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Phil. Trans. R. Soc. A (2007) 365, 1281–1291
doi:10.1098/rsta.2006.1967
Published online 9 February 2007
Swift-BAT results on the prompt emission
of short bursts
B Y S COTT D. B ARTHELMY *
NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA
This is a brief review of short hard bursts (SHBs) from previous missions and from SwiftBAT; in particular, a review of the developing class of gamma-ray bursts which are
similar to SHBs in that they have the short hard initial spike (0.1 to a few seconds), but
that they also have a long extended phase of soft emission (50–200 s). Further, we
suggest that a class of events discovered by Horvath in the T90 versus hardness ratio
plane is this SHB with extended emission.
Keywords: gamma-ray burst; SHB; extended emission; long GRB
1. Introduction
This will be an abbreviated version of the talk given at the Royal Society
discussion meeting. This proceedings paper will concentrate only on the
observations made on short hard bursts (SHBs) and, in particular, those SHBs
with the so-called extended emission (SHBwEE). There will be a brief review of
the T90 versus hardness ratio (HR) and spectral lag work, mostly to provide a
groundwork for the Burst Alert Telescope (BAT) SHBwEE results. There will
also be a brief section on the status of the BAT instrument and its performance.
It will not cover (i) the redshift distribution results, (ii) the BAT burst catalogue
results that allow a pseudo-redshift determination or (iii) the transition from the
prompt gamma-ray emission phase into the X-ray emission phase.
2. Burst duration distributions
The work of Norris (1984) with KONUS-13/14 and ISEE-3 using the durations of
bursts gave indications that there were two distinct types or populations of
bursts. The work by Kouveliotou et al. (1993) using the much larger BATSE
sample of bursts plus spectral information confirmed this. Figure 1 shows the
distribution of T90 durations for bursts detected by the CGRO-BATSE
instrument. The distribution clearly shows two peaks: one peaking at
approximately 0.5 s and the other at approximately 30 s. These are the so-called
SHBs and the long bursts (sometimes also called the long soft bursts; LSBs). The
hard versus soft spectral aspects will be discussed later. The minimum between
these two peaks is at approximately 2 s. It is this minimum that has led to a
*scott@milkyway.gsfc.nasa.gov
One contribution of 35 to a Discussion Meeting Issue ‘Gamma-ray bursts’.
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S. D. Barthelmy
80
number of bursts
60
40
20
short
0
10–3
10–2
10–1
1
duration (s)
long
10
102
103
Figure 1. A histogram of the T90 durations for 586 bursts detected by CGRO-BATSE. The
distribution clearly shows the two populations of bursts: short bursts and long bursts. The
minimum between these two peaks is approximately 2 s.
general working approximation within the burst community that SHBs have
durations less than 2 s and that LSBs have durations longer than 2 s. It should be
noted that these two distributions have a significant overlap.
It is important to note that the details of this duration distribution are
dependent on the detecting instrument. In particular, they are dependent on the
energy window of the instrument and the trigger criteria used. As an example,
the BeppoSAX Wide Field Camera did not detect any short bursts during the 6
years of operations. The KONUS-13/14 distribution is similar (Norris 1984).
Figure 2 shows the T90 distribution for the Swift-BAT instrument. The ratio of
SHB to LSB is much lower in BAT than for BATSE. This ratio difference is
generally due to the softer energy range of the BAT trigger (15–150 keV)
compared with the BATSE one (50–300 keV).
The spectral differences between the short and the long bursts are seen in
figure 3 (ignore the ellipses for the moment; they will be covered in §4c). Here,
the HR is plotted against the T90 duration. While the separation of the two
populations in the duration (T90) dimension is clearly seen, the separation of the
two populations in the spectral (HR) dimension is not so strong. There is a much
greater overlap in the HR for the two populations.
3. Short hard bursts
As already mentioned, there are two classical populations of gamma-ray bursts
(GRBs): SHB and LSB. Swift-BAT has confidentally detected 15 (and maybe as
many as 18) SHBs. They are listed in table 1 along with some related parameters
(redshift, lag, host galaxy and comments). For a complete discussion of this table
and results, please see Gehrels (2007).
