arXiv subjects

D. Z. Li

Publications and source records attributed to D. Z. Li.

At least 19 recordsLinked to original sources

arXiv2026

A sudden dramatic change and recovery of magneto-environment of a repeating fast radio burst

Fast radio bursts (FRBs) are millisecond-duration radio bursts with unidentified extra-galactic origin. Some FRBs exhibit mild magneto-ionic environmental variations, possibly attributed to plasma turbulence or binary configuration. We report an abrupt magneto-ionic variation of FRB 20220529, a repeating FRB from a disk galaxy at redshift $0.1839 \pm 0.0001$. Initially, the Faraday rotation measure (RM) had a median of $17~{\rm rad~m^{-2}}$ and a scatter of $101~{\rm rad~m^{-2}}$ over 17 months. In December 2023, it jumped to $1977 \pm 84~{\rm rad~m^{-2}}$, and returned to typical values within two weeks. This drastic RM variation suggests that a dense magnetized clump enters and exits the line of sight in week timescales. One plausible scenario invokes a coronal mass ejection from a companion star, while other scenarios invoking extreme turbulence or binary orbital motion are also possible.

astro-ph.HE↗

arXiv2025

Constraining the origin of the long term periodicity of FRB 20180916B with Polarization Position Angle

FRB 20180916B is a repeating Fast Radio Burst (FRB) which produces bursts in a 5.1 day active window which repeats with a 16.34 day period. Models have been proposed to explain the periodicity using dynamical phenomena such as rotation, precession or orbital motion. Polarization Position Angle (PA) of the bursts can be used to distinguish and constraint the origin of the long term periodicity of the FRB. We aim to study the PA variability on short (within an observation) and long timescales (from observation to observation). We aim to compare the observed PA variability with the predictions of various dynamical progenitor models for the FRB. We use the calibrated burst dataset detected by uGMRT in Band 4 (650 MHz) which have been published in arXiv:2409.12584 . We transform the PA measured at 650 MHz to infinite frequency such that PAs measured in different observations are consistent, and finally measure the changes within and across active windows. We find that PA of the bursts vary according to the periodicity of the source. We constrain the PA variability to be within seven degrees on timescales less than four hours for all MJDs. In addition, we also tentatively note the PA measured at the same phase in the active window varies from one cycle to another. Using the findings, we constrain rotational, precession and binary progenitor models. Rotational model partially agrees with observed PA variability but requires further study to fully constrain. We robustly rule out all flavors of precessional models where either precession explains the periodicity of the FRB or the variability from one cycle to another. Lastly, we draw similarities between FRB 20180916B and a X-ray binary system, Her X 1, and explicitly note that both the sources exhibit a similar form of PA variability.

astro-ph.HE↗

arXiv2025

Ninety percent circular polarization detected in a repeating fast radio burst

Fast radio bursts (FRBs) are extra-galactic sources with unknown physical mechanisms. They emit millisecond-duration radio pulses with isotropic equivalent energy of $10^{36}\sim10^{41}$ ergs. This corresponds to a brightness temperature of FRB emission typically reaching the level of $10^{36}$ K, but can be as high as above $10^{40}$ K for sub-microsecond timescale structures, suggesting the presence of underlying coherent relativistic radiation mechanisms. Polarization carries the key information to understand the physical origin of FRBs, with linear polarization usually tracing the geometric configuration of magnetic fields and circular polarization probing both intrinsic radiation mechanisms and propagation effects. Here we show that the repeating sources FRB 20201124A emits $90.9\pm 1.1\%$ circularly polarized radio pulses. Such a high degree of circular polarization was unexpected in theory and unprecedented in observation in the case of FRBs, since such a high degree of circular polarization was only common among Solar or Jovian radio activities, attributed to the sub-relativistic electrons. We note that there is no obvious correlation between the degree of circular polarization and burst fluence. Besides the high degree of circular polarization, we also detected rapid swing and orthogonal jump in the position angle of linear polarization. The detection of the high degree circular polarization in FRB 20201124A, together with its linear polarization properties that show orthogonal modes, place strong constraints on FRB physical mechanisms, calling for an interplay between magnetospheric radiation and propagation effects in shaping the observed FRB radiation.

