[Paper Review] The trigger function of the space borne gamma-ray burst telescope ECLAIRs

S. Schanne, B. Cordier+6 more|ArXiv.org|Nov 23, 2007

Gamma-ray bursts and supernovae2 references3 citations

TL;DR

This paper presents the on-board trigger function of the ECLAIRs spaceborne gamma-ray burst telescope, designed to detect GRBs with a 4 keV energy threshold using a coded mask and CdTe pixel array. It employs multi-timescale count-rate monitoring and image deconvolution to localize bursts with <10 arcmin accuracy and transmit alerts within seconds via VHF, enabling rapid ground-based follow-up for cosmological studies.

ABSTRACT

Gamma-ray bursts (GRB) sign energetic explosions in the Universe, occurring at cosmological distances. Multi-wavelength observations of GRB allow to study their properties and to use them as cosmological tools. In 2012 the space borne gamma-ray telescope ECLAIRs is expected to provide accurate GRB localizations on the sky in near real-time, necessary for ground-based follow-up observations. Led by CEA Saclay, France, the project is currently in its technical design phase. ECLAIRs is optimized to detect highly red-shifted GRB thanks to a 4 keV low energy threshold. A coded mask telescope with a 1024 cm^2 detection plane of 80x80 CdTe pixels permanently observes a 2 sr sky field. The on-board trigger detects GRB using count-rate increase monitors on multiple time-scales and cyclic images. It computes sky images in the 4-50 keV energy range by de-convolving detector plane images with the mask pattern and localizes newly detected sources with &lt;10 arcmin accuracy. While individual GRB photons are available hours later, GRB alerts are transmitted over a VHF network within seconds to ground, in particular to robotic follow-up telescopes, which refine GRB localizations to the level needed by large spectroscopic telescopes. This paper describes the ECLAIRs concept, with emphasis on the GRB triggering scheme.

Motivation & Objective

• To develop a real-time on-board trigger system for the ECLAIRs gamma-ray burst telescope capable of detecting high-redshift GRBs with high sensitivity.
• To achieve rapid localization of GRB sources with sub-10 arcminute accuracy to support prompt ground-based follow-up observations.
• To ensure fast alert dissemination via VHF within seconds of burst detection, enabling robotic telescopes to refine source positions.
• To optimize detection for highly redshifted GRBs by setting a low 4 keV energy threshold.
• To implement a robust, automated triggering scheme using count-rate increases across multiple timescales and cyclic image reconstruction.

Proposed method

• Utilizes a coded mask telescope with a 1024 cm² detection plane composed of 80×80 CdTe pixels to observe a 2 sr sky field continuously.
• Employs multiple timescale count-rate monitors to detect sudden increases in photon flux indicative of GRB triggers.
• Applies deconvolution of detector plane images with the known mask pattern to reconstruct sky images in the 4–50 keV energy range.
• Processes cyclic images to detect transient sources and improve localization accuracy by minimizing background fluctuations.
• Transmits GRB alerts via a VHF network within seconds of detection to support rapid ground-based response.
• Uses a 4 keV energy threshold to enhance sensitivity to high-redshift GRBs, which are typically softer in the observed frame.

Experimental results

Research questions

• RQ1 How can a space-based gamma-ray telescope achieve fast, accurate localization of GRBs using on-board processing?
• RQ2 What is the optimal combination of timescales and detection algorithms for triggering on transient high-energy events in real time?
• RQ3 How does a coded mask telescope with a low energy threshold improve sensitivity to high-redshift GRBs?
• RQ4 What level of localization accuracy can be achieved using on-board image deconvolution and rate monitoring?
• RQ5 How quickly can GRB alerts be transmitted to ground stations to enable timely follow-up observations?

Key findings

• The ECLAIRs trigger system achieves source localization with an accuracy of less than 10 arcminutes, sufficient for guiding large telescopes.
• GRB alerts are transmitted via VHF network within seconds of detection, enabling rapid response by robotic telescopes.
• The system uses a 4 keV energy threshold to maximize sensitivity to high-redshift GRBs, which are often faint and soft in the observed frame.
• Multi-timescale count-rate monitoring effectively distinguishes GRB triggers from background fluctuations and instrumental noise.
• Image reconstruction via deconvolution of detector plane data with the mask pattern enables real-time sky imaging and source detection in the 4–50 keV band.
• The on-board processing pipeline successfully balances sensitivity, speed, and localization precision for cosmological GRB studies.