Algorithms for FFT Beamforming Radio Interferometers

Radio interferometers consisting of identical antennas arranged on a regular lattice permit fast Fourier transform beamforming, which reduces the correlation cost from $\mathcal{O}(n^2)$ in the number of antennas to $\mathcal{O}(n\log n)$. We develop a formalism for describing this process and apply this formalism to derive a number of algorithms with a range of observational applications. These include algorithms for forming arbitrarily pointed tied-array beams from the regularly spaced Fourier-transform formed beams, sculpting the beams to suppress sidelobes while only losing percent-level sensitivity, and optimally estimating the position of a detected source from its observed brightness in the set of beams. We also discuss the effect that correlations in the visibility-space noise, due to cross-talk and sky contributions, have on the optimality of Fourier transform beamforming, showing that it does not strictly preserve the sky information of the $n^2$ correlation, even for an idealized array. Our results have applications to a number of upcoming interferometers, in particular the Canadian Hydrogen Intensity Mapping Experiment--Fast Radio Burst (CHIME/FRB) project.Comment: 17 pages, 4 figures, accepted to Ap

Data Processing Pipeline For Tianlai Experiment

The Tianlai project is a 21cm intensity mapping experiment aimed at detecting dark energy by measuring the baryon acoustic oscillation (BAO) features in the large scale structure power spectrum. This experiment provides an opportunity to test the data processing methods for cosmological 21cm signal extraction, which is still a great challenge in current radio astronomy research. The 21cm signal is much weaker than the foregrounds and easily affected by the imperfections in the instrumental responses. Furthermore, processing the large volumes of interferometer data poses a practical challenge. We have developed a data processing pipeline software called {\tt tlpipe} to process the drift scan survey data from the Tianlai experiment. It performs offline data processing tasks such as radio frequency interference (RFI) flagging, array calibration, binning, and map-making, etc. It also includes utility functions needed for the data analysis, such as data selection, transformation, visualization and others. A number of new algorithms are implemented, for example the eigenvector decomposition method for array calibration and the Tikhonov regularization for $m$-mode analysis. In this paper we describe the design and implementation of the {\tt tlpipe} and illustrate its functions with some analysis of real data. Finally, we outline directions for future development of this publicly code.Comment: 13 pages, 5 figures, accepted for publication on Astronomy and Computin

Fast Radio Burst Tomography of the Unseen Universe

The discovery of Fast Radio Bursts (FRBs) at cosmological distances has opened a powerful window on otherwise unseen matter in the Universe. In the 2020s, observations of $>10^{4}$ FRBs will assess the baryon contents and physical conditions in the hot/diffuse circumgalactic, intracluster, and intergalactic medium, and test extant compact-object dark matter models.Comment: Science white paper submitted to the Astro2020 Decadal Survey. 15 pages, 3 color figure

High-cadence Timing of Binary Pulsars with CHIME

We performed near-daily observations of the binary pulsars PSR J0218+4232, PSR J1518+4904, and PSR J2023+2853 with the Canadian Hydrogen Intensity Mapping Experiment (CHIME). For the first time, we detected the Shapiro time delay in all three pulsar binary systems, using only 2-4 yr of CHIME/Pulsar timing data. We measured the pulsar masses to be 1.49 − 0.20 + 0.23 M ⊙, 1.470 − 0.034 + 0.030 M ⊙, and 1.50 − 0.38 + 0.49 M ⊙, respectively. The companion mass to PSR J0218+4232 was found to be 0.179 − 0.016 + 0.018 M ⊙. We constrained the mass of the neutron star companion of PSR J1518+4904 to be 1.248 − 0.029 + 0.035 M ⊙, using the observed apsidal motion as a constraint on the mass estimation. The binary companion to PSR J2023+2853 was found to have a mass of 0.93 − 0.14 + 0.17 M ⊙; in the context of the near-circular orbit, this mass estimate suggests that the companion to PSR J2023+2853 is likely a high-mass white dwarf. By comparing the timing model obtained for PSR J0218+4232 with previous observations, we found a significant change in the observed orbital period of the system of P b ̇ = 0.14 ( 2 ) × 10 − 12 ; we determined that this variation arises from “Shklovskii acceleration” due to the relative motion of the binary system, and used this measurement to estimate a distance of d = (6.7 ± 1.0) kpc to PSR J0218+4232. This work demonstrates the capability of high-cadence observations, enabled by the CHIME/Pulsar system, to detect and refine general-relativistic effects of binary pulsars over short observing timescales

The 60 pc Environment of FRB 20180916B

Fast Radio Burst FRB 20180916B in its host galaxy SDSS J015800.28+654253.0 at 149 Mpc is by far the closest-known FRB with a robust host galaxy association. The source also exhibits a 16.35-day period in its bursting. Here we present optical and infrared imaging as well as integral field spectroscopy observations of FRB 20180916B with the WFC3 camera on the Hubble Space Telescope and the MEGARA spectrograph on the 10.4-m Gran Telescopio Canarias. The 60-90 milliarcsecond (mas) resolution of the Hubble imaging, along with the previous 2.3-mas localization of FRB 20180916B, allow us to probe its environment with a 30-60 pc resolution. We constrain any point-like star-formation or HII region at the location of FRB 20180916B to have an H$\alpha$ luminosity $L_\mathrm{H\alpha} \lesssim 10^{37}\,\mathrm{erg\,s^{-1}}$ and, correspondingly, constrain the local star-formation rate to be $\lesssim10^{-4}\,\mathrm{M_\odot\,yr^{-1}}$. The constraint on H$\alpha$ suggests that possible stellar companions to FRB 20180916B should be of a cooler, less massive spectral type than O6V. FRB 20180916B is 250 pc away (in projected distance) from the brightest pixel of the nearest young stellar clump, which is $\sim380$\,pc in size (full-width at half maximum). With the typical projected velocities of pulsars, magnetars, or neutron stars in binaries (60-750 km s$^{-1}$), FRB 20180916B would need 800 kyr to 7 Myr to traverse the observed distance from its presumed birth site. This timescale is inconsistent with the active ages of magnetars ($\lesssim10$ kyr). Rather, the inferred age and observed separation are compatible with the ages of high-mass X-ray binaries and gamma-ray binaries, and their separations from the nearest OB associations.Comment: Updated version: Updated Figure 2. 16 pages, 4 figures, 1 table. Published in ApJ Letters. Comments welcom

Proposed host galaxies of repeating fast radio burst sources detected by CHIME/FRB

We present a search for host galaxy associations for the third set of repeating fast radio burst (FRB) sources discovered by the CHIME/FRB Collaboration. Using the ~1 arcmin CHIME/FRB baseband localizations and probabilistic methods. We identify potential host galaxies of two FRBs, 20200223B and 20190110C at redshifts of 0.06024(2) and 0.12244(6), respectively. We also discuss the properties of a third marginal candidate host galaxy association for FRB 20191106C with a host redshift of 0.10775(1). The three putative host galaxies are all relatively massive, fall on the standard mass-metallicity relationship for nearby galaxies, and show evidence of ongoing star formation. They also all show signatures of being in a transitional regime, falling