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A radio transient with unusually slow periodic emission

Abstract

The high-frequency radio sky is bursting with synchrotron transients from massive stellar explosions and accretion events, but the low-frequency radio sky has, so far, been quiet beyond the Galactic pulsar population and the long-term scintillation of active galactic nuclei. The low-frequency band, however, is sensitive to exotic coherent and polarized radio-emission processes, such as electron-cyclotron maser emission from flaring M dwarfs1, stellar magnetospheric plasma interactions with exoplanets2 and a population of steep-spectrum pulsars3, making Galactic-plane searches a prospect for blind-transient discovery. Here we report an analysis of archival low-frequency radio data that reveals a periodic, low-frequency radio transient. We find that the source pulses every 18.18 min, an unusual periodicity that has, to our knowledge, not been observed previously. The emission is highly linearly polarized, bright, persists for 30–60 s on each occurrence and is visible across a broad frequency range. At times, the pulses comprise short-duration (<0.5 s) bursts; at others, a smoother profile is observed. These profiles evolve on timescales of hours. By measuring the dispersion of the radio pulses with respect to frequency, we have localized the source to within our own Galaxy and suggest that it could be an ultra-long-period magnetar.

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Fig. 1: Sixty-four of the 71 detected pulses of GLEAM-X J162759.5-523504.3 aligned to its measured period P and period derivative \(\dot{{\boldsymbol{P}}}\).
Fig. 2: Maximum brightness of each pulse and total pulse fluence as a function of time across the two observed intervals of activity.
Fig. 3: Dynamic spectrum of the observation recorded at 03:59 on 10 January 2018.
Fig. 4: Transient parameter space populated with common known radio transients.

Data availability

Data that support this paper are available at the following public repository: https://github.com/nhurleywalker/GLEAM-X_Periodic_Transient. Further data products can be supplied by the authors on reasonable request.

Code availability

Code that supports this paper is available at the following public repository: https://github.com/nhurleywalker/GLEAM-X_Periodic_Transient. Figure 3 was generated using https://github.com/nhurleywalker/Transient_Phase_Space. Further code can be supplied by the authors on reasonable request.

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Acknowledgements

N.H.-W. is the recipient of an Australian Research Council Future Fellowship (project number FT190100231) and G.E.A. is the recipient of an Australian Research Council Discovery Early Career Researcher Award (project number DE180100346) funded by the Australian Government. This scientific work makes use of the Murchison Radio-astronomy Observatory, operated by CSIRO. We acknowledge the Wajarri Yamatji people as the traditional owners of the observatory site. Support for the operation of the Murchison Widefield Array is provided by the Australian Government (NCRIS), under a contract to Curtin University administered by Astronomy Australia Limited. We acknowledge the Pawsey Supercomputing Centre, which is supported by the Western Australian Government and the Australian Government. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. This project was supported by resources and expertise provided by CSIRO IMT Scientific Computing. This work used resources of China SKA Regional Centre prototype funded by the National Key R&D Programme of China (2018YFA0404603) and Chinese Academy of Sciences (114231KYSB20170003).

Author information

Authors and Affiliations

Authors

Contributions

N.H.-W. calibrated and processed the data for the observations described herein, determined the position and flux density of the source and prepared the manuscript, with contributions from all co-authors. X.Z. processed all polarization data, including performing polarization calibration and analysis. A.B. and S.J.M. performed the analysis to derive the period and period derivative. A.B. performed the X-ray observations and analysis. S.J.M. calculated and applied the dispersion measure and barycentric corrections. T.N.O. developed the original detection methodology, performed the original archive search and made the initial discovery. P.J.H. helped to develop the detection methodology and provided supercomputing support. G.E.A. contributed to astrophysical calculations and interpretation of the data. T.J.G., G.H.H., J.S.M. and X.Z. determined polarization calibration methods. T.J.G. performed early refinement of the period estimate.

Corresponding author

Correspondence to N. Hurley-Walker.

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Extended data figures and tables

Extended Data Fig. 1 Dynamic spectra of the observations used to calculate the dispersion measure.

