GCN Circular 2568

A. Rau, A. von Kienlin, G. Lichti on behalf of the INTEGRAL SPI-ACS GRB
team (Garching);

K. Hurley on behalf of the IPN team;

M. Beck on behalf of the INTEGRAL Science Data Center (ISDC) Geneva;

report:

The Anti-Coincidence Shield (ACS) of the Spectrometer onboard INTEGRAL
(SPI) has provided more than one hundred GRB triggers for the 3rd
Interplanetary Network (IPN) over the last 16 months.  By studying IPN
triangulations involving the ACS and one or more distant spacecraft,
such as Ulysses and Mars Odyssey, it was determined that the absolute
timing uncertainty of the ACS lightcurves was of the order of 100 ms at
most.  These triangulations involved bursts with known positions, such
as SGRs and GRBs with long-wavelength counterparts.  Accordingly, IPN
triangulations up to now involving the ACS and distant spacecraft have
included a conservative estimate of the systematic uncertainty of this
order.

Recently, for cases involving precisely known IPN positions, an
additional triangulation of some of these bursts using Konus-Wind
and/or Helicon-Coronas-F data in conjunction with ACS INTEGRAL revealed  a
systematic inaccuracy in the ACS timing such that the ACS was lagging
behind UTC by 134 +- 12 ms (S. Golenetskii, E. Mazets, V. Pal'shin and D.
Frederiks, private communication).  Further investigations by the
INTEGRAL SPI/ACS Team confirmed this timing error by comparing the rise
times of (most likely) particle-induced short events in the ACS and
corresponding saturation signatures in the Ge-detectors of SPI.
Using this method, the error was found to be 125 +/- 10 ms.
The cause of this error is still unresolved.

Starting with bursts which occurred after April 2 2004 11:00 UTC, a
correction of 125 ms has been added to the publicly available ACS light
curve files (http://isdcarc.unige.ch/arc/FTP/ibas/spiacs/).  Bursts which
occurred before this date remain uncorrected at this time, but it
is emphasized that a timing correction of ~125 ms should be added to
all data files since the start of the mission.

Application of this correction to IPN bursts with known positions
yields better agreement with the triangulated positions.  Accordingly,
the value of the ACS systematic uncertainty will now be reduced.


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