research Center of Science, Tokyo Institute of Technology, Meguro City, Tokyo 152-8551,
Japan
4Department of Astronomy, Stockholm University, SE-106 91 Stockholm, Sweden
5Lund Observatory, SE-221 00 Lund, Sweden
6The Oskar Klein Centre for Cosmoparticle Physics, AlbaNova, SE-106 91 Stockholm, Sweden
7Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, I-56127 Pisa, Italy
8Laboratoire AIM, CEA-IRFU/CNRS/Universit´e Paris Diderot, Service d’Astrophysique, CEA Saclay,
91191 Gif sur Yvette, France
9Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, I-34127 Trieste, Italy
10Dipartimento di Fisica, Universit`a di Trieste, I-34127Trieste, Italy
11Rice University, Department of Physics and Astronomy, MS-108, P. O. Box 1892, Houston, TX 77251,
USA
12Istituto Nazionale di Fisica Nucleare, Sezione di Padova,I-35131 Padova, Italy
13Dipartimento di Fisica “G. Galilei”, Universit`a di Padova,I-35131 Padova, Italy
14Center for Space Plasma and Aeronomic Research (CSPAR), University of Alabama in Huntsville,
Huntsville, AL 35899,USA
15Max-Planck Institut fu¨r extraterrestrische Physik, 85748 Garching, Germany
16Istituto Nazionale di Fisica Nucleare, Sezione di Perugia,I-06123 Perugia, Italy
17Dipartimento di Fisica, Universit`a degli Studi di Perugia, I-06123 Perugia,Italy
18Dipartimento di Fisica “M. Merlin” dell’Universit`a e del Politecnico di Bari, I-70126 Bari, Italy
19Istituto Nazionale di Fisica Nucleare, Sezione di Bari, 70126 Bari, Italy
20Laboratoire Leprince-Ringuet, E´cole polytechnique, CNRS/IN2P3, Palaiseau, France
21Institut de Ciencies de l’Espai (IEEC-CSIC), Campus UAB, 08193 Barcelona, Spain
22INAF-Istituto di Astrofisica Spaziale e Fisica Cosmica, I-20133 Milano, Italy
23NASA Goddard Space Flight Center, Greenbelt, MD 20771,USA
24Center for Research and Exploration in Space Science and Technology (CRESST) and NASA Goddard
– 4 –
Space Flight Center, Greenbelt, MD 20771,USA
25DepartmentofPhysicsandCenterforSpaceSciencesandTechnology,UniversityofMarylandBaltimore
County, Baltimore, MD 21250,USA
26Space Science Division, Naval Research Laboratory,Washington, DC 20375, USA
27George Mason University, Fairfax, VA 22030,USA
28Laboratoire de Physique Th´eorique et Astroparticules, Universit´e Montpellier 2, CNRS/IN2P3, Mont-
pellier, France
29Department of Physics, Stockholm University, AlbaNova, SE-106 91 Stockholm, Sweden
30Royal Swedish Academy of Sciences Research Fellow, funded by a grant from the K. A. Wallenberg
Foundation
31Agenzia Spaziale Italiana (ASI) Science Data Center, I-00044 Frascati (Roma), Italy
32DipartimentodiFisica,Universit`adiUdineandIstitutoNazionalediFisicaNucleare,SezionediTrieste,
Gruppo Collegato di Udine, I-33100Udine, Italy
33Los Alamos National Laboratory, Los Alamos, NM 87545, USA
34Osservatorio Astronomico di Trieste, Istituto Nazionale di Astrofisica, I-34143 Trieste, Italy
35Department of Physical Sciences, Hiroshima University, Higashi-Hiroshima,Hiroshima 739-8526,Japan
36INAF Istituto di Radioastronomia,40129 Bologna, Italy
37Centre for Astrophysics Research, Science and Technology Research Institute, University of Hertford-
shire, Hatfield AL10 9AB, UK
38Department of Physics, Center for Cosmology and Astro-Particle Physics, The Ohio State University,
Columbus, OH 43210,USA
39Research Institute for Science and Engineering, Waseda University, 3-4-1, Okubo, Shinjuku, Tokyo,
169-8555Japan
40Centre d’E´tude Spatiale des Rayonnements,CNRS/UPS, BP 44346,F-30128Toulouse Cedex 4, France
41NASA Marshall Space Flight Center, Huntsville, AL 35812,USA
42University College Dublin, Belfield, Dublin 4, Ireland