From top to bottom, separated by horizontal black lines, we show the observations 1205008192 (72–103 MHz), 1205007112 (103–134 MHz), 1205011432 (139–170 MHz), 1205010352 (170–200 MHz) and 1205009272 (200–231 MHz). These observations were taken on 13 March 2018 between 20:12 and 21:24 (see Fig. 1). The left panel shows the data aligned using a period of 1091.1690 s, whereas the right panel shows the same, including a dispersion correction of 57 pc cm−3. Strong ionospheric scintillation is visible in the 72–103-MHz data, causing ripples in the brightness of the source over time.

Extended Data Fig. 2 The explored search space in P and \(\dot{{\boldsymbol{P}}}\) for the pulses recorded from GLEAM-X J162759.5-523504.3.

The contours show the peak flux density of the mean profile at 154 MHz recovered at each combination of P (y axis) and \(\dot{P}\) (x axis), in levels of 15, 14, 13, 12 and 11 Jy. The best-fit values of P = 1,091.1690 s and \(\dot{P}=6\times {10}^{-10}{{\rm{s}}{\rm{s}}}^{-1}\) are marked with a dark red ‘+’.

Extended Data Fig. 3 The flux density of GLEAM-X J162759.5-523504.3 as a function of frequency.

This is derived from the same observations shown in Extended Data Fig. 1. Points are determined via an average of the source profile in each frequency bin, weighting by the signal-to-noise ratio of the frequency-averaged profile. A power-law fit using the data spanning 95–195 MHz is shown in blue, with α = −1.16 ± 0.04.

Extended Data Fig. 4 X-ray luminosity and spectral properties of magnetars15 compared with the X-ray luminosity limits of GLEAM-X J162759.5-523504.3.

The two faintest known magnetars are labelled. The coloured contours represent expected Swift XRT count rates for putative X-ray luminosities and blackbody temperatures for a source at 1.3 kpc, assuming a hydrogen column density of NH ≈ 2 × 1021 cm−2. The grey dashed line represents the implied luminosity upper limit for GLEAM-X J162759.5-523504.3 based on the 3 − σ upper limit obtained from the Swift XRT observation. From the magnetar fundamental plane, we predict GLEAM-X J162759.5-523504.3 to have a quiescent luminosity 4.5 orders of magnitude lower than our limit from the Swift XRT.

Extended Data Fig. 5 Pulse profiles for the three detections on 3 January 2018 compared with a wide pulse detected on 14 March 2018.

Barycentric corrections and dedispersion have been applied. The data are all taken at the same frequency, 170–200 MHz. Vertical dashed lines encapsulating the profile found on 13 March 2018 are overplotted to guide the eye.

Extended Data Fig. 6 Images in full Stokes and polarized intensity (in Jy beam−1), and RM (in rad m−2), of the region 5° × 5° around GLEAM-X J162759.5-523504.3.

The images were made using observation 1200354592 on 18 January 2018 at 23:49, using only the interval where the source was producing emission. Faraday rotation over the imaged bandwidth of 30 MHz causes the Stokes Q, U and V emission to average to zero. The polarized intensity shows the maximum value of the RM spectrum. Where the polarized intensity is less than seven times the local noise, the corresponding RM value has been masked.

Extended Data Fig. 7 A scatter plot of period derivative \(\dot{{\boldsymbol{P}}}\) against period P.

GLEAM-X J162759.5-523504.3 is shown as a blue arrow with an upper limit on \(\dot{P}\) (see Methods) in context with the known pulsars40 (black dots), X-ray-detected magnetars15 (red dots and arrows) and magnetars known to emit in both X-ray and radio frequencies (red circles around black dots). The slowest (and radio-quiet) X-ray magnetar 1E 161348–5055 is also shown with an upper limit on \(\dot{P}\). The green dashed and dot-dashed lines correspond to the theoretical ‘death lines’ for pulsar radio emission for cases I and III calculated by Zhang et al.47.

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Hurley-Walker, N., Zhang, X., Bahramian, A. et al. A radio transient with unusually slow periodic emission. Nature 601, 526–530 (2022). https://doi.org/10.1038/s41586-021-04272-x

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