GTC Optical/Near-infrared Upper Limits and NICER X-Ray Analysis of SGR J1935+2154 for the Outburst in 2022

Abstract The Galactic magnetar SGR J1935+2154 has undergone another outburst since 2022 October 10. We present the results of searching for an optical/near-infrared (NIR) counterpart of SGR J1935+2154 before and during this outburst. No counterpart was detected at the magnetar’s position in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>r</mml:mi> <mml:mo accent="false">′</mml:mo> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>z</mml:mi> <mml:mo accent="false">′</mml:mo> </mml:math> bands, providing stringent upper limits of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>r</mml:mi> <mml:mo accent="false">′</mml:mo> <mml:mo>≳</mml:mo> <mml:mn>28.65</mml:mn> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>z</mml:mi> <mml:mo accent="false">′</mml:mo> <mml:mo>≳</mml:mo> <mml:mn>26.27</mml:mn> </mml:math> . 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 ∝ F −0.76±0.10 for flux ≳2.6 × 10 −9 erg cm −2 s −1 , while the temperature and radius followed a log-normal distribution with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="italic">kT</mml:mi> <mml:mo>=</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>1.63</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.50</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.73</mml:mn> </mml:mrow> </mml:msubsup> <mml:mspace width="0.33em"/> <mml:mi>keV</mml:mi> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>R</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>bb</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>4.35</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1.35</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>1.95</mml:mn> </mml:mrow> </mml:msubsup> <mml:mspace width="0.33em"/> <mml:mi>km</mml:mi> </mml:math> , respectively. The persistent flux evolution experienced a quick decay and an enhancement ∼27 days after the Burst Alert Telescope trigger. Using the NIR and X-ray emission, together with the Gran Telescopio Canarias 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.