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1. A Clock Stabilization System for CHIME/FRB Outriggers

   https://doi.org/10.3847/1538-3881/ac397a  

   Mena-Parra, J. ; Leung, C. ; Cary, S. ; Masui, K. W. ; Kaczmarek, J. F. ; Amiri, M. ; Bandura, K. ; Boyle, P. J. ; Cassanelli, T. ; Cliche, J. -F. ; et al ( January 2022 , The Astronomical Journal)

   The Canadian Hydrogen Intensity Mapping Experiment (CHIME) has emerged as the prime telescope for detecting fast radio bursts (FRBs). CHIME/FRB Outriggers will be a dedicated very-long-baseline interferometry (VLBI) instrument consisting of outrigger telescopes at continental baselines working with CHIME and its specialized real-time transient-search backend (CHIME/FRB) to detect and localize FRBs with 50 mas precision. In this paper, we present a minimally invasive clock stabilization system that effectively transfers the CHIME digital backend reference clock from its original GPS-disciplined ovenized crystal oscillator to a passive hydrogen maser. This enables us to combine the long-term stability and absolute time tagging of the GPS clock with the short- and intermediate-term stability of the maser to reduce the clock timing errors between VLBI calibration observations. We validate the system with VLBI-style observations of Cygnus A over a 400 m baseline between CHIME and the CHIME Pathfinder, demonstrating agreement between sky-based and maser-based timing measurements at the 30 ps rms level on timescales ranging from one minute to up to nine days, and meeting the stability requirements for CHIME/FRB Outriggers. In addition, we present an alternate reference clock solution for outrigger stations that lack the infrastructure to support a passive hydrogen maser.
2. The host galaxy and persistent radio counterpart of FRB 20201124A

   https://doi.org/10.1093/mnras/stac465  

   Ravi, Vikram ; Law, Casey J. ; Li, Dongzi ; Aggarwal, Kshitij ; Bhardwaj, Mohit ; Burke-Spolaor, Sarah ; Connor, Liam ; Lazio, T. Joseph W. ; Simard, Dana ; Somalwar, Jean ; et al ( April 2022 , Monthly Notices of the Royal Astronomical Society)

   The physical properties of fast radio burst (FRB) host galaxies provide important clues towards the nature of FRB sources. The 16 FRB hosts identified thus far span three orders of magnitude in mass and specific star formation rate, implicating a ubiquitously occurring progenitor object. FRBs localized with ∼arcsecond accuracy also enable effective searches for associated multiwavelength and multi-time-scale counterparts, such as the persistent radio source associated with FRB 20121102A. Here we present a localization of the repeating source FRB 20201124A, and its association with a host galaxy (SDSS J050803.48+260338.0, z = 0.098) and persistent radio source. The galaxy is massive (${\sim}3\times 10^{10}\, \text{M}_{\odot }$), star-forming (few solar masses per year), and dusty. Very Large Array and Very Long Baseline Array observations of the persistent radio source measure a luminosity of 1.2 × 1029 erg s−1 Hz−1, and show that is extended on scales ≳50 mas. We associate this radio emission with the ongoing star formation activity in SDSS J050803.48+260338.0. Deeper, high-resolution optical observations are required to better utilize the milliarcsecond-scale localization of FRB 20201124A and determine the origin of the large dispersion measure (150–220 pc cm−3) contributed by the host. SDSS J050803.48+260338.0 is an order of magnitude more massive than any galaxy or stellar system previously associated with a repeating FRB source, butmore »is comparable to the hosts of so far non-repeating FRBs, further building the link between the two apparent populations.

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3. PSR J2222−0137: I. Improved physical parameters for the system

   https://doi.org/10.1051/0004-6361/202141450  

   Guo, Y. J. ; Freire, P. C. ; Guillemot, L. ; Kramer, M. ; Zhu, W. W. ; Wex, N. ; McKee, J. W. ; Deller, A. ; Ding, H. ; Kaplan, D. L. ; et al ( October 2021 , Astronomy & Astrophysics)

