UvA-DARE is a service provided by the library of the University of Amsterdam (http
s
://dare.uva.nl)
UvA-DARE (Digital Academic Repository)
The Fermi GBM Gamma-Ray Burst Spectral Catalog: Four Years of Data
Gruber, D.; Goldstein, A.; Weller von Ahlefeld, V.; Bhat, N.P.; Bissaldi, E.; Briggs, M.S.;
Byrne, D.; Cleveland, W.H.; Connaughton, V.; Diehl, R.; Fishman, G.J.; Fitzpatrick, G.; Foley,
S.; Gibby, M.; Giles, M.M.; Greiner, J.; Guiriec, S.; van der Horst, A.J.; von Kienlin, A.;
Kouveliotou, C.; Layden, E.; Lin, L.; Meegan, C.A.; McGlynn, S.; Paciesas, W.S.; Pelassa, V.;
Preece, R.D.; Rau, A.; Wilson-Hodge, C.A.; Xiong, S.; Younes, G.; Yu, H-F.
DOI
10.1088/0067-0049/211/1/12
Publication date
2014
Document Version
Final published version
Published in
The Astrophysical Journal. Supplement Series
Link to publication
Citation for published version (APA):
Gruber, D., Goldstein, A., Weller von Ahlefeld, V., Bhat, N. P., Bissaldi, E., Briggs, M. S.,
Byrne, D., Cleveland, W. H., Connaughton, V., Diehl, R., Fishman, G. J., Fitzpatrick, G.,
Foley, S., Gibby, M., Giles, M. M., Greiner, J., Guiriec, S., van der Horst, A. J., von Kienlin, A.,
... Yu, H-F. (2014). The Fermi GBM Gamma-Ray Burst Spectral Catalog: Four Years of Data.
The Astrophysical Journal. Supplement Series
,
211
(1), [12]. https://doi.org/10.1088/0067-
0049/211/1/12
General rights
It is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s)
and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open
content license (like Creative Commons).
Disclaimer/Complaints regulations
If you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please
let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material
inaccessible and/or remove it from the website. Please Ask the Library: https://uba.uva.nl/en/contact, or a letter
to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You
will be contacted as soon as possible.
The Astrophysical Journal Supplement Series, 211:12 (27pp), 2014 March doi:10.1088/0067-0049/211/1/12
C
2014. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
THE FERMI GBM GAMMA-RAY BURST SPECTRAL CATALOG: FOUR YEARS OF DATA
David Gruber
1,2
, Adam Goldstein
3
, Victoria Weller von Ahlefeld
1,4
, P. Narayana Bhat
3
, Elisabetta Bissaldi
5,6
,
Michael S. Briggs
3
, Dave Byrne
7
, William H. Cleveland
8
, Valerie Connaughton
3
, Roland Diehl
1
,
Gerald J. Fishman
9
, Gerard Fitzpatrick
7
, Suzanne Foley
7
, Melissa Gibby
10
, Misty M. Giles
10
,
Jochen Greiner
1
, Sylvain Guiriec
11
, Alexander J. van der Horst
12
, Andreas von Kienlin
1
,
Chryssa Kouveliotou
9
, Emily Layden
3
,LinLin
3,13
, Charles A. Meegan
3
,Sin
´
ead McGlynn
7
,
William S. Paciesas
3
,V
`
eronique Pelassa
3
, Robert D. Preece
3
, Arne Rau
1
, Colleen A. Wilson-Hodge
9
,
Shaolin Xiong
3
, George Younes
8
, and Hoi-Fung Yu
1
1
Max-Planck-Institut f
¨
ur extraterrestrische Physik, Giessenbachstrasse 1, D-85748 Garching, Germany
2
Planetarium S
¨
udtirol, Gummer 5, I-39053 Karneid, Italy
3
University of Alabama in Huntsville, 320 Sparkman Drive, Huntsville, AL 35805, USA
4
School of Physics and Astronomy, University of Edinburgh, James Clerk Maxwell Building, Mayfield Road, EH9 3JZ Edinburgh, UK
5
Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, I-34127 Trieste, Italy
6
Dipartimento di Fisica, Universita’ di Trieste, I-34127 Trieste, Italy
7
School of Physics, University College Dublin, Belfield, Stillorgan Road, Dublin 4, Ireland
8
Universities Space Research Association, 320 Sparkman Drive, Huntsville, AL 35805, USA
9
Space Science Office, VP62, NASA/Marshall Space Flight Center, Huntsville, AL 35812, USA
10
Jacobs Technology, Inc., Huntsville, AL, USA
11
NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
12
Astronomical Institute, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
13
SabancıUniversity, Faculty of Engineering and Natural Sciences, Orhanlı–Tuzla,
˙
Istanbul 34956, Turkey
Received 2013 October 7; accepted 2014 January 10; published 2014 February 25
ABSTRACT
In this catalog we present the updated set of spectral analyses of gamma-ray bursts (GRBs) detected by the Fermi
Gamma-Ray Burst Monitor during its first four years of operation. It contains two types of spectra, time-integrated
spectral fits and spectral fits at the brightest time bin, from 943 triggered GRBs. Four different spectral models
were fitted to the data, resulting in a compendium of more than 7500 spectra. The analysis was performed similarly
but not identically to Goldstein et al. All 487 GRBs from the first two years have been re-fitted using the same
methodology as that of the 456 GRBs in years three and four. We describe, in detail, our procedure and criteria for
the analysis and present the results in the form of parameter distributions both for the observer-frame and rest-frame
quantities. The data files containing the complete results are available from the High-Energy Astrophysics Science
Archive Research Center.
