arXiv subjects

Calvin Leung

Publications and source records attributed to Calvin Leung.

At least 37 records · Page 2Linked to original sources

arXiv2026

Constraining the mass of the M31 ionized baryon Halo using CHIME/FRB Catalog 2

The circumgalactic medium (CGM) surrounding galaxies is believed to be a significant reservoir of baryons, yet its total mass remains poorly constrained. We present a novel approach to probe the CGM of the Andromeda galaxy (M31) using fast radio bursts (FRBs) from the CHIME/FRB Catalog 2. By comparing the dispersion measures (DMs) of 171 FRBs whose sightlines intersect M31's halo (within $r_{\rm vir}= 302\,\mathrm{kpc}$) to a control sample of 684 FRBs, we estimate the DM contribution from M31's CGM. We find evidence for an excess DM of $δ\mathrm{DM} = 5.9$--$59.6\,\mathrm{pc\,cm^{-3}}$ in the inner halo ($\approx 0$--$151\,\mathrm{kpc}$) and $δ\mathrm{DM} = 25.7$--$64.6\,\mathrm{pc\,cm^{-3}}$ in the outer halo ($\approx 151$--$302\,\mathrm{kpc}$). Using a generalized halo model parameterized by a closure radius $r_{\mathrm{close}}$, we constrain the baryon distribution and infer a best-fit value for $r_{\mathrm{close}}$ to be $9.2^{+9.9}_{-4.9}\,r_{\rm vir}$ ($1σ$), corresponding to a total CGM mass of $M_{b,\mathrm{halo}} = 18.6^{+7.9}_{-8.4} \times 10^{10}\,M_\odot$. Our results suggest that M31 may harbor a substantial fraction of its cosmic baryon budget in diffuse, ionized gas. This work demonstrates the potential of FRBs as a powerful tool for studying the CGM of nearby galaxies, with future larger samples expected to provide tighter constraints on the baryon content of galactic halos.

astro-ph.GA↗

arXiv2026

A possible wave-optical effect in lensed FRBs

Context: Fast Radio Bursts (FRBs) are enigmatic extragalactic bursts whose properties are still largely unknown, but based on their extremely small time duration, they are proposed to have a compact structure, making them candidates for wave-optical effects if gravitational lensed. If an FRB is lensed into multiple-images bursts at different times by a galaxy or cluster, a likely scenario is that only one image is detected, because the others fall outside the survey area and time frame. Aims: In this work we explore the FRB analog of quasar microlensing, namely the collective microlensing by stars in the lensing galaxy, now with wave optics included. The eikonal regime is applicable here. Methods. We study the voltage (rather than the intensity) in a simple simulation consisting of (a) microlensing stars, and (b) plasma scattering by a turbulent interstellar medium. Results: The auto-correlation of the voltage shows peaks (at order-microsecond separations) corresponding to wave-optical interference between lensed micro-images. The peaks are frequency dependent if plasma-scattering is significant. While qualitative and still in need of more realistic simulations, the results suggest that a strongly-lensed FRB could be identified from a single image. Conclusions: Microlensing could sniff out macro-lensed FRBs

astro-ph.HE↗

arXiv2026

The Second CHIME/FRB Catalog of Fast Radio Bursts

We present a catalog of 4539 fast radio bursts (FRBs) observed with the Canadian Hydrogen Intensity Mapping Experiment (CHIME) telescope between 25 July 2018 and 15 September 2023. These bursts originate from 3641 unique sources, including 981 bursts from 83 known repeating sources. For each FRB, the catalog provides a $O(10')$ estimate of sky location along with corresponding measurements of cumulative exposure time and survey sensitivity over the observing period. It includes a total-intensity dynamic spectrum between 400 and 800 MHz at 0.983 ms resolution. From this spectrum, we constrain a model of the burst morphology and measure key parameters such as arrival time, intrinsic temporal width, dispersion measure, scattering time, and flux density. This second catalog includes all FRBs from the first catalog, with every event reprocessed using a uniform and improved analysis framework. We show that previously published inferences remain valid under the updated measurements. We assess consistency of the detection rate across observational parameters, present initial distributions of burst properties, and outline ongoing and future studies that will use this catalog to investigate the nature of FRBs and their utility as astrophysical and cosmological probes.

