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Compact radio sources exhibit scintillation, an interference pattern arising from propagation through inhomogeneous plasma, where scintillation patterns encode the relative distances and velocities of the source, scattering material, and Earth. In previous work, we showed that the scintillation velocity of the repeating fast radio burst FRB20201124A could be measured by correlating burst spectra pairs, with low values of the scintillation velocity and scattering timescale suggesting scattering nearby the Earth at $\sim0.4\,$kpc. In this work, we have measured the scintillation velocity at 10 epochs spanning a year, observing an annual variation which strongly implies the screen is within the Milky Way. Modelling the annual variation with a 1D anisotropic or 2D isotropic screen results in a screen distance $d_{l} = 0.40\pm0.04\,$kpc or $d_{l} = 0.46\pm0.06\,$kpc from Earth respectively, possibly associated with material outside of the Local Bubble or the edge of the Orion-Eridanus Superbubble. Additional measurements particularly at times of low effective velocity will help probe changes in screen properties, and distinguish between screen models. Where scintillation of an FRB originates in its host galaxy or local environment, these techniques could be used to detect orbital motion, and probe the FRB's local ionised environment., Comment: 5 pages, 5 Figures, accepted in MNRAS Letters. Replacement, fixed typo in abstract: 0.4kpc instead of 0.4pc

We report the observations of the highly active FRb20201124A with the upgraded Giant Metrewave Radio Telescope at 550-750~MHz. These observations in the incoherent array mode simultaneously provided an arcsecond localization of bursts from \rss, the discovery of persistent radio emission associated with the host galaxy, and the detection of 48 bursts. Using the brightest burst in the sample ($F= 108~{\rm Jy~ms}$) we find a structure-maximizing dispersion measure of $410.8 \pm 0.5~{\rm pc~cm}^{-3}$. We find that our observations are complete down to a fluence level of $10~{\rm Jy~ms}$, above which the cumulative burst rate scales as a power-law $R(>\!F) = 10~{\rm hr}^{-1} \left(F/10\mathrm{~Jy~ms}\right)^{\gamma}$ with $\gamma = -1.2 \pm 0.2$. We find that the bursts are on average wider than those reported for other repeating FRBs. We find that the waiting time between bursts is well approximated by an exponential distribution with a mean of $\sim 2.9$ min during our observations. We searched for periodicities using both a standard Fourier domain method and the Fast Folding Algorithm, but found no significant candidates. We measure bulk spectro-temporal drift rates between $-0.75$ and $-20~{\rm MHz~ms}^{-1}$. Finally, we use the brightest burst to set an upper limit to the scattering time of 11.1~ms at 550~MHz. The localization of FRB20201124A adds strength to the proof-of-concept method described in our earlier work and serves as a potential model for future localizations and follow-up of repeating FRBs with the uGMRT., Comment: 11 pages, 8 figures, accepted for publication in MNRAS

Scintillation of compact radio sources results from the interference between images caused by multipath propagation, and probes the intervening scattering plasma and the velocities of the emitting source and scattering screen. In FRB20201124A, a repeating fast radio burst (FRB) which entered a period of extreme activity, we obtained many burst detections in observations at the upgraded Giant Metrewave Radio Telescope (uGMRT) and the Effelsberg 100\,m Radio Telescope. Bursts nearby in time show similar scintillation patterns, and we measure a scintillation timescale of $14.3\pm1.2$\,min and $7\pm2$\,min at Effelsberg (1370\,MHz) and uGMRT (650\,MHz), respectively, by correlating burst pair spectra. The scintillation bandwidth scaled to 1\,GHz is $0.5\pm0.1$\,MHz, and the inferred scintillation velocity at Effelsberg is $V_{\mathrm{ISS}}\approx (59\pm7) \sqrt{d_{l}/2\,\rm{kpc}}~{\rm km~s}^{-1}$, higher than Earth's velocity for any screen beyond a lens distance of $d_{l} \gtrsim 400\,$pc. From the measured scintillation bandwidth, FRB20201124A has comparatively lower scattering than nearby pulsars, and is underscattered by a factor of $\sim 30$ or $\sim 1200$ compared to the NE2001 and YMW16 model predictions respectively. This underscattering, together with the measured scintillation velocity are consistent with a scattering screen more nearby the Earth at $d_{l} \sim 400\,$pc, rather than at the 2\,kpc spiral arm which NE2001 predicts to be the dominant source of scattering. With future measurements, the distance, geometry, and velocity of the scattering screen could be obtained through modelling of the annual variation in $V_{\rm ISS}$, or through inter-station time delays or interferometric observations. Scintillation/scattering measurements of FRBs could help improve Galactic electron density models, particularly in the Galactic halo or at high Galactic latitudes., Comment: 9 pages, 9 figures, Accepted for publication in MNRAS

