The anticoincidence system of the INTEGRAL spectrometer has been an essential component of the

interplanetary network since launch. It has observed about 600 events which have been confirmed as

either soft gamma repeaters or cosmic gamma ray bursts by other instruments in the IPN. It has also

observed over 130 events which are unconfirmed, but which are almost certainly weak bursts below the

thresholds of the other IPN experiments. We review the highlights of these observations, which include

gamma-ray bursts, soft gamma repeaters, and one or two extragalactic giant magnetar flares

Hurley, Kevin

Lichti, Giselher

Rau, Arne

von Kienlin, Andreas

Caltech Authors - Main

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Six Years of Bursts with the SPI-ACS 
Kevin Hurley1 
University of California Space Sciences Laboratory 
7 Gauss Way, Berkeley, CA, 94720-7450 U.S.A. 
E-mail: khurley@ssl.berkeley.edu 
Arne Rau 
California Institute of Technology 
1200 California Blvd., MS 105-24, Pasadena, CA, 91125 U.S.A. 
E-mail: arne@astro.caltech.edu 
Giselher Lichti 
Max-Planck-Institut für extraterrestrische Physik 
Giessenbachstrasse Postfach 1312, Garching, 85748 Germany 
E-mail: grl@mpe.mpg.de 
Andreas von Kienlin 
Max-Planck-Institut für extraterrestrische Physik 
Giessenbachstrasse Postfach 1312, Garching, 85748 Germany 
E-mail: azk@mpe.mpg.de 
 
The anticoincidence system of the INTEGRAL spectrometer has been an essential component of the 
interplanetary network since launch.  It has observed about 600 events which have been confirmed as 
either soft gamma repeaters or cosmic gamma ray bursts by other instruments in the IPN.  It has also 
observed over 130 events which are unconfirmed, but which are almost certainly weak bursts below the 
thresholds of the other IPN experiments.  We review the highlights of these observations, which include 
gamma-ray bursts, soft gamma repeaters, and one or two extragalactic giant magnetar flares. 
 
 
 
 
 
 
 
 
 
 
 
The 7th INTEGRAL Workshop 
Copenhagen, Denmark 
September 8– 11 2008
                                                 
 
1  Speaker 
© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it 
 
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1.Introduction 
The SPI anticoincidence system was first proposed as a gamma-ray burst (GRB) detector 
at the 2nd INTEGRAL workshop in  St. Malo in 1996 (Hurley 1997).  Its BGO shield, which 
surrounds the spectrometer on the bottom and sides, has a maximum effective area of up to 
5250 cm2 at 100 keV, and a thickness of 1.6 to 5 cm.  For comparison, a BATSE module had 
4000 cm2, the Swift BAT has 5240 cm2, and the Fermi Burst Monitor has ~200 cm2.  The SPI-
ACS has a superior response above 100 keV.  Its effective area is a function of azimuthal and 
zenithal angles, and drops to about 500 cm2 on-axis.  When triggered, it produces 105 s long 
GRB time histories with 50 ms resolution in a single energy channel which ranges from ~100 
keV to 10 MeV.  It does not record energy spectra, nor does it have an independent GRB 
localization capability.   
2. The Score So Far 
The SPI-ACS has now detected about 550 confirmed cosmic gamma-ray bursts (1 every 3 
days); these are events which have been observed by at least one other experiment in the 
interplanetary network (IPN).  The weakest confirmed event is GRB071112, with a fluence of 
4.8 x 10-8 erg cm-2.  Its time history is shown in figure 1.  The total trigger rate is extremely 
variable, ranging from one to several hundred triggers per day.  Some of these are random 
fluctuations, while others are due to noise or particles.  Others, however, are almost certainly 
weak gamma-ray bursts which are unconfirmed because they are below the thresholds of the 
other IPN experiments; 135 have been detected so far.  The SPI ACS has detected about 45 
short bursts and one giant flare from the magnetar SGR1806-20 (Mereghetti et al. 2005), and 
one short burst from SGR1900+14.  No events have been detected from the ‘new’ magnetar 
SGR0501+4516, probably because the bursts have been relatively weak.  
 
