Fast X-ray Transient Detection with AXIS: application to Magnetar Giant Flares

TL;DR

MGFs probe extreme physics and heavy-element production, and this study develops a framework to assess AXIS detectability across three emission phases. Serendipitous prompt-spike detections are possible but expected to be rare, with $N^{MGFs}_{AXIS\,5\,yr} \approx 0.21$, while pulsating tails could be detected and studied out to $\sim 20$ Mpc if AXIS achieves rapid repointing, enabling first extragalactic magnetar pulsation measurements. X-ray signatures from r-process nucleosynthesis are predicted to be too faint for extragalactic detection, making Galactic observations the feasible path to constraining MGFs in heavy-element production. The authors advocate a strategy of wide-field monitoring plus targeted observations of nearby, star-forming galaxies within $<100$ Mpc to maximize AXIS’s scientific return on MGFs.

Abstract

Magnetar giant flares (MGFs) are among the most luminous high-energy transients in the local universe, consisting of a short, intense MeV gamma-ray spike followed by a softer, pulsating X-ray tail and possibly delayed radioactive emission. While only three Galactic events have been firmly detected, several extragalactic candidates have recently been reported, motivating the need for sensitive, rapid-response gamma- and X-ray facilities to constrain their rates and energetics. We present a feasibility study of detecting MGFs with the Advanced X-ray Imaging Satellite (AXIS), focusing on two complementary pathways: (i) serendipitous discovery of the prompt gamma-ray spike within the field of view, and (ii) rapid follow-up of MGF tails in nearby galaxies. Using sensitivity rescaling and volumetric rate estimates, we find that serendipitous detection of prompt spikes during the mission lifetime is possible but unlikely, primarily because of their short duration and primarily because of their short duration and hard spectrum, in the assumption that the hard gamma-ray spectrum can be reliably extrapolated to the instrument's energy range. In contrast, AXIS's superior sensitivity, if accompanied by fast repointing capabilities, offer an extraordinary opportunity to detect pulsating X-ray tails out to about 20 Mpc, enabling the first extragalactic measurements of periodic modulations from a magnetar and potentially constraining emission geometry and fireball physics. Finally, we evaluate the detectability of soft X-ray line emission from r-process nucleosynthesis in MGFs, finding that such signals are extremely faint and confining the detection to Galactic distances. Our study offer a general framework for assessing the detectability of short transients with future missions.

Figures (6)

• Figure 1: Schematic light curve of a magnetar giant flare. The emission proceeds in three phases: (A) an initial $\gamma$-ray spike of sub-second duration, (B) a pulsating X-ray tail modulated at the neutron star spin period, and (C) a delayed nuclear $\gamma$-ray component peaking at $t_{\rm peak}$ and decaying approximately as a power law of index $-1.2$. Credits: Jakub Cehula and Anirudh Patel.
• Figure 2: Left: Fraction of AXIS detectable events as function of the distance. Right: Cumulative number of MGFs as seen by AXIS in 5 years mission as a function of the distance.
• Figure 3: Left: Lightcurve of an MGF tail mimicking the one observed during the 2004 event from SGR 1806–20 Hurley+05. Right: Simulated AXIS light curve of a decaying tail similar to the 2004 event from SGR 1806–20.
• Figure 4: Left: Simulated AXIS-detected tails of a giant flare similar to the 2004 SGR 1806–20 event, scaled to different extragalactic distances. Right Power spectra of simulated AXIS light curves for a magnetar giant flare tail observed at increasing distances. The evident peak $f=0.20$ corresponds to the 5 s periodicity that we input in the simulation.
• Figure 5: Left: Periodicity detection significance as a function of distance for simulated AXIS observations of magnetar giant flare tails. Right: Maximum time-to-target on a magnetar giant flare as a function of distance, based on simulated AXIS observations.