Tomas Cassanelli

Context. Two fast radio bursts, FRB 20121102A and FRB 20180916B, show periodic activity with cycles of 159.3 and 16.33 days, respectively. These cycles consist of active and inactive windows, with the peak activity centred within the active phase. For FRB 20180916B, studies reported a frequency-dependent or chromatic behaviour, for which the activity window starts earlier and becomes narrower at higher frequencies. The activity across frequencies is typically modelled with a power law. Aims. We developed a simple model that combines the phase and frequency dependence of the activity windows of FRB 20121102A and FRB 20180916B. Our goal was to perform a chromaticity study for FRB 20121102A that incorporates model improvements to account for the cyclic nature of its activity window and to compare the chromatic behaviour of the two periodic FRBs. Methods. We standardised the detections from the 425 observing epochs for FRB 20121102A and the 214 epochs for FRB 20180916B to account for differences in the radio telescope sensitivity. To the normalised detection-rate phase distribution, we fitted a von Mises distribution and extracted the peak activity phase and activity width. These quantities as a function of frequency were then modelled as power laws to construct the chromatic model. Results. The activity window starts earlier at higher frequencies for the two sources. The activity window of FRB 20121102A broadens at higher frequencies, however, and that of FRB 20180916B broadens at lower frequencies. Interestingly, it appears to remain active during at least 80% of the cycle at C band. Conclusions. The observed chromatic behaviour of FRB 20180916B is consistent with previous findings. For FRB 20121102A, a chromaticity in its activity window is also seen, but the source appears to be active for longer at higher frequencies, which is different from the behaviour of FRB 20180916B.

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 ( 1 0 ′ ) 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.

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 ∼50 mas 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.

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.

One major goal in fast radio burst science is to detect fast radio bursts (FRBs) over a wide field of view without sacrificing the angular resolution required to pinpoint them to their host galaxies. Wide-field detection and localization capabilities have already been demonstrated using connected-element interferometry; the CHIME/FRB Outriggers project will push this further using widefield cylindrical telescopes as widefield outriggers for very long baseline interferometry (VLBI). This paper describes an offline VLBI software correlator written in Python for the CHIME/FRB Outriggers project. It includes features well-suited to modern widefield instruments like multibeaming/multiple phase center correlation, pulse gating including coherent dedispersion, and a novel correlation algorithm based on the quadratic estimator formalism. This algorithm mitigates sensitivity loss that arises in instruments where the windowing and channelization is done outside the VLBI correlator at each station, which accounts for a 30% sensitivity drop away from the phase center. Our correlation algorithm recovers this sensitivity on both simulated and real data. As an end-to-end check of our software, we have written a preliminary pipeline for VLBI calibration and single-pulse localization, which we use in Lanman et al. to verify the astrometric accuracy of the CHIME/FRB Outriggers array.

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.

As part of the Canadian Hydrogen Intensity Mapping Experiment Fast Radio Burst (CHIME/FRB) project, we report 41 new rotation measures (RMs) from 20 repeating fast radio bursts (FRBs) obtained between 2019 and 2023 for which no previous RM was determined. We also report 22 additional RM measurements for eight further repeating FRBs. We observe temporal RM variations in practically all repeating FRBs. Repeaters appear to be separated into two categories: those with dynamic and those with stable RM environments, differentiated by the ratios of RM standard deviations to the averaged RM magnitudes. Sources from stable RM environments likely have small RM contributions from the interstellar medium of their host galaxies, whereas sources from dynamic RM environments share some similarities with Galactic pulsars in eclipsing binaries but appear distinct from solitary pulsars in the Galactic centre. We observe a new stochastic, secular, and again stochastic trend in the temporal RM variation of FRB 20180916B, which does not support binary orbit modulation being its cause. We highlight two more repeaters that show RM sign change, namely FRBs 20290929C and 20190303A. We perform an updated comparison of polarization properties between repeating and nonrepeating FRBs, which show a marginal dichotomy in their distribution of electron-density-weighted parallel-component line-of-sight magnetic fields.

The discovery and localization of FRB 20240209A by the Canadian Hydrogen Intensity Mapping Fast Radio Burst (CHIME/FRB) experiment marks the first repeating FRB localized with the CHIME/FRB Outriggers and adds to the small sample of repeating FRBs with associated host galaxies. Here we present Keck and Gemini observations of the host that reveal a redshift z = 0.1384 ± 0.0004. We perform stellar population modeling to jointly fit the optical through mid-IR data of the host and infer a median stellar mass log(M _*/M _⊙) = 11.35 ± 0.01 and a mass-weighted stellar population age ~11 Gyr, corresponding to the most massive and oldest FRB host discovered to date. Coupled with a star formation rate M _⊙ yr⁻¹, the specific star formation rate −11.9 yr⁻¹ classifies the host as quiescent. Through surface brightness profile modeling, we determine an elliptical galaxy morphology, marking the host as the first confirmed elliptical FRB host. The discovery of a quiescent early-type host galaxy within a transient class predominantly characterized by late-type star-forming hosts is reminiscent of short-duration gamma-ray bursts, Type Ia supernovae, and ultraluminous X-ray sources. Based on these shared host demographics, coupled with a large offset as demonstrated in our companion Letter, we conclude that preferred sources for FRB 20240209A include magnetars formed through merging binary neutron stars/white dwarfs or the accretion-induced collapse of a white dwarf, or a luminous X-ray binary. Together with FRB 20200120E localized to a globular cluster in M81, our findings provide strong evidence that some fraction of FRBs may arise from a process distinct from the core collapse of massive stars.