We investigate whether the sky rate of Fast Radio Bursts depends on Galactic latitude using the first catalog of Fast Radio Bursts (FRBs) detected by the Canadian Hydrogen Intensity Mapping Experiment Fast Radio Burst (CHIME/FRB) Project. We first select CHIME/FRB events above a specified sensitivity threshold in consideration of the radiometer equation, and then compare these detections with the expected cumulative time-weighted exposure using Anderson-Darling and Kolmogrov-Smirnov tests. These tests are consistent with the null hypothesis that FRBs are distributed without Galactic latitude dependence ($p$-values distributed from 0.05 to 0.99, depending on completeness threshold). Additionally, we compare rates in intermediate latitudes ($|b| < 15^\circ$) with high latitudes using a Bayesian framework, treating the question as a biased coin-flipping experiment -- again for a range of completeness thresholds. In these tests the isotropic model is significantly favored (Bayes factors ranging from 3.3 to 14.2). Our results are consistent with FRBs originating from an isotropic population of extragalactic sources., Comment: Accepted for publication in ApJ

Fast radio bursts (FRBs) are exceptionally luminous flashes of unknown physical origin, reaching us from other galaxies (Petroff et al. 2019). Most FRBs have only ever been seen once, while others flash repeatedly, though sporadically (Spitler et al. 2016, CHIME/FRB Collaboration et al. 2021). Many models invoke magnetically powered neutron stars (magnetars) as the engines producing FRB emission (Margalit & Metzger 2018, CHIME/FRB Collaboration et al. 2020). Recently, CHIME/FRB announced the discovery (Bhardwaj et al. 2021) of the repeating FRB 20200120E, coming from the direction of the nearby grand design spiral galaxy M81. Four potential counterparts at other observing wavelengths were identified (Bhardwaj et al. 2021) but no definitive association with these sources, or M81, could be made. Here we report an extremely precise localisation of FRB 20200120E, which allows us to associate it with a globular cluster (GC) in the M81 galactic system and to place it ~2pc offset from the optical center of light of the GC. This confirms (Bhardwaj et al. 2021) that FRB 20200120E is 40 times closer than any other known extragalactic FRB. Because such GCs host old stellar populations, this association strongly challenges FRB models that invoke young magnetars formed in a core-collapse supernova as powering FRB emission. We propose, instead, that FRB 20200120E is a highly magnetised neutron star formed via either accretion-induced collapse of a white dwarf or via merger of compact stars in a binary system (Margalit et al. 2019). Alternative scenarios involving compact binary systems, efficiently formed inside globular clusters, could also be responsible for the observed bursts., Comment: Submitted. Comments welcome

