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Abstract For many pulsars, the scattering structures responsible for scintillation are typically dominated by a single, thin screen along the line of sight, which persists for years or decades. In recent years, an increasing number of doubly lensed events have been observed, where a secondary lens crosses the line of sight. This causes additional or distorted scintillation arcs over timescales ranging from days to months. In this work, we report such a transient event for pulsar B1737+13 and propose a possible lensing geometry including the distance to both lenses and the orientation of the main screen. Using phase retrieval techniques to separate the two lenses in the wavefield, we report the curvature and rate of motion of features associated with the secondary lens as it passed through the line of sight. By fitting the annual variation of the curvature, we report a possible distance and orientation for the main screen. The distance of the secondary lens is found by mapping the secondary feature onto the sky and tracking its position over time for different distances. We validate this method using B0834+06, for which the screen solutions are known through VLBI, and successfully recover the correct solution for the secondary feature. With the identified lensing geometry, we are able to estimate the size of the secondary lens, 1–3 au. Although this is an appropriate size for a structure that could cause an extreme scattering event, we do not have conclusive evidence for or against that possibility. 

Abstract We present the first results of the holographic beam-mapping program for the Canadian Hydrogen Intensity Mapping Experiment (CHIME). We describe the implementation of a holographic technique as adapted for CHIME, and introduce the processing pipeline which prepares the raw holographic timestreams for analysis of beam features. We use data from six bright sources across the full 400–800 MHz observing band of CHIME to provide measurements of the copolar and cross-polar beam response in both amplitude and phase for all 1024 dual-polarized feeds in the array. In addition, we present comparisons with independent probes of the CHIME beam, which indicate the presence of polarized beam leakage. Holographic measurements of the beam have already been applied in science with CHIME, e.g., in estimating the detection significance of far-sidelobe fast radio bursts, and in validating the beam models used for CHIME’s first detections of 21 cm emission (in cross-correlation with measurements of large-scale structure from galaxy surveys and the Lyαforest). Measurements presented in this paper, and future holographic results, will provide a unique data set to characterize the CHIME beam and improve the experiment’s prospects for a detection of the baryon acoustic oscillation signal. 

Abstract 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 5σupper limit ofL_{9.9 GHz} < 2.1 × 10²⁵erg s⁻¹Hz⁻¹on 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. 

Abstract In extreme scattering events, the brightness of a compact radio source drops significantly, as light is refracted out of the line of sight by foreground plasma lenses. Despite recent efforts, the nature of these lenses has remained a puzzle, because any roughly round lens would be so highly overpressurized relative to the interstellar medium that it could only exist for about a year. This, combined with a lack of constraints on distances and velocities, has led to a plethora of theoretical models. We present observations of a dramatic double-lensing event in pulsar PSR B0834+06 and use a novel phase-retrieval technique to show that the data can be reproduced remarkably well with a two-screen model: one screen with many small lenses and another with a single, strong one. We further show that the latter lens is so strong that it would inevitably cause extreme scattering events. Our observations show that the lens moves slowly and is highly elongated on the sky. If similarly elongated along the line of sight, as would arise naturally from a sheet of plasma viewed nearly edge-on, no large overpressure is required and hence the lens could be long-lived. 

Abstract We report 10 fast radio bursts (FRBs) detected in the far sidelobe region (i.e., ≥5° off-meridian) of the Canadian Hydrogen Intensity Mapping Experiment (CHIME) from August 28 2018 to August 31 2021. We localize the bursts by fitting their spectra with a model of the CHIME/FRB synthesized beam response. We find that the far sidelobe events have on average ∼500 times greater fluxes than events detected in CHIME’s main lobe. We show that the sidelobe sample is therefore statistically ∼20 times closer than the main lobe sample. We find promising host galaxy candidates (P_cc< 1%) for two of the FRBs, 20190112B and 20210310B, at distances of 38 and 16 Mpc, respectively. CHIME/FRB did not observe repetition of similar brightness from the uniform sample of 10 sidelobe FRBs in a total exposure time of 35,580 hr. Under the assumption of Poisson-distributed bursts, we infer that the mean repetition interval above the detection threshold of the far sidelobe events is longer than 11,880 hr, which is at least 2380 times larger than the interval from known CHIME/FRB detected repeating sources, with some caveats, notably that very narrowband events could have been missed. Our results from these far sidelobe events suggest one of two scenarios: either (1) all FRBs repeat and the repetition intervals span a wide range, with high-rate repeaters being a rare sub-population, or (2) non-repeating FRBs are a distinct population different from known repeaters. 

Abstract We report the detection of 21 cm emission at an average redshift $\overline{z}=2.3$in the cross-correlation of data from the Canadian Hydrogen Intensity Mapping Experiment (CHIME) with measurements of the Lyαforest 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 withk_∥≲ 0.13 Mpc⁻¹at the average redshift. Line-of-sight spectra to the eBOSS background quasar locations are extracted from the CHIME maps and combined with the Lyαforest flux transmission spectra to estimate the 21 cm–Lyαcross-correlation function. Fitting a simulation-derived template function to this measurement results in a 9σdetection significance. The coherent accumulation of the signal through cross-correlation is sufficient to enable a detection despite excess variance from foreground residuals ∼6–10 times brighter than the expected thermal noise level in the correlation function. These results are the highest-redshift measurement of 21 cm emission to date, and they set the stage for future 21 cm intensity mapping analyses atz> 1.8. 