43DepartmentofPhysicsandDepartmentofAstronomy,UniversityofMaryland,CollegePark,MD20742,
USA
44DepartmentofPhysics,RoyalInstitute ofTechnology(KTH),AlbaNova,SE-10691Stockholm,Sweden
45Universities Space Research Association (USRA), Columbia, MD 21044,USA
46DepartmentofAstronomyandAstrophysics,PennsylvaniaStateUniversity,UniversityPark,PA16802,
USA
47Istituto Nazionale di Fisica Nucleare, Sezione di Roma “Tor Vergata”, I-00133 Roma, Italy
– 5 –
ABSTRACT
We report on the observation of the bright, long gamma-ray burst,
GRB 090926A, by the Gamma-ray Burst Monitor (GBM) and Large Area Tele-
48Department of Physics, Tokyo Institute of Technology, Meguro City, Tokyo 152-8551,Japan
49Department of Physics and Astronomy, University of Denver, Denver, CO 80208,USA
50InstituteofSpaceandAstronauticalScience,JAXA,3-1-1Yoshinodai,Sagamihara,Kanagawa229-8510,
Japan
51HiroshimaAstrophysicalScienceCenter,HiroshimaUniversity,Higashi-Hiroshima,Hiroshima739-8526,
Japan
52National Research Council Research Associate, National Academy of Sciences, Washington, DC 20001,
USA
53Institut fu¨r Astro- und Teilchenphysik and Institut fu¨r Theoretische Physik, Leopold-Franzens-
Universit¨at Innsbruck, A-6020 Innsbruck, Austria
54Universit´e Bordeaux 1, CNRS/IN2p3, Centre d’E´tudes Nucl´eaires de Bordeaux Gradignan, 33175
Gradignan, France
55Kavli Institute for Cosmological Physics, University of Chicago, Chicago, IL 60637, USA
56Santa Cruz Institute for Particle Physics, Department of Physics and Department of Astronomy and
Astrophysics, University of California at Santa Cruz, Santa Cruz, CA 95064,USA
57Department of Physics, University of Washington, Seattle, WA 98195-1560,USA
58Space Sciences Division, NASA Ames Research Center, Moffett Field, CA 94035-1000,USA
59NYCB Real-Time Computing Inc., Lattingtown, NY 11560-1025,USA
60Department of Chemistry and Physics, Purdue University Calumet, Hammond, IN 46323-2094,USA
61Partially supported by the International Doctorate on Astroparticle Physics (IDAPP) program
62Institucio´ Catalana de Recerca i Estudis Avan¸cats (ICREA), Barcelona,Spain
63Consorzio Interuniversitario per la Fisica Spaziale (CIFS), I-10133 Torino, Italy
64INTEGRAL Science Data Centre, CH-1290 Versoix, Switzerland
65NASA PostdoctoralProgramFellow, USA
66Dipartimento di Fisica, Universit`a di Roma “Tor Vergata”,I-00133 Roma, Italy
67Joint Center for Particle Nuclear Physics and Cosmology (J-CPNPC), Nanjing 210093,China
68Purple Mountain Observatory,Chinese Academy of Sciences, Nanjing 210008,China
69Aoyama Gakuin University, Sagamihara-shi,Kanagawa 229-8558,Japan
70School of Pure and Applied Natural Sciences, University of Kalmar, SE-391 82 Kalmar, Sweden
– 6 –
scope (LAT) instruments on board the Fermi Gamma-ray Space Telescope.
GRB 090926A shares several features with other bright LAT bursts. In par-
ticular, it clearly shows a short spike in the light curve that is present in all
detectors that see the burst, and this in turn suggests that there is a common
region of emission across the entire Fermi energy range. In addition, while a
separate high-energy power-law component has already been observed in other
GRBs, here we report for the first time the detection with good significance
of a high-energy spectral break (or cutoff) in this power-law component around
1.4 GeV in the time-integrated spectrum. If the spectral break is caused by opac-
ity to electron-positron pair production within the source, then this observation
allows us to compute the bulk Lorentz factor for the outflow, rather than a lower
limit.