Key words: gamma-ray burst: general – methods: data analysis
Online-only material: color figures
1. INTRODUCTION
During its first four years of operation, the Fermi Gamma-
Ray Burst Monitor (GBM; Meegan et al. 2009) provided the
scientific community with an enormous sample of gamma-ray
burst (GRB) data, significantly expanding our understanding of
the physical properties and characteristics of GRBs. In addition,
discoveries of new and intriguing phenomena were associated
with many individual GRBs (e.g., Ackermann et al. 2010;
Guiriec et al. 2011; Ackermann et al. 2011; Axelsson et al.
2012; Guiriec et al. 2013).
Here, we present the second GBM GRB spectral catalog
which will provide the most comprehensive resource of GRB
spectral properties to date. In order to be as complete and
uniform as possible, our methodology follows closely, but is not
identical to, the procedures employed in the GRB catalog from
the Burst And Transient Source Experiment (BATSE; Kaneko
et al. 2006) and the first GBM GRB spectral catalog (Goldstein
et al. 2012). We include representative spectral fits for all GBM
bursts from the first four years of operation (2008 July 14–2012
July 13).
For each GRB, we show two types of spectra: time-integrated
spectra (henceforth labeled F for fluence) and spectra at the
brightest time bin (henceforth labeled P for peak flux). A set
of four empirical models was applied to the data in both cases.
The selection of these model functions is based on tradition
(Band et al. 1993; Kaneko et al. 2006; Goldstein et al. 2012)
and mathematical complexity. The signal-to-noise ratios (S/Ns)
in the data of GBM bursts rarely support models with more than
four free parameters, which is why we resort to models with
two, three or four free fit parameters and only in exceptional
cases additive terms (e.g., a blackbody component; Axelsson
et al. 2012) can be added. Making use of the Castor statistics
(Ackermann et al. 2011), we derive the best model for each GRB
and present the distribution and characteristics of the model
parameters.
This catalog is organized as follows. In Section 2 we present
a short overview of the GBM; in Section 3 we describe the
methodology used in the production of this catalog, including
detector selection, data types, energy selection and background
fitting, and the source selection. We then offer a description of
the spectral models used in this catalog in Section 4, present the
spectral analysis methods in Section 5, and present the results
1
The Astrophysical Journal Supplement Series, 211:12 (27pp), 2014 March Gruber et al.
in Section 6. Finally, in Section 7 we conclude with a summary
and a discussion.
2. FERMI GBM
The Fermi
14
Gamma-Ray Space Telescope was successfully
launched on 2008 June 11 into a Low Earth orbit of ∼565 km
altitude at an 25.
◦
6 inclination. Its payload comprises two
instruments, the GBM and the Large Area Telescope (LAT;
Atwood et al. 2009). The goal of GBM is to augment the
science return from Fermi with its prime objective being joint
spectral and timing analyses of GRBs seen in common with the
LAT. In addition, GBM provides near real-time burst locations
which permit (1) the Fermi spacecraft to repoint the LAT toward
the observed GRB and (2) to perform follow-up observations
with ground-based facilities. Compared to other high-energy
spacecraft, the great advantage of GBM is its capability to
observe the whole unocculted sky at any given time with a
field of view of 8 sr and its very broad energy coverage.
Therefore, along with GRBs, GBM offers great capabilities to
observe all kinds of high-energy astrophysical phenomena, such
as, e.g., solar flares (e.g., Gruber et al. 2011b; Ackermann et al.
