The first observation of an intermediate flare from SGR 1935+2154

TL;DR

This paper reports the first observation of an intermediate flare from SGR 1935+2154, analyzing its spectral and temporal properties, and estimating its distance based on spectral fits, marking a significant addition to SGR flare studies.

Contribution

It provides the first detection and detailed spectral analysis of an intermediate flare from SGR 1935+2154, expanding understanding of SGR flare characteristics.

Findings

The flare lasted about 1.7 seconds with an energy fluence of ~2.5×10⁻⁵ erg/cm².

Spectral analysis favors complex models like CPL and 2BB over simpler ones.

Estimated distance to SGR 1935+2154 is less than 10 kpc, consistent with the associated supernova remnant.

Abstract

We report on the bright burst detected by four Interplanetary network (IPN) spacecraft on 2015 April 12. The IPN localization of the source is consistent with the position of the recently discovered soft gamma-repeater SGR 1935+2154. From the Konus-Wind (KW) observation, we derive temporal and spectral parameters of the emission, and the burst energetics. The rather long duration of the burst ($\sim$1.7 s) and the large measured energy fluence ($\sim2.5\times10^{-5}$ erg cm$^{-2}$) put it in the class of rare "intermediate" SGR flares, and this is the first one observed from SGR 1935+2154. A search for quasi-periodic oscillations in the KW light curve yields no statistically significant signal. Of four spectral models tested, optically thin thermal bremsstrahlung and a single blackbody (BB) function can be rejected on statistical grounds; two more complex models, a cutoff power law (CPL) and a sum of two BB functions (2BB), fit the burst spectra well and neither of them may be ruled out by the KW observation. The CPL and 2BB model parameters we report for this bright flare are typical of SGRs; they are also consistent with those obtained from observations of much weaker and shorter SGR 1935+2154 bursts with other instruments. From the distribution of double blackbody spectral fit parameters we estimate the SGR 1935+2154 distance to be $<$10.0 kpc, in agreement with that of the Galactic supernova remnant G57.2+0.8 at 9.1 kpc.