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Ue-Li Pen

Publications and source records attributed to Ue-Li Pen.

At least 55 records · Page 3Linked to original sources
arXiv2024

Updating the first CHIME/FRB catalog of fast radio bursts with baseband data

In 2021, a catalog of 536 fast radio bursts (FRBs) detected with the Canadian Hydrogen Intensity Mapping Experiment (CHIME) radio telescope was released by the CHIME/FRB Collaboration. This large collection of bursts, observed with a single instrument and uniform selection effects, has advanced our understanding of the FRB population. Here we update the results for 140 of these FRBs for which channelized raw voltage ('baseband') data are available. With the voltages measured by the telescope's antennas, it is possible to maximize the telescope sensitivity in any direction within the primary beam, an operation called 'beamforming'. This allows us to increase the signal-to-noise ratio (S/N) of the bursts and to localize them to sub-arcminute precision. The improved localization is also used to correct the beam response of the instrument and to measure fluxes and fluences with a ~10% uncertainty. Additionally, the time resolution is increased by three orders of magnitude relative to that in the first CHIME/FRB catalog, and, applying coherent dedispersion, burst morphologies can be studied in detail. Polarization information is also available for the full sample of 140 FRBs, providing an unprecedented dataset to study the polarization properties of the population. We release the baseband data beamformed to the most probable position of each FRB. These data are analyzed in detail in a series of accompanying papers.

astro-ph.HE↗
arXiv2024

On the cusp of cusps: a universal model for extreme scattering events in the ISM

The scattering structures in the ISM responsible for so-called ``extreme scattering events" (ESEs), observed in quasars and pulsars, remain enigmatic. Current models struggle to explain the high-frequency light curves of ESEs, and a recent analysis of a double lensing event in PSR\,B0834+06 reveals features of ESEs that may also be challenging to accommodate via existing models. We propose that these features arise naturally when the lens has a cusp-like profile, described by the elementary $A_3$ cusp catastrophe. This is an extension of previous work describing pulsar scintillation as arising from $A_2$ fold catastrophes in thin, corrugated plasma sheets along the line of sight. We call this framework of describing the lens potentials via elementary catastrophes ``doubly catastrophic lensing", as catastrophes (e.g. folds and cusps) have long been used to describe universal features in the light curves of lensing events that generically manifest, regardless of the precise details of the lens. Here, we argue that the lenses themselves may be described by these same elementary structures. If correct, the doubly catastrophic lensing framework would provide a unified description of scintillation and ESEs, where the lenses responsible for these scattering phenomena are universal and can be fully described by a small number of unfolding parameters. This could enable their application as giant cosmic lenses for precision measurements of coherent sources, including FRBs and pulsars.

astro-ph.HE↗
arXiv2024

Scintillated microlensing: measuring cosmic distances with fast radio bursts

We propose a novel means of directly measuring cosmological distances using scintillated microlensing of fast radio bursts (FRBs). In standard strong lensing measurements of cosmic expansion, the main source of systematic uncertainty lies in modeling the mass profile of galactic halos. Using extra-galactic stellar microlensing to measure the Hubble constant avoids this systematic uncertainty as the lens potential of microlenses depends only on a single parameter: the mass of the lens. FRBs, which may achieve nanosecond precision on lensing time delays, are well-suited to precision measurements of stellar microlensing, for which the time delays are on the order of milliseconds. However, typical angular separations between the microlensed images on the order of microarcseconds make the individual images impossible to spatially resolve with ground-based telescopes. We propose leveraging scintillation in the ISM to resolve the microlensed images, effectively turning the ISM into an astrophysical-scale interferometer. Using this technique, we estimate a 6\% uncertainty on $H_0$ from a single observed scintillated microlensing event, with a sub-percent uncertainty on $H_0$ achievable with only 30 such events. With an optical depth for stellar microlensing of $10^{-3}$, this may be achievable in the near future with upcoming FRB telescopes.

astro-ph.IM↗
arXiv2023

Complex classical paths in quantum reflections and tunneling

The real-time propagator of the symmetric Rosen-Morse, also known as the symmetric modified Pöschl-Teller, barrier is expressed in the Picard-Lefschetz path integral formalism using real and complex classical paths. We explain how the interference pattern in the real-time propagator and energy propagator is organized by caustics and Stoke's phenomena, and list the relevant real and complex classical paths as a function of the initial and final position. We discover the occurrence of singularity crossings, where the analytic continuation of the complex classical path no longer satisfies the boundary value problem and needs to be analytically continued. Moreover, we demonstrate how these singularity crossings play a central role in the real-time description of quantum tunneling.

