GTC optical/NIR upper limits and NICER X-ray analysis of SGR J1935+2154 for the outburst in 2022

TL;DR

This study reports on the optical, NIR, and X-ray observations of the magnetar SGR J1935+2154 during its 2022 outburst, providing upper limits on optical/NIR emission and analyzing X-ray burst properties and spectral evolution.

Contribution

It presents the first combined optical/NIR upper limits and detailed X-ray spectral analysis of SGR J1935+2154 during its 2022 outburst, exploring emission mechanisms.

Findings

No optical/NIR counterpart detected, setting stringent upper limits.

X-ray burst flux follows a power-law distribution.

Persistent emission showed rapid decay and later enhancement.

Abstract

The Galactic magnetar SGR J1935+2154 has undergone another outburst since 2022 October 10. We present the results of searching for an optical/NIR counterpart of SGR J1935+2154 before and during this outburst. No counterpart was detected at the magnetar's position in ${r'}$ and ${z'}$ bands, providing stringent upper limits of $r'\gtrsim 28.65$ and $z'\gtrsim 26.27$. Using archival X-ray data from NICER, we investigated the properties of the bursts and the spectral evolution of persistent emission. The burst flux $F$ showed a power-law distribution of $N\propto F^{-0.76\pm0.10}$ for flux $\gtrsim 2.6\times 10^{-9}\rm{\ erg\ cm^{-2}\ s^{-1}}$, while the temperature and radius followed a lognormal distribution with $kT=1.63^{+0.73}_{-0.50}\ \rm{keV}$ and $R_{\rm bb}=4.35_{-1.35}^{+1.95}\ \rm{km}$, respectively. The persistent flux evolution experienced a quick decay and an enhancement $\sim 27$ days after the BAT trigger. Using the near-infrared (NIR) and X-ray emission, together with the GTC optical/NIR upper limits, we discussed the origin of the NIR emission from the magnetar based on the fallback disk model and magnetosphere model. We found that either model cannot be ruled out with currently available data. Further mid-infrared observations are needed to find out the mechanism for producing the NIR emission from SGR J1935+2154.