We present the design, implementation and performance of a digital backend constructed for the Canadian Hydrogen Intensity Mapping Experiment (CHIME) that uses accelerated computing to observe radio pulsars and transient radio sources. When operating, the CHIME correlator outputs 10 independent streams of beamformed data for the CHIME/Pulsar backend that digitally track specified celestial positions. Each of these independent streams are processed by the CHIME/Pulsar backend system which can coherently dedisperse, in real-time, up to dispersion measure values of 2500 pc/cm$^{-3}$ . The tracking beams and real-time analysis system are autonomously controlled by a priority-based algorithm that schedules both known sources and positions of interest for observation with observing cadences as small as one day. Given the distribution of known pulsars and radio-transient sources, the CHIME/Pulsar system can monitor up to 900 positions once per sidereal day and observe all sources with declinations greater than $-20^\circ$ once every $\sim$2 weeks. We also discuss the science program enabled through the current modes of data acquisition for CHIME/Pulsar that centers on timing and searching experiments., Comment: 18 pages, 9 figures, 1 table. Submitted to ApJS

Magnetars are highly magnetized young neutron stars that occasionally produce enormous bursts and flares of X-rays and gamma-rays. Of the approximately thirty magnetars currently known in our Galaxy and Magellanic Clouds, five have exhibited transient radio pulsations. Fast radio bursts (FRBs) are millisecond-duration bursts of radio waves arriving from cosmological distances. Some have been seen to repeat. A leading model for repeating FRBs is that they are extragalactic magnetars, powered by their intense magnetic fields. However, a challenge to this model has been that FRBs must have radio luminosities many orders of magnitude larger than those seen from known Galactic magnetars. Here we report the detection of an extremely intense radio burst from the Galactic magnetar SGR 1935+2154 using the Canadian Hydrogen Intensity Mapping Experiment (CHIME) FRB project. The fluence of this two-component bright radio burst and the estimated distance to SGR 1935+2154 together imply a 400-800 MHz burst energy of $\sim 3 \times 10^{34}$ erg, which is three orders of magnitude brighter than those of any radio-emitting magnetar detected thus far. Such a burst coming from a nearby galaxy would be indistinguishable from a typical FRB. This event thus bridges a large fraction of the radio energy gap between the population of Galactic magnetars and FRBs, strongly supporting the notion that magnetars are the origin of at least some FRBs., Comment: Submitted to Nature. This version: Geocentric arrival time corrected

We report on the detection of seven bursts from the periodically active, repeating fast radio burst (FRB) source FRB 180916.J0158+65 in the 300-400-MHz frequency range with the Green Bank Telescope (GBT). Emission in multiple bursts is visible down to the bottom of the GBT band, suggesting that the cutoff frequency (if it exists) for FRB emission is lower than 300 MHz. Observations were conducted during predicted periods of activity of the source, and had simultaneous coverage with the Low Frequency Array (LOFAR) and the FRB backend on the Canadian Hydrogen Intensity Mapping Experiment (CHIME) telescope. We find that one of the GBT-detected bursts has potentially associated emission in the CHIME band (400-800 MHz) but we detect no bursts in the LOFAR band (110-190 MHz), placing a limit of $\alpha > -1.0$ on the spectral index of broadband emission from the source. We also find that emission from the source is severely band-limited with burst bandwidths as low as $\sim$40 MHz. In addition, we place the strictest constraint on observable scattering of the source, $<$ 1.7 ms, at 350 MHz, suggesting that the circumburst environment does not have strong scattering properties. Additionally, knowing that the circumburst environment is optically thin to free-free absorption at 300 MHz, we find evidence against the association of a hyper-compact HII region or a young supernova remnant (age $<$ 50 yr) with the source., Comment: Accepted for publication in ApJL

Fast radio bursts (FRBs) are bright, millisecond-duration radio transients originating from extragalactic distances. Their origin is unknown. Some FRB sources emit repeat bursts, ruling out cataclysmic origins for those events. Despite searches for periodicity in repeat burst arrival times on time scales from milliseconds to many days, these bursts have hitherto been observed to appear sporadically, and though clustered, without a regular pattern. Here we report the detection of a $16.35\pm0.15$ day periodicity (or possibly a higher-frequency alias of that periodicity) from a repeating FRB 180916.J0158+65 detected by the Canadian Hydrogen Intensity Mapping Experiment Fast Radio Burst Project (CHIME/FRB). In 38 bursts recorded from September 16th, 2018 through February 4th, 2020, we find that all bursts arrive in a 5-day phase window, and 50% of the bursts arrive in a 0.6-day phase window. Our results suggest a mechanism for periodic modulation either of the burst emission itself, or through external amplification or absorption, and disfavour models invoking purely sporadic processes.