quant-ph↗
arXiv2023

Crossing singularities in the saddle point approximation

We describe a new phenomenon in the study of the real-time path integral, where complex classical paths hit singularities of the potential and need to be analytically continued beyond the space for which they solve the boundary value problem. We show that the behavior is universal and central to the problem of quantum tunneling. These analytically continued complex classical paths enrich the study of real-time Feynman path integrals.

quant-ph↗
arXiv2023

FRBs from rapid spindown neutron stars

A fast radio burst (FRB) localized to a globular cluster (GC) challenges FRB models involving ordinary young magnetars. In this paper, we examine the rapid spindown millisecond neutron star (NS) scenario, which favours the dynamic environment in GCs. Fast spindown corresponds to a larger magnetic field than regular millisecond pulsars, which empirically favours giant pulse (GP) emission. The kinetic energy in millisecond NSs can readily exceed the magnetic energy in magnetars. The high inferred isotropic luminosity of most FRBs is challenging to explain in spin-down powered pulsars. A recent observation of a GP from the Crab pulsar, on the other hand, suggests highly Doppler-beamed emission, making the required energy orders of magnitude smaller than estimated with isotropic assumptions. Considering this strong beaming effect, GPs from a recycled pulsar with a modest magnetic field could explain the energetics and burst rates for a wide range of FRBs. The short life span accounts for a paucity of bright FRBs in the Milky Way neighbourhood. We point out that tidal disruption spin-up from a main sequence star can provide sufficient accretion rate to recycle a NS with mild magnetic field. It can also explain the observed source density and the spatial offset in the GC for FRB 20200120E. Frequency variation in the scattering tail for some of the brightest FRBs is expected in this scenario.

astro-ph.HE↗
arXiv2023

A Detection of Cosmological 21 cm Emission from CHIME in Cross-correlation with eBOSS Measurements of the Lyman-$α$ Forest

We report the detection of 21 cm emission at an average redshift $\bar{z} = 2.3$ in the cross-correlation of data from the Canadian Hydrogen Intensity Mapping Experiment (CHIME) with measurements of the Lyman-$α$ forest from eBOSS. Data collected by CHIME over 88 days in the $400-500$~MHz frequency band ($1.8 < z < 2.5$) are formed into maps of the sky and high-pass delay filtered to suppress the foreground power, corresponding to removing cosmological scales with $k_\parallel \lesssim 0.13\ \text{Mpc}^{-1}$ at the average redshift. Line-of-sight spectra to the eBOSS background quasar locations are extracted from the CHIME maps and combined with the Lyman-$α$ forest flux transmission spectra to estimate the 21 cm-Lyman-$α$ cross-correlation function. Fitting a simulation-derived template function to this measurement results in a $9σ$ detection significance. The coherent accumulation of the signal through cross-correlation is sufficient to enable a detection despite excess variance from foreground residuals $\sim6-10$ times brighter than the expected thermal noise level in the correlation function. These results are the highest-redshift measurement of \tcm emission to date, and set the stage for future 21 cm intensity mapping analyses at $z>1.8$.

astro-ph.CO↗
arXiv2023

A search for pulsars around Sgr A* in the first Event Horizon Telescope dataset

The Event Horizon Telescope (EHT) observed in 2017 the supermassive black hole at the center of the Milky Way, Sagittarius A* (Sgr A*), at a frequency of 228.1 GHz ($λ$=1.3 mm). The fundamental physics tests that even a single pulsar orbiting Sgr A* would enable motivate searching for pulsars in EHT datasets. The high observing frequency means that pulsars - which typically exhibit steep emission spectra - are expected to be very faint. However, it also negates pulse scattering, an effect that could hinder pulsar detections in the Galactic Center. Additionally, magnetars or a secondary inverse Compton emission could be stronger at millimeter wavelengths than at lower frequencies. We present a search for pulsars close to Sgr A* using the data from the three most-sensitive stations in the EHT 2017 campaign: the Atacama Large Millimeter/submillimeter Array, the Large Millimeter Telescope and the IRAM 30 m Telescope. We apply three detection methods based on Fourier-domain analysis, the Fast-Folding-Algorithm and single pulse search targeting both pulsars and burst-like transient emission; using the simultaneity of the observations to confirm potential candidates. No new pulsars or significant bursts were found. Being the first pulsar search ever carried out at such high radio frequencies, we detail our analysis methods and give a detailed estimation of the sensitivity of the search. We conclude that the EHT 2017 observations are only sensitive to a small fraction ($\lesssim$2.2%) of the pulsars that may exist close to Sgr A*, motivating further searches for fainter pulsars in the region.