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Swift-BAT results and short bursts
25
number of bursts
20
15
10
5
0
10–3
0.01
0.1
1
T90 (s)
10
100
1000
Figure 2. The T90 distribution of bursts detected by Swift-BAT. The SHB peak is only weakly
present in the BAT-detected bursts.
2
1.5
HR
1
–0.5
–0
–0.5
–1
–2
–1.5
–1
–0.5
0
0.5
T90
1
1.5
2
2.5
3
Figure 3. BATSE T90–HR scatterplot with the three populations fitted with two-dimensional
Gaussians (shown with ellipses). The original SHB population (upper left), the LSB population
(lower right) and the third population fitted by Horvath (middle ellipse). Note that the SHB and
LSB have no correlation of T90 with HR, but that the ‘third’ population has T90 anti-correlated
with HR. Adapted from Horvath et al. (2006).
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Table 1. Short bursts detected by Swift-BAT. Question marks indicate uncertainty on that
particular entry.
name
redshift
afterglow host
050509B
050709b
050724
050813
050906
060614
050925d
051105A
051210
051221A
051227
060121e
060313
060502B
060505
060614
060801
061006
0.225
0.161
0.258
0.7 (?) 1.8
—
0.165?
—
—
—
0.547
—
1.5 (?) 4.5
—
—
0.089
0.125
1.131
—
X
X, O
X, O, R
X
—
late slew
—
—
X
X, O, R
X
X, O
X, O
X
X, O
X, O
X
X, O?
Eiso/
(1050 erg)
elliptical
0.01
SF galaxy
0.6
elliptical
3
—
galaxyc
galaxy?
—
0.9?
clusterc
in galaxy plane
—
—
0.098
—
cluster?c
SF galaxy
0.2
—
—
galaxy
—
—
cluster?c
galaxy?
—
galaxy
w0.5
dwarf galaxy
3.7
galaxy?
w13
no bright galaxy
—
duration
(s)a
lag (ms)
origin?
0.042
0.07
0.276
0.012
0.162
16
0.056
0.028
0.852
1.35
0.9
1.97
0.736
0.048
4.0
103(5)
0.5
130
4.3G3.2
0.0G2.0
K4.2G8.2
K9.7G14.0
—
K20G20
—
6.3G5.3
K5.3G24.0
0.8G0.5
2G10?
2G29
0.3G0.7
K0.2G2.8
—
3G6
K8G8
5G3
NS–NS
NS–NS
NS–NS
—
—
is it short?
new SGR?
—
—
—
—
—
—
—
is it short?
—
—
—
a
T90 in 30–350 (400) keV. bHETE GRB. cGalaxy in cluster. dSoft spectrum. eNot related to
massive stars, consistent with NS–NS merger models.
(a ) Redshift distribution
Figure 4 shows the distribution of redshifts of bursts detected by BAT. The
short bursts have a clearly lower distribution than the long bursts. This is an
instrumental effect based on the lower fluence of soft photons available for the
SHB and thus the reduced sensitivity of the BAT triggers.
On a related topic to SHB and distance distributions, soft gamma-ray
repeaters (SGRs) superflares are also short in duration. On 27 December 2004,
BAT detected a superflare from SGR1806-20. Even though these SGR superflare
events tend to be softer than SHBs (as are regular SGR outbursts), many authors
have noted that some of the SHBs in the upper left cluster in figure 3 (at least the
lower portion thereof) could be due to SGR superflares. This is an improbable
situation for the BAT instrument because the 041227 superflare event, which is
already 100 times more energetic than any of the other two superflares, could
not be seen by BAT beyond 40 Mpc (Palmer et al. 2005). All of the detected
Swift-BAT SHBs are well beyond the Virgo cluster of galaxies.
(b ) Fast temporal variations in short hard burst
Unlike many SHBs which have a single spike of emission in their lightcurves,
figure 5 shows Swift-BAT GRBs 051221a and 060313, which have many separate,
statistically significant peaks in their lightcurves. Many of these spikes have rise
times less than 1 ms and a few have fall times less than 1 ms.