Figure 1.  The weakest confirmed cosmic event observed by SPI-ACS.  This burst, which 
resembles many of the weak unconfirmed events, was also detected by Swift.   
 
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3. Science With The SPI-ACS Data 
 
The science which can be done with the SPI-ACS data falls into three broad categories: 
1. detailed light curve studies which take advantage of the excellent statistics and 
high energy sensitivity of the ACS, 
2. Obtaining precise burst localizations with the IPN, and searching for 
electromagnetic counterparts, and 
3. Searching for gravitational radiation, neutrino emission, and very high gamma-ray 
emission from bursts, more or less independently of the rapidity and accuracy of 
the localizations. 
We discuss each of these in turn. 
 
3.1 Light Curve Studies 
 
Figure 2 shows the light curve of GRB021206.  With a fluence of 1.6 x 10-4 erg cm-2, this 
is one of the most intense events observed by the ACS.  The statistics are excellent: up to 
60,000 counts are recorded in a single 50 ms time bin.  A gamma-ray afterglow is evident in this 
light curve; these are only detected in the strongest bursts, when they are observed with 
excellent statistics. 
 
Figure 2.  The main figure shows the light curve of GRB021206, with the background 
indicated by a dashed line.  The inset shows the later part of the light curve after 
background subtraction.  A gamma-ray afterglow is evident, which can be fit with a 
power law of index ~-1.1. 
 
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Gamma-ray afterglows following intense GRBs have been observed by other instruments 
prior to GRB021206.  However, the observation by the SPI-ACS of a gamma-ray afterglow 
following the giant flare from SGR1806-20 was completely unexpected.  This phenomenon, 
which was confirmed by RHESSI and Konus, lasted about one hour, and its spectrum extended 
to at least 1 MeV.  The light curve is shown in figure 3.   
 
Figure 3.  From Mereghetti et al. 2005.  The SPI-ACS light curve of the SGR1806-20 
giant flare with 50 s binning.  The flare itself is evident starting around 0 seconds.  The 
afterglow starts around the end of the flare, peaks around 500 s, and continues for about 
one hour.  Reproduced by permission of the AAS.  DOI: 10.1086/430669  
 
Since radio data on giant flares provide evidence for an expanding outflow from the 
magnetar, the afterglow may be related to it.  
 
3.2 SPI-ACS in the 3rd Interplanetary Network. 
 
The SPI-ACS is one of nine instruments in the IPN.  The others are RHESSI, the Suzaku 
WAM, AGILE (Super-AGILE and the mini-calorimeter), and the Swift BAT, all in low Earth 
orbit.  (The Fermi GBM is being added to the network currently.)  Konus-Wind is at about 5 
light-seconds from Earth.  Finally, MESSENGER, on its way to Mercury, and Mars Odyssey, in 
orbit around Mars, provide the interplanetary baselines.  The configuration of the network is 
shown in figure 4. 
 
 
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Figure 4.  The present configuration of the Interplanetary Network. 
 
The INTEGRAL spacecraft occupies a unique position in the network: it is far enough 
from Earth to provide a statistically independent vertex for GRB localization.  About 200 bursts 
per year are detected by the IPN, and in general, they are not the same ones that narrow-field 
instruments such as IBIS and the Swift BAT observe.  They are the more intense events, with 
fluences greater than 10-6 erg cm-2, and/or peak fluxes above 0.4 photon cm-2.  Because 
INTEGRAL, Wind, and MESSENGER have fields of view that are unocculted by planetary 
bodies, the IPN constitutes an all-sky monitor, and it is in continuous operation, when the duty 
cycles of all its instruments are considered.   
Two interesting events which were detected by the ACS are bursts on November 3 2005 
and February 1 2007.  Their light curves are shown in figure 5.   
 
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Figure 5.  The SPI-ACS light curves of the November 3 2005 (left) and February 1 2007 
(right) events. 
 
The IPN localizations of these events are shown in figure 6.  Because of their overlaps 
with bright galaxies, as well as their light curves and energy spectra, these bursts are quite likely 
to be extragalactic giant magnetar flares from M81 and M31 respectively (Frederiks et al. 2007, 
Mazets et al. 2008).  If so, their isotropic gamma-ray energies were 7 x 1046 and 1.5 x 1045 erg, 
respectively. 
 
