Line intensity mapping using atomic hydrogen (HI) has the potential to efficiently map large volumes of the universe if the signal can be successfully separated from overwhelmingly bright radio foreground emission. This motivates cross-correlations, to ascertain the cosmological nature of measured HI fluctuations, and to study their connections with galaxies and the underlying matter density field. However, these same foregrounds render the cross-correlation with projected fields such as the lensing of the cosmic microwave background (CMB) difficult. Indeed, the correlated Fourier modes vary slowly along the line of sight, and are thus most contaminated by the smooth-spectrum radio continuum foregrounds. In this paper, we implement a method that avoids this issue by attempting to measure the non-linear gravitational coupling of the small-scale 21cm power from the Canadian Hydrogen Intensity Mapping Experiment (CHIME) with large-scale Planck CMB lensing. This measurement is a position-dependent power spectrum, i.e. a squeezed integrated bispectrum. Using 94 nights of CHIME data between 1.0 < z < 1.3 and aggressive foreground filtering, we find that the expected signal is five times smaller than the current noise. We forecast that incorporating the additional nights of CHIME data already collected would enable a signal-to-noise ratio of 3, without any further improvements in filtering for foreground cleaning.

Fast radio bursts (FRBs) are a similar to 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 no evidence for a signal with a nonzero expectation in the phase correlation, (C(f, t)), but we do find an excess correlation signature is present in the absolute power of the phase correlation, (C(f, t)) 2. 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.

We present a catalog of 4539 fast radio bursts (FRBs) observed with the Canadian Hydrogen Intensity Mapping Experiment telescope between 2018 July 25 and 2023 September 15. These bursts originate from 3641 unique sources, including 981 bursts from 83 known repeating sources. For each FRB, the catalog provides an 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.

Observations with the Canadian Hydrogen Intensity Mapping Experiment (CHIME) have been used to measure the 21 cm intensity mapping auto power spectrum, at z∼ 1, over a frequency range from 608.2 MHz to 707.8 MHz at wavenumbers 0.4 h Mpc^-1≲ k ≲ 1.5 h Mpc^-1. In this paper, we present the results of two different approaches to interpreting this measurement. In the first approach, we use a parametric power spectrum model to constrain an amplitude parameter, defined as 𝒜^2_ HI≡ 10^6 Ω_ HI^2(b^2_ HI+⟨ f μ^2⟩)^2, where Ω_ HI is the cosmological density parameter for atomic hydrogen (HI), b_ HI is the linear bias for HI, and ⟨ f μ^2⟩ incorporates the dominant large-scale impact of redshift-space distortions on the angle-averaged power spectrum. Imposing an additional prior on either Ω_ HI or b_ HI, based on values in the literature, allows us to break the pairwise degeneracy between those two parameters. In the second approach, we compare CHIME's measurement with predictions for the power spectrum of HI from the IllustrisTNG simulations, finding that the measurement disagrees with the TNG100 run at 3.1σ and the TNG300 run at 4.0σ. This disagreement is most likely attributable to the strength of nonlinear redshift-space clustering of HI in the simulations, rather than the total abundance of HI, and invites further investigation of the physical processes in the simulations that determine the behavior of HI at nonlinear scales. These results exemplify the ability of 21 cm intensity mapping to provide astrophysical information using measurements at nonlinear scales.

The sensitivity of radio telescopes is becoming increasingly limited by the presence of radio frequency interference (RFI), which will worsen as the radio spectrum becomes more crowded. One context where this poses a challenge is the field of fast radio burst (FRB) science, where there is increasing scientific interest in capturing as large of a population of bursts as possible and accurately measuring their celestial coordinates using interferometry. With several modern radio facilities actively collecting data for large FRB surveys that will be transformative to the field, properly mitigating unwanted interference is essential for the science goals of these surveys to be met. In this work, we present variations of a spatial filter based on the Karhunen-Loeve (KL) Transform to enhance the sensitivity of radio interferometers and demonstrate its applicability to FRB detection and localization. We derive a particular variation of the filter for the case of point-like radio pulses, which we show reduces to the maximum-signal-to-noise beamformer. We apply this filter to CHIME/FRB baseband data and demonstrate its capability to enhance the sensitivity and overall localization rate of CHIME/FRB Outriggers. We compare the cross-correlation signal-to-noise obtained using the spatial filter with that obtained using a spectral-kurtosis RFI flagger for a sample of 100 FRBs recorded by CHIME and its Outriggers, and show that this filter will double the total number of FRBs successfully localized with the CHIME/FRB Outrigger telescopes. While demonstrated here in the context of CHIME/FRB Outriggers, the spatial filter presented in this work–which we have made publicly available–is broadly applicable to other interferometric radio facilities engaged in FRB science and transient detection, including next-generation telescopes such as CHORD, DSA-2000, BURSTT, and CHARTS.

