2020ApJ...905...99Z

We have performed a multiwavelength study of supernova remnant (SNR) G57.2+0.8 and its environment. The SNR hosts the magnetar SGR 1935+2154, which emitted an extremely bright millisecond-duration radio burst on 2020 April 28. We used the ¹²CO and ¹³CO J = 1-0 data from the Milky Way Image Scroll Painting CO line survey to search for molecular gas associated with G57.2+0.8, in order to constrain the physical parameters (e.g., the distance) of the SNR and its magnetar. We report that SNR G57.2+0.8 is likely impacting the molecular clouds (MCs) at the local standard of rest (LSR) velocity V_LSR∼30kms^–1 and excites a weak 1720 MHz OH maser with a peak flux density of 47 mJy beam^–1. The chance coincidence of a random OH spot falling in the SNR is <=12%, and the OH-CO correspondence chance is 7% at the maser spot. This combines to give <1% false probability of the OH maser, suggesting a real maser detection. The LSR velocity of the MCs places the SNR and magnetar at a kinematic distance of 6.6 ± 0.7 kpc. The nondetection of thermal X-ray emission from the SNR and the relatively dense environment suggests G57.2+0.8 be an evolved SNR with an age t≳1.6×10⁴(d/6.6kpc)yr. The explosion energy of G57.2+0.8 is lower than 2×10⁵¹(n₀/10cm^–3)^1.16(d/6.6kpc)^3.16erg, which is not very energetic even assuming a high ambient density n₀=10cm^–3. This reinforces the opinion that magnetars do not necessarily result from very energetic supernova explosions.