Systematic Study of the Simultaneous Events Detected by GECAM

TL;DR

This paper performs the first systematic analysis of STE detected by GECAM, leveraging sub-microsecond timing to show that STE arise from high-energy cosmic-ray cascades in satellite material rather than atmospheric or random backgrounds. By examining zenith and non-zenith pointing, geomagnetic-latitude modulation, field-line angular correlations, and detector clustering, the authors demonstrate a satellite-cascade origin with energy spectra evolving with multiplicity and a persistent 511 keV line. The results substantiate STE as a physical, isotropic, and calibration-friendly phenomenon and propose GECAM as a Micro Cosmic-Ray Observatory, enabling in-situ study of near-Earth cosmic-ray interactions and offering a framework to distinguish STE from astrophysical transients. The work thus expands the scientific utility of all-sky gamma-ray monitors beyond transients toCosmic-ray environment characterization and instrument timing calibration.

Abstract

GECAM is a constellation of all-sky monitors in hard X-ray and gamma-ray band primarily aimed at high energy transients such as gamma-ray bursts, soft gamma-ray repeaters, solar flares and terrestrial gamma-ray flashes. As GECAM has the highest temporal resolution (0.1~$μ$s) among instruments of its kind, it can identify the so-called simultaneous events (STE) that deposit signals in multiple detectors nearly at the same time (with a 0.3~$μ$s window). However, the properties and origin of STE have not yet been explored. In this work, we implemented, for the first time, a comprehensive analysis of the STE detected by GECAM, including their morphology, energy deposition, and the dependence on the geomagnetic coordinates. We find that these STE probably result from direct interactions between high-energy charged cosmic rays and satellite. These results demonstrate that GECAM can detect, identify, and characterize high-energy cosmic rays, making it a Micro Cosmic-Ray Observatory (MICRO) in low Earth orbit.

Figures (17)

• Figure 1: The structural layout of the GECAM payload is shown from different perspectives and comprises 25 GRDs (designated G01 through G25) and eight CPDs (designated C01 through C08). The payload comprises two main components: the detector dome housing and the EBOX.The payload coordinate system is defined such that the $+X$, $+Y$, and $+Z$ axes align with the central normals of detectors G18, G24, and G01, respectively ZhaoYi2023.
• Figure 2: Attitude variations of GECAM during its monitoring mission are shown. The angle $\theta$ denotes the angle between the payload coordinate system’s $+Z$ axis and the vector toward the Earth’s center. At $\theta = 0^\circ$, GECAM observes toward the Earth. At $\theta = 180^\circ$, GECAM observes directly away from the Earth.
• Figure 3: Schematic diagram illustrating STE. Within a $2\,\mu\mathrm{s}$ interval, two-detector and seven-detector STE were detected in time windows (a) and (b), respectively. Outside these windows, no events satisfy the simultaneity criterion. Colored spheres indicate the triggered GRDs and their identifiers.
• Figure 4: Statistical distribution of STE observed on-orbit by GECAM-B. The figure illustrates the average number of STE per hour for varying numbers of detectors triggered ($N$), depicting the observational characteristics of events that simultaneously trigger multiple detectors.
• Figure 5: Multi-detector random coincidence probability theoretical calculations (blue circles) and simulation results (orange squares) within a $0.3\,\mu\mathrm{s}$ time window.