Abstract
Fast radio bursts (FRBs) are extragalactic astrophysical transients1 whose brightness requires emitters that are highly energetic yet compact enough to produce the short, millisecond-duration bursts. FRBs have thus far been detected at frequencies from 8 gigahertz (ref. 2) down to 300 megahertz (ref. 3), but lower-frequency emission has remained elusive. Some FRBs repeat4,5,6, and one of the most frequently detected, FRB 20180916B7, has a periodicity cycle of 16.35 days (ref. 8). Using simultaneous radio data spanning a wide range of wavelengths (a factor of more than 10), here we show that FRB 20180916B emits down to 120 megahertz, and that its activity window is frequency dependent (that is, chromatic). The window is both narrower and earlier at higher frequencies. Binary wind interaction models predict a wider window at higher frequencies, the opposite of our observations. Our full-cycle coverage shows that the 16.3-day periodicity is not aliased. We establish that low-frequency FRB emission can escape the local medium. For bursts of the same fluence, FRB 20180916B is more active below 200 megahertz than at 1.4 gigahertz. Combining our results with previous upper limits on the all-sky FRB rate at 150 megahertz, we find there are 3–450 FRBs in the sky per day above 50 Jy ms. Our chromatic results strongly disfavour scenarios in which absorption from strong stellar winds causes FRB periodicity. We demonstrate that some FRBs are found in ‘clean’ environments that do not absorb or scatter low-frequency radiation.
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Data availability
Raw data were generated by the Apertif system on the Westerbork Synthesis Radio Telescope and by the International LOFAR Telescope. The Apertif data that support the findings of this study are available through the ALERT archive, http://www.alert.eu/FRB20180916B. The LOFAR data are available through the LOFAR Long Term Archive, https://lta.lofar.eu/, by searching for ‘Observations’ at J2000 coordinates RA = 01:57:43.2000, Dec. = +65:42:01.020, or by selecting COM_ALERT in ‘Other projects’ and downloading data which includes R3 in the ‘Observation description’.
Code availability
The custom code used to generate these results is publicly available at https://doi.org/10.5281/zenodo.4559593 (ref. 80).
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Acknowledgements
This research was supported by the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013)/ERC grant agreement no. 617199 (‘ALERT’), and by Vici research programme ‘ARGO’ with project number 639.043.815, financed by the Dutch Research Council (NWO). Instrumentation development was supported by NWO (grant 614.061.613 ‘ARTS’) and the Netherlands Research School for Astronomy (‘NOVA4-ARTS’, ‘NOVA-NW3’ and ‘NOVA5-NW3-10.3.5.14’). PI of aforementioned grants is J.v.L. We further acknowledge funding from an NWO Veni Fellowship to E.P.; from Netherlands eScience Center (NLeSC) grant ASDI.15.406 to D.V. and A.S.; from National Aeronautics and Space Administration (NASA) grant number NNX17AL74G issued through the NNH16ZDA001N Astrophysics Data Analysis Program (ADAP) to S.S.; by the WISE research programme, financed by NWO, to E.A.K.A.; from FP/2007-2013 ERC grant agreement no. 291531 (‘HIStoryNU’) to T.v.d.H.; and from VINNOVA VINNMER grant 2009-01175 to V.M.I. I.P.-M. and Y.M. thank M. A. Krishnakumar for providing a software module that was useful in estimating the scatter-broadening timescale. This work makes use of data from the Apertif system installed at the Westerbork Synthesis Radio Telescope owned by ASTRON. ASTRON, the Netherlands Institute for Radio Astronomy, is an institute of NWO. This paper is based (in part) on data obtained with the International LOFAR Telescope (ILT) under project code COM_ALERT. These data are accessible through the LOFAR Long Term Archive, https://lta.lofar.eu/. LOFAR (Methods) is the low frequency array designed and constructed by ASTRON. It has observing, data processing and data storage facilities in several countries, that are owned by various parties (each with their own funding sources), and that are collectively operated by the ILT foundation under a joint scientific policy. The ILT resources have benefitted from the following recent major funding sources: CNRS-INSU, Observatoire de Paris and Université d’Orléans, France; BMBF, MIWF-NRW, MPG, Germany; Science Foundation Ireland (SFI), Department of Business, Enterprise and Innovation (DBEI), Ireland; NWO, The Netherlands; The Science and Technology Facilities Council, UK; Ministry of Science and Higher Education, Poland. We acknowledge use of the CHIME/FRB Public Database, provided at https://www.chime-frb.ca/ by the CHIME/FRB Collaboration.
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I.P.-M., L.C., J.v.L., Y.M., S.t.V., A.B., L.O., E.P., S.S. and D.V. analysed and interpreted the data. I.P.-M., L.C., J.v.L., Y.M. and S.t.V. contributed to the LOFAR data acquisition, and to the conception, design and creation of LOFAR analysis software. I.P.-M., L.C. and J.v.L. conceived and drafted the work, and Y.M., S.t.V., A.B., L.O., E.P., S.S. and D.V. contributed substantial revisions. L.O., J.A., O.M.B., E.K., D.v.d.S., A.S., R.S., E.A.K.A., B.A., W.J.G.d.B., A.H.W.M.C., S.D., H.D., K.M.H., T.v.d.H., B.H., V.M.I., A.K., G.M.L., D.M.L., A.M., V.A.M., H.M., M.J.N., T.O., E.O., M.R. and S.J.W. contributed to the conception, design and creation of the Apertif hardware, software and firmware used in this work, and to the Apertif data acquisition.
