SVOM/VT: On-ground processing of VT-VHF data

TL;DR

This paper details the architecture and performance of the ground processing system for VT--VHF data from SVOM, enabling rapid optical afterglow identification and redshift estimation for gamma-ray bursts.

Contribution

It introduces the design and operational assessment of the VT--VHF ground processing pipelines used in SVOM for GRB afterglow detection.

Findings

The system successfully identifies optical afterglow candidates in real-time.

Early detections facilitate spectroscopic redshift measurements.

The processing system demonstrates robustness over the first year of operations.

Abstract

The VT--VHF data comprise three types of onboard-processed data results generated from four sequential observational sequences and transmitted to the ground via a Very High Frequency (VHF) downlink. On the ground, these data are processed by three successive pipelines: the pre-processing pipeline, the VT--VHF data processing pipeline (VVPP), and the VT afterglow candidate pipeline (VTAC). These pipelines perform packet decoding, astrometric and photometric calibration, and afterglow candidate identification, respectively. This paper describes the architecture and operational implementation of the VT--VHF ground processing system and assesses its end-to-end performance using the first year of SVOM operations. These data enable rapid identification of GRB optical counterparts. Early detections, while the source is still optically bright, facilitate spectroscopic redshift measurements. Dual-band colors provide preliminary redshift constraints and help identify high-redshift candidates, whereas non-detections in both bands may indicate very high redshift, significant extinction, or intrinsically dark bursts. In-orbit operations show that the VT--VHF ground processing system successfully identifies optical afterglow candidates for a significant fraction of ECLAIRs triggers with available VT--VHF data, demonstrating its robustness and readiness.