Subject headings: gamma rays: bursts
1. Introduction
Gamma-Ray Bursts (GRBs) are the most energetic transients in the universe. The first
brief and intense flash, the so-called prompt emission, has been observed in the X-ray and
gamma-ray bands, while subsequent long-lived afterglow emission has so far been observed
mainly at energies in the X-ray band and below. The prompt emission is thought to be
producedinanultra-relativisticoutflow, butitsdetailedemissionmechanismhasbeenalong-
standing problem. It has been widely believed that the afterglow is the synchrotron emission
from the forward shock that propagates in the external medium, but Swift observations
have pointed out some difficulties in this model (for recent reviews, Zhang 2007; M´esz´aros
2006). The study of the gamma-ray emission in the GeV energy range is expected to give
us important information on these issues and even on the nature of the progenitors and
the ultra-relativistic outflows of GRBs (Band et al. 2009; Falcone et al. 2008; Fan & Piran
2008).
The Fermi Gamma-ray Space Telescope hosts two instruments, the Large Area Tele-
scope (LAT, 20 MeV to more than 300 GeV; Atwood et al. 2009) and the Gamma-ray Burst
Monitor (GBM, 8 keV–40 MeV; Meegan et al. 2009), which together are capable of measur-
ingthespectral parametersofGRBsacrossseven decadesinenergy. Sincethestartofscience
operations in early August 2008, the Fermi LAT has significantly detected 16 GRBs. These
events, including the very bright long-duration and short-duration bursts GRB 080825C,
GRB 080916C, GRB 081024B, GRB 090510, and GRB 090902B, have revealed many im-
portant, seemingly common, features of GRB GeV emission (Abdo et al. 2009a,b,c,d, 2010;
– 7 –
Ackermann et al. 2010a,b): (1) the GeV emission onsets of many LAT GRBs are delayed
with respect to the MeV emission onsets; (2) some LAT GRBs have extra hard components
apart from the canonical Band function (Band et al. 1993), which typically peaks in νF
ν
between around 100 keV–1 MeV; (3) the GeV emission lasts longer than the prompt MeV
emission, showing power-law temporal decays at late times.
In this paper, we report on the analysis of the bright, long GRB 090926A detected by
Fermi LAT/GBM. The light curve of this burst above 100 MeV shows a sharp spike with a
width of 0.15 s, fast variability that we use to constrain the origin of the high-energy photons
within the spike. Furthermore, from the detection of a break in the >100MeV gamma-ray
spectrum, we derive constraints on the bulk Lorentz factor and the distance of the emitting
region from the central source. Section 2 summarizes the detections of GRB 090926A by the
GBM and the LAT, and the follow-up observations. Section 3 presents the light curves of the
promptemissionasseenbybothinstrumentsanddescribesasharppulseseeninalldetectors.
In section 4, we detail the spectral analysis of the burst through time-resolved spectroscopy,
the measurement of a break in the extra-component, and the extended emission found in
the LAT data out to 4.8 ks after the trigger. These last two points are at the center of the
physical interpretation of the observations that is developed in section 5.1. Throughout this
paper, we adopt a Hubble constant of H = 72 km s−1 Mpc−1 and cosmological parameters
0
of Ω = 0.73 and Ω = 0.27.
Λ M
2. Observations
At 04:20:26.99 (UT) on Sept. 26, 2009 (hereafter T = 275631628.98s mission elapsed
0
time), the Fermi Gamma-ray Burst Monitor (GBM) triggered on and localized the long
GRB090926Aat(RA,Dec)=(354.5◦,−64.2◦),inJ2000coordinates(Bissaldi2009;Uehara et al.
