SGR 1935+2154's Quiet Local Environment: Clues for Its Progenitor

TL;DR

This study investigates the local environment of magnetar SGR 1935+2154 to infer its progenitor, finding it likely originated from a non-very massive star or binary merger, based on its quiet stellar surroundings.

Contribution

The paper provides new evidence linking magnetar progenitors to less massive stars by analyzing the quiet environment of SGR 1935+2154 and ruling out massive star companions.

Findings

The environment around SGR 1935+2154 has a low density of massive stars.

The magnetar likely originated from a non-very massive star or binary merger.

No massive companions were found in the trajectory analysis.

Abstract

Magnetars are highly magnetized neutron stars (NSs) whose evolution and radiation are governed by the decay and/or reconfiguration of their magnetic fields. The origin of magnetars remains an open question, with proposed progenitor scenarios including core-collapse (CC) of very massive stars ($\ge 25~M_\odot$) or non-very massive stars ($8<M_*<25~M_\odot$), mergers of stellar systems, and accretion-induced collapse (AIC) of white dwarfs (WDs). Investigating the environments of magnetars can offer valuable clues to this issue. In this work, we study the local (a radius of $0.87^\circ$, $\sim 100$ pc at 6.6 kpc) stellar environment of SGR 1935+2154, which is spatially associated with the supernova remnant (SNR) G57.2+0.8, based on astrometry from Gaia DR3 and multi-band photometry from optical to infrared (IR). We discover that the upper limit of the surface density of massive stars around SGR 1935+2154 is only a quarter of that of the solar neighborhood, where the star formation rate is modest in the Galaxy. This quiet environment implies that the magnetar was likely formed by the CC of either a non-very massive star or a binary merger product rather than the CC of a very massive star. Although alternative channels cannot be excluded, their probabilities may be substantially lower. The studies of magnetars associated with SNRs consistently favor non-very massive progenitors, implying that such progenitors may produce a considerable fraction of magnetars. We also backtrack the trajectories of SGR 1935+2154 and its surrounding stars to search for its potential massive companions, yet no such companions are found.