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Swift-BAT results and short bursts
10
Noccur
long GRBs
5
diamonds:
short GRBs
0
0.01
0.1
1
10
redshift
Figure 4. Distribution of redshifts of BAT bursts with measured redshifts. The long bursts are
shown in the red histogram. The SHBs are shown as the blue diamonds. This was adapted from
Norris & Bonnell (2006).
(a)
(b)
counts /1–ms bin
200
100
150
80
60
100
40
50
0
20
0.10
0.20
0.30
0.5 s
0.40
0.50
0
0.2
0.4
0.6
0.8
1.0
1.0 s
Figure 5. Lightcurve for Swift-BAT (a) GRB 051221A and (b) GRB 060613 in the 15–350 keV
band. Note the many peaks and the very fine temporal structure (some of the peaks have rise times
less than 1 ms). These are mask-weighted lightcurves which means background has been
subtracted. The error bars are approximately 5 counts sK1 for both lightcurves.
(c ) Spectral lag
Norris & Bonnell (2006) have done extensive work on the spectral lag
properties of GRBs, in particular, the spectral lag as a tool for separating short
from long bursts. Figure 6 shows the lag values for 260 BATSE bursts that have
‘zero lag’ (i.e. anything with a lag in the G20 ms range). Figure 6a shows that
there is no correlation of lag with peak flux (i.e. neither correlation with an
intrinsic brightness of the SHB nor with distance). Figure 6b collapses figure 6a
in the flux dimension to produce a histogram of lags for SHB with the x -axis now
in sigma units. The distribution is tightly confined around zero with only a small
tail on the positive side (demarcated by the blue arrow). These extra bursts with
small positive lag values are due to long bursts with extremely small lag values.
Norris & Bonnell (2006) estimate that only one or two out of the 20 events seen
in this tail are true SHB, yielding that approximately 18 are not true SHBs. The
level of contamination is 7%.
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S. D. Barthelmy
(b) 30
25
20
Noccur
Fp (ph cm–2 s–1, 50–300 kev)
(a) 50
10
15
10
5
2
–0.050
–0.025
0
0.025
0.050
lag (s): 25–50 keV, 100–300 keV
0
–10
–5
0
lag in sigmas
5
10
Figure 6. Spectral lag distribution for short GRBs (adapted from Norris & Bonnell (2006)). (a) The
flux versus lag. The grey diamonds are 260 BATSE bursts with ‘zero lag’. The black and open
diamonds are eight bursts from figure 9 (TTE and PREBCDISCSC data, respectively). (b)
collapses (a) to make a histogram of the lag values with the x -axis now in sigma units. The arrow
shows the long tail of positive lags which is a contamination of long bursts (7%).
4. Short hard bursts with extended emission
There is a growing body of evidence that there is a class of events that is at least
observationally different, if not intrinsically, from the SHB and LSB classical
populations of GRBs. This putative new class starts out like a SHB (the initial
short pulse), but then it has ongoing emission—well beyond the 2 s, or even few
seconds, durations that have previously been defined as the duration limit for
SHBs. This ongoing (extended) emission is softer than the initial emission of the
initial spike. Using this observational-based definition, I will call these events
SHBs with extended emission (SHBwEE).
(a ) Swift-BAT SHBwEE
Figures 7 and 8 show the lightcurves for GRBs 050724 and 060614, respectively.
Swift-BAT has detected two other SHBwEEs: GRBs 051227 and 061006, plus 10
traditional SHBs (i.e. no extended emission above the BAT background level). This
4 out of 14 ratio is much higher than the 8 out of 260 ratio that Norris & Bonnell
(2006) found in BATSE. This difference is probably due to the somewhat softer
energy window that BAT has over BATSE, and that BAT has an image-trigger
capability which BATSE did not (note that GRB 061006 was not detected by BAT
using a rate trigger on the initial spike, but rather using an image trigger on the
extended emission phase because the initial spike occurred during a spacecraft slew
manoeuvre when all triggers are disabled).
