NICER View of the 2020 Burst Storm and Persistent Emission of SGR 1935+2154

Abstract

We report on NICER observations of the magnetar SGR1935+2154, covering its 2020 burst storm and long-term persistent emission evolution up to ∼90 days postoutburst. During the first 1120s taken on April 28 00:40:58 UTC, we detect over 217 bursts, corresponding to a burst rate of >0.2 burstss −1. Three hours later, the rate was 0.008burstss −1 , remaining at a comparatively low level thereafter. The T 90 burst duration distribution peaks at 840 ms; the distribution of waiting times to the next burst is fit with a lognormal with an average of 2.1 s. The 1-10 keV burst spectra are well fit by a blackbody, with an average temperature and area of kT=1.7 keV and R 2 =53 km 2. The differential burst fluence distribution over ∼3 orders of magnitude is well modeled with a power-law form dN/dF∝F −1.5±0.1. The source persistent emission pulse profile is double-peaked hours after the burst storm. We find that the burst peak arrival times follow a uniform distribution in pulse phase, though the fast radio burst associated with the source aligns in phase with the brighter peak. We measure the source spin-down from heavy-cadence observations covering days 21-39 postoutburst, n =-´-3.72 3 10 12 ()  Hzs −1 , a factor of 2.7 larger than the value measured after the 2014 outburst. Finally, the persistent emission flux and blackbody temperature decrease rapidly in the early stages of the outburst, reaching quiescence 40 days later, while the size of the emitting area remains unchanged.

Broadband X-ray burst spectroscopy of the fast-radio-burst-emitting Galactic magnetar

“…It comprised a long-lasting burst storm, with at least a few hundred bursts observed within a few hours 6-8 . The 1-25 keV persistent emission of the source was significantly enhanced subsequent to this storm over a period lasting several weeks 7,9 .We observed SGR J1935+2154 with the NICER X-ray Timing Instrument 10 (0.2-12 keV) onboard the International Space Station on April 28, from 00:40:58 UTC until 00:59:36 UTC (~19 minutes), covering just the tail end of the storm. This NICER observation revealed over 200 bursts 7 emitted by SGR J1935+2154, which was also visible to the Fermi Gamma-ray Burst Monitor (GBM; 8 keV -30 MeV).…”

“…It comprised a long-lasting burst storm, with at least a few hundred bursts observed within a few hours 6-8 . The 1-25 keV persistent emission of the source was significantly enhanced subsequent to this storm over a period lasting several weeks 7,9 .…”

Multiwavelength Observations of Fast Radio Bursts

“…Since FRB 20200428A is aligned with this peak, its observation could be interpreted with the magnetar being instantaneously viewed down the polar axis and, as such, connected with the polar magnetic field lines. The rarity of FRB 20200428A could be therefore explained as the result of the coincidence between the burst emission and the polar field lines aligned with the line of sight, whereas the other sporadic and much less bright radio bursts occur at different spin phases [222]. Among the several mechanisms by which magnetars can emit FRBs, two classes are worth mentioning-in one case, the flaring magnetar emits a plasmoid of relativistic particles which then shocks the external medium at outer radii (10 14 -10 16 cm) causing synchrotron maser emission which manifests itself as the FRB [126][127][128]214,232].…”

Periodicity Search on X-ray Bursts of SGR J1935+2154 Using 8.5 yr of Fermi/GBM Data

“…Many searches have been carried out (e.g. Lin et al 2020b,c;Younes et al 2020;Mereghetti et al 2020;Yang et al 2021). We independently searched the continuous timetagged event (CTTE) data of Fermi/GBM NaI detectors within the range of 8 keV -900 keV from 2013-01-01T00:00:00 (UTC) to 2021-07-08T23:59:59 (UTC), including the latest explosive activity of SGR J1935+2154 in July by the following three steps:…”