Double Mode Cepheids from the Zwicky Transient Facility Survey

Multi-mode Cepheids pulsate simultaneously in more than one mode of oscillation. They provide an independent means to test stellar models and pulsation theories. They can also be used to derive metallicities. In recent years, the number of known multi-mode Cepheids has increased dramatically with the discovery of a large number of Galactic double-mode Cepheids. To date, 209 double-mode Cepheids have been detected in the Galactic bulge and disk, mostly based on the Optical Gravitational Lensing Experiment's (OGLE) catalog. In this paper, we conduct a comprehensive search for double-mode Cepheids in the northern sky based on Zwicky Transient Facility Data Release 5. We found 72 such objects in the Milky Way. The periods of the 30 sample objects already included in the OGLE catalog show excellent agreement with the OGLE periods. The period ratios of our new Cepheids are consistent with those of known double-mode Cepheids, as evidenced by their loci in the so-called `Petersen diagram'. Compared with OGLE, the completeness of our double-mode Cepheid sample is around 71\%. The much improved temporal sampling of the Zwicky Transient Facility offers significant scope to find more double-mode Cepheids, especially at the distribution's short-period end.Comment: 11 pages, 3 figures, 3 tables; accepted for publication in ApJ

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 and find that it tends to an equilibrium of $2.6_{-2.6}^{+2.9}$% over our exposure thus far. We also report on 14 more sources which are promising repeating FRB candidates and which merit follow-up observations for confirmation.Comment: Submitted to ApJ. Comments are welcome and follow-up observations are encouraged

A VLBI Software Correlator for Fast Radio Transients

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 which arises in instruments where the windowing and channelization is done outside the VLBI correlator at each station, which accounts for a 30 percent 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. (2024) to verify the astrometric accuracy of the CHIME/FRB Outriggers array.Comment: 24 pages, 7 figures. Comments, reference suggestions, and PRs welcome

Updating the First CHIME/FRB Catalog of Fast Radio Bursts with Baseband Data

International audienceIn 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