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Overview of the ECLAIRs Trigger for SVOM gamma-ray burst detection

This paper provides an overview of the ECLAIRs onboard trigger system for the SVOM satellite mission, detailing how its two simultaneous algorithms (CRT and IMT) autonomously detect, localize, and alert the scientific community to Gamma-Ray Bursts to enable rapid multi-wavelength follow-up.

Original authors: S. Schanne, F. Château, N. Dagoneau, F. Daly, H. Le Provost, P. Kestener

Published 2026-04-28

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CC BY 4.0

Original authors: S. Schanne, F. Château, N. Dagoneau, F. Daly, H. Le Provost, P. Kestener

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Cosmic "Security Guard": How SVOM Catches Space Explosions

Imagine you are sitting in a dark, massive stadium at night. Suddenly, a tiny firework goes off in the far distance. It’s so brief and faint that if you were just staring aimlessly, you’d miss it. But you have a high-tech security system designed to catch even the smallest spark.

That is essentially what the ECLAIRs instrument on the SVOM satellite does for the universe. This paper explains how this "security system" works to catch Gamma-Ray Bursts (GRBs)—the most violent, explosive events in the cosmos.

1. The Mission: Catching the "Flash"

Gamma-Ray Bursts are like the universe’s ultimate fireworks. They happen when massive stars collapse or when two neutron stars crash into each other. They are incredibly bright but happen very fast.

The SVOM satellite is a joint French-Chinese mission launched in 2024. Its job is to act like a cosmic paparazzi: it waits for a flash, snaps a picture, and immediately tells every telescope on Earth, "Hey! Something just exploded over there! Look now!"

2. The Brains: Two Different "Detectives"

The satellite doesn't just have one way of looking for trouble; it has two specialized "detectives" running at the same time inside its computer (the UGTS).

Detective #1: The "Quick-Reaction" Scout (The CRT)
Think of this detective as someone watching a stopwatch. They aren't looking at pictures; they are just counting. If they suddenly see a massive spike in "clicks" (photons) in a fraction of a second, they shout, "Wait! I felt a bump!" Once they feel the bump, they quickly take a snapshot to see where it came from. This is perfect for short, sudden explosions.
Detective #2: The "Patient Observer" (The IMT)
This detective is more like a photographer using a long exposure. They aren't looking for sudden bumps; they are looking for a slow, steady glow that might last for minutes. They constantly stitch together many small pictures to create one big, clear image. This is how they catch the "slow burners"—the long-lasting, massive explosions.

3. The "Slew": The Automatic Pivot

One of the coolest parts of this system is its autonomy.

Imagine you are looking through a narrow straw. If a bird flies by, you can't see it clearly. But if your straw was attached to a robot that could instantly whip around to face the bird, you’d catch it.

When ECLAIRs detects a burst, it doesn't wait for a human on Earth to wake up and press a button. It sends a command to the satellite itself: "Slew!" The entire spacecraft physically turns to point its high-powered telescopes (the MXT and VT) directly at the explosion. This allows scientists to see the "afterglow" before it fades away.

4. The "Neighborhood Watch": Telling the World

The satellite doesn't keep the secret. It uses a special radio network (VHF) to broadcast an "Alert" to Earth. Within about 15 seconds of seeing a burst, scientists in France and China receive a digital package containing:

Where it happened (the coordinates).
How bright it was (the light curve).
A "sketch" of the area (the sub-image).

This allows ground-based telescopes all over the world to join the hunt immediately.

5. The Challenges: Dealing with "Noise"

Space isn't a quiet place. The satellite has to deal with:

The Earth: The Earth is a giant, bright object that can block the view. The software is smart enough to ignore "flashes" coming from the Earth itself.
Solar Storms: When the Sun gets grumpy and sends radiation our way (the South Atlantic Anomaly), the satellite has to "shield its eyes" to prevent its electronics from getting fried.
False Alarms: Sometimes, a stray particle hits the detector and looks like a burst. The software uses complex math (like comparing the "peak" of a signal to the "background noise") to make sure it doesn't cry wolf.

Summary

In short, this paper describes a highly sophisticated, automated, and incredibly fast cosmic alarm system. By using two different mathematical approaches to "watch" the sky, the SVOM satellite can catch everything from a millisecond-long spark to a massive, minutes-long glow, ensuring that humanity never misses the most spectacular shows in the universe.