(b ) BATSE, KONUS and HETE SHBwEEs
Figure 9 shows the lightcurves of eight SHBwEEs that Norris & Bonnell
(2006) identified from the BATSE catalogue. All eight have zero lag (figure 6)
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Swift-BAT results and short bursts
15–25 keV (counts/s/det)
0.02
0.01
0
–50
0
50
100
time (s)
150
200
250
Figure 7. Lightcurve of Swift-BAT GRB 050724, a SHBwEE. The initial spike at TC0 s is
suppressed in amplitude compared with the extended emission phase owing to the time binning.
The extended emission starts to rise at approximately TC20 s, peaks around TC80 s and returns
to instrumental background around TC200 s.
15–150 keV (counts/s/det)
2.5
2.0
1.5
1.0
0.5
0
0
100
50
time since BAT trigger (s)
150
Figure 8. Lightcurve of Swift-BAT GRB 060614, a SHBwEE. The extended emission starts to rise
at approximately TC10 s, peaks around TC45 s and returns to instrumental background around
TC150 s. The spectral lag for the spike phase is 9G6 and 3G6 ms for the extended emission phase.
The apparent periodic structure in the extended emission is not statistically significant (Gehrels
et al. 2006).
and are therefore, in the Norris & Bonnell (2006) definition, true short bursts.
These eight bursts were previously classified as long bursts by the BATSE team
because they were using only T90 and HR; no spectral lag information was used.
It is only recently that the spectral lag factor is being incorporated into the
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S. D. Barthelmy
105
104
103
1025
10
104
103
102
GRB 9210226
trigg 1997
GRB 9907126
trigg 7547
104
GRB 961225
trigg 5725
counts (s –1)
103
102
104
GRB 910709
trigg 503
103
102
104
GRB 920525
trigg 1625
103
102
104
GRB 931222
trigg 2703
103
102
GRB 000107
trigg 7934
103
102
GRB 951007
trigg 3853
103
102
0
10
20
30
40
50
time (s)
60
70
80
90
100
Figure 9. BATSE SHBwEE. Out of the 260 short bursts found by using the ‘zero-lag’ test on the
BATSE catalogue of burst, these are eight that have extended emission and still have a zero-lag
value. Adapted from Norris & Bonnell (2006).
classification method. Given that there is a 7% chance of contamination (§3c),
it is improbable that any of these eight are long bursts. Note that all eight have
the same basic lightcurve envelope: they start with an initial intense and short
spike (0.5–2 s), then a hiatus of emission, followed by the onset of the soft
emission for 50–100 s. HETE GRB 050709 (figure 10) shows the identical
temporal envelope. Figure 11 shows three KONUS bursts that are clearly in the
same SHBwEE temporal and spectral envelope. Additionally, KONUS has six
more bursts with nearly the same signature (Mazets et al. 2002)—the only
difference is that there is no clear hiatus of emission immediately after the
initial spike. Given that four instruments have seen these SHBwEEs and at
least one BAT SHBwEE would have been classified as a traditional SHB (i.e.
the extended emission would not have been detectable in BATSE; Barthelmy
et al. 2005b), it seems probable that this is a new (sub)class of bursts.
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Swift-BAT results and short bursts
WXM counts (s) (2–25 keV)
1400
1200
1000
800
600
400
200
0
0
100
time (s)
200
300
Figure 10. HETE SHBwEE. This is the lightcurve of GRB 050709 that was detected by HETE.
This burst has the same profile of a SHB (the initial spike of intense spectrally hard emission,
followed by a hiatus of emission, then increasing spectrally soft emission, peaking and then
decreasing back to instrumental background). Note that in this plot, there is variable-width time
binning to preserve the narrowness of the initial spike.
(b)
3500
(c)
G1+G2+G3
3500
3000
4500
2500
4000
2000
3500
1500
–20
0
20
40
60
–100 –50
G1+G2+G3
8500
coumts/2.048 s
coumts/1.024 s
G1+G2+G3
coumts/4.086 s
(a)
8000
7500
7000
0
50
100 150
–100 –50
T–T0, s
T–T0, s
0
50
100 150
T–T0, s
Figure 11. KONUS SHBwEE. This shows three of the best examples of SHBwEE from the KONUS
catalogue (Mazets et al. 2002). These three bursts also show the profile of initial spike, followed by
a hiatus of emission, then increasing soft emission, peaking and then decreasing emission.