astro-ph.HE↗

arXiv2025

A Spatial Gap in the Sky Distribution of Fast Radio Burst Detections Coinciding with Galactic Plasma Overdensities

We analyze the positional and morphological properties of about 3600 unique fast radio burst (FRB) sources reported in the second FRB catalog generated by the Canadian Hydrogen Intensity Mapping Experiment (CHIME) telescope. We find a two-dimensional dependence of FRB detections on sky position, and identify a significant absence of detections in a roughly circular region centered at Galactic coordinates (77.7$^\circ$, 0.9$^\circ$), spanning an area of 213.6 deg$^2$. This detection gap spatially coincides with the Cygnus X region $--$ a plasma-rich star-forming region in the Milky Way. This feature is most likely the result of increased sky temperature and strong multi-path scattering by turbulent ionized plasma, which broadens the FRB signals beyond detectability in the CHIME band. Our simulations yield a mean of 6 expected FRB detections within the gap when accounting for the elevated sky temperature in the direction of the detection gap. We infer that a lower limit of the maximum scattering timescale $τ_{\rm sc,\, 1\,GHz} \geq 5.59$ ms, obtained without assuming a model of the Galactic electron distribution, is sufficient to suppress the brightness of all coincident FRBs. A similar suppression is seen in Catalog 2 along other high-emission measure (EM) sightlines (i.e., EM$\geq$2900 pc cm$^{-6}$), further supporting a broader trend of suppression due to Galactic scattering. Future very long baseline interferometry (VLBI) measurements of scattering disks with CHIME Outriggers could help confirm our interpretation. Our work highlights the notion that FRBs can serve as new, model-independent tracers of the warm ionized medium within our Milky Way Galaxy.

astro-ph.HE↗

arXiv2025

Detection of Partial Coherence due to Multipath Propagation for FRB 20220413B with CHIME/FRB

Fast radio bursts (FRBs) are a $\sim$ millisecond-long transient phenomenon that propagate across extragalactic distances and are effectively a point source. Radio wave propagation through inhomogeneous distributions of plasma can act as a lens, generating multiple images of the emitted electric field. A lens can produce images of a point source where the phase of the electric field between images remains coherent when observed by a radio telescope. FRB 20220413B shows a complicated pulse structure with time separated components that may be image copies of the main components due to plasma lensing. We perform several analyses to determine if FRB 20220413B is consistent with expectations of a plasma lensed FRB. We analyze and fit the morphology of the burst to a plasma lens model and find consistency in the spectro-temporal profile but not the observed flux. Using the complex-valued channelized voltage data from the CHIME telescope, we perform a time-lag correlation analysis and report correlation signatures present in the electric field of FRB 20220413B. We find that there exists an excess correlation signature only in absolute power and not in phase. We perform a frequency-lag correlation analysis on the spectra of all subcomponents of the burst and find a consistent scintillation bandwidth across all components. We find the scintillation bandwidth is consistent with expectations of scattering due to the Milky Way. We interpret this as all burst components propagating through the same scintillation screen located in the Milky Way, which would generate the excess variance signature observed, even in the absence of phase coherence between burst components. We find that while the burst morphology can be modeled by a plasma lens, the coherent signature present in the time-lag correlation is consistent with the expectations of a common scattering screen, but not coherent plasma lensing.

astro-ph.HE↗

arXiv2025

A milliarcsecond localization associates FRB 20190417A with a compact persistent radio source and an extreme magneto-ionic environment

