Scientific performance of on-board analyses for the SVOM X-ray telescope MXT

Abstract

The Microchannel X-ray Telescope on board the Space-based multi-band astronomical Variable Objects Monitor (SVOM) satellite detects and localizes the X-ray afterglow of gamma-ray bursts. One year after the launch, this paper presents the in-flight performance of the scientific analyses conducted by the on-board computer. After summarizing the analysis steps, the paper reviews the on-board results obtained with 15 gamma-ray burst afterglows detected by the telescope between October 2024 and August 2025. For all bursts, the localization uncertainty is estimated to be below 2 arcmin, as required by the mission design. On average, the measured position is found to be 40 arcsec away from the position measured by other experiments with a better sky resolution. Moreover, we show that the on-board analysis provides a precise sky location for the burst only a few seconds after the beginning of the observation. Taking advantage of an efficient very-high-frequency antenna network, this information is quickly collected on the ground and disseminated to other observation facilities. This low-latency strategy is critical for the multi-wavelength and multi-instrument follow-up program of SVOM.

Figures (5)

• Figure 1: Photon cumulative maps for a bright source (Vela X-1 high-mass X-ray binary, Jan. 12, 2025) in the top panel and a weak source (S5 0716+714 blazar, Jan. 30, 2025) in the bottom panel. The reconstructed source position is represented with a yellow cross and its $R90$ uncertainty with a full yellow circle. The true source position is indicated by a red point.
• Figure 2: From top to bottom, the cumulative source counts (red), the cumulative background counts (blue), the signal-to-noise ratio (black) and the localization uncertainty $R90$ (green) are plotted as a function of the integration time for a bright source (Vela X-1 high-mass X-ray binary, Jan. 12, 2025) in the left-hand panel and a weak source (S5 0716+714 blazar, Jan. 30, 2025) in the right-hand panel. The vertical magenta dashed lines indicate when the photon cumulative maps in figure \ref{['fig:cum_map']} were extracted.
• Figure 3: Observation time needed to confidently detect 15 gamma-ray burst afterglows with the MXT. It is plotted as a function of the time since the original ECLAIRs trigger.
• Figure 4: Localization accuracy associated with the 15 first gamma-ray burst afterglows detected by the MXT. The points indicate the angular difference between the position measured on board and the position measured by other instruments (Swift/XRT Gehrels:2004qma, SVOM/VT Wang:2009pn, EP/FXT Yuan:2022fpjZhang:2025vgv) known to localize a source with a precision of a few arcsec. The error bars are computed as the quadratic sum of uncertainties at a 90% confidence level, including systematics. The horizontal dashed line (40.3 arcsec) marks the median value of angular difference for all gamma-ray bursts.
• Figure 5: MXT observation of GRB 241217A. The background-subtracted light curve, as calculated on board, is plotted as a function of time (red points). The corresponding background contribution is indicated with the blue points. The hatched areas mark the observation interruptions. During the second interruption, the SVOM satellite was slewed a second time (green vertical line) to center the source in the field of view.