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

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.

Figures (5)

• Figure 1: Minimum detectable initial mass towards SGR 1935+2154 in each band. We consider three different combinations of distance and extinction: $(6.6~\mathrm{kpc},~7~\mathrm{mag})$, $(10~\mathrm{kpc},~8~\mathrm{mag})$, and $(15~\mathrm{kpc},~9~\mathrm{mag})$, where $(6.6~\mathrm{kpc},~7~\mathrm{mag})$ represents a plausible location of SGR 1935+2154 (see Sec. \ref{['sec:Astrometry']} and Sec. \ref{['sec: SGR extinction']}). Even at $(15~\mathrm{kpc},~9~\mathrm{mag})$, massive stars would in principle still be included in our sample of massive star candidates (see Sec. \ref{['sec: photometry']}).
• Figure 2: The distribution of reduced chi-square $\chi^2_r$ for the median fitted results (top left) along with the fitted results for different values of $\chi^2_r$ (others). Top left: Sources are retained based on the cut $\chi^2_r \leq 15$, as indicated by the black dashed line. A considerable number of sources show large $\chi^2_r$ values since we use TLUSTY models (a hot star model) and $A_{\rm V}$ is limited to $0-10$ mag in order to identify massive star candidates around SGR 1935+2154, so cooler or more highly extincted stars cannot be fitted. Others: The fitted results are sorted in ascending order of $\chi^2_r$, along with the corresponding residuals (defined as [observed - fitted] / error) for each band. The two black dashed lines in the residual plots designate the $+3\sigma$ and $-3\sigma$ levels.
• Figure 3: Cumulative counts of the required distances for the sources to be considered as massive star candidates towards SGR 1935+2154. The blue shaded region denotes the reference distance range adopted for SGR 1935+2154 2020ApJ...905...99Z. A considerable fraction of the sources would need to be placed at distances far beyond that of SGR 1935+2154, even outside the Galactic disk (we truncate the plot at 30 kpc), to be considered massive star candidates. These sources are therefore very likely not massive stars in the vicinity of the SGR 1935+2154, so they are not the focus of our study.
• Figure 4: Spatial distribution of massive star candidates around SGR 1935+2154 within $6.6$ kpc (left), $10$ kpc (middle), and $15$ kpc (right). The green star marks the position of SGR 1935+2154, while the circles indicate the massive star candidates, color-coded by their $A_{\rm V}$ values.
• Figure 5: The median results of the backtracked time to closet approach of the samples with respect to SGR 1935+2154 versus the closest angular distance. The gray points in background are all the sample selected in Sect. \ref{['sec:Astrometry']}. Massive star candidates are marked in blue ($\leq 6.6$ kpc), green (6.6--10 kpc), and orange (10--15 kpc), with the dashed line showing the angular separation subtended by 1 pc at 6.6 kpc.