We report the milliarcsecond localization of a high (1379 pc/cc) dispersion measure (DM) repeating fast radio burst, FRB 20190417A. Combining European VLBI Network detections of five repeat bursts, we confirm the FRB's host to be a low-metallicity, star-forming dwarf galaxy at z = 0.12817, similar to the hosts of FRBs 20121102A, 20190520B and 20240114A. We also confirm that it is associated with a previously reported persistent radio source (PRS), which is compact on milliarcsecond scales. Visibility-domain model fitting constrains the transverse physical size of the PRS to < 23 pc and yields an integrated flux density of 191(39) microJy at 1.4 GHz. Though we do not find significant evidence for DM evolution, FRB 20190417A exhibits a time-variable rotation measure (RM) ranging between +3958(11) and +5061(24) rad/m2 over three years. We find no evidence for intervening galaxy clusters in the FRB's line-of-sight and place a conservative lower limit on the rest-frame host DM contribution of 1228 pc/cc (90% confidence) -- the largest known for any FRB so far. This system strengthens the emerging picture of a rare subclass of repeating FRBs with large and variable RMs, above-average host DMs, and luminous PRS counterparts in metal-poor dwarf galaxies. Our results suggest that these systems are the result of environmental selection, or a distinct engine for FRB emission.

astro-ph.HE↗

arXiv2025

Morphology of 35 Repeating Fast Radio Burst Sources at Microsecond Time Scales with CHIME/FRB

The Canadian Hydrogen Intensity Mapping Experiment Fast Radio Burst (CHIME/FRB) project has discovered the most repeating fast radio burst (FRB) sources of any telescope. However, most of the physical conclusions derived from this sample are based on data with a time resolution of $\sim$1 ms. In this work, we present for the first time a morphological analysis of the raw voltage data for 124 bursts from 35 of CHIME/FRB's repeating sources. We do not find any significant correlations amongst fluence, dispersion measure (DM), burst rate, and burst duration. Performing the first large-scale morphological comparison at timescales down to microseconds between our repeating sources and 125 non-repeating FRBs, we find that repeaters are narrower in frequency and broader in duration than non-repeaters, supporting previous findings. However, we find that the duration-normalized sub-burst widths of the two populations are consistent, possibly suggesting a shared physical emission mechanism. Additionally, we find that the spectral fluences of the two are consistent. When combined with the larger bandwidths and previously found larger DMs of non-repeaters, this suggests that non-repeaters may have higher intrinsic specific energies than repeating FRBs. We do not find any consistent increase or decrease in the DM ($\lessapprox 1$ pc cm$^{-3}$ yr$^{-1}$) and scattering timescales ($\lessapprox 2$ ms yr$^{-1}$) of our sources over $\sim2-4$ year periods.

astro-ph.HE↗

arXiv2025

Stellar Mass-Dispersion Measure Correlations Constrain Baryonic Feedback in Fast Radio Burst Host Galaxies

Low redshift fast radio bursts (FRBs) provide robust measurements of the host-galaxy contribution to the dispersion measure (DM), which can constrain the circumgalactic medium (CGM) of the hosts. We curate a sample of 20 nearby FRBs with low scattering timescales and face-on host galaxies with stellar masses ranging from $10^9 < M^* / M_\odot < 10^{11}$. We fit the distribution of the host galaxy DM to a quadratic model as a function of stellar mass with a mass-independent scatter and find that the more massive the host, the lower its host DM. We report that this relation has a negative slope of $m = -97 \pm 44$ pc/cm$^{-3}$ per dex in stellar mass. We compare this measurement to similar fits to three sub-grid models implemented in the CAMELS suite of simulations from Astrid, IllustrisTNG, and SIMBA and find that fine-tuning of the host ISM contribution as a function of stellar mass is required in order to reconcile the observational data with the predictions of the fiducial CAMELS-Astrid model. More generally, models which attribute a positive correlation between stellar mass and host dispersion measure ($m > 0$) to the CGM are in tension with our measurement. We show that this conclusion is robust to a wide range of assumptions, such as the offset distribution of FRBs from their hosts and the statistics of the cosmic contribution to the DM budget along each sightline. Our results indirectly imply a lower limit on the strength of baryonic feedback in the Local Universe $(z < 0.2)$ in isolated $\sim L^*$ halos, complementing results from weak lensing surveys and kSZ observations which target higher halo mass and redshift ranges.