astro-ph.HE↗

arXiv2024

Rotation Measure study of FRB 20180916B with the uGMRT

Context. Fast Radio Burst 20180916B is a repeating FRB whose activity window has a 16.34 day periodicity that also shifts and varies in duration with the observing frequency. Recently, arxiv:2205.09221 reported the FRB has started to show secular Rotation Measure (RM) increasing trend after only showing stochastic variability around a constant value of $-114.6$ rad m$^{-2}$ since its discovery. Aims. We aim to further study the RM variability of FRB 20180916B. The data comes from the ongoing campaigns of FRB 20180916B using the upgraded Giant Metrewave Radio Telescope (uGMRT). The majority of the observations are in Band 4, which is centered at 650 MHz with 200 MHz bandwidth. Methods. We apply a standard single pulse search pipeline to search for bursts. In total, we detect 116 bursts with $\sim$36 hours of on-source time spanning 1200 days, with two bursts detected during simultaneous frequency coverage observations. We develop and apply a polarization calibration strategy suited for our dataset. On the calibrated bursts, we use QU-fitting to measure RM. Lastly, we also measure various other properties such as rate, linear polarization fraction and fluence distribution. Results. Of the 116 detected bursts, we could calibrate 79 of them. From which, we observed in our early observations the RM continued to follow linear trend as modeled by arxiv:2205.09221. However, our later observations suggest the source switch from the linear trend to stochastic variations around a constant value of $-58.75$ rad m$^{-2}$. We also study cumulative rate against fluence and note that rate at higher fluences (> 1.2 Jy ms) scales as $γ= -1.09(7)$ whereas that at lower fluences (between 0.2 and 1.2 Jy ms) only scales as $γ= -0.51(1)$, meaning rate at higher fluence regime is steeper than at lower fluence regime.

astro-ph.HE↗

arXiv2024

Constraints on the Intergalactic and Local Dispersion Measure of Fast Radio Bursts with the CHIME/FRB far side-lobe events

We study the 10 fast radio bursts (FRBs) detected in the far side-lobe region of the CHIME telescope from 2018 August 28 to 2021 August 31. We find that the far side-lobe events have on average $\sim$500 times greater fluxes than events detected in CHIME's main lobe. We show that the side-lobe sample is therefore statistically $\sim$20 times closer than the main-lobe sample. The median dispersion measure (DM) excess, after removing the Galactic disk component using the NE2001 for the free electron density distribution of the Milky Way, of the 10 far side-lobe and 471 non-repeating main-lobe FRBs in the first CHIME/FRB catalog is 183.0 and 433.9 pc\;cm$^{-3}$, respectively. By comparing the DM excesses of the two populations under reasonable assumptions, we statistically constrain that the local degenerate contributions (from the Milky Way halo and the host galaxy) and the intergalactic contribution to the excess DM of the 471 non-repeating main-lobe FRBs for the NE2001 model are 131.2$-$158.3 and 302.7$-$275.6 pc cm$^{-3}$, respectively, which corresponds to a median redshift for the main-lobe FRB sample of $\sim$0.3. These constraints are useful for population studies of FRBs, and in particular for constraining the location of the missing baryons.

