GCN Circular 11766

S.Mereghetti (INAF IASF-Milano), D.Gotz (CEA-Saclay), J. Borkowski (CAMK, 
Torun), M. Beck, C.Ferrigno (ISDC, Versoix), on behalf of the IBAS 
Localization Team announce:

A new version of the IBAS Alert Distribution system is operational at the 
INTEGRAL Science Data Center. In addition to the already available 
functionalities, the new version distributes in real time also Alert 
Packets for low significance triggers discovered in the field of view of 
the IBIS imager.

The new Alert Packets are called WEAK Alerts and are identified by the 
field PKT_TYPE=6 (see http://www.isdc.unige.ch/integral/science/grb#IBAS 
for details on the different Alert types).

Users interested to receive the Alert Packets of the new type "WEAK" 
should contact the ISDC IBAS team (isdc-ibas@unige.ch).

The threshold for the distribution of the usual high significance IBAS 
triggers remains unchanged at

SIGMA= 8.0   (Alert Packets with PKT_TYPE= 3 (WAKEUP))

while the threshold for the distribution of low significance IBAS triggers is 
currently set at

SIGMA= 6.5   (Alert Packets with PKT_TYPE= 6 (WEAK))

This threshold corresponds to a rate of a few WEAK Alert Packets per week, 
but it could be adjusted in the future.

Many WEAK alerts will be produced by statistical fluctuations, but there 
is also a non negligible probability that some of them are due to real 
astrophysical events. In most cases it is not possible to recognize the 
GRBs based only on the INTEGRAL data, but the detection of afterglows 
could confirm some of these events as genuine GRBs.

Several bursts that were confirmed by off-line interactive analysis of the 
INTEGRAL data produced triggers above the threshold currently adopted for 
the WEAK alerts. Recent examples include the two short bursts GRB 100703A 
(GCN Circ. n.10936) and GRB 110112B (GCN Circ. n.11562) and the long burst 
GRB 100909A (GCN Circ. n.11260).

WEAK Alert Packets provide a single sky position, with error radius 
smaller than 4 arcmin. They are intended for robotic telescopes which can 
react without a negative impact on scheduled observing programmes. The 
value of SIGMA in the WEAK Alert Packet can be used as a figure of merit.

WEAK Alert Packets may or may not be followed by the usual WAKEUP and/or 
REFINED Alert Packets, depending on the evolution of the detected signal.
WEAK alerts for events belows WAKEUP threshold are neither confirmed nor
cancelled with an OFFLINE packet.

For further details see
http://ibas.iasf-milano.inaf.it/IBAS_News.html

It is recommended that current ibas clients upgrade their IBAS Client 
Software to the version 2.1.0 (available at 
http://www.isdc.unige.ch/integral/analysis#Software), although the 
previous versions should process WEAK alert without problems.