astro-ph.HE↗
arXiv2023

Regimes in astrophysical lensing: refractive optics, diffractive optics, and the Fresnel scale

Astrophysical lensing has typically been studied in two regimes: diffractive optics and refractive optics. Diffractive optics is characterized by a perturbative expansion of the Kirchhoff-Fresnel diffraction integral, while refractive optics is characterized by the stationary phase approximation. Previously, it has been assumed that the Fresnel scale, $R_F$ , is the relevant physical scale that separates these two regimes. With the recent introduction of Picard-Lefschetz theory to the field of lensing, it has become possible to generalize the refractive description of discrete images to all wave parameters, and, in particular, exactly evaluate the diffraction integral at all frequencies. In this work, we assess the regimes of validity of refractive and diffractive approximations for a simple one-dimensional lens model through comparison with this exact evaluation. We find that, contrary to previous assumptions, the true separation scale between these regimes is given by $R_F / \sqrtκ$, where $κ$ is the convergence of the lens. Thus, when the lens is strong, refractive optics can hold for arbitrarily small scales. We also argue that intensity variations in diffractive optics are generically small, which has implications for the study of strong diffractive scintillation (DISS).

astro-ph.HE↗
arXiv2023

High Resolution VLBI Astrometry of pulsar scintillation screens with the $θ-θ$ Transform

The recent development of $θ\mhyphenθ$ techniques in pulsar scintillometry has opened the door for new high resolution imaging techniques of the scattering medium. By solving the phase retrieval problem and recovering the wavefield from a pulsar dynamic spectrum, the Doppler shift, time delay, and phase offset of individual images can be determined. However, the results of phase retrieval from a single dish are only known up to a constant phase rotation, which introduces extra parameters when doing astrometry using Very Long Baseline Interferometry. We present an extension to previous $θ\mhyphenθ$ methods using the interferometric visibilities between multiple stations to calibrate the wavefields. When applied to existing data for PSR B0834+06 we measure the effective screen distance and lens orientation with five times greater precision than previous works.

astro-ph.IM↗
arXiv2023

DM-power: an algorithm for high precision dispersion measure with application to fast radio bursts

We present DM-power, a new method for precisely determining the dispersion measure (DM) of radio bursts, and apply it to the Fast Radio Burst (FRB) source FRB~20180916B. Motivated by the complex structure on multiple time scales seen in FRBs, DM-power optimizes the DM by combining measurements at multiple Fourier frequencies in the power spectrum of the burst. By optimally weighting the measurements at each Fourier frequency, DM-power finds a burst DM that effectively incorporates information on many different burst timescales. We validate this technique on simulated Gaussian pulse profiles with a precision down to $σ_{\rm DM} \sim 0.001~{\rm pc~cm}^{-3}$, and then apply it to bursts from pulsar B0329+54 and FRB~20180916B. The precision of these DM measurements are sufficient to measure a statistically significant variation in DM over a $\approx 2$ hr span. While this variation could be the result of electron density variations along the line of sight, it is more like that the observed variation is the result of intrinsic frequency-dependent burst structure that can mimic a dispersive delay.

astro-ph.HE↗
arXiv2023

The Radio Parallax of the Crab Pulsar: A First VLBI Measurement Calibrated with Giant Pulses

We use four observations with the European VLBI network to measure the first precise radio parallax of the Crab Pulsar. We found two in-beam extragalactic sources just outside the Crab Nebula, with one bright enough to use as a background reference source in our data. We use the Crab Pulsar's giant pulses to determine fringe and bandpass calibration solutions, which greatly improved the sensitivity and reliability of our images and allowed us to determine precise positional offsets between the pulsar and the background source. From those offsets, we determine a parallax of $π=0.53\pm0.06\rm{\;mas}$ and proper motion of $(μ_α,μ_δ)=(-11.34\pm0.06,2.65\pm0.14)\rm{\;mas\;yr^{-1}}$, yielding a distance of $d=1.90^{+0.22}_{-0.18}\rm{\;kpc}$ and transverse velocity of $v_{\perp}=104^{+13}_{-11}\rm{\;km\;s^{-1}}$. These results are consistent with the Gaia 3 measurements, and open up the possibility of far more accurate astrometry with further VLBI observations.