Superconducting transition-edge sensor (TES) detectors have been the standard in Cosmic Microwave Background experiments for almost two decades and are now being adapted for use in nuclear physics, such as neutrinoless double beta decay searches. In this paper we focus on a new high-bandwidth frequency multiplexed TES readout system developed for CUPID, a neutrinoless double beta decay experiment that will replace CUORE. In order to achieve the high energy resolution requirements for CUPID, the readout noise of the system must be kept to a minimum. Low TES operating resistance and long wiring between the readout SQUID and the warm electronics are needed for CUPID, prompting a careful consideration of the design parameters of this application of frequency multiplexing. In this work, we characterize the readout noise of the newly designed frequency multiplexed TES readout system for CUPID and construct a noise model to understand it. We find that current sharing between the SQUID coil impedance and other branches of the circuit, as well as the long output wiring, worsen the readout noise of the system. To meet noise requirements, a SQUID with a low input inductance, high transimpedance and/or low dynamic impedance is needed, and the wiring capacitance should be kept as small as possible. Alternatively, the option of adding a cryogenic low-noise amplifier at the output of the SQUID should be explored.

The LiteBIRD satellite will map the polarisation of the Cosmic Microwave Background (CMB) with unprecedented sensitivity by means of antenna-coupled transition edge sensors (TESs). Its current design features single SQUID-Array Amplifiers (SAA) at a temperature of about 400 mK in order to read out up to 64 TESs in a frequency division multiplexing scheme. The front-end part of the electronics (the SQUID Controller Assembly -SCA) was designed to handle this configuration. Nevertheless, alternative configurations of the experiment are being studied that might require a dual-stage SQUID (DSS) amplification chain in order to meet thermal and noise requirements. In light of this possibility, we have successfully investigated the feasibility of controlling two individual SAAs in a DSS Amplifier (DSSA) configuration with two independent channels of the present SCA. The SQUID directly connected to the sensor, commonly referred to as ‘front-end’, behaves as an emitter-follower, or buffer, circuit and it decouples the impedance of the TES(s) from the rest of the readout chain whilst producing a moderate amplification. The second SQUID, ‘booster’, is responsible for a high-gain amplification of the signal and can be thermally decoupled from the front-end allowing it to dissipate a lot more power and thus exhibiting a higher gain. We characterised a breadboard model in which two SQUID amplifiers are cascaded but hosted on the same circuit board and thus not thermally decoupled. This was intended to perform functional tests and to assess the feasibility of controlling a DSSA without introducing major changes in the already developed LiteBIRD SCA. We present the typical I-V and I-$\Phi$ curves of the two SQUIDs along with the characterisation of the DSSA in terms of bandwidth, linearity and total harmonic distortion (THD).

High redshift Fast Radio Bursts (FRBs) are expected to be extremely powerful probes of our Universe. However, while a significant number of FRBs are expected to exist at high redshift, detecting them has been difficult, with only a handful robustly confirmed at redshifts greater than one. In many other fields, gravitational lensing from galaxy clusters has enabled high redshift detections by magnifying background sources. In this work we forecast the populations of FRBs expected to be detected by CHIME and upcoming instrument CHORD, for blank fields and by lensing through a range of strong lensing galaxy clusters, based on existing, observationally driven cluster models. We find that the presence of a galaxy cluster of mass M≥5×10^14 M_⊙ within the detection beam of a transit telescope will approximately double the rate of detected high redshift (z≥1 CHIME, z≥2 CHORD) FRBs for that beam. Consequently, we find that knowledge of cluster positions can be used by instruments like CHIME or CHORD in tandem with novel observational strategies to isolate a sample of high redshift FRBs with ≳50% purity at rate of ≲3 per year. This would provide a statistically high redshift sample of mostly gravitationally lensed FRBs, that would be ideal candidates for optical follow-up, constraining the FRB-star formation relation and for use in cosmological studies including measuring H_0, characterising dark matter substructures and probing reionization.

