A fast radio burst source at a complex magnetised site in a barred galaxy

Fast radio bursts (FRBs) are highly dispersed radio bursts prevailing in the universe. The recent detection of FRB~200428 from a Galactic magnetar suggested that at least some FRBs originate from magnetars, but it is unclear whether the majority of cosmological FRBs, especially the actively repeating ones, are produced from the magnetar channel. Here we report the detection of 1863 polarised bursts from the repeating source FRB~20201124A during a dedicated radio observational campaign of Five-hundred-meter Aperture Spherical radio Telescope (FAST). The large sample of radio bursts detected in 88 hr over 54 days indicate a significant, irregular, short-time variation of the Faraday rotation measure (RM) of the source during the first 36 days, followed by a constant RM during the later 18 days. Significant circular polarisation up to 75\% was observed in a good fraction of bursts. Evidence suggests that some low-level circular polarisation originates from the conversion from linear polarisation during the propagation of the radio waves, but an intrinsic radiation mechanism is required to produce the higher degree of circular polarisation. All of these features provide evidence for a more complicated, dynamically evolving, magnetised immediate environment around this FRB source. Its host galaxy was previously known. Our optical observations reveal that it is a Milky-Way-sized, metal-rich, barred-spiral galaxy at redshift z=0.09795+-0.00003, with the FRB residing in a low stellar density, interarm region at an intermediate galactocentric distance, an environment not directly expected for a young magnetar formed during an extreme explosion of a massive star.

Analysis of Type II radio burst relationship with CME driven shocks

Type II radio bursts are known to be the signatures of coronal shocks. In this paper we examine the relationship between 129 type II bursts in the frequency range 35 – 450 MHz observed at Culgooora observatory during May 2002 – October 2015 and the associated CMEs. We apply Newkirk (1961) density model to determine the formation height of type IIs. We find that in 109/129 cases, type II bursts were preceded/ succeeded by CMEs. The CME associated type II events in which the CME height is above the type II burst source are categorized as group I events (91/129 cases). 91% of the bursts in this group are also associated with flares and 58% of these bursts originate during decaying phase of the flare. The correlation between CME speed and type II shock speed for limb events in this group is 0.33.The CME associated type IIs in which the CME height is below the type II source are categorized as group II (18/129 cases). CME driven shock could have been the exciter of these type II bursts.88% of this group events are associated with flares and 62% of these bursts originate during the rising phase of the flare. The correlation between CME speed and type II shock speed for limb events in this group is 0.96. In 20/129 cases of our data set, type II bursts are not associated with CME and are categorized as group III. 90% of the bursts in this group are associated with flares. 77% of the bursts in the group are originating in the decaying phase of flares. Poor temporal association (9/69 cases) between type IIs and flares of X class during this period. Our results suggest that inspite of temporal association with metric type II bursts, majority of the CME driven shocks (84%) are not successful in exciting type II bursts in 35-450 MHz domain. The type II bursts temporally correlated with CMEs and likely to have been excited by CMEs (type II height > CME height) are originating during the rising phase of the flares in majority of the events. In case of type II bursts temporally correlated with CMEs supposedly not excited by the CMEs (type II height < CME height) ,majority of them are originating in the decaying phase of flares.

LOFAR observations of radio burst source sizes and scattering in the solar corona

Low frequency radio wave scattering and refraction can have a dramatic effect on the observed size and position of radio sources in the solar corona. The scattering and refraction is thought to be due to fluctuations in electron density caused by turbulence. Hence, determining the true radio source size can provide information on the turbulence in coronal plasma. However, the lack of high spatial resolution radio interferometric observations at low frequencies, such as with the LOw Frequency ARray (LOFAR), has made it difficult to determine the true radio source size and level of radio wave scattering. Here we directly fit the visibilities of a LOFAR observation of a Type IIIb radio burst with an elliptical Gaussian to determine its source size and position. This circumvents the need to image the source and then de-convolve LOFAR’s point spread function, which can introduce spurious effects to the source size and shape. For a burst at 34.76 MHz, we find full width at half maximum (FWHM) heights along the major and minor axes to be 18.8′ ± 0.1′ and 10.2′ ± 0.1′, respectively, at a plane of sky heliocentric distance of 1.75 R⊙. Our results suggest that the level of density fluctuations in the solar corona is the main cause of the scattering of radio waves, resulting in large source sizes. However, the magnitude of ε may be smaller than what has been previously derived in observations of radio wave scattering in tied-array images.

Extremely band-limited repetition from a fast radio burst source

ABSTRACT The fast radio burst (FRB) population is observationally divided into sources that have been observed to repeat and those that have not. There is tentative evidence that the bursts from repeating sources have different properties than the non-repeating ones. In order to determine the occurrence rate of repeating sources and characterize the nature of repeat emission, we have been conducting sensitive searches for repetitions from bursts detected with the Australian Square Kilometre Array Pathfinder (ASKAP) with the 64-m Parkes radio telescope, using the recently commissioned Ultra-wideband Low (UWL) receiver system, over a band spanning 0.7–4.0 GHz. We report the detection of a repeat burst from the source of FRB 20190711A. The detected burst is 1 ms wide and has a bandwidth of just 65 MHz. We find no evidence of any emission in the remaining part of the 3.3 GHz UWL band. While the emission bandwidths of the ASKAP and UWL bursts show ν−4 scaling consistent with a propagation effect, the spectral occupancy is inconsistent with diffractive scintillation. This detection rules out models predicting broad-band emission from the FRB 20190711A source and puts stringent constraints on the emission mechanism. The low spectral occupancy highlights the importance of sub-banded search methods in detecting FRBs.

A Bimodal Burst Energy Distribution of a Repeating Fast Radio Burst Source

Fast radio bursts (FRBs) are cosmic sources that emit millisecond-duration radio pulses with a wide range of luminosities and yet unknown origin(s) (Petroff et al. 2019; cordes et al. 2019). A subset of FRBs were found to repeat, the prototype of which is the first precisely-located FRB 121102 (Spitler et al. 2016), residing in a dwarf galaxy at redshift z=0.193 (Chatterjee 2017; Tendulkar et al. 2017). The source has been observed by most major telescopes and shows non-Poisson clustering of bursts over time, the hitherto highest burst rate, and a burst isotropic equivalent energy largely consistent with a power-law (Law et al. 2017; zhang et al. 2018; Gourdji et al. 2019), all of which are crucial characteristics to be compared to non-repeating sources. However, due to sensitivity limits, no true energy distribution of any FRB is known. Here we report the detection of 1652 independent bursts, more than quadruple the total of all previously published ones combined, in a total of 59.5 observing hours spanning 47 days using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The peak burst rate of 122 hr-1is by far the highest ever observed of any FRB. A characteristic peak in the isotropic equivalent energy distribution is found to be ~4.8×1037 erg at 1.25 GHz, suggesting a possible threshold for producing abundant coherent radio bursts from FRBs. The burst energy distribution is optimally described by a bimodal distribution consisting of a log-normal function plus a Cauchy function. While no periodicity was found between 1 ms and 1000 s, and the majority of the burst arrival times are consistent with being random, there exists a visible peak in the waiting time distribution at about 3.4 ms, corresponding to significant clustering. Our results start to reveal the stochastic nature of abundant weaker bursts, which could be present in other FRB sources, apparently repeating or not. FRB generation mechanisms must be efficient and economical. Expensive triggers and/or contrived conditions for burst production seem unlikely.