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Extended data figures and tables
Extended Data Fig. 1 The FRB 20180916B fluence distribution at Apertif and at LOFAR.
For each fluence F we plot how many brighter bursts are detected per hour, Rate (>F) (h−1). The light green data points show the cumulative distribution function (CDF) of all Apertif bursts, with dash-dotted, dotted and dashed lines giving the power-law fit respectively to bursts with fluences lower than 3.2 Jy ms, between 3.2 Jy ms and 7.8 Jy ms, and above 7.8 Jy ms. The coloured solid lines correspond to different phase ranges within the active window, with no discernible difference between them other than the rate scaling. The LOFAR fluence distribution is shown in crimson. The fit to a broken power law (‘broken pl’) with a fluence turnover at 104 Jy ms is shown as a grey dotted line. For the same fluence, FRB 20180916B is more active at 150 MHz than 1,370 MHz, even at the peak activity phases observed by Apertif.
Extended Data Fig. 2 Dynamic spectra of Apertif bursts A01–A27.
We display PA (top panel), Stokes parameters I, L and V (central panel) and dynamic spectra (bottom panel), for bursts with full Stokes data (for example, panel A01 at top left). Bursts with only intensity data, such as A02, are limited to the total intensity profile. Burst identifiers are given in the top left corners, and activity cycle number in the top-right corners. Data have been dedispersed to DM = 348.75 pc cm−3, and downsampled 2× in time and 8× in frequency.
Extended Data Fig. 3 Dynamic spectra of Apertif bursts A28–A54.
As in Extended Data Fig. 2.
Extended Data Fig. 4 Observations and detections as a function of phase.
a, b, Shown are histograms of burst detections (‘N. Bursts’; a) and of observation duration (‘Obs. Duration’; b), both as a function of phase for the best period fitted to Apertif and CHIME/FRB data (16.29 days). In both panels, instruments are colour-coded by central frequency, with blue for high frequencies and red for low frequencies. This figure was generated using an adaptation of the frbpa package20.
Extended Data Fig. 5 Comparison of simulated and observed activity window P values.
a–c, Each panel compares the P value obtained through the Kolmogorov–Smirnov statistic on two instrument burst samples. The vertical black lines give the observed P value, whereas the histograms correspond to 105 simulations of the P value that would be obtained if both instrument burst samples were drawn from the same distribution. N is the number of resulting simulations per P value. Shown are comparisons of burst samples from Apertif and LOFAR (a), Apertif and CHIME/FRB (b), and CHIME/FRB and LOFAR (c). In all panels, the vertical grey dotted, dash-dotted and dashed lines show respectively the P value where 68.27% (1σ), 95.45% (2σ) and 99.73% (3σ) of the simulations give a larger P value.
Extended Data Fig. 6 Stacked LOFAR bursts.
After dedispersion to the S/N-maximizing DM of 349.00 pc cm−3, the individual bursts were co-added. a, The pulse profiles in eight different frequency bands of the co-added total, and fits to the scattering tail. The central frequency of the band is indicated on the vertical-axis labels. b, The dynamic spectrum of the stacked bursts.
Extended Data Fig. 7 Apertif burst properties against phase.
a, The structure-optimized DM, with the 348.75 pc cm−3 average as a reference. b, The drift rate of bursts with multiple components. c, d, The fluence (c) and the average polarization position angle (PA) (d) of each burst. In all panels, bursts are colour-coded by activity cycle. Each colour corresponds to a different activity cycle (see key at bottom left), and the data points with a black edge represent bursts with S/N > 20. The error bars represent 1σ (s.d.) errors.
Extended Data Fig. 8 Five of the bursts with a measurable drift rate.
a–e, For each burst, the top panel shows the pulse profile as a solid black line and the fitted multi-component Gaussian in grey (the burst name is given at top left.). Coloured regions indicate the subcomponent position. The main panels show the dynamic spectra, the subcomponent centroids with 1σ (s.d.) errors and the fitted drift rate \(\dot{\nu }\) (white line). The right panels display the spectra and the fitted Gaussian of each subcomponent, with the same colour as the shaded region of the pulse profile.
Extended Data Fig. 9 Finding the best period.
a−l, The periodograms between 0.03 day and 20 day periods of four instrument combinations and three different period searching techniques. Each column corresponds, from left to right, to all detections combined (blue), Apertif detections (green), CHIME/FRB detections (yellow) and CHIME/FRB and Apertif detections combined (red). Each row corresponds to a different search technique, with Pearson’s χ2 test7 at the top, maximum continuous fraction in the centre21, and the normalized QMIEU method54 at the bottom. The vertical grey lines mark the position of the aliased periods, solid lines for fN = (Nfsid + f0) and dotted lines for fN = (Nfsid−f0). The number in the top left corner of each plot indicates the best period using the given burst data set and periodicity search method, with errors giving the full-width at half-maximum.
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Pastor-Marazuela, I., Connor, L., van Leeuwen, J. et al. Chromatic periodic activity down to 120 megahertz in a fast radio burst. Nature (2021). https://ift.tt/3mBZ1If
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