   Context. The PSR J2222−0137 binary system has a set of features that make it a unique laboratory for tests of gravity theories. Aims. To fully exploit the system’s potential for these tests, we aim to improve the measurements of its physical parameters, spin and orbital orientation, and post-Keplerian parameters, which quantify the observed relativistic effects. Methods. We describe an improved analysis of archival very long baseline interferometry (VLBI) data, which uses a coordinate convention in full agreement with that used in timing. We have also obtained much improved polarimetry of the pulsar with the Five hundred meter Aperture Spherical Telescope (FAST). We provide an improved analysis of significantly extended timing datasets taken with the Effelsberg, Nançay, and Lovell radio telescopes; this also includes previous timing data from the Green Bank Telescope. Results. From the VLBI analysis, we have obtained a new estimate of the position angle of the ascending node, Ω = 189 −18 +19 deg (all uncertainties are 68% confidence limits), and a new reference position for the pulsar with an improved and more conservative uncertainty estimate. The FAST polarimetric results, and in particular the detection of an interpulse, yield much improved estimates for the spin geometry of themore »pulsar, in particular an inclination of the spin axis of the pulsar of ∼84 deg. From the timing, we obtain a new ∼1% test of general relativity (GR) from the agreement of the Shapiro delay parameters and the rate of advance of periastron. Assuming GR in a self-consistent analysis of all effects, we obtain much improved masses: 1.831(10)  M ⊙ for the pulsar and 1.319(4)  M ⊙ for the white dwarf companion; the total mass, 3.150(14)  M ⊙ , confirms this as the most massive double degenerate binary known in the Galaxy. This analysis also yields the orbital orientation; in particular, the orbital inclination is 85.27(4) deg – indicating a close alignment between the spin of the pulsar and the orbital angular momentum – and Ω = 187.7(5.7) deg, which matches our new VLBI estimate. Finally, the timing also yields a precise measurement of the variation in the orbital period, Ṗ b = 0.251(8) × 10 −12 ss −1 ; this is consistent with the expected variation in the Doppler factor plus the orbital decay caused by the emission of gravitational waves predicted by GR. This agreement introduces stringent constraints on the emission of dipolar gravitational waves.« less
4. A Sudden Period of High Activity from Repeating Fast Radio Burst 20201124A

   https://doi.org/10.3847/1538-4357/ac4bc7  

   Lanman, Adam E. ; Andersen, Bridget C. ; Chawla, Pragya ; Josephy, Alexander ; Noble, Gavin ; Kaspi, Victoria M. ; Bandura, Kevin ; Bhardwaj, Mohit ; Boyle, P. J. ; Brar, Charanjot ; et al ( March 2022 , The Astrophysical Journal)

   The repeating FRB 20201124A was first discovered by CHIME/FRB in November of 2020, after which it was seen to repeat a few times over several months. It entered a period of high activity in April of 2021, at which time several observatories recorded tens to hundreds more bursts from the source. These follow-up observations enabled precise localization and host-galaxy identification. In this paper, we report on the CHIME/FRB-detected bursts from FRB 20201124A, including their best-fit morphologies, fluences, and arrival times. The large exposure time of the CHIME/FRB telescope toward the location of this source allows us to constrain its rates of activity. We analyze the repetition rates over different spans of time, constraining the rate prior to discovery to <3.4 day⁻¹(at 3σ), and demonstrate a significant change in the event rate following initial detection. Lastly, we perform a maximum-likelihood estimation of a power-law luminosity function, finding a best-fit indexα= −4.6 ± 1.3 ± 0.6, with a break at a fluence threshold of$F_{\min }\sim 16.6$Jy ms, consistent with the fluence completeness limit of the observations. This index is consistent within uncertainties with those of other repeating FRBs for which it has been determined.
5. An Overview of CHIME, the Canadian Hydrogen Intensity Mapping Experiment

   https://doi.org/10.3847/1538-4365/ac6fd9  

   Amiri, Mandana ; Bandura, Kevin ; Boskovic, Anja ; Chen, Tianyue ; Cliche, Jean-François ; Deng, Meiling ; Denman, Nolan ; Dobbs, Matt ; Fandino, Mateus ; Foreman, Simon ; et al ( July 2022 , The Astrophysical Journal Supplement Series)

   The Canadian Hydrogen Intensity Mapping Experiment (CHIME) is a drift scan radio telescope operating across the 400–800 MHz band. CHIME is located at the Dominion Radio Astrophysical Observatory near Penticton, BC, Canada. The instrument is designed to map neutral hydrogen over the redshift range 0.8–2.5 to constrain the expansion history of the universe. This goal drives the design features of the instrument. CHIME consists of four parallel cylindrical reflectors, oriented north–south, each 100 m × 20 m and outfitted with a 256-element dual-polarization linear feed array. CHIME observes a two-degree-wide stripe covering the entire meridian at any given moment, observing three-quarters of the sky every day owing to Earth’s rotation. An FX correlator utilizes field-programmable gate arrays and graphics processing units to digitize and correlate the signals, with different correlation products generated for cosmological, fast radio burst, pulsar, very long baseline interferometry, and 21 cm absorber back ends. For the cosmology back end, the$N_{\mathrm{feed}}^2$correlation matrix is formed for 1024 frequency channels across the band every 31 ms. A data receiver system applies calibration and flagging and, for our primary cosmological data product, stacks redundant baselines and integrates for 10 s. We present an overview of themore »instrument, its performance metrics based on the first 3 yr of science data, and we describe the current progress in characterizing CHIME’s primary beam response. We also present maps of the sky derived from CHIME data; we are using versions of these maps for a cosmological stacking analysis, as well as for investigation of Galactic foregrounds.

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