astro-ph.HE↗

arXiv2024

Magnetospheric origin of a fast radio burst constrained using scintillation

Fast radio bursts (FRBs) are micro-to-millisecond duration radio transients that originate mostly from extragalactic distances. The emission mechanism responsible for these high luminosity, short duration transients remains debated. The models are broadly grouped into two classes: physical processes that occur within close proximity to a central engine; and central engines that release energy which moves to large radial distances and subsequently interacts with surrounding media producing radio waves. The expected emission region sizes are notably different between these two types of models. FRB emission size constraints can therefore be used to distinguish between these competing models and inform on the physics responsible. Here we present the measurement of two mutually coherent scintillation scales in the frequency spectrum of FRB 20221022A: one originating from a scattering screen located within the Milky Way, and the second originating from a scattering screen located within its host galaxy or local environment. We use the scattering media as an astrophysical lens to constrain the size of the lateral emission region, $R_{\star\mathrm{obs}} \lesssim 3\times10^{4}$ km. We find that this is inconsistent with the expected emission sizes for the large radial distance models, and is more naturally explained with an emission process that operates within or just beyond the magnetosphere of a central compact object. Recently, FRB 20221022A was found to exhibit an S-shaped polarisation angle swing, supporting a magnetospheric emission process. The scintillation results presented in this work independently support this conclusion, while highlighting scintillation as a useful tool in our understanding of FRB emission physics and progenitors.

astro-ph.HE↗

arXiv2023

Change of rotation measure during eclipse of a black widow PSR J2051$-$0827

Black widows are millisecond pulsars ablating their companions. The material blown from the companion blocks the radio emission, resulting in radio eclipses. The properties of the eclipse medium are poorly understood. Here, we present direct evidence of the existence of magnetic fields in the eclipse medium of the black widow PSR J2051$-$0827 using observations made with the Five-hundred-meter Aperture Spherical radio Telescope (FAST). We detect a regular decrease in rotation measure (RM) in the egress of eclipse, changing from $60\,\rm rad\,m^{-2}$ to $-28.7\,\rm rad\,m^{-2}$. The RM gradually changes back to normal when the line-of-sight moves away from the eclipse. The estimated line-of-sight magnetic field strength in the eclipse medium is $\sim 0.1$ G. The RM reversal could be caused by a change of the magnetic field strength along the line of sight due to binary orbital motion. The RM reversal phenomenon has also been observed in some repeating fast radio bursts (FRBs), and the study of spider pulsars may provide additional information about the origin of FRBs.

astro-ph.HE↗

arXiv2023

Modelling Annual Scintillation Velocity Variations of FRB 20201124A

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.

astro-ph.HE↗

arXiv2023

Revealing the Dynamic Magneto-ionic Environments of Repeating Fast Radio Burst Sources through Multi-year Polarimetric Monitoring with CHIME/FRB

Fast radio bursts (FRBs) display a confounding variety of burst properties and host galaxy associations. Repeating FRBs offer insight into the FRB population by enabling spectral, temporal and polarimetric properties to be tracked over time. Here, we report on the polarized observations of 12 repeating sources using multi-year monitoring with the Canadian Hydrogen Intensity Mapping Experiment (CHIME) over 400-800 MHz. We observe significant RM variations from many sources in our sample, including RM changes of several hundred $\rm{rad\, m^{-2}}$ over month timescales from FRBs 20181119A, 20190303A and 20190417A, and more modest RM variability ($\rm{ΔRM \lesssim}$ few tens rad m$^{-2}$) from FRBs 20181030A, 20190208A, 20190213B and 20190117A over equivalent timescales. Several repeaters display a frequency dependent degree of linear polarization that is consistent with depolarization via scattering. Combining our measurements of RM variations with equivalent constraints on DM variability, we estimate the average line-of-sight magnetic field strength in the local environment of each repeater. In general, repeating FRBs display RM variations that are more prevalent/extreme than those seen from radio pulsars in the Milky Way and the Magellanic Clouds, suggesting repeating FRBs and pulsars occupy distinct magneto-ionic environments.