2009). This position was ∼52◦ with respect to the LAT boresight at the time of the trigger
and well within the field of view. An Autonomous Repoint Request (ARR) was gener-
ated, but the spacecraft initially remained in survey mode as the Earth avoidance angle
condition was not satisfied by the burst pointing direction. The on-board GBM position of
GRB 090926Awas occulted by the Earth at roughly T +500s until it rose above the horizon
0
at approximately T +3000s. At that time, the spacecraft slewed to GRB 090926A and kept
0
it close to the center of the LAT field of view until T +18000s, though the source location
0
was occulted by the Earth several times over that time period.
Emission fromGRB090926Awas evident inthe Fermi LAT rawtrigger event rates, and
thenumberofLATevents(∼ 200photoncandidatesabove100MeV)iscomparabletothatof
the other bright LAT bursts, GRB 080916C, GRB 090510 and GRB 090902B. The increase
– 8 –
in the photon count rate during the prompt phase is spatially and temporally correlated
with the GBM emission with high significance, and extended emission is observed until
T +4800s. The best LAT on-ground localization is (RA, Dec) = (353.56◦, −66.34◦), with
0
a 90% containment radius of 0.07◦ (statistical; 68% containment radius: 0.04◦, systematic
error is less than 0.1◦) and is consistent with the XRT localization.
Indeed, based upon the GCN report issued for the LAT detection, a Swift TOO ob-
servation was performed, and an afterglow for GRB 090926A was detected with XRT and
UVOT at T +47ks and localized at (RA, Dec) = (353.40070◦, -66.32390◦) with an uncer-
0
tainty of 1.5 ′′(90% confidence) (Vetere et al. 2009). VLT observations determined a redshift
for GRB 090926A of z = 2.1062, using the X-shooter spectrograph (Malesani et al. 2009).
Suzaku/WAM and Skynet/PROMPT also detected the soft gamma-ray prompt and optical
afterglow emission, respectively (Noda et al. 2009).
3. Light Curves
In figure 1, we show the GBM and LAT light curves in several energy bands. The
highest energy photon is a 19.6 GeV event, observed at T +25s within 0.03◦ from the LAT
0
position of GRB 090926A, well within the 68% containment of the point spread function at
that energy. The light curves show that the onset of the LAT emission is delayed by 3.3
s with respect to the GBM emission, similar to other LAT GRBs (Abdo et al. 2009a,b,c,d,
2010; Ackermann et al. 2010a,b). Detailed analysis of the GBM data results in a formal
T90 duration1 (Kouveliotou et al. 1993) of 13.1±0.2 s, with a start time of T +2.2s and
0
a stop time of T +15.3s. The emission measured in the LAT above 100MeV has a similar
0
duration; however, owing to the efficient background rejection applied to the LAT data, the
signal is clearly visible in the light curve well after this time range.
The time intervals chosen for spectroscopy are indicated by the vertical lines in figure 1,
withboundariesatT +(0,3.3,9.8,10.5,21.6)s. TheendofthelasttimeintervalatT +21.6s
0 0
was chosen somewhat arbitrarily as the end of the prompt phase, but we carefully verified
that our results are not affected by a slightly different choice. Figure 2 shows a zoom of
some of the light curves between T + 2.2s and T + 15.3s, with a binning of 0.05s, and
0 0
highlights the presence of the sharp peak seen in each of the NaI, BGO, and LAT light
curves at T +10s. As seen on figure 2, the peak is clearly in coincidence in all of the light
0
curves, indicating a strong correlation of the emission from a few keV to energies > 100
1The T90 duration is the time over which the central 90% of the counts between 50 and 300 keV have
been accumulated.
– 9 –
MeV. Because this peak is the only one evident at all energies, we chose to run a dedicated
spectral analysis between T +9.8s and T +10.5s as described in section 4.1.
0 0
We estimated the variability time scale using the full width at half maximum (FWHM)
of the bright pulse seen around T +10s. A combination of exponential functions is used to
0
fit the light curve as performed in Norris et al. (1996). The light curve fitting is performed
for all bright NaI detectors (N6, N7, N8) with a 2ms time resolution. Two exponential
functions are used to represent the weak and main bright peaks and include a quadratic
function to fit the longer timescale variations. As a result, we obtain a FWHM of the main
peak of 0.15 ± 0.01s for the bright pulse.