astro-ph.HE↗
arXiv2023

Detections of 21-cm absorption with a blind FAST survey at z $\leqslant$ 0.09

We present the early science results from a blind search of the extragalactic HI 21-cm absorption lines at z $\leqslant$ 0.09 with the drift-scan observation of the Five-hundred-meter Aperture Spherical radio Telescope (FAST). We carried out the search using the data collected in 643.8 hours by the ongoing Commensal Radio Astronomy FasT Survey (CRAFTS), which spans a sky area of 3155 deg$^{2}$ and covers 44827 radio sources with a flux density greater than 12 mJy. Due to the radio frequency interference (RFI), only the relatively clean data in the frequency range of 1.3-1.45 GHz are used in the present work. Under the assumption of $T_{s}/c_{f}$ = 100 K, the total completeness-corrected comoving absorption path length spanned by our data and sensitive to Damped Lyman $α$ Absorbers (DLAs) are $ΔX^{inv}$ = 8.33$\times10^3$ ($Δz^{inv} = 7.81\times10^{3}$) for intervening absorption. For associated absorption, the corresponding values are $ΔX^{asc}$ = 12.8 ($Δz^{asc} = 11.9$). Three known HI absorbers (UGC 00613, 3C 293 and 4C +27.14) and two new HI absorbers (towards NVSS J231240-052547 and NVSS J053118+315412) are detected blindly. We fit the HI profiles with multi-components Gaussian functions and calculate the redshift (0.063, 0.066), width, flux density, optical depth and HI column densities for each absorption. Our results demonstrate the power of FAST in blindly searching HI absorbers. For absorption towards NVSS J231240-052547, the optical counterparts are faint and currently lack existing spectra. The most likely interpretation is that a radio-loud active galactic nucleus (AGN) is faint in the optical as the background source, with a faint optical absorber in between. NVSS J053118+315412 exhibits an associated absorption with a complex profile, which may suggest unsettled gas structures or gas accretion onto the supermassive black hole (SMBH).

astro-ph.GA↗
arXiv2023

Plasma lensing with magnetic field and a small correction to the Faraday rotation measurement

Plasma lensing displays interesting characteristics that set it apart from gravitational lensing. The magnetised medium induces birefringence in the two polarisation modes. As the lensing deflection grows stronger, e.g. when images form near the critical curve, the geometric delay of the signal can cause rotation in linear polarisation, in addition to Faraday rotation. This rotation has a frequency dependence to the power of four. We study the geometric rotation of the lensed image in a Gaussian density model and find that it is necessary to take into account the geometric rotation when estimating magnetised media, especially in the under-dense lens. At frequencies of $\sim 1$ GHz or lower, the geometric rotation can dominate. We simulate the flux of lensed images and find that when the image forms near the lensing critical curve, the birefringence can convert the linear polarisation and un-polarisation pulse into a circular mode. The lensing magnification has the potential to increase the probability of detecting such events.

astro-ph.CO↗
arXiv2023

TONE: A CHIME/FRB Outrigger Pathfinder for localizations of Fast Radio Bursts using Very Long Baseline Interferometry

The sensitivity and field of view of the Canadian Hydrogen Intensity Mapping Experiment (CHIME) has enabled its fast radio burst (FRB) backend to detect thousands of FRBs. However, the low angular resolution of CHIME prevents it from localizing most FRBs to their host galaxies. Very long baseline interferometry (VLBI) can readily provide the subarcsecond resolution needed to localize many FRBs to their hosts. Thus we developed TONE: an interferometric array of eight $6~\mathrm{m}$ dishes to serve as a pathfinder for the CHIME/FRB Outriggers project, which will use wide field of view cylinders to determine the sky positions for a large sample of FRBs, revealing their positions within their host galaxies to subarcsecond precision. In the meantime, TONE's $\sim3333~\mathrm{km}$ baseline with CHIME proves to be an excellent testbed for the development and characterization of single-pulse VLBI techniques at the time of discovery. This work describes the TONE instrument, its sensitivity, and its astrometric precision in single-pulse VLBI. We believe that our astrometric errors are dominated by uncertainties in the clock measurements which build up between successive Crab pulsar calibrations which happen every $\approx 24~\mathrm{h}$; the wider fields of view and higher sensitivity of the Outriggers will provide opportunities for higher-cadence calibration. At present, CHIME-TONE localizations of the Crab pulsar yield systematic localization errors of ${0.1}-{0.2}~\mathrm{arcsec}$ - comparable to the resolution afforded by state-of-the-art optical instruments ($\sim 0.05 ~\mathrm{arcsec}$).