(c ) The Horvath third class of bursts
Horvath has performed fits in the HR–T90 plane with two-dimensional Gaussians
(the three ellipses in figure 3) and finds a third population of burst to be 11% of the
total. While the traditional SHB and LSB populations have no significant
correlation between T90 and HR, his third population clearly shows an anticorrelation. The longer the burst (i.e. the larger the T90 value), the softer the HR.
While Horvath states that he does not know of a reason for this anticorrelation, it seems reasonable that this third population is SHBwEE. The
extended emission part of the burst is softer than the initial spike phase of the
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S. D. Barthelmy
Table 2. BAT basic characteristics.
item
value
energy range
energy resolution
location accuracy
field of view
detectors
12–300 keV
5 keV (at 60 keV)
1–3 arcmin (1s, depends on rising edge of burst)
2 sr
5200 cm2 of CZT, 32 768 elements
GRB. For a given short burst, as the soft extended emission phase extends
longer and longer, the total HR will be decreased. This is the same as the anticorrelation behaviour seen in the Horvath third population.
(d ) Conclusions on short hard bursts
We have shown that there is a sizable and distinct population of bursts
whose lightcurves and spectral evolutions are different than the traditional
short and long bursts. There are two differences. Firstly, they have an initial
spectral hard and very short (less than a couple of seconds) emission phase
which is identical to the SHBs. And secondly, they have an extended emission
phase (lasting as much as 100–200 s) which is spectrally softer and is very
close to the duration and spectral properties of the LSBs. Owing to this
combination of properties of both the short and the long bursts, these
SHBwEEs have been misidentified (owing to a lack of sufficient low-energy
response) as to a type in previous missions (i.e. they were most commonly
classified as LSB). This plus the results of Horvath (§4c) suggest that there is
a third class or population of bursts (which we call SHBwEE). However, we
cannot rule out the possibility that (i) there are still only two populations,
(ii) all SHBs have extended emission, and (iii) the ratio of initial, hard
emission to extended, soft emission has a large dynamic range such that most
SHBs have such a low ratio that the extended emission is not detected by the
past and the current missions.
5. Status of the Swift-BAT instrument
The BAT on Swift is fully described in Barthelmy et al. (2005a) and the top-level
instrumental parameters are shown in table 2. BAT has and continues to operate
flawlessly since it was turned on and commissioned on 17 December 2004. Between
then and 16 September 2006, there have been a total of 12.7 days (2% downtime)
when BAT was not in a fully operational and triggerable state. This downtime was
due to two computer crashes (both early in the mission and the cause has been
fixed), two flight software upload sessions, a time when one of the two redundant
loop heatpipes temporarily shutdown, and finally due to a handful of times when
the spacecraft went into safehold state. In addition to these system downtimes, the
BAT instrument is not triggerable to GRB (or hard X-ray transients) while in the
SAA or while the spacecraft is in slew mode (each approximately 15% of the total
time). There have been 173 GRBs detected, localized and distributed to the world
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Swift-BAT results and short bursts
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community which yields an annual rate of 100G8 GRBs yrK1. Of these,
approximately 10 per year are short GRBs. There have been slight increases
due to two refinements in the triggering system. The first involved a change in the
threshold levels in the short rate trigger criteria (4–32 ms duration triggers) and
other trigger control parameters. The second involved a bug in the BAT flight
software that allowed the long image triggers (5–43 min) to be enabled. Since
launch, approximately 16% of the 32 768 CZT detectors have become
permanently or semi-permanently noisy and had to be disabled.
I wish to acknowledge and thank Jay Norris for many useful conversations that contributed
significantly to this paper. I also wish to acknowledge and thank John K. Cannizzo and John
D. Myers for their help in the preparation of this manuscript.
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