In this page we give a brief description IBAS and its performances for the general interested reader (preceded by some information on the INTEGRAL satellites and its instruments used by IBAS). More detailed information can be found in the publications listed at the end of this page. All the technical aspects relevant for the users IBAS are described in the Users Page.
The updated IBAS results can be found in the Results Page.
The INTEGRAL Mission
INTEGRAL is a satellite of the European Space Agency. It was launched on October 17 2002.
The satellite is in a highly eccentric orbit with period of 72 hours. Observations are performed while the satellite is above the radiation belts (~90% of the orbit).
During the observations the data are transmitted continuously to the ground and reach the ISDC only a few seconds after they have been collected by the instruments on board.
The instrument used for the GRB localizations is IBIS/ISGRI, which provides images in the 15 keV-1 MeV range over a large field of view of 30x30 degrees using the coded mask technique. Its sensitivity is nearly constant in the central 9x9 degrees and it worsens at larger off-axis angles. ISGRI is based on an array of 128x128 pixels of CdTe with a thickness of 2 mm and a total geometric area of ~2600 square centimeters.
Currently the other INTEGRAL instruments are not used by IBAS, with the exception of the Anticoincidence Shield (ACS) of SPI. The ACS, a large well of BGO scintillators surrounding the Ge spectrometer, has a maximum sensitivity to GRBs at ~90 degrees from the satellite pointing direction. The ACS provides light curves in 50 ms bins without any directional information.
GRB detection and localization
The search for GRBs is done on ground, at the ISDC. No on-board triggering system is present on INTEGRAL and the operating modes of IBIS/ISGRI do not change when a GRB occurs. In nominal conditions, the IBIS/ISGRI data reach the ISDC with an average delay of ~10 s, due to on-board buffering and data transmission between the different components of the ground segment.
IBAS uses two different methods to look for GRBs in the IBIS/ISGRI data.
In the first method the overall counting rate is monitored to look for significant excesses with respect to a running average of the background. Different energy ranges and integration times (from a few milliseconds to several seconds) are sampled in parallel. A rapid imaging analysis is performed only when a significant counting rate excess is detected. Imaging allows to eliminate many false triggers caused, e.g., by instrumental effects or background variations that do not produce a point source in the reconstructed sky images.
The second method is entirely based on imaging. Images of the sky are continuously produced (integration times of 10, 20, 40 and 100 s) and compared with the previous ones to search for new sources.
Distribution of GRB coordinates
The Alert messages with the GRB positions are delivered via Internet to all the users who subscribe to IBAS.
This is done exclusively by means of Alert Packets sent using the Internet UDP transport protocol. No e-mail delivery of Alerts is foreseen since IBAS is mainly designed for use by automatic telescopes.
The Alert Packet distribution is regulated by two parameters: a “STRONG” threshold and a “POSSIBLE” threshold.
For the GRBs detected with significance above the STRONG level, the Alert Packets are immediately distributed by the automatic software to all the registered users. In case of events with lower statistical significance (above the POSSIBLE threshold but below the STRONG one) the Alerts are sent only to the members of the IBAS Localization Team, who perform further analysis and, if the GRB is confirmed, can manually send an appropriate Alert Packet to all the registered users.
The GCN is registered as an IBAS user. As such it receives the IBAS Alerts (exactly as any other user) and redistributes them to its clients. Thus many astronomers are receiving information on the INTEGRAL GRBs indirectly through the GCN system. Although this might be convenient, (e.g. to get the alerts by e-mail or to have a unique interface), we suggest to the users concerned with speed and reliability to receive the Alerts by subscribing directly to the IBAS system, as described in the Users Page.
Distribution of ACS light curves
The light curves of the triggers in the SPI Anticoincidence Shield are automatically posted on the ISDC web pages within a few minutes after the trigger. Both plots and ascii files with the binned light curves can be retrieved. Note that only a fraction of these lightcurves are due to GRBs. No directional or spectral information is available.
IBAS has been designed and developed in collaboration between ISDC, IASF
Milano (CNR) and MPE Garching.
The following people built and run the IBAS system:
IBAS PublicationsThe "official" reference for the IBAS system is:
S. Mereghetti, D. Götz, J. Borkowski, R. Walter and H. Pedersen, The INTEGRAL Burst Alert System (IBAS), 2003, A&A, 411, L291 (astro-ph/0308173)
Other journal and proceeding papers on IBAS are listed below (most recent first):
S.Mereghetti, D.Götz, J.Borkowski, M.Beck, A.von Kienlin, N.Lund The INTEGRAL Burst Alert System: Results and Future Perspectives, Proceedings 5th INTEGRAL Workshop: "The INTEGRAL Universe", February 16-20,2004, astro-ph/0404019
S.Mereghetti, Gamma-Ray Bursts observed by INTEGRAL Proceedings of the Gamma-ray Burst Symposium 2003, Santa Fe astro-ph/0312340
S. Mereghetti, D. Götz, J. Borkowski Real time localization of Gamma Ray Bursts with INTEGRAL Adv. Sp. Res.in press, Proceedings of the 34th COSPAR Scientific Assembly, Houston, 10-19 October 2002, astro-ph/0302194
D. Götz, S. Mereghetti, 2002 Observation of
Gamma-ray Bursts with INTEGRAL
Contribution to the XXII Moriond Astrophysics Meeting, The Gamma Ray Universe, Les Arcs 9-16 March 2002,
S.Mereghetti, D.I.Cremonesi & J.Borkowski, 2001 The
INTEGRAL Burst Alert System
In: Exploring the gamma-ray Universe. Proceedings of the Fourth INTEGRAL Workshop, 4-8 September 2000, Alicante, Spain. Editor: B. Battrick, Scientific editors: A. Gimenez, V. Reglero & C. Winkler. ESA SP-459, Noordwijk: ESA Publications Division, ISBN 92-9092-677-5, 2001, p. 513 - 516
& J.Borkowski, 2001 The
INTEGRAL Burst Alert System
In: Gamma-Ray Bursts in the Afterglow Era, Proceedings of the International workshop held in Rome, CNR headquarters, 17-20 October, 2000. Edited by E. Costa, F. Frontera, and J.Hjorth. Berlin Heidelberg: Springer, 2001, p. 363.
A. von Kienlin, N.Arend & G.G. Lichti, 2001 A GRB Detection
System Using the BGO-Shield of the INTEGRAL-Spectrometer SPI
In: Gamma-Ray Bursts in the Afterglow Era, Proceedings of the International workshop held in Rome, CNR headquarters, 17-20 October, 2000. Edited by E. Costa, F. Frontera, and J.Hjorth. Berlin Heidelberg: Springer, 2001, p. 427.
S.Mereghetti, S.Brandt, D.Jennings, J.Borkowski
& R.Walter 2000 The
INTEGRAL Burst Alert System
Gamma-ray Bursts, 5th Huntsville Symposium, Editors: R.M. Kippen, R. S. Mallozzi, G.J. Fishman. AIP Conference Series, Vol. 526, p.686
S.Mereghetti & B.Teegarden, 1997 INTEGRAL
Burst Alert Service
in Proc. 2nd INTEGRAL Workshop "The Transparent Universe", St.Malo September 1996, eds. C.Winkler, T.Courvoisier & Ph. Durouchoux, ESA SP-382
ISDC Technical Documents on IBAS: