A Search for Radio Bursts and Periodic Emission from SGR 1935+214 at High Radio Frequencies using the Deep Space Network

ATel #13713; Aaron B. Pearlman (Caltech), Walid A. Majid (JPL, Caltech), Thomas A. Prince (Caltech, JPL), Charles J. Naudet (JPL), Jonathon Kocz (Caltech)
on 7 May 2020; 23:30 UT
Credential Certification: Aaron B. Pearlman (aaron.b.pearlman@caltech.edu)

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SGR 1935+2154 is a Galactic magnetar that has recently emitted a series of short X-ray bursts (e.g., see GCN Circular #27657; GCN Circular #27659; GCN Circular #27661; ATel #13675; ATel #13678; ATel #13682; ATel #13685; ATel #13687; ATel #13696), indicating that the source has entered into an active state. CHIME/FRB and STARE2 both detected bright, high fluence radio bursts, which were coincident with an X-ray burst detected during this recent period of X-ray flaring activity (ATel #13681; ATel #13684; ATel #13687; ATel #13692; ATel #13696). Radio observations of the magnetar were subsequently performed at a variety of wavelengths (e.g., see ATel #13690; ATel #13693; ATel #13697; ATel #13699; ATel #13707), which all yielded null detections except for a single highly polarized, 30 mJy radio burst detected by FAST at 1.25 GHz (ATel #13699).

We carried out Target of Opportunity (ToO) radio observations of SGR 1935+2154 using the Deep Space Network (DSN) 34 m diameter radio telescopes (DSS-55 and DSS-54) in Madrid, Spain at center frequencies of 2.3 GHz (S-band) and 8.4 GHz (X-band). The observations were performed during three separate epochs: Epoch 1 (start time: 2020 April 29, 22:54:25 UTC; duration: 9159 s), Epoch 2 (start time: 2020 April 30, 22:53:09 UTC; duration: 11063 s), and Epoch 3 (start time: 2020 May 1, 23:55:01 UTC, duration: 3420 s). We used the pulsar backend to simultaneously record data at S-band and X-band during Epochs 2 and 3. Only X-band data were recorded during Epoch 1. The data were recorded using a single circular polarization channel with bandwidths of ~110 MHz at S-band and ~396 MHz at X-band. Power spectral density measurements were channelized and saved in a digital polyphase filterbank with a frequency and time resolution of 0.46 MHz and 2.2 ms, respectively.

We dedispersed the S-band and X-band data from each epoch with trial DMs between 0 and 3,000 pc cm^-3 and carried out a search for radio bursts using a Fourier domain matched filtering algorithm, where each dedispersed time-series was convolved with boxcar functions with logarithmically spaced widths between 2.2 ms and 662.3 ms. Candidates with detection signal-to-noise ratios above 6.0 were saved and classified using a GPU-accelerated machine learning pipeline based on the FETCH (Fast Extragalactic Transient Candidate Hunter) package (Agarwal et al. 2019). The dynamic spectra of the candidates were also visually inspected for verification. No radio bursts were detected at either S-band or X-band during Epochs 1-3.

The dedispersed S-band and X-band data from each epoch were also independently searched for periodic radio emission. We used a GPU-accelerated Fast Folding Algorithm (FFA) algorithm to search for pulsed radio emission with periods between 3 ms and 100 s. We also used a GPU-accelerated Fourier Domain Acceleration Search (FDAS) pipeline, which employs a matched filtering algorithm to correct for Doppler smearing, to search for periodicity. No statistically significant periods, with a signal-to-noise (S/N) ratio above 7.0, were found after folding the data modulo each of the period candidates identified by the two algorithms. Moreover, we find no evidence of radio pulsations at or near the previously reported 3.2 s rotational period (Israel et al. 2016).

We place the following upper limits (6-sigma) on the magnetar's radio flux density during each of our observations: Epoch 1 (< 0.2 Jy at 8.4 GHz), Epoch 2 (< 0.5 Jy at 2.3 GHz; < 0.3 Jy at 8.4 GHz), and Epoch 3 (< 0.5 Jy at 2.3 GHz; < 0.3 Jy at 8.4 GHz), for an assumed duty cycle of 10%.

We thank the DSN team for scheduling and carrying out these observations.