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$^{-3}$)., Comment: Accepted for publication in ApJL

Fast radio bursts (FRBs) are brief, bright, extragalactic radio flashes. Their physical origin remains unknown, but dozens of possible models have been postulated. Some FRB sources exhibit repeat bursts. Though over a hundred FRB sources have been discovered to date, only four have been localised and associated with a host galaxy, with just one of the four known to repeat. The properties of the host galaxies, and the local environments of FRBs, provide important clues about their physical origins. However, the first known repeating FRB has been localised to a low-metallicity, irregular dwarf galaxy, and the apparently non-repeating sources to higher-metallicity, massive elliptical or star-forming galaxies, suggesting that perhaps the repeating and apparently non-repeating sources could have distinct physical origins. Here we report the precise localisation of a second repeating FRB source, FRB 180916.J0158+65, to a star-forming region in a nearby (redshift $z = 0.0337 \pm 0.0002$) massive spiral galaxy, whose properties and proximity distinguish it from all known hosts. The lack of both a comparably luminous persistent radio counterpart and a high Faraday rotation measure further distinguish the local environment of FRB 180916.J0158+65 from that of the one previously localised repeating FRB source, FRB 121102. This demonstrates that repeating FRBs have a wide range of luminosities, and originate from diverse host galaxies and local environments., Comment: 61 pages, 11 figures, 3 tables. Published in Nature

Manipulating and coupling molecule gears is the first step towards realizing molecular-scale mechanical machines. Here, we theoretically investigate the behavior of such gears using molecular dynamics simulations. Within a nearly rigid-body approximation we reduce the dynamics of the gears to the rotational motion around the orientation vector. This allows us to study their behavior based on a few collective variables. Specifically, for a single hexa (4-tert-butylphenyl) benzene molecule we show that the rotational-angle dynamics corresponds to the one of a Brownian rotor. For two such coupled gears, we extract the effective interaction potential and find that it is strongly dependent on the center of mass distance. Finally, we study the collective motion of a train of gears. We demonstrate the existence of three different regimes depending on the magnitude of the driving-torque of the first gear: underdriving, driving and overdriving, which correspond, respectively, to no collective rotation, collective rotation and only single gear rotation. This behavior can be understood in terms of a simplified interaction potential.

We report on the discovery of eight 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 103.5 to 1281 pc cm$^{-3}$. They display varying degrees of activity: six sources were detected twice, another three times, and one ten times. These eight repeating FRBs likely represent the bright and/or high-rate end of a distribution of infrequently repeating sources. For all sources, we determine sky coordinates with uncertainties of $\sim$10$^\prime$. FRB 180916.J0158+65 has a burst-averaged DM = $349.2 \pm 0.3$ pc cm$^{-3}$ and a low DM excess over the modelled Galactic maximum (as low as $\sim$20 pc cm$^{-3}$); this source also has a Faraday rotation measure (RM) of $-114.6 \pm 0.6$ rad m$^{-2}$, much lower than the RM measured for FRB 121102. FRB 181030.J1054+73 has the lowest DM for a repeater, $103.5 \pm 0.3$ pc cm$^{-3}$, with a DM excess of $\sim$ 70 pc cm$^{-3}$. Both sources are interesting targets for multi-wavelength follow-up due to their apparent proximity. The DM distribution of our repeater sample is statistically indistinguishable from that of the first 12 CHIME/FRB sources that have not repeated. We find, with 4$\sigma$ significance, that repeater bursts are generally wider than those of CHIME/FRB bursts that have not repeated, suggesting different emission mechanisms. Our repeater events show complex morphologies that are reminiscent of the first two discovered repeating FRBs. The repetitive behavior of these sources will enable interferometric localizations and subsequent host galaxy identifications., Comment: 40 pages, 11 figures; accepted by ApJL on 28 September 2019; added analysis of correlation between width and max. flux density

