CTAO LST-1 observations of magnetar SGR 1935+2154: Deep limits on sub-second bursts and persistent tera-electronvolt emission

TL;DR

This study uses 25.5 hours of high-quality CTAO LST-1 observations to search for TeV counterparts to both the persistent and sub-second bursting emission from the Galactic magnetar SGR 1935+2154 during 2021–2022 activity. Employing robust, low-photon-statistics analyses and full instrument simulations, the authors derive stringent TeV upper limits in the 0.1–10 TeV range for both persistent and burst emission, finding no significant detection and constraining the TeV-to-X-ray flux ratio for the brightest burst to be below approximately $10^{-3}$. The results support models in which the magnetar’s hard X-ray tail does not extend to TeV energies, consistent with a spectral break around the MeV range, and place meaningful constraints on FRB-related high-energy processes that could yield TeV photons. The work demonstrates the effectiveness of LST-1 and future CTAO capabilities for fast transient TeV astronomy, particularly in the low-photon-statistics regime, and motivates rapid-targeting strategies and real-time analyses for magnetar and FRB counterpart searches.

Abstract

The Galactic magnetar SGR 1935+2154 has exhibited prolific high-energy (HE) bursting activity in recent years. Investigating its potential tera-electronvolt counterpart could provide insights into the underlying mechanisms of magnetar emission and very high-energy (VHE) processes in extreme astrophysical environments. We aim to search for a possible tera-electronvolt counterpart to both its persistent and sub-second-scale burst emission. We analysed over 25 h of observations from the Large-Sized Telescope prototype (LST-1) of the Cherenkov Telescope Array Observatory (CTAO) during periods of HE activity from SGR 1935+2154 in 2021 and 2022 to search for persistent emission. For bursting emission, we selected and analysed nine 0.1 s time windows centred around known short X-ray bursts, targeting potential sub-second-scale tera-electronvolt counterparts in a low-photon-statistics regime. While no persistent or bursting emission was detected in our search, we establish upper limits for the tera-electronvolt emission of a short magnetar burst simultaneous to its soft gamma-ray flux. Specifically, for the brightest burst in our sample, the ratio between tera-electronvolt and X-ray flux is less than $10^{-3}$. The non-detection of either persistent or bursting tera-electronvolt emission from SGR 1935+2154 suggests that if such components exist, they may occur under specific conditions not covered by our observations. This aligns with theoretical predictions of VHE components in magnetar-powered fast radio bursts and the detection of MeV - GeV emission in giant magnetar flares. These findings underscore the potential of magnetars, fast radio bursts, and other fast transients as promising candidates for future observations in the low-photon-statistics regime with Imaging Atmospheric Cherenkov Telescopes, particularly with the CTAO.

Figures (5)

• Figure 1: $\theta^2$ plot on SGR 1935$+$2154 persistent emission. We used $25.5h$ of high-quality data (see Sect. \ref{['sec:PersistentEmission']}), which we show here in a single energy bin from $\qtyrange{0.1}{10}{\tera\electronvolt}$. We show the distribution of ON counts in blue and the background distribution in orange. The dashed line represents the $\theta^2$ cut used to evaluate the significance. No emission is detected.
• Figure 2: Multi-band SED of the persistent emission of SGR 1935$+$2154. Black lines show the best fit emission model in the X-ray and soft gamma-ray bands borghese_sgr1935_2022. In the VHE band, our $95\%$ confidence level ULs confirm the non-detection obtained by hess_sgr1935_2021. The tera-electronvolt ULs are about the same order of magnitude as the X-ray emission and confirm previous studies, which suggest that the power-law component observed above $>10keV$ must have a break at $MeV$ energies.
• Figure 3: Excess (left) and statistical significance (right) maps in a $\qtyproduct{3 x 3}{\deg}$ region centred on SGR 1935$+$2154 in the $\qtyrange[range-phrase = \text{ and }]{0.1}{10}{\tera\electronvolt}$ energy range. The ON region is shown as a white circle, while the background ring is delimited by the two dashed circles. The maps do not show any significant excess on the source.
• Figure 4: Distribution of binned OFF counts (black points and labels) in the observation run of burst $\#1$ of Table \ref{['table:BurstUpperLimits']}. The data are consistent with the expectation from a Poisson distribution (blue points, labels and line) with background rate $R_\text{BKG}=0.81s^{-1}$, using bins of width $\delta t=0.1s$. The total duration of the run was $1443s$.
• Figure 5: Soft and VHE gamma-ray SED of the bursting emission of SGR 1935$+$2154 during the burst on July 7 2021 00:33:31.670, measured on a $\sim 0.1s$ timescale.