astro-ph.IM↗
arXiv2023

CHIME/FRB Discovery of 25 Repeating Fast Radio Burst Sources

We present the discovery of 25 new repeating fast radio burst (FRB) sources found among CHIME/FRB events detected between 2019 September 30 and 2021 May 1. The sources were found using a new clustering algorithm that looks for multiple events co-located on the sky having similar dispersion measures (DMs). The new repeaters have DMs ranging from $\sim$220 pc cm$^{-3}$ to $\sim$1700 pc cm$^{-3}$, and include sources having exhibited as few as two bursts to as many as twelve. We report a statistically significant difference in both the DM and extragalactic DM (eDM) distributions between repeating and apparently nonrepeating sources, with repeaters having lower mean DM and eDM, and we discuss the implications. We find no clear bimodality between the repetition rates of repeaters and upper limits on repetition from apparently nonrepeating sources after correcting for sensitivity and exposure effects, although some active repeating sources stand out as anomalous. We measure the repeater fraction over time and find that it tends to an equilibrium of $2.6_{-2.6}^{+2.9}$% over our total time-on-sky thus far. We also report on 14 more sources which are promising repeating FRB candidates and which merit follow-up observations for confirmation.

astro-ph.HE↗
arXiv2023

Resolving the Emission Regions of the Crab Pulsar's Giant Pulses II. Evidence for Relativistic Motion

The Crab Pulsar is the prime example of an emitter of giant pulses. These short, very bright pulses are thought to originate near the light cylinder, at $\sim\!1600{\rm\;km}$ from the pulsar. The pulsar's location inside the Crab Nebula offers an unusual opportunity to resolve the emission regions, using the nebula, which scatters radio waves, as a lens. We attempt to do this using a sample of 61998 giant pulses found in coherently combined European VLBI network observations at $18{\rm\;cm}$. These were taken at times of relatively strong scattering and hence good effective resolution, and from correlations between pulse spectra, we show that the giant pulse emission regions are indeed resolved. We infer apparent diameters of $\sim\!2000$ and $\sim\!2400{\rm\;km}$ for the main and interpulse components, respectively, and show that with these sizes the correlation amplitudes and decorrelation timescales and bandwidths can be understood quantitatively, both in our observations and in previous ones. Using pulse-spectra statistics and correlations between polarizations, we also show that the nebula resolves the nanoshots that comprise individual giant pulses. The implied diameters of $\sim\!1100{\rm\;km}$ far exceed light travel-time estimates, suggesting the emitting plasma is moving relativistically, with $γ\simeq10^{4}$, as inferred previously from drifting bands during the scattering tail of a giant pulse. If so, the emission happens over a region extended along the line of sight by $\sim\!10^{7}{\rm\;km}$. We conclude that relativistic motion likely is important for producing giant pulses, and may be similarly for other sources of short, bright radio emission, such as fast radio bursts.

astro-ph.HE↗
arXiv2023

The First CHIME/FRB Fast Radio Burst Catalog

We present a catalog of 536 fast radio bursts (FRBs) detected by the Canadian Hydrogen Intensity Mapping Experiment Fast Radio Burst (CHIME/FRB) Project between 400 and 800 MHz from 2018 July 25 to 2019 July 1, including 62 bursts from 18 previously reported repeating sources. The catalog represents the first large sample, including bursts from repeaters and non-repeaters, observed in a single survey with uniform selection effects. This facilitates comparative and absolute studies of the FRB population. We show that repeaters and apparent non-repeaters have sky locations and dispersion measures (DMs) that are consistent with being drawn from the same distribution. However, bursts from repeating sources differ from apparent non-repeaters in intrinsic temporal width and spectral bandwidth. Through injection of simulated events into our detection pipeline, we perform an absolute calibration of selection effects to account for systematic biases. We find evidence for a population of FRBs - comprising a large fraction of the overall population - with a scattering time at 600 MHz in excess of 10 ms, of which only a small fraction are observed by CHIME/FRB. We infer a power-law index for the cumulative fluence distribution of $α=-1.40\pm0.11(\textrm{stat.})^{+0.06}_{-0.09}(\textrm{sys.})$, consistent with the $-3/2$ expectation for a non-evolving population in Euclidean space. We find $α$ is steeper for high-DM events and shallower for low-DM events, which is what would be expected when DM is correlated with distance. We infer a sky rate of $[525\pm30(\textrm{stat.})^{+140}_{-130}({\textrm{sys.}})]/\textrm{sky}/\textrm{day}$ above a fluence of 5 Jy ms at 600 MHz, with scattering time at $600$ MHz under 10 ms, and DM above 100 pc cm$^{-3}$.

astro-ph.HE↗