Figure 6.  IPN error boxes for the November 3 2005 (left) and February 1 2007 (right) 
bursts.  The outer contours of M81 are not visible in the left figure, but they overlap the 
IPN error box. 
 
3.3 Searches for Exotic Counterparts 
 
All-sky experiments, and experiments which view a large fraction of the sky, and record 
their data, often do not require fast or precise GRB localizations to determine whether an event 
can be correlated with a GRB.  The trigger time alone is often sufficient to search for more 
exotic radiation, such as gravitational and neutrino radiation, and very high energy gamma-ray 
 
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emission.  LIGO, AMANDA, Pi of the Sky, and Milagro are in this category.  These 
experiments benefit from having a large number of bright (and presumably nearby), 
isotropically distributed GRBs to study.  Some results of these studies have appeared in Abbott 
et al. (2008), Achterberg et al. (2008), and Abdo et al. (2007).  So far, they are all upper limits, 
but the Abbott et al. LIGO result is particularly interesting, since it demonstrates that the 
070201 event could not have been caused by a binary merger in M31. 
 
4. Summary 
 
The INTEGRAL SPI-ACS is a sensitive monitor of fast gamma-ray transients in the 
fluence range 4.8 x 10-8 erg cm-2 and above.  Using these data, over 175 GCN Circulars have 
been issued, and GCN Notices have been issued starting around October 2008.  About 90 of the 
bursts detected by the ACS have radio, optical, and/or X-ray counterparts, and for 38 events, a 
spectroscopic or photometric redshift has been determined; the redshifts lie in the range 0.105 to 
3.35.  Both the ACS light curve data, as well as the IPN data, are made publicly available at 
ftp://isdcarc.unige.ch/arc/FTP/ibas/spiacs/ and ssl.berkeley.edu/ipn3/index.html, respectively. 
 
Acknowledgment.  KH is grateful for support under the U.S. INTEGRAL Guest Observer 
program, NASA grant NNX08AC90G. 
References 
[1] K. Hurley, INTEGRAL as a Gamma-Ray Burst Detector in the Fourth Interplanetary Network, in 
The Transparent Universe, Proc. 2nd INTEGRAL Workshop, Eds. C. Winkler, T. Courvoisier, and 
Ph. Durouchoux, ESA SP 382, p. 491, 1997 
[2] S. Mereghetti, D. Gotz, A.  von Kienlin, A. Rau, G. Lichti, G. Weidenspointner, G., and P. Jean, 
The First Giant Flare from SGR1806-20: Observations Using the Anticoincidence Shield of the 
Spectrometer on INTEGRAL, Ap. J. 624, L105, 2005 
[3] D. Frederiks, V. Palshin, R. Aptekar, S. Golenetskii, T. Cline, and E. Mazets, On the Possibility of 
Identifying the Short Hard Burst GRB051103 with a Giant Flare from a Soft Gamma Repeater in 
the M81 Group of Galaxies, Astron. Lett. 33(1), 19, 2007 
[4] E. Mazets, R. Aptekar, T. Cline, D. Frederiks, J. Goldsten, S. Golenetskii, K. Hurley, A. von 
Kienlin, and V. Palshin, A Giant Flare from a Soft Gamma Repeater in the Andromeda Galaxy 
(M31), Ap. J. 680, 545, 2008 
[5] A., Achterberg et al., The Search for Muon Neutrinos from Northern Hemisphere Gamma-Ray 
Bursts with AMANDA, Ap. J. 674, 357, 2008  
[6] B. Abbott et al., Implications for the Origin of GRB 070201 from LIGO Observations, Ap. J. 681, 
1419, 2008 
[7] A. Abdo et al., Milagro Constraints on Very High Energy Emission from Short-Duration Gamma-
Ray Bursts, Ap. J. 666, 361, 2007 
 


Six Years of Bursts with the SPI-ACS

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Six Years of Bursts with the SPI-ACS

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