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Register a new userHighly polarised microstructure from the repeating FRB 20180916B
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Fast radio bursts (FRBs) are bright, coherent, short-duration radio transients of as-yet unknown extragalactic origin. FRBs exhibit a wide variety of spectral, temporal and polarimetric properties, which can unveil clues into their emission physics and propagation effects in the local medium. Here we present the high-time-resolution (down to 1 $mu$s) polarimetric properties of four 1.7-GHz bursts from the repeating FRB 20180916B, which were detected in voltage data during observations with the European VLBI Network (EVN). We observe a range of emission timescales spanning three orders of magnitude, with the shortest component width reaching 3-4 $mu$s (below which we are limited by scattering). This is the shortest timescale measured in any FRB, to date. We demonstrate that all four bursts are highly linearly polarised ($gtrsim 80%$), show no evidence for significant circular polarisation ($lesssim 15%$), and exhibit a constant polarisation position angle (PPA) during and between bursts. On short timescales ($lesssim 100$ $mu$s), however, there appear to be subtle (few degree) PPA variations across the burst profiles. These observational results are most naturally explained in an FRB model where the emission is magnetospheric in origin, as opposed to models where the emission originates at larger distances in a relativistic shock.
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Fast Radio Burst FRB 20180916B in its host galaxy SDSS J015800.28+654253.0 at 149 Mpc is by far the closest-known FRB with a robust host galaxy association. The source also exhibits a 16.35-day period in its bursting. Here we present optical and infrared imaging as well as integral field spectroscopy observations of FRB 20180916B with the WFC3 camera on the Hubble Space Telescope and the MEGARA spectrograph on the 10.4-m Gran Telescopio Canarias. The 60-90 milliarcsecond (mas) resolution of the Hubble imaging, along with the previous 2.3-mas localization of FRB 20180916B, allow us to probe its environment with a 30-60 pc resolution. We constrain any point-like star-formation or HII region at the location of FRB 20180916B to have an H$alpha$ luminosity $L_mathrm{Halpha} lesssim 10^{37},mathrm{erg,s^{-1}}$ and, correspondingly, constrain the local star-formation rate to be $lesssim10^{-4},mathrm{M_odot,yr^{-1}}$. The constraint on H$alpha$ suggests that possible stellar companions to FRB 20180916B should be of a cooler, less massive spectral type than O6V. FRB 20180916B is 250 pc away (in projected distance) from the brightest pixel of the nearest young stellar clump, which is $sim380$,pc in size (full-width at half maximum). With the typical projected velocities of pulsars, magnetars, or neutron stars in binaries (60-750 km s$^{-1}$), FRB 20180916B would need 800 kyr to 7 Myr to traverse the observed distance from its presumed birth site. This timescale is inconsistent with the active ages of magnetars ($lesssim10$ kyr). Rather, the inferred age and observed separation are compatible with the ages of high-mass X-ray binaries and gamma-ray binaries, and their separations from the nearest OB associations.
The observed Fast Radio Burst (FRB) population can be divided into one-off and repeating FRB sources. Either this division is a true dichotomy of the underlying sources, or selection effects and low activity prohibit us from observing repeat pulses from all constituents making up the FRB source population. We attempt to break this degeneracy through FRB population synthesis. With that aim we extend frbpoppy, which earlier only handled one-off FRBs, to also simulate repeaters. We next model the Canadian Hydrogen Intensity Mapping Experiment FRB survey (CHIME/FRB). Using this implementation, we investigate the impact of luminosity functions on the observed dispersion measure (DM) and distance distributions of both repeating and one-off FRBs. We show that for a single, intrinsically repeating source population with a steep luminosity function, selection effects should shape the DM distributions of one-off and repeating FRB sources differently. This difference is not yet observed. We next show how the repeater fraction over time can help in determining the repetition rate of an intrinsic source population. We simulate this fraction for CHIME/FRB, and show that a source population comprised solely of repeating FRBs can describe CHIME/FRB observations with the use of a flat luminosity function. From the outcome of these two methods we thus conclude that all FRBs originate from a single and mostly uniform population of varying repeaters. Within this population, the luminosity function cannot be steep, and there must be minor differences in physical or behaviour parameters that correlate with repeat rate.
A repeating fast radio burst (FRB), FRB 20180916B (hereafter FRB 180916), was reported to have a 16.35-day period. This period might be related to a precession period. In this paper, we investigate two precession models to explain the periodic activity of FRB 180916. In both models, the radio emission of FRB 180916 is produced by a precessing jet. For the first disk-driven jet precession model, an extremely low viscous parameter (i.e., the dimensionless viscosity parameter $alpha lesssim 10^{-8}$) is required to explain the precession of FRB 180916, which implies its implausibility. For the second tidal force-driven jet precession model, we consider a compact binary consists of a neutron star/black hole and a white dwarf; the white dwarf fills its Roche lobe and mass transfer occurs. Due to the misalignment between the disk and orbital plane, the tidal force of the white dwarf can drive jet precession. We show that the relevant precession periods are several days to hundreds of days, depending on the specific accretion rates and component masses. The duration of FRB 180916 generation in the binary with extremely high accretion rate will be several thousand years.
We report on the discovery and analysis of bursts from nine new repeating fast radio burst (FRB) sources found using the Canadian Hydrogen Intensity Mapping Experiment (CHIME) telescope. These sources span a dispersion measure (DM) range of 195 to 1380 pc cm$^{-3}$. We detect two bursts from three of the new sources, three bursts from four of the new sources, four bursts from one new source, and five bursts from one new source. We determine sky coordinates of all sources with uncertainties of $sim$10$^prime$. We detect Faraday rotation measures for two sources, with values $-20(1)$ and $-499.8(7)$ rad m$^{-2}$, that are substantially lower than the RM derived from bursts emitted by FRB 121102. We find that the DM distribution of our events, combined with the nine other repeaters discovered by CHIME/FRB, is indistinguishable from that of thus far non-repeating CHIME/FRB events. However, as previously reported, the burst widths appear statistically significantly larger than the thus far non-repeating CHIME/FRB events, further supporting the notion of inherently different emission mechanisms and/or local environments. These results are consistent with previous work, though are now derived from 18 repeating sources discovered by CHIME/FRB during its first year of operation. We identify candidate galaxies that may contain FRB 190303.J1353+48 (DM = 222.4 pc cm$^{-3}$).
The discovery that at least some Fast Radio Bursts (FRBs) repeat has ruled out cataclysmic events as the progenitors of these particular bursts. FRB~121102 is the most well-studied repeating FRB but despite extensive monitoring of the source, no underlying pattern in the repetition has previously been identified. Here, we present the results from a radio monitoring campaign of FRB~121102 using the 76-m Lovell telescope. Using the pulses detected in the Lovell data along with pulses from the literature, we report a detection of periodic behaviour of the source over the span of five years of data. We predict that the source is currently `off and that it should turn `on for the approximate MJD range $59002-59089$ (2020-06-02 to 2020-08-28). This result, along with the recent detection of periodicity from another repeating FRB, highlights the need for long-term monitoring of repeating FRBs at a high cadence. Using simulations, we show that one needs at least 100 hours of telescope time to follow-up repeating FRBs at a cadence of 0.5--3 days to detect periodicities in the range of 10--150 days. If the period is real, it shows that repeating FRBs can have a large range in their activity periods that might be difficult to reconcile with neutron star precession models.
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