astro-ph.HE↗

arXiv2022

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

Fast radio bursts (FRBs) are highly dispersed millisecond-duration radio bursts. Recent observations of a Galactic FRB suggest that at least some FRBs originate from magnetars, but the origin of cosmological FRBs is still not settled. Here we report the detection of 1863 bursts in 82 hr over 54 days from the repeating source FRB~20201124A. These observations show irregular short-time variation of the Faraday rotation measure (RM), which probes the density-weighted line-of-sight magnetic field strength, of individual bursts during the first 36 days, followed by a constant RM. We detected circular polarisation in more than half of the burst sample, including one burst reaching a high fractional circular polarisation of 75%. Oscillations in fractional linear and circular polarisations as well as polarisation angle as a function of wavelength were detected. All of these features provide evidence for a complicated, dynamically evolving, magnetised immediate environment within about an astronomical unit (au; Earth-Sun distance) of the source. Our optical observations of its Milky-Way-sized, metal-rich host galaxy reveal a barred spiral, with the FRB source residing in a low stellar density, interarm region at an intermediate galactocentric distance. This environment is inconsistent with a young magnetar engine formed during an extreme explosion of a massive star that resulted in a long gamma-ray burst or superluminous supernova.

astro-ph.HE↗

arXiv2022

High frequency study of FRB 20180916B using the 100-m Effelsberg radio telescope

FRB 20180916B is a repeating fast radio burst (FRB) with an activity period of 16.33 days. In previous observations ranging from $\sim 150-1400$ MHz, the activity window was found to be frequency dependent, with lower frequency bursts occurring later. In this work, we present the highest-frequency detections of bursts from this FRB, using the 100-m Effelsberg Radio Telescope at 4$-$8 GHz. We present the results from two observing campaigns. We performed the first campaign over an entire activity period which resulted in no detections. The second campaign was in an active window at 4$-$8 GHz which we predicted from our modelling of chromaticity, resulting in eight burst detections. The bursts were detected in a window of 1.35 days, 3.6 days preceding the activity peak seen by CHIME, suggesting the chromaticity extends to higher frequency. The detected bursts have narrower temporal widths and larger spectral widths compared to lower frequencies. All of them have flat polarization position angle sweeps and high polarization fractions. The bursts also exhibit diffractive scintillation due to the Milky Way, following a $f^{3.90\pm0.05}$ scaling, and vary significantly over time. We find that burst rate across frequency scales as $f^{-2.6\pm0.2}$. Lastly, we examine implications of the frequency dependency on the source models.

astro-ph.HE↗

arXiv2022

Sub-second periodicity in a fast radio burst

Fast radio bursts (FRBs) are millisecond-duration flashes of radio waves that are visible at distances of billions of light-years. The nature of their progenitors and their emission mechanism remain open astrophysical questions. Here we report the detection of the multi-component FRB 20191221A and the identification of a periodic separation of 216.8(1) ms between its components with a significance of 6.5 sigmas. The long (~3 s) duration and nine or more components forming the pulse profile make this source an outlier in the FRB population. Such short periodicity provides strong evidence for a neutron-star origin of the event. Moreover, our detection favours emission arising from the neutron-star magnetosphere, as opposed to emission regions located further away from the star, as predicted by some models.

astro-ph.HE↗

arXiv2022

A Large Scale Magneto-ionic Fluctuation in the Local Environment of Periodic Fast Radio Burst Source, FRB 20180916B

