[Paper Review] On the Distance of SGR 1935+2154 Associated with FRB 200428 and Hosted in SNR G57.2+0.8

Shu-Qing Zhong, Zi-Gao Dai+2 more|arXiv (Cornell University)|May 22, 2020

Pulsars and Gravitational Waves Research24 references10 citations

TL;DR

This paper estimates the distance to the magnetar SGR 1935+2154, associated with FRB 200428 and hosted in supernova remnant G57.2+0.8, by combining dispersion measure (DM) contributions from the Galaxy, magnetar wind nebula, and SNR, assuming physical association between the SGR and SNR. It finds a distance of $9.0 \pm 2.5$ kpc and SNR radius of 10–18 pc, consistent with prior studies, and estimates a local Faraday rotation measure of $\sim -110$ rad m$^{-2}$.

ABSTRACT

Owing to the detection of an extremely bright fast radio burst (FRB) 200428 associated with a hard X-ray counterpart from the magnetar soft gamma-ray repeater (SGR) 1935+2154, the distance of SGR 1935+2154 potentially hosted in the supernova remnant (SNR) G57.2+0.8 can be revisited. Under the assumption that the SGR and the SNR are physically related, in this Letter, by investigating the dispersion measure (DM) of the FRB contributed by the foreground medium of our Galaxy and the local environments and combining with other observational constraints, we find that the distance of SGR 1935+2154 turns out to be $9.0\pm2.5\,$kpc and the SNR radius falls into $10$ to $18\,$pc since the local DM contribution is as low as $0-18\,$pc cm$^{-3}$. These results are basically consistent with the previous studies. In addition, an estimate for the Faraday rotation measure of the SGR and SNR is also carried out.

Motivation & Objective

• To re-evaluate the distance to SGR 1935+2154 using dispersion measure (DM) constraints, assuming physical association with SNR G57.2+0.8.
• To assess the consistency of the SNR radius and DM contribution from the local environment with observational data.
• To estimate the Faraday rotation measure (RM) contribution from the SNR environment using polarization data and theoretical models.
• To reconcile conflicting distance estimates for the SGR and SNR using multi-wavelength constraints.

Proposed method

• Uses the observed DM of FRB 200428 (332.7 pc cm⁻³) and decomposes it into contributions from the Galaxy (DM_Gal), magnetar wind nebula (DM_MWN), and SNR (DM_SNR).
• Applies the YMW16 Galactic electron density model to compute DM_Gal as a function of distance D.
• Employs theoretical models for DM_MWN based on magnetar properties (B_p, P, μ_±) and DM_SNR based on SNR evolution in constant ISM or wind environments.
• Derives SNR radius and DM_SNR using energy-density relations and SNR expansion models, varying explosion energy and ambient density.
• Estimates RM_SNR using Piro & Gaensler (2018) formalism for both ISM and wind scenarios, linking it to magnetic energy fraction and explosion energy.
• Compares predicted RM values with observed RM of +112.3 rad m⁻² from SGR 1935+2154 and foreground RM of +223 ± 2 rad m⁻² for G57.2+0.8 to infer local RM contribution.

Experimental results

Research questions

• RQ1 What is the distance to SGR 1935+2154 if it is physically associated with SNR G57.2+0.8 and FRB 200428 is linked to the magnetar?
• RQ2 How do the DM contributions from the Galaxy, magnetar wind nebula, and SNR collectively constrain the SGR distance?
• RQ3 What is the expected SNR radius and DM_SNR contribution for plausible SNR evolution models?
• RQ4 What is the Faraday rotation measure (RM) contribution from the local SNR environment, given observed RM values?
• RQ5 How do the results compare with previous distance estimates from radio, X-ray, and HI/CO measurements?

Key findings

• The distance to SGR 1935+2154 is estimated at $9.0 \pm 2.5$ kpc, based on DM decomposition and observational constraints.
• The SNR radius is constrained to $10$ to $18$ pc, consistent with the SGR's age of $\sim 3.6$ kyr and physical association with the SNR.
• The DM contribution from the SNR is estimated at $0$ to $18$ pc cm⁻³, depending on ambient density and explosion energy.
• The local Faraday rotation measure (RM) contribution from the SNR environment is estimated at $\sim -110$ rad m⁻², implying an explosion energy of $\sim (1-5) \times 10^{51}$ erg in the ISM scenario.
• The results are consistent with previous distance estimates from HI, CO, and X-ray observations, supporting the physical association between SGR 1935+2154 and SNR G57.2+0.8.
• The wind scenario yields lower RM values ($<8$ rad m⁻²), but the ISM scenario better matches the observed RM, favoring a constant ISM environment for the SNR.