astro-ph.GA↗

arXiv2025

Accretion from a Shock-Inflated Companion: Spinning Down Neutron Stars to Hour-Long Periods

Recent observations have unveiled a population of pulsars with spin periods of a few minutes to hours that lie beyond the traditional ``death line.'' If they originate from neutron stars (NSs), the existence of such ultra-long period pulsars (ULPs) challenges our current understanding of NS evolution and emission. In this work, we propose a new channel for disk formation based on NSs born in close binaries with main-sequence companion stars. Using a hydrodynamic simulation of supernova-companion interactions, we show that a newborn NS may gravitationally capture gas as it moves through the complex density field shaped by the explosion. For a binary separation of $20\rm~R_\odot$ and a companion mass of $4\rm~M_\odot$, we find the occurrence fraction for disk formation around unbound NSs to be $\sim10\%$. By modeling the disk evolution and its interaction with the NS, we find a bimodal distribution in spin periods: canonical pulsars with $P\lesssim10\rm\,s$ are the ones who lack disks or whose magnetospheres never interacted with the disk, and ULPs with $10^3\lesssim P<10^5\rm\,s$ are produced when the system undergoes a short-lived ``propeller'' phase during which the NS undergoes rapid spin-down. Such ULPs are formed under strong initial dipolar magnetic field strengths $B_0\gtrsim10^{14}\rm\,G$, with a formation rate of $10^{-4}\rm\,yr^{-1}$ in the Milky Way. We also find that a small population of pulsars with moderate magnetic field strengths ($10^{13}\lesssim~B_0\lesssim10^{14}\rm\,G$) and relatively slow initial periods ($P_0\gtrsim0.1\rm\,s$) evolve to $P\sim10^2\rm\,s$, filling the gap between the bimodal distribution. Thus, our model provides a unified explanation for pulsars beyond the ``death line.''

astro-ph.HE↗

arXiv2025

CHIME/FRB Discovery of an Unusual Circularly Polarized Long-Period Radio Transient with an Accelerating Spin Period

We report the discovery of CHIME J1634+44, a Long Period Radio Transient (LPT) unique for two aspects: it is the first known LPT to emit fully circularly polarized radio bursts, and it is the first LPT with a significant spin-up. Given that high circular polarization ($>90$\%) has been observed in FRB~20201124A and in some giant pulses of PSR~B1937+21, we discuss the implications of the high circular polarization of CHIME J1634+44 and conclude its emission mechanism is likely to be ``pulsar-like''. While CHIME J1634+44 has a pulse period of 841 s, its burst arrival patterns are indicative of a secondary 4206 s period, probably associated with binary activity. The timing properties suggest it has a significantly negative period derivative of $\dot{P}=-9.03(0.11)\times 10^{-12}$ s s$^{-1}$. Few systems have been known to spin-up, most notably transitional millisecond pulsars and cataclysmic binaries, both of which seem unlikely progenitors for CHIME J1634+44. If the period was only associated with the spin of the object, then the spin up is likely generated by accretion of material from a companion. If, however, the radio pulse period and the orbital period are locked, as appears to be the case for two other LPTs, the spin up of CHIME J1634+44 could be driven by gravitational wave radiation.

astro-ph.HE↗

arXiv2025

FRB 20250316A: A Brilliant and Nearby One-Off Fast Radio Burst Localized to 13 parsec Precision

