The trigger and localization system of SVOM-GRM

TL;DR

This paper presents the design and preliminary testing of the localization and spectral analysis system for the SVOM-GRM satellite's gamma-ray detectors, including an on-board algorithm and a ground-based MCMC method.

Contribution

It introduces a joint spectral and localization analysis method using MCMC, improving accuracy over the on-board algorithm for gamma-ray burst detection.

Findings

Localization error of about 4.14 degrees for GRB 240629A

On-ground MCMC method reduces systematic biases in localization

Preliminary results demonstrate effective spectral and localization capabilities

Abstract

The Space multi-band Variable Object Monitor (SVOM) is an astronomical satellite jointly developed by China and France, primarily focused on the detection of gamma-ray bursts (GRBs) and transient sources. The SVOM satellite was launched on 22nd June, 2024 with four payloads installed onboard. As one of payload, GRM comprises 3 gamma-ray detectors (each detector has an effective area of approximately 200~cm$^{2}$) with distinct pointing directions, enabling the temporal and spectral measurements as well as localization of GRBs in the energy range of 15-5000 keV. This article firstly introduces the on-board localization algorithm design for GRM and presents preliminary test results. Then, leveraging abundant ground-based computational resources, a joint fitting method for spectral and localization analysis using Monte Carlo Markov Chain (MCMC) is implemented. In contrast to the on-board localization algorithm, the on-ground MCMC method comprehensively considers the influence of spectral characteristics, thereby mitigating systematic biases. Finally, a systematic analysis based on this method is provided, highlighting the localization and spectral measurement capabilities of GRM. The preliminary localization analysis result for the on-board detected GRB 240629A by both GRM and Fermi/GBM shows that the localization result (error$\sim$4.14$^{\circ}$) of GRM is consistent with the Fermi/GBM result.