2012a), soft gamma repeaters (e.g., Lin et al. 2011; von Kienlin
et al. 2012) and terrestrial gamma-ray flashes (e.g., Briggs et al.
2013).
Designed to study the gamma-ray sky in the energy band of
∼8 keV–40 MeV, GBM is composed of 12 sodium iodide (Na i)
and two bismuth germanate (BGO) scintillation detectors. With
a thickness of 1.27 cm and a diameter of 12.7 cm, the Na i
crystals cover an energy range from 8 keV–1 MeV. They are
oriented around the spacecraft such that the position of the GRB
can be determined.
The two BGO crystals have a diameter and thickness of
12.7 cm, covering an energy range of 200 keV–40 MeV, and are
located on opposite sides of the spacecraft so that at least one is
illuminated from any direction. A source location is calculated in
spacecraft coordinates and used in the production of the detector
response matrices (DRMs; see Section 5).
For more details on the GBM detectors and their calibration,
refer to Meegan et al. (2009), Bissaldi et al. (2009), and Paciesas
et al. (2012).
3. METHOD
During the first four years of operation, GBM triggered on
a total of 954 GRBs (von Kienlin et al. 2014), 943 of which
are presented in this catalog. The remaining bursts are excluded
due to a low accumulation of counts or a lack of spectral/
temporal coverage. In order to deliver the most useful analysis
to the community, we have attempted to make the method as
systematic and uniform as possible; circumstances under which
deviations were employed are clearly indicated. Details of the
detector and data selection as well as the process used to fit
the data are described in this section. Many of the criteria are
adopted from the GBM Burst Catalog (Paciesas et al. 2012) and
we have attempted to maintain this in all aspects. However, due
to the nature of spectral analysis we demand stricter criteria to
ensure that we have adequate signal in all energy channels. This
effectively reduces the GRB sample from that used in the burst
catalog.
We highlight that this catalog only presents the analysis of
GRBs that triggered the GBM. There is a non-negligible amount
14
Formerly known as the Gamma-Ray Large Area Space Telescope or
GLAST.
of GRBs that did not trigger GBM whose temporal and spectral
properties are presented elsewhere (Gruber et al. 2012). These
GRBs do not have different properties compared to the triggered
GRB sample but simply occurred during times when the GBM
triggering algorithm was switched off (e.g., when the spacecraft
was at latitudes of high geomagnetic activity).
3.1. Detector Selection
The detector selection is consistent with Goldstein et al.
(2012), i.e., a maximum of three Na i detectors together with
one BGO detector were used for the spectral analysis. Since
the effective area (i.e., detection efficiency) of the Na i detectors
decreases rapidly for high incidence angles (Bissaldi et al. 2009)
only detectors with source angles 60
◦
are used for the spectral
analysis. In addition, it has been verified that the detectors were
neither obstructed by the spacecraft nor by the solar panels of
Fermi. However, due to small inaccuracies in the spacecraft
mass model or location uncertainties, the blockage code does
not always return a subset of detectors that is free from blockage.
This is evident when the low-energy data deviate strongly from
the fit model (Goldstein et al. 2012). When this occurs we
remove these detectors from the selected sample. If more than
three Na i detectors are qualified for the spectral fitting, the Na i
detectors with the smallest source angles were used to avoid a
fitting bias toward lower energies.
3.2. Data Types
GBM persistently records two different types of science data,
called CTIME (fine time resolution, coarse spectral resolution
of 8 energy channels) and CSPEC (coarse time resolution, full
spectral resolution of 128 energy channels). CTIME (CSPEC)
data have a nominal time resolution of 0.256 s (4.096 s) which is
increased to 64 ms (1.024 s) whenever GBM triggers on an event.
After 600 s in triggered mode, both data types return to their
non-triggered time resolution. The third and primary data type
used in this catalog is the “time tagged events” (TTEs) which
consist of individual events, each tagged with arrival time (2
μs precision), energy (128 channels) and detector number. The
TTE data are generated and stored on-board in a continuously
recycling buffer. When GBM enters trigger mode, the buffered
pre-trigger TTE are transmitted as science data along with
∼300 s of post-trigger TTE.
For the purpose of this catalog, we choose a standard time
binning of 1024 ms for bursts longer than 2 s in duration as
defined by the burst T
90
(Kouveliotou et al. 1993) presented in
von Kienlin et al. (2014) and 64 ms for bursts of duration 2 s
and shorter. The time history of TTE typically starts at ∼30 s
before trigger and extends to ∼300 s after trigger. This TTE
data time span is adequate for the analysis of most GRBs. For
GRBs that have evident precursors or emissions that last more
than 300 s after trigger, we use the CSPEC data, which extend
∼4000 s before and after the burst for triggered events. CSPEC
data were used for 76 GRBs in this catalog.