Precise localizations of a small number of repeating fast radio bursts (FRBs) using very long baseline interferometry (VLBI) have enabled multiwavelength follow-up observations revealing diverse local environments. However, the 2--3\% of FRB sources that are observed to repeat may not be representative of the full population. Here we use the VLBI capabilities of the full CHIME Outriggers array for the first time to localize a nearby (40 Mpc), bright (kJy), and apparently one-off FRB source, FRB 20250316A, to its environment on 13-pc scales. We use optical and radio observations to place deep constraints on associated transient emission and the properties of its local environment. We place a $5σ$ upper limit of $L_{\mathrm{9.9~\mathrm{GHz}}} < 2.1\times10^{25}~\mathrm{erg~s^{-1}~Hz^{-1}}$ on spatially coincident radio emission, a factor of 100 lower than any known compact persistent radio source associated with an FRB. Our KCWI observations allow us to characterize the gas density, metallicity, nature of gas ionization, dust extinction and star-formation rate through emission line fluxes. We leverage the exceptional brightness and proximity of this source to place deep constraints on the repetition of FRB 20250316A, and find it is inconsistent with all well-studied repeaters given the non-detection of bursts at lower spectral energies. We explore the implications of a measured offset of 190$\pm20$ pc from the center of the nearest star-formation region, in the context of progenitor channels. FRB 20250316A marks the beginning of an era of routine localizations for one-off FRBs on tens of mas-scales, enabling large-scale studies of their local environments.

astro-ph.HE↗

arXiv2025

James Webb Space Telescope Observations of the Nearby and Precisely-Localized FRB 20250316A: A Potential Near-IR Counterpart and Implications for the Progenitors of Fast Radio Bursts

We present deep James Webb Space Telescope near-infrared imaging to search for a quiescent or transient counterpart to FRB 20250316A, which was precisely localized with the CHIME/FRB Outriggers array to an area of $11\times13$ pc in the outer regions of NGC 4141 at $d\approx40$ Mpc. Our F150W2 image reveals a faint source near the center of the FRB localization region ("NIR-1"; $M_{\rm F150W2}\approx-2.5$ mag; probability of chance coincidence $\approx0.36$), the only source within $\approx2.7σ$. We find that it is too faint to be a globular cluster, young star cluster, red supergiant star, or a giant star near the tip of the red giant branch (RGB). It is instead consistent with a red giant near the RGB "clump" or a massive ($\gtrsim20$ M$_{\odot}$) main sequence star, although the latter explanation is less likely. The source is too bright to be a supernova remnant, Crab-like pulsar wind nebula, or isolated magnetar. Alternatively, NIR-1 may represent transient emission, namely a dust echo from an energetic outburst associated with the FRB, in which case we would expect it to fade in future observations. We explore the stellar population near the FRB and find that it is composed of a mix of young massive stars ($\sim10-100$ Myr) in a nearby HII region that extends to the location of FRB 20250316A, and old evolved stars ($\gtrsim$ Gyr). The overlap with a young stellar population, containing stars of up to $\approx20$ M$_\odot$, may implicate a neutron star / magnetar produced in the core collapse of a massive star as the source of FRB 20250316A.

astro-ph.HE↗

arXiv2025

Discovery and Localization of the Swift-Observed FRB 20241228A in a Star-forming Host Galaxy

On 2024 December 28, CHIME/FRB detected the thus-far non-repeating FRB 20241228A with a real-time signal-to-noise ratio of $>50$. Approximately 112~s later, the X-ray Telescope onboard the Neil Gehrels Swift Observatory was on source, the fastest follow-up to-date of a non-repeating FRB (Tohuvavohu et al. in prep.). Using CHIME/FRB and two of the three CHIME/FRB Outriggers, we obtained a Very Long Baseline Interferometry localization for FRB 20241228A with a 1$σ$ confidence ellipse of 11$^{\prime\prime}$ by 0.2$^{\prime\prime}$. This represents the first published localization using both the CHIME-KKO and CHIME-GBO Outriggers. We associate FRB 20241228A with a star-forming galaxy at a redshift of $z = 0.1614\pm0.0002$. The persistent X-ray luminosity limit at this source's location and distance is $<1.2 \times 10^{43}$ erg s$^{-1}$ in the $0.3-10$ keV band, the most stringent limit of any non-repeating FRB to-date (Tohuvavohu et al. in prep.). The stellar mass ($\sim 2.6 \times 10^{10}\,M_{\odot}$) and star formation rate ($\sim 2.9\,M_{\odot}$~yr$^{-1}$) of the host galaxy of FRB 20241228A are consistent with the broader FRB host galaxy population. We measure significant scattering ($\sim$1ms) and scintillation ($\sim$20 kHz at 600 MHz) along the line of sight to this source, and suggest the scintillation screen is Galactic while the scattering screen is extragalactic. FRB 20241228A represents an exciting example of a new era in which we can harness VLBI-localizations and rapid high-energy follow-up to probe FRB progenitors.