This technical summary provides an overview of the paper "Overview of the ECLAIRs Trigger for SVOM gamma-ray burst detection," which details the onboard detection and localization system for the French-Chinese SVOM satellite mission.

1. Problem Statement

The primary challenge in Gamma-Ray Burst (GRB) science is the rapid detection and localization of transient high-energy events to enable immediate multi-wavelength follow-up. For a satellite mission like SVOM, this requires an onboard system capable of:

Real-time processing: Analyzing massive volumes of photon data that are too large to downlink instantly.
Spectral and Temporal Diversity: Detecting a wide range of bursts, from millisecond-duration events to ultra-long bursts (up to 22 minutes), and from standard GRBs to soft, X-ray-rich, or high-redshift bursts.
Robustness in Harsh Environments: Managing high background noise caused by the Cosmic X-ray Background (CXB), Earth occultation, and the South Atlantic Anomaly (SAA).
Resource Constraints: Operating within the limited CPU and telemetry power available in a low-Earth orbit (LEO) environment.

2. Methodology

The ECLAIRs instrument utilizes a coded-mask imaging technique (4–150 keV) and employs a dual-algorithm trigger system running on the Scientific Processing and Control Unit (UGTS).

A. Dual-Algorithm Trigger Design

To balance CPU efficiency with sensitivity, the system splits tasks between two parallel algorithms:

Count-Rate Trigger (CRT): Optimized for short-duration events (10 ms to 20.48 s). It uses a two-step process: first, it scans detector zones for count-rate excesses using a sliding temporal background model; second, it performs a targeted sky reconstruction only for the most significant excess to confirm the source.
Image Trigger (IMT): Optimized for long-duration events (up to 22 minutes). It performs systematic sky reconstructions every ~20 seconds. It uses a "sky-history summation" scheme, where reconstructed images are summed pairwise (dyadic scales) to build cumulative integrations, allowing the detection of very slow-evolving transients.

B. Data Pre-processing and Background Management

Pre-processing (PRP): Filters photon data into "Single Events" (SEB) to reduce noise and uses ring buffers for rapid count computation.
Background Modeling: The system uses 2D quadratic polynomial fitting to remove non-uniform detector illumination and manages Earth occultation by masking regions of the sky blocked by the planet.
SAA Protection: The system implements "Xoff" logic, which automatically disables data acquisition during extreme particle flux periods to protect the hardware while allowing the trigger to continue running transparently.

C. Alert Dissemination

Upon detection, the system generates a VHF Alert Sequence containing:

Alert/Slew Requests: Coordinates and SNR metrics to trigger autonomous spacecraft slews for narrow-field follow-up (MXT/VT).
Light Curves and Sub-images: Rapidly transmitted data to the ground via a VHF network to allow the global community to perform immediate follow-up.

3. Key Contributions

Innovative Dual-Path Architecture: The simultaneous use of CRT (rate-based) and IMT (image-based) allows the mission to cover the entire temporal spectrum of GRBs without overloading the onboard Leon3 CPU.
Ultra-Long Exposure Capability: The IMT’s ability to sum cleaned sky images enables the detection of ultra-long GRBs, a relatively unexplored area of high-energy astrophysics.
Advanced Background Subtraction: The implementation of onboard spatial and temporal background modeling (including catalog source subtraction) ensures high sensitivity even in variable environments.
Integrated Autonomous Response: The seamless link between the ECLAIRs trigger, the spacecraft platform, and the ground-based VHF network minimizes the latency between detection and multi-wavelength observation.

4. Results

In-flight Performance: Since activation in July 2024, the trigger has produced 804 alert sequences over 17 months.
Detection Rate: The system has confirmed 71 GRBs, yielding an average rate of approximately 50.1 GRBs per year.
Scientific Success: The trigger successfully detected GRB 250314A, one of the most distant GRBs observed in 12 years ( $z = 7.3$ ), facilitating follow-up by the James Webb Space Telescope (JWST).
Reliability: Despite high solar activity and variable backgrounds, the system maintains a manageable false-alert rate, primarily caused by detector plane inhomogeneities.

5. Significance

The ECLAIRs trigger represents a significant advancement in space-based transient astronomy. By lowering the energy threshold (down to 4 keV) and providing rapid, sub-arcminute localization through autonomous slews, it significantly enhances the probability of capturing the early afterglow of high-redshift and X-ray-rich bursts. This capability is vital for understanding the evolution of the early universe and the physics of the most energetic explosions in the cosmos.