3.3. Energy Selection and Background Fitting
With the optimum subset of detectors selected, the best time
and energy selections are chosen to fit the data. The available
and reliable energy channels in the Na i detectors lie between
∼8 keV and ∼1 MeV. This selection excludes the overflow
channel at high energies and those channels <8 keV where the
transmission of gamma-rays is poor due to the silicone pad in
front of the Na i crystal and the multi layer insulation around the
2
The Astrophysical Journal Supplement Series, 211:12 (27pp), 2014 March Gruber et al.
detectors (Bissaldi et al. 2009). We perform a similar selection
to the BGO detector for each burst, selecting channels between
∼300 keV and ∼38 MeV. We select enough pre- and post-burst
data to sufficiently model the background and fit a single energy
dependent polynomial (choosing up to fourth order) to the
background. For each detector the time selection and polynomial
order are varied until the χ
2
statistic map over all energy
channels is minimized, resulting in an adequate background fit.
This approach is rather subjective in that it is dependent on the
observer’s choice of the background intervals. In the future, it
may be advantageous to implement more objective background
selection methods such as the “direction dependent background
fitting” method presented in Sz
´
ecsi et al. (2013).
3.4. Source Selection
Knowing the background model, the background-subtracted
count rates are summed over all Na i detectors for a given burst
to produce a single GRB count light curve. Using this light
curve, only time bins (1.024 s for long burst and 64 ms for short
bursts) with an S/N greater or equal to 3.5 were selected, in
agreement with Goldstein et al. (2012). This time selection is
then applied to all detectors for a given burst.
This criterion ensures that there is adequate signal to suc-
cessfully perform a spectral fit and constrain the parameters of
the fit. This does however eliminate some faint bursts from the
catalog sample (i.e., those with no time bins with signal above
3.5σ ). While the possibility remains that not all signal from the
GRB was selected, this method nevertheless provides the most
objective way to obtain a selection of intrinsic GRB counts as in-
cluding less significant bins would only increase the uncertainty
in the measurements.
This selection is what we refer to as the F selection, since it
is a time-integrated selection and the derived photon and energy
fluences are representative of but not equal to the fluence over
the total duration of the burst. We draw attention to the fact that
time bins with an S/N less than 3.5, which were not included
in the fitting process, also contribute to the photon and energy
fluence. For more than 80% of the GRBs in the catalogue the
ratio of count fluence (without the intervals with S/N < 3.5)
versus the total counts (with the intervals with S/N < 3.5) is
larger than 0.8. So while there are some bursts for which the
fluence can be considerably underestimated, the other option
would be to overestimate the fluence of those bursts by including
background that contaminates the spectral fits.
The other selection performed is a 1.024 s peak photon flux
selection for long bursts (T
90
> 2 s) and 64 ms peak count rate
flux selection for short bursts (T
90
2 s). This selection is made
by adding the count rates in the Na i detectors again and selecting
the single bin of signal with the highest background-subtracted
count rate. This selection is a snapshot of the energetics at the
most intense part of the burst and is depicted as the P selection.
Figure 1 shows the distribution of accumulation times used
for the fitting process based on the signal-to-noise selection
criteria. The distribution of accumulation times reported is
similar to the observed emission time of the burst, excluding
quiescent periods, (e.g., Mitrofanov et al. 1999) and peaks
at 0.26 s and ∼15 s for short and long GRBs, respectively.
The dividing duration time scale between the two classes of
GRBs is ∼1.27 s and is, as expected from the employed source
selection methodology, somewhat smaller than the canonical
2 s (Kouveliotou et al. 1993). Figure 1 also includes the
comparisons of the model photon fluence and model photon
flux compared to the accumulation time. In both cases two
specific regions are visible for long and short GRBs. In addition,
Figures 1(b) and 1(c) show a clear correlation between the
photon fluence (flux) and accumulation time, indicating the
existence of two different burst groups, similar to the ones
delineated by the hardness–duration relationship found by
Kouveliotou et al. (1993).
4. MODELS
We chose four spectral models to fit the spectra of GRBs
in our selection sample. These models include a single power
law (Pl), Band’s GRB function (Band), an exponential cut-
off power law (Comp), and a smoothly broken power law
(Sbpl). All models are formulated in units of photon flux with
energy (E) in keV and multiplied by a normalization constant A
(photon s
−1
cm
−2
keV
−1
). Below we describe each model and
its features in detail.