astro-ph.HE↗

arXiv2025

Measurement of the Dispersion$\unicode{x2013}$Galaxy Cross-Power Spectrum with the Second CHIME/FRB Catalog

The dispersion of extragalactic fast radio bursts (FRBs) can serve as a powerful probe of the diffuse plasma between and surrounding galaxies, which contains most of the Universe's baryons. By cross-correlating the dispersion of background FRBs with the locations of foreground galaxies, we can study the relative spatial distributions of plasma and galaxies on scales of 0.1 to 50 Mpc, which are strongly affected by feedback processes in galaxy formation. Here we present the measurement of the dispersion$\unicode{x2013}$galaxy angular cross-power spectrum between 2873 FRBs from the Second CHIME/FRB Catalog and nearly 6 million galaxies from the Dark Energy Spectroscopic Instrument (DESI) Legacy Imaging Survey. Over five photometric galaxy redshift bins spanning $0.05 < z <0.5$ and at 5.1$σ$ significance, we make the first definitive detection of spatial correlations in FRB dispersion measure due to cosmic structure. While parameter inferences should be interpreted with caution because of incomplete modelling of both the signal and systematic errors, our data indicate that the plasma$\unicode{x2013}$galaxy cross-power spectrum cuts off relative to the matter power spectrum at a scale $k_\textrm{cut}^{-1}=0.9^{+0.4}_{-0.4}\,\textrm{Mpc}$. This scale is consistent with those X-ray stacking analyses that suggest dark-matter halos with group-scale masses are largely evacuated of their baryons by feedback processes. Our study demonstrates that FRBs are promising tools to discern the physics of baryonic structure formation and will only become more powerful as FRB surveys expand.

astro-ph.CO↗

arXiv2025

The CHIME/FRB Discovery of the Extremely Active Fast Radio Burst Source FRB 20240114A

Among the thousands of observed fast radio bursts (FRBs), a few sources exhibit exceptionally high burst activity observable by many telescopes across a broad range of radio frequencies. Almost all of these highly active repeaters have been discovered by CHIME/FRB, due to its daily observations of the entire Northern sky as a transit radio telescope. FRB 20240114A is a source discovered and reported by CHIME/FRB to the community in January 2024; given its low declination, even the detection of a few bursts hints at a high burst rate. Following the community announcement of this source as a potentially active repeater, it was extensively followed up by other observatories and has emerged as one of the most prolific FRB repeaters ever observed. This paper presents the five bursts CHIME/FRB observed from FRB 20240114A, with channelized raw voltage data saved for two bursts. We do not observe changes in the DM of the source greater than ~1.3 pc cm$^{-3}$ in our observations over nearly a year baseline. We find an RM of ~ +320 rad m$^{-2}$. We do not find evidence for scattering at the level of < 0.3 ms in the bursts, and we find no evidence for astrophysical scintillation. In our observations of FRB 20240114A, we see a burst rate ~49x higher than the median burst rate of apparent non-repeaters also discovered by CHIME/FRB. Each discovery of highly active FRBs provides a valuable opportunity to investigate whether there is a fundamental difference between repeating and apparently non-repeating sources.