4. Spectral Analysis
4.1. LAT and GBM spectral fitting
We performed a time-integrated joint spectral analysis of the LAT and GBM data for
the prompt phase defined as T + 3.3s to T + 21.6s in figure 1. For the GBM, we used
0 0
‘Time Tagged Events’ (TTE) data from the NaI detectors 6, 7, 8 and BGO detector 1. As
in Abdo et al. (2009d), background rates and errors are estimated during the prompt phase
by fitting background regions of the light curve before and after the burst. We derived our
background estimates using the time intervals [T −44;T −8]s and [T +36;T +100]s for
0 0 0 0
the NaI detectors, and [T − 43;T − 16]s and [T + 43;T + 300]s for the BGO detector.
0 0 0 0
For the LAT, we extracted ‘transient’ class data from an energy-dependent acceptance cone
around the burst position, as described in Abdo et al. (2009d), and considered front- and
back-converting events separately (Atwood et al. 2009). The data files for the analysis were
prepared using the LAT ScienceTools-v9r15p2 package, which is available from the Fermi
Science Support Center (FSSC), and the P6 V3 TRANIENT response functions.71 A synthetic
background was derived for the LAT data using an empirical model of the rates expected for
the position of the source in the sky and for the position and orientation of the spacecraft
during the burst interval.
The joint spectral fitting of GBM and LAT data was performed using rmfit version 3.2
(Kaneko et al. 2006; Abdo et al. 2009d), which estimates the goodness-of-fit in terms of the
Castor Statistic (C-STAT) to handle correctly the small number of events at the highest
energies. The Castor statistic (Dorman 2003) is similar to the Cash statistic (Cash 1979)
except foranoffset thatisconstant fora particulardataset. Aglobaleffective areacorrection
71http://fermi.gsfc.nasa.gov/ssc/
– 10 –
ts/Bin 234000000 G BN(Ma8 I 7Nk ae+IV 6-N 1a+4I .83 keV) Counts/bin 2460000000000 5 10 15 20 234000000000 E [Hz]
Coun 100 Time since T0 (s) 1000 RAT
0 aaaa bbb cc d 0
-20 0 20 40 60 80 100
3000 GBM NaI6 + bin100000
nts/Bin 2000 (N1a4I.73 +k eNVa-I286 0 keV) Counts/500000 5 10 15 20 20000E [Hz]
ou 1000 Time since T0 (s) 10000AT
C R
0 0
300-20 0 20 40 60 80 100
GBM BGO1 bin10000
nts/Bin 120000 (260 keV-5 MeV) Counts/ 50000 5 10 15 20 12000000 TE [Hz]
u Time since T (s) A
Co 0 R
0 0
-20 0 20 40 60 80 100
LAT
n 200 (All events) 2000
Counts/Bi 100 1000 RATE [Hz]
0 0
30-20 0 20 40 60 80 10060
LAT in
ounts/Bin 1200 (> 100 MeV) Counts/b 150000 5Time s1i0nce T0 (1s5) 20 2400 RATE [Hz]
C
0 0
-20 0 20 40 60 80 100
V] LAT EVENTS
[Me 104 (> 1 GeV)
y
g
r
e
n
E
103
-20 0 20 40 60 80 100
Time since T (s)
0
Fig. 1.— GBM and LAT light curves for the gamma-ray emission of GRB 090926A. The
data from the GBM NaI detectors were divided into soft (8–14.3keV) and hard (14.3–
260keV) bands to reveal similarities between the light curve at the lowest energies and that
of the LAT data. The fourth panel shows all LAT events that pass the on-board GAMMA
filter (Atwood et al. 2009). The first four light curves are background-subtracted and are
shown for 0.1s time bins. The fifth and sixth panels show LAT data ‘transient’ class events
for energies >100MeV and >1GeV respectively, both using 0.5s time bins. The vertical
lines indicate the boundaries of the intervals used for the time-resolved spectral analysis,
T + (0,3.3,9.8,10.5,21.6) s. The insets show the counts for each data set, binned using
0
these intervals, to illustrate the numbers of counts considered in each spectral fit.