Cosmic shear, galaxy clustering, and the abundance of massive halos each probe the large-scale structure of the Universe in complementary ways. We present cosmological constraints from the joint analysis of the three probes, building on the latest analyses of the lensing-informed abundance of clusters identified by the South Pole Telescope (SPT) and of the auto- and cross-correlation of galaxy position and weak lensing measurements (3 x 2pt) in the Dark Energy Survey (DES). We consider the cosmological correlation between the different tracers and we account for the systematic uncertainties that are shared between the large-scale lensing correlation functions and the small-scale lensing-based cluster mass calibration. Marginalized over the remaining Lambda cold dark matter (Lambda CDM) parameters (including the sum of neutrino masses) and 52 astrophysical modeling parameters, we measure Omega(m) = 0.300 +/- 0.017 and sigma(8) = 0.797 +/- 0.026. Compared to constraints from Planck primary cosmic microwave background (CMB) anisotropies, our constraints are only 15% wider with a probability to exceed of 0.22 (1.2 sigma) for the two-parameter difference. We further obtain S-8 equivalent to sigma(8)(Omega(m)/0.3)(0.5) = 0.796 +/- 0.013 which is lower than the Planck measurement at the 1.6 sigma level. The combined SPT cluster, DES 3 x 2pt, and Planck datasets mildly prefer a nonzero positive neutrino mass, with a 95% upper limit Sigma m(nu) < 0.25 eV on the sum of neutrino masses. Assuming a wCDM model, we constrain the dark energy equation of state parameter w = -1.15(-0.17)(+0.23) and when combining with Planck primary CMB anisotropies, we recover w = -1.20(-0.09)(+0.15), a 1.7 sigma difference with a cosmological constant. The precision of our results highlights the benefits of multiwavelength multiprobe cosmology and our analysis paves the way for upcoming joint analyses of next-generation datasets.

We report the discovery of the repeating fast radio burst (FRB) source FRB 20240209A using the Canadian Hydrogen Intensity Mapping Experiment (CHIME)/FRB telescope. We detected 22 bursts from this repeater between 2024 February and July, 6 of which were also recorded at the Outrigger station k’niʔatn k’l ⌣ stk’masqt (KKO). The multiple very long baseline interferometry localizations using the 66 km long CHIME–KKO baseline, each with a different baseline vector orientation due to the repeater’s high decl. of ∼86°, enabled the combined localization region to be constrained to 1″ × 2″. We present deep Gemini optical observations that, combined with the FRB localization, enabled a robust association of FRB 20240209A to the outskirts of a luminous galaxy (P(O∣x) = 0.99; L ≈ 5.3 × 10 10 L ⊙ ). FRB 20240209A has a projected physical offset of 40 ± 5 kpc from the center of its host galaxy, making it the FRB with the largest host galaxy offset to date. When normalized by the host galaxy size, the offset of FRB 20240209A (5.1 R eff ) is comparable to that of FRB 20200120E (5.7 R eff ), the only FRB source known to originate in a globular cluster. We consider several explanations for the large offset, including a progenitor that was kicked from the host galaxy or in situ formation in a low-luminosity satellite galaxy of the putative host, but find the most plausible scenario to be a globular cluster origin. This, coupled with the quiescent, elliptical nature of the host as demonstrated in our companion Letter, provides strong evidence for a delayed formation channel for the progenitor of the FRB source.

In this paper, we present improvements to the pointing accuracy of the South Pole Telescope (SPT) using machine learning. The ability of the SPT to point accurately at the sky is limited by its structural imperfections, which are impacted by the extreme weather at the South Pole. Pointing accuracy is particularly important during SPT participation in observing campaigns with the Event Horizon Telescope (EHT), which requires stricter accuracy than typical observations with the SPT. We compile a training dataset of historical observations of astronomical sources made with the SPT-3G and EHT receivers on the SPT. We train two XGBoost models to learn a mapping from current weather conditions to two telescope drive control arguments — one which corrects for errors in azimuth and the other for errors in elevation. Our trained models achieve root mean squared errors on withheld test data of 2[Formula: see text]14 in cross-elevation and 3[Formula: see text]57 in elevation, well below our goal of 5[Formula: see text] along each axis. We deploy our models on the telescope control system and perform further in situ test observations during the EHT observing campaign in April 2024. Our models result in significantly improved pointing accuracy: for sources within the range of input variables where the models are best trained, average combined pointing error improved 33%, from 15[Formula: see text]9 to 10[Formula: see text]6. These improvements, while significant, fall shy of our ultimate goal, but they serve as a proof of concept for the development of future models. Planned upgrades to the EHT receiver on the SPT will necessitate even stricter pointing accuracy which will be achievable with our methods.