astro-ph.HE↗

arXiv2025

CHIME/FRB Outriggers: Design Overview

The Canadian Hydrogen Intensity Mapping Experiment (CHIME) has emerged as the world's premier facility for studying fast radio bursts (FRBs) through its fast transient search backend CHIME/FRB\@. The CHIME/FRB Outriggers project will augment this high detection rate of 2--3 FRBs per day with the ability to precisely localize them using very long baseline interferometry (VLBI). Using three strategically located stations in North America and deploying recently developed synoptic VLBI observing techniques, the Outriggers will provide $\sim 50$~milliarcsecond localization precision for the majority of detected FRBs. This paper presents an overview of the design and implementation of the Outriggers, covering their geographic distribution, structural design, and observational capabilities. We detail the scientific objectives driving the project, including the characterization of FRB populations, host galaxy demographics, and the use of FRBs as cosmological probes. We also discuss the calibration strategies available to mitigate ionospheric and instrumental effects, ensuring high-precision localization. With two stations currently in science operations, and the third in commissioning, the CHIME/FRB Outriggers project is poised to become a cornerstone of the FRB field, offering unprecedented insights into this enigmatic cosmic phenomenon.

astro-ph.HE↗

arXiv2025

A Catalog of Local Universe Fast Radio Bursts from CHIME/FRB and the KKO Outrigger

We present the first catalog of fast radio burst (FRB) host galaxies from CHIME/FRB Outriggers, selected uniformly in the radio and the optical by localizing 81 new bursts to 2'' x ~60'' accuracy using CHIME and the KKO Outrigger, located 66 km from CHIME. Of the 81 localized bursts, we use the Probabilistic Association of Transients to their Hosts (PATH) algorithm to securely identify 21 new FRB host galaxies, and compile spectroscopic redshifts for 19 systems, 15 of which are newly obtained via spectroscopic observations. The most nearby source is FRB 20231229A, at a distance of 90 Mpc. One burst in our sample is from a previously reported repeating source in a galaxy merger (FRB 20190303A). Three new FRB host galaxies (FRBs 20230203A, 20230703A, and 20231206A) are found towards X-ray and optically selected galaxy clusters, potentially doubling the sample of known galaxy cluster FRBs. A search for radio counterparts reveals that FRB 20231128A is associated with a luminous persistent radio source (PRS) candidate with high significance ($P_{cc} \sim 10^{-2}$). If its compactness is confirmed, it would be the nearest known compact PRS at $z = 0.1079$. Our catalog significantly increases the statistics of the Macquart relation at low redshifts ($z < 0.2$). In the near future, the completed CHIME/FRB Outriggers array will produce hundreds of FRBs localized with very long baseline interferometry (VLBI). This will significantly expand the known sample and pave the way for future telescopes relying on VLBI for FRB localization.

astro-ph.HE↗

arXiv2025

Searching for axion dark matter gegenschein of the Vela supernova remnant with FAST

Axions are one of the leading dark matter candidates. If we are embedded in a Milky Way dark matter halo comprised of axions, their stimulated decay would enable us to observe a counterimage (``axion gegenschein") with a frequency equal to half the axion mass in the opposite direction of a bright radio source. This spectral line emission will be broadened to $Δν/ν\sim σ_d/c \sim 10^{-3}$ due to the velocity dispersion of dark matter, $σ_d$. In this pilot study, we perform the first search for the expected axion gegenschein image of Vela supernova remnant (SNR) with 26.4 hours of effective ON-OFF data from the Five-hundred-meter Aperture Spherical radio Telescope (FAST) L-band (1.0 - 1.5~GHz) 19-beam receiver. Our null detection limits the axion-photon coupling strength to be $g_{aγγ} \lesssim 2 \times 10^{-10} \mathrm{GeV}^{-1}$ in the mass ranges of $8.7\,μ\mathrm{eV} \leq m_a \leq 9.44\,μ\mathrm{eV}$ and $10.85\,μ\mathrm{eV} \leq m_a \leq 12.01\,μ\mathrm{eV} $. These results provide a stronger constraint on $g_{aγγ}$ in this axion mass range than the current limits obtained by the direct search of axion decay signal from galaxy clusters which uses FAST observations, but is a factor of $\sim 3$ times weaker than the current CAST limit.Based on our observation strategy, data processing methods, and results, the expected sensitivity will reach $\sim 10^{-11}\mathrm{GeV}^{-1}$ with $\sim 2000$ hours of observation in the future.

astro-ph.CO↗