4.1. Power-law Model
The single power law with two free parameters is defined as
f
PL
(E) = A
E
E
piv
λ
, (1)
where A is the amplitude and λ is the spectral index. The
pivot energy (E
piv
) normalizes the model to the energy range
under consideration and helps reduce cross-correlation of other
parameters. In all cases, E
piv
is held fixed at 100 keV. While
most GRBs exhibit a spectral break in the GBM passband, some
GRBs are too weak to adequately constrain this break in the fits.
These bursts are well fit by the single power law.
4.2. Band’s GRB Function
Band’s GRB function (Band et al. 1993) has become the
standard for fitting GRB spectra, and therefore we include it in
our analysis:
f
BAND
(E) = A
⎧
⎪
⎪
⎪
⎨
⎪
⎪
⎪
⎩
E
100 keV
α
exp
−
(α+2)E
E
peak
,E<
(α−β) E
peak
α+2
E
100 keV
β
exp(β − α)
(α−β)E
peak
100 keV (α+2)
α−β
.
E
(α−β) E
peak
α+2
(2)
The four free parameters are the amplitude, A, the low- and high-
energy spectral indices, α and β, respectively, and the νF
ν
peak
energy, E
peak
. This function is essentially a smoothly broken
power law with a curvature defined by its spectral indices. The
low-energy index spectrum is a power-law only asymptotically.
4.3. Comptonized Model
The Comptonized model is an exponentially cutoff power
law, which is a subset of the Band function in the limit that
β →−∞:
f
COMP
(E) = A
E
E
piv
α
exp
−
(α +2)E
E
peak
(3)
The three free parameters are the amplitude A, the low-energy
spectral index α and E
peak
. E
piv
is again fixed to 100 keV, as for
the power-law model.
3
The Astrophysical Journal Supplement Series, 211:12 (27pp), 2014 March Gruber et al.
0
50
100
150
200
0.01 0.10 1.00 10.00 100.00 1000.00
3.5 - σ Accumulation Time [s]
0.01 0.10 1.00 10.00 100.00
Accumulation time [s]
0.1
1.0
10.0
100.0
1000.0
Photon fluence [ph cm
-2
]
0.01 0.10 1.00 10.00 100.00
Accumulation time [s]
1
10
Photon flux [ph cm
-2
s
-1
]
(a)
(b) (c)
Figure 1. Panel (a) shows the distribution of the accumulation times based on the 3.5σ signal-to-noise selections. Note the similarity to the traditional t
90
distribution,
with the minimum near 1 s. No other estimation of the duration was factored into the production of the accumulation time. Panels (b) and (c) show the comparison of
the model photon fluence and model photon flux to the accumulation time respectively. The photon fluences and fluxes shown in these figures are from the estimated
BEST model fits.
4.4. Smoothly Broken Power Law
The final model that we consider in this catalog is a broken
power law characterized by one break with flexible curvature
and is able to fit spectra with sharp or smooth transitions
between the low- and high-energy power laws. This model,
first published in Ryde (1999), where the logarithmic derivative
of the photon flux is a continuous hyperbolic tangent, has been
re-parameterized (Kaneko et al. 2006) as given below:
f
SBPL
(E) = A
E
E
piv
b
10
(a−a
piv
)
, (4)
where
a = mΔ ln
e
q
+ e
−q
2
,
a
piv
= mΔ ln
e
q
piv
+ e
−q
piv
2
,
q =
log(E/E
b
)
Δ
,q
piv
=
log(E
piv
/E
b
)
Δ
,
m =
λ
2
− λ
1
2
,b=
λ
1
+ λ
2
2
. (5)
In the above relations, the low- and high-energy power-law
indices are λ
1
and λ
2
, respectively, E
b
is the break energy in keV,
and Δ is the break scale in decades of energy. The break scale is
independent and not coupled to the power-law indices as for the
Band function, and represents an additional degree of freedom.
However, Kaneko et al. (2006) found that an appropriate value
for Δ for GRB spectra is 0.3; therefore, we fix Δ at this value.
In addition, we tested the behavior of Δ for some bright GRBs
by letting it vary during the fit process. The results of this study
are presented in Section 6.
We choose to fit these four different functions because the
measurable spectrum of GRBs is dependent on intensity. Less
intense bursts (in the observer frame) provide less data to support
a large number of parameters. This may appear obvious, but
it allows us to determine why in many situations a particular
4