From CMB polarization data alone, we reconstruct the CMB lensing power spectrum, comparable in overall constraining power to previous temperature-based reconstructions, and an unlensed E-mode power spectrum, with clear detections of the third through the tenth acoustic peaks. The observations, taken in 2019 and 2020 with the South Pole Telescope (SPT) and the SPT-3G camera, cover 1500 deg(2) at 95, 150, and 220 GHz with arcminute resolution and roughly 4.9 mu K-arcmin coadded noise in polarization. The power spectrum estimates, together with systematic parameter estimates and a joint covariance matrix, follow from a Bayesian analysis using the marginal unbiased score expansion (MUSE) method. The E-mode spectrum at l > 2000 and lensing spectrum at L > 350 are the most precise to date. Assuming the Lambda CDM model, and using only these SPT data and priors on tau and absolute calibration from Planck, we find H-0 = 66.81 +/- 0.81 km/s/Mpc, comparable in precision to the Planck determination and in 5.4 sigma tension with the most precise H-0 inference derived via the distance ladder. We also find S-8 = sigma(8) (Omega(m)/0.3)(0.5) = 0.850 +/- 0.017, providing further independent evidence of a slight tension with low-redshift structure probes. The Lambda CDM model provides a good simultaneous fit to the combined Planck, ACT, and SPT data, and thus passes a powerful test. Combining these CMB datasets with BAO observations, we explore extensions to the Lambda CDMmodel. We find that the effective number of neutrino species, spatial curvature, and primordial helium fraction are consistent with standard model values, and that the 95% confidence upper limit on the neutrino mass sum is 0.075 eV, close to the minimum sum expected from observations of solar and atmospheric neutrino oscillations. The SPT data are consistent with the somewhat weak (< 3 sigma) preference for excess lensing power seen in Planck and ACT data relative to predictions of the.CDM model given the combined Planck, ACT, and BAO datasets. We also detect at greater than 3 sigma the influence of nonlinear evolution in the CMB lensing power spectrum and discuss it in the context of the S-8 tension. Forthcoming SPT-3G analyses will feature deeper and wider observations in temperature and polarization, providing even tighter constraints and more powerful tests of the Lambda CDM model.

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.

The detection of satellite thermal emission at millimeter wavelengths is presented using data from the 3rd-Generation receiver on the South Pole Telescope (SPT-3G). This represents the first reported detection of thermal emission from artificial satellites at millimeter wavelengths. Satellite thermal emission is shown to be detectable at high signal-to-noise on timescales as short as a few tens of milliseconds. An algorithm for downloading orbital information and tracking known satellites given observer constraints and time-ordered observatory pointing is described. Consequences for cosmological surveys and short-duration transient searches are discussed, revealing that the integrated thermal emission from all large satellites does not contribute significantly to the SPT-3G survey intensity map. Measured satellite positions are found to be discrepant from their two-line element (TLE) derived ephemerides up to several arcminutes which may present a difficulty in cross-checking or masking satellites from short-duration transient searches.

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 Outrigger 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 5 σ upper limit of L _9.9 GHz < 2.1 × 10 ^25 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 Keck Cosmic Webb Imager 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 that it is inconsistent with all well-studied repeaters given the nondetection of bursts at lower spectral energies. We explore the implications of a measured offset of 190 ± 20 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 milliarcseconds scales, enabling large-scale studies of their local environments.

We present the first detection of the cosmological 21 cm intensity mapping signal in auto-correlation at z 1 with the Canadian Hydrogen Intensity Mapping Experiment (CHIME). Using 94 nights of observation, we have measured the 21 cm auto-power spectrum over a frequency range from 608.2 MHz to 707.8 MHz (z = 1.34 to 1.01) at 0.4 h Mpc^-1 < k < 1.5 h Mpc^-1, with a detection significance of 12.5 sigma. Our analysis employs significant improvements to the CHIME data processing pipeline compared to previous work, including novel radio frequency interference (RFI) detection and masking algorithms, achromatic beamforming techniques, and foreground filtering before time averaging to minimize spectral leakage. We establish the robustness and reliability of our detection through a comprehensive suite of validation tests. We also measure the 21 cm signal in two independent sub-bands centered at z 1.08 and z 1.24 with detection significance of 8.7 sigma and 9.2 sigma, respectively. We briefly discuss the theoretical interpretation of these measurements in terms of a power spectrum model, deferring the details to a companion paper. This auto-power spectrum detection demonstrates CHIME's capability to probe large-scale structure through 21 cm intensity mapping without reliance on external galaxy surveys.