Fast radio burst (FRB) source 20180916B exhibits a 16.33-day periodicity in its burst activity. It is as of yet unclear what proposed mechanism produces the activity, but polarization information is a key diagnostic. Here, we report on the polarization properties of 44 bursts from FRB 20180916B detected between 2018 December and 2021 December by CHIME/FRB, the FRB project on the Canadian Hydrogen Intensity Mapping Experiment the Canadian Hydrogen Intensity Mapping Experiment. In contrast to previous observations, we find significant variations in the Faraday rotation measure (RM) of FRB 20180916B. Over the nine month period 2021 April$-$2021 December we observe an apparent secular increase in $\rm{RM}$ of $\sim 50 \; \rm{rad\, m^{-2}}$ (a fractional change of over $40\%$) that is accompanied by a possible drift of the emitting band to lower frequencies. This interval displays very little variation in the dispersion measure ($Δ\rm{DM}\lesssim 0.8\; \rm{pc\, cm^{-3}}$) which indicates that the observed RM evolution is likely produced from coherent changes in the Faraday-active medium's magnetic field. Burst-to-burst RM variations appear unrelated to the activity cycle phase. The degree of linear polarization of our burst sample ($\gtrsim 80\%$) is consistent with the negligible depolarization expected for this source in the 400-800 MHz bandpass of CHIME. FRB 20180916B joins other repeating FRBs in displaying substantial RM variations between bursts. This is consistent with the notion that repeater progenitors may be associated with young stellar populations by their preferential occupation of dynamic magnetized environments commonly found in supernova remnants, pulsar wind nebulae or near high mass stellar companions.

astro-ph.HE↗

arXiv2021

Scintillation timescale measurement of the highly active FRB20201124A

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.

astro-ph.HE↗

arXiv2021

Polarization properties of FRB 20201124A from detections with the 100-m Effelsberg Radio Telescope

The repeating FRB source, FRB 20201124A, was found to be highly active in March and April 2021. We observed the source with the Effelsberg 100-m radio telescope at 1.36 GHz on 9 April 2021 and detected 20 bursts. A downward drift in frequency over time is clearly seen from the majority of bursts in our sample. A structure-maximizing dispersion measure (DM) search on the multi-component bursts in our sample yields a DM of 411.6$\pm$0.6 pc/cm$^3$. We find that the rotation measure (RM) of the bursts varies around their mean value of -605 rad/m$^2$ with a standard deviation of 11.1 rad/m$^2$. This RM magnitude is 10 times larger than the expected Galactic contribution along this line of sight (LoS). We estimate a LoS magnetic field strength of 4--6 $μ$G, assuming that the entire host galaxy DM contributes to the RM. Further polarization measurements will help determine FRB 20201124A's RM stability. The bursts are highly linearly polarized, with some showing signs of circular polarization, the first for a repeating FRB. Their polarization position angles (PAs) are flat across the burst envelopes and vary between bursts. We argue that the varying polarization fractions and PAs of FRB 20201124A are similar to known magnetospheric emission from pulsars, while the observed circular polarization, combined with the RM variability, is hard to explain with Faraday conversion. The high linear polarization fractions, flat PAs, and downward drift from FRB 20201124A bursts are similar to previous repeating sources, while the observed circular polarization is a newly seen behaviour among repeaters.

astro-ph.HE↗

arXiv2021

Burst properties of the highly active FRB 20201124A using uGMRT

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 $R(>\!F) = 10~{\rm hr}^{-1} \left(F/10\mathrm{~Jy~ms}\right)^γ$ with $γ= -1.2 \pm 0.2$. 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.

astro-ph.HE↗

arXiv2021

A repeating fast radio burst source in a globular cluster

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.

astro-ph.HE↗

arXiv2021

Observation of high efficiency Betatron radiation from femtosecond petawatt laser irradiated near critical plasmas

We present an experimental demonstration of high conversion efficiency Betatron x-ray radiation from petawatt laser irradiated near critical plasmas. Direct laser acceleration serves as the dominant regime when laser pulse of ~5e20 W/cm2 intensity is focused into plasmas with electron density of 3e20 /cm3. Electron beam with a charge of ~35 nC is accelerated up to a maximum energy of 70 MeV and emit x-rays when oscillating in the laser field. The deduced energy conversion efficiency from laser to x-rays is up to 1e-4, orders of magnitude higher than other betatron regimes. Enhancement of acceleration and radiation with sharp plasma density boundary is also obtained and further interpretated with 2D particle-in-cell simulations.

physics.plasm-ph↗