Revisit the periodicity of SGR J1935+2154 bursts with updated sample

TL;DR

The study investigates whether bursts from the magnetar SGR J1935+2154 exhibit an active periodic window, motivated by FRB associations and earlier suggestions of a ~238-day cycle. It expands the burst dataset by incorporating Fermi/GBM and GECAM records up to 2022 and adds a targeted GBM search for 2008–2014 bursts, analyzed with Period Folding and Lomb-Scargle methods. The results favor a periodicity near $126.88 \pm 2.05$ days, while showing that previous ~238-day claims are likely artifacts of sample selection and observational windows; a ~55-day peak is identified as a fake signal from window effects, and other peaks (158, 238, 498 days) can arise from burst-rate variation. The paper discusses physical interpretations, including magnetar free precession, and highlights that the burst history remains complex, necessitating further monitoring to robustly test periodicity hypotheses.

Abstract

Since FRB 200428 has been found to be associated with an X-ray burst from the Galactic magnetar SGR J1935+2154, it is interesting to explore whether the magnetar bursts also follow the similar active periodic behavior as some repeating FRBs. Previous studies show that there is possible period about 230 day in SGR J1935+2154 bursts. Here, we collected an updated burst sample from SGR J1935+2154, including all bursts reported by Fermi/GBM and GECAM till 2022 January. We also developed a targeted search pipeline to reveal more bursts from SGR J1935+2154 in the Fermi/GBM data from 2008 August to 2014 December (i.e. before the first burst detected by Swift/BAT). With this burst sample, we re-analyzed the possible periodicity of SGR J1935+2154 bursts using the Period Folding and Lomb-Scargle Periodogram methods. Our results show that the periodicity $\sim$238 day reported in literature is probably fake and the observation effects may introduce false periods (i.e. 55 day) according to simulation tests. We find that, for the current burst sample, the most probable period is 126.88$\pm$2.05 day, which could be interpreted as the precession of the magnetar. However, we note that the whole burst history is very complicated and difficult to be perfectly accommodated with any period reported thus far, therefore more monitoring observations of SGR J1935+2154 are required to test any periodicity hypothesis.

Figures (9)

• Figure 1: The location of candidate bursts. The red star marks the accurate position of SGR J1935+2154. The black and blue represent the location of candidate bursts and real (confirmed) bursts (i.e. which are also detected by Swift/BAT), respectively. The gray lines mean 1-$\sigma$ confidence level.
• Figure 2: The visible time windows of Fermi/GBM and GECAM to SGR J1935+2154. The exposure time is the union of the Fermi/GBM and GECAM visible time interval in that day.
• Figure 3: The light curve of candidate bursts from SGR J1935+2154. The blue curves represent the summed light curves of those NaI detectors with significant excess. The red dotted line represent the trigger time and pink shadow is the duration of candidate burst.
• Figure 4: The period of SGR J1935+2154 derived from sample A. The left two panels show the result of reduced $\chi^{2}$ and $(C-C_{\mathrm e})/\sqrt{C_{\mathrm v}}$ of period folding. The Lomb-Scargle periodogram of observed bursts data and simulated data of observation windows shows in the right upper panel with blue line and lightcoral dashed line, respectively. We normalize these two periodogram with maximum power value, and subtract the normalized periodogram of simulated data from the observed one. The result is shown in the right lower panel. The vertical red dotted line indicate the peak of relative periodogram which is 126.88 day, which is also significant in the period folding results with the reduced $\chi^{2}$ and $(C-C_{\mathrm e})/\sqrt{C_{\mathrm v}}$ method. The vertical orange dotted and the vertical green dotted line represent 158.15 day and 238 day, respectively.
• Figure 5: The period of SGR J1935+2154 derived from sample B. The left two panel show the result of reduced $\chi^{2}$ and $(C-C_{\mathrm e})/\sqrt{C_{\mathrm v}}$ of period folding. The Lomb-Scargle periodogram of observed bursts data and simulated data of observation windows shows in the right upper panel with blue line and lightcoral dashed line, respectively. We normalize these two periodogram with maximum power value, and subtract normalized periodogram of simulated data from observed one. The result show in the right lower panel. The vertical red dotted line indicate the peak of relative periodogram which is 127.02 day. It is consistent with the peak of reduced $\chi^{2}$ and $(C-C_{\mathrm e})/\sqrt{C_{\mathrm v}}$. The vertical orange dotted and the vertical green dotted line represent 158.15 day and 238 day, respectively