Abstract We present the localization and host galaxy of FRB 20190208A, a repeating source of fast radio bursts (FRBs) discovered using CHIME/FRB. As part of the Pinpointing REpeating ChIme Sources with EVN dishes repeater localization program on the European VLBI Network (EVN), we monitored FRB 20190208A for 65.6 hr at ∼1.4 GHz and detected a single burst, which led to its very long baseline interferometry localization with 260 mas uncertainty (2σ). Follow-up optical observations with the MMT Observatory (i≳ 25.7 mag (AB)) found no visible host at the FRB position. Subsequent deeper observations with the Gran Telescopio Canarias, however, revealed an extremely faint galaxy (r= 27.32 ± 0.16 mag), very likely (99.95%) associated with FRB 20190208A. Given the dispersion measure of the FRB (∼580 pc cm⁻³), even the most conservative redshift estimate ( $z_{\max }\sim 0.83$) implies that this is the lowest-luminosity FRB host to date (≲10⁸L_⊙), even less luminous than the dwarf host of FRB 20121102A. We investigate how localization precision and the depth of optical imaging affect host association and discuss the implications of such a low-luminosity dwarf galaxy. Unlike the other repeaters with low-luminosity hosts, FRB 20190208A has a modest Faraday rotation measure of a few tens of rad m⁻², and EVN plus Very Large Array observations reveal no associated compact persistent radio source. We also monitored FRB 20190208A for 40.4 hr over 2 yr as part of the Extragalactic Coherent Light from Astrophysical Transients repeating FRB monitoring campaign on the Nançay Radio Telescope and detected one burst. Our results demonstrate that, in some cases, the robust association of an FRB with a host galaxy will require both high localization precision and deep optical follow-up. 

Abstract 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 ratios of the bursts and to localize them to subarcminute precision. The improved localizations are also used to correct the beam response of the instrument and to measure fluxes and fluences with an ∼10% uncertainty. Additionally, the time resolution is increased by 3 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 data set 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. 

ABSTRACT We present the joint analysis of Neutral Hydrogen (H i) Intensity Mapping observations with three galaxy samples: the Luminous Red Galaxy (LRG) and Emission Line Galaxy (ELG) samples from the eBOSS survey, and the WiggleZ Dark Energy Survey sample. The H i intensity maps are Green Bank Telescope observations of the redshifted $$21\rm cm$$ emission on $$100 \, {\rm deg}^2$$ covering the redshift range 0.6 < z < 1.0. We process the data by separating and removing the foregrounds present in the radio frequencies with FastI ICA. We verify the quality of the foreground separation with mock realizations, and construct a transfer function to correct for the effects of foreground removal on the H i signal. We cross-correlate the cleaned H i data with the galaxy samples and study the overall amplitude as well as the scale dependence of the power spectrum. We also qualitatively compare our findings with the predictions by a semianalytical galaxy evolution simulation. The cross-correlations constrain the quantity $$\Omega _{\rm {H\,\small {I}}} b_{\rm {H\,\small {I}}} r_{\rm {H\,\small {I}},{\rm opt}}$$ at an effective scale keff, where $$\Omega _\rm {H\,\small {I}}$$ is the H  i density fraction, $$b_\rm {H\,\small {I}}$$ is the H i bias, and $$r_{\rm {H\,\small {I}},{\rm opt}}$$ the galaxy–hydrogen correlation coefficient, which is dependent on the H  i content of the optical galaxy sample. At $$k_{\rm eff}=0.31 \, h\,{\rm Mpc^{-1}}$$ we find $$\Omega _{\rm {H\,\small {I}}} b_{\rm {H\,\small {I}}} r_{\rm {H\,\small {I}},{\rm Wig}} = [0.58 \pm 0.09 \, {\rm (stat) \pm 0.05 \, {\rm (sys)}}] \times 10^{-3}$$ for GBT-WiggleZ, $$\Omega _{\rm {H\,\small {I}}} b_{\rm {H\,\small {I}}} r_{\rm {H\,\small {I}},{\rm ELG}} = [0.40 \pm 0.09 \, {\rm (stat) \pm 0.04 \, {\rm (sys)}}] \times 10^{-3}$$ for GBT-ELG, and $$\Omega _{\rm {H\,\small {I}}} b_{\rm {H\,\small {I}}} r_{\rm {H\,\small {I}},{\rm LRG}} = [0.35 \pm 0.08 \, {\rm (stat) \pm 0.03 \, {\rm (sys)}}] \times 10^{-3}$$ for GBT-LRG, at z ≃ 0.8. We also report results at $$k_{\rm eff}=0.24$$ and $$k_{\rm eff}=0.48 \, h\,{\rm Mpc^{-1}}$$. With little information on H i parameters beyond our local Universe, these are amongst the most precise constraints on neutral hydrogen density fluctuations in an underexplored redshift range.