We report the detection of a single burst from the first-discovered repeating Fast Radio Burst source, FRB 121102, with CHIME/FRB, which operates in the frequency band 400-800 MHz. The detected burst occurred on 2018 November 19 and its emission extends down to at least 600 MHz, the lowest frequency detection of this source yet. The burst, detected with a significance of 23.7$\sigma$, has fluence 12$\pm$3 Jy ms and shows complex time and frequency morphology. The 34 ms width of the burst is the largest seen for this object at any frequency. We find evidence of sub-burst structure that drifts downward in frequency at a rate of -3.9$\pm$0.2 MHz ms$^{-1}$. Our best fit tentatively suggests a dispersion measure of 563.6$\pm$0.5 pc cm$^{-3}$, which is ${\approx}$1% higher than previously measured values. We set an upper limit on the scattering time at 500 MHz of 9.6 ms, which is consistent with expectations from the extrapolation from higher frequency data. We have exposure to the position of FRB 121102 for a total of 11.3 hrs within the FWHM of the synthesized beams at 600 MHz from 2018 July 25 to 2019 February 25. We estimate on the basis of this single event an average burst rate for FRB 121102 of 0.1-10 per day in the 400-800 MHz band for a median fluence threshold of 7 Jy ms in the stated time interval., Comment: Accepted in Astrophysical Journal Letters

The discovery of a repeating Fast Radio Burst (FRB) source, FRB 121102, eliminated models involving cataclysmic events for this source. No other repeating FRB has yet been detected in spite of many recent FRB discoveries and follow-ups, suggesting repeaters may be rare in the FRB population. Here we report the detection of six repeat bursts from FRB 180814.J0422+73, one of the 13 FRBs detected by the Canadian Hydrogen Intensity Mapping Experiment (CHIME) FRB project during its pre-commissioning phase in July and August 2018. These repeat bursts are consistent with originating from a single position on the sky, with the same dispersion measure (DM), ~189 pc cm-3. This DM is approximately twice the expected Milky Way column density, and implies an upper limit on the source redshift of 0.1, at least a factor of ~2 closer than FRB 121102. In some of the repeat bursts, we observe sub-pulse frequency structure, drifting, and spectral variation reminiscent of that seen in FRB 121102, suggesting similar emission mechanisms and/or propagation effects. This second repeater, found among the first few CHIME/FRB discoveries, suggests that there exists -- and that CHIME/FRB and other wide-field, sensitive radio telescopes will find -- a substantial population of repeating FRBs., Comment: accepted by Nature

Magneto-transport measurements are performed on the two-dimensional electron system (2DES) in an AlGaAs/GaAs heterostructure. By increasing the magnetic field perpendicular to the 2DES, magnetoresistivity oscillations due to Landau quantisation can be identified just near the direct insulator-quantum Hall (I-QH) transition. However, different mobilities are obtained from the oscillations and transition point. Our study shows that the direct I-QH transition does not always correspond to the onset of strong localisation., Comment: 11 pages, 7 figures

Magnetic-field-induced phase transitions were studied with a two-dimensional electron AlGaAs/GaAs system. The temperature-driven flow diagram shows the features of the $\Gamma$(2) modular symmetry, which includes distorted flowlines and shiftted critical point. The deviation of the critical conductivities is attributed to a small but resolved spin splitting, which reduces the symmetry in Landau quantization. [B. P. Dolan, Phys. Rev. B 62, 10278.] Universal scaling is found under the reduction of the modular symmetry. It is also shown that the Hall conductivity could still be governed by the scaling law when the semicircle law and the scaling on the longitudinal conductivity are invalid. *corresponding author:yhchang@phys.ntu.edu.tw, Comment: The revised manuscript has been published in J. Phys.: Condens. Matter