We present a spectro-temporal analysis of 137 fast radio bursts (FRBs) from the first CHIME/FRB baseband catalog, including 125 one-off bursts and 12 repeat bursts, down to microsecond resolution using the least-squares optimization fitting routine fitburst . We measure subburst components within burst envelopes as narrow as ~23 μ s (FWHM), with 20% of the sample displaying substructures narrower than 100 μ s, offering constraints on emission mechanisms. Scattering timescales in the sample range from 30 μ s to 13 ms at 600 MHz. Our measured values are compared with those in the first CHIME/FRB intensity catalog, revealing that nearly one-third of our sample exhibits additional burst components at higher time resolutions. We observe no correlations between scattering time and dispersion measure, rotation measure, or linear polarization fraction, with the latter suggesting that depolarization due to multipath propagation is negligible in our sample. Bursts with narrower envelopes (≤1 ms) in our sample exhibit higher flux densities, indicating the potential presence of submillisecond FRBs that are being missed by our real-time system below a brightness threshold. Most multicomponent bursts in our sample exhibit subburst separations of ≤1 ms, with no bursts showing separations < 41 μ s, even at a time resolution of 2.56 μ s, but both scattering and low signal-to-noise ratio can hinder the detection of additional components. Lastly, given the morphological diversity of our sample, we suggest that one-off and repeating FRBs can come from different classes but have overlapping property distributions.

Large multiplexing factors are a primary advantage of kinetic inductance detectors (KIDs), but the implementation of high density arrays still presents significant challenges. Deviations between designed and achieved resonant frequencies are common, and differential loading and responsivity variation across an array may lead to dynamic inter-resonator interactions. It is therefore valuable to be able to both set and maintain the resonant frequency of a KID in situ, using the readout system. We show that it is possible to alter the resonant frequency of the devices by multiple linewidths through the application of readout current, and establish a new stable operational bias point at the driven frequency by making use of the hysteretic bistability commonly seen as bifurcation in frequency-domain measurements. We examine this interaction using a readout tone at fixed frequency positioned near or within the unbiased resonant bandwidth. Development of a control methodology based on this principle remains in an early stage, but a foundational step is understanding the interaction of the readout current with the resonator, in particular its influence on the resonant frequency. In this work, we study conventional KIDs with no physical isolation from the substrate, so we posit that the readout current primarily interacts with the resonator via non-thermal mechanisms, resulting in a predominantly reactive response. This behaviour is reproduced by a simple lumped-element circuit model of the resonance and readout system, providing a straightforward framework for analysis and interpretation. This demonstration is an important early step in the development of techniques which seek to dynamically alter the resonant frequencies of conventional KID arrays, and sets the stage for fast active resonant frequency control under operational conditions.

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″ × ∼ 60″ accuracy using CHIME and the k’niʔatn k’l ⌣ stk’masqt Outrigger station, located 66 km from CHIME. Of the 81 localized bursts, we use the probabilistic association of transients to their hosts 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 toward 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 ∼ 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.

The main challenge of 21-cm cosmology experiments is astrophysical foregrounds that are difficult to separate from the signal due to telescope systematics. An earlier study has shown that foreground residuals induced by antenna gain errors can be estimated and subtracted using the hybrid foreground residual subtraction (HyFoReS) technique, which relies on cross-correlating linearly filtered data. In this paper, we apply a similar technique to the CHIME stacking analysis to subtract beam-induced foreground contamination. Using a linear high-pass delay filter for foreground suppression, the CHIME collaboration reported a 11.1 sigma detection in the 21-cm signal stacked on eBOSS quasar locations, despite foreground residual contamination mainly due to the instrument chromatic transfer function. Following the HyFoReS formalism, we cross-correlate the foreground-dominated data at low delay with the contaminated signal at high delay to estimate residual foregrounds and subtract them from the signal. We find that HyFoReS can improve the signal-to-noise ratio of the stacked 21-cm signal by 10%-20% after the delay foreground filter, although some of the improvement can also be achieved with an alternative flagging technique. We have shown that it is possible to use HyFoReS to reduce beam-induced foreground contamination, benefiting the analysis of the HI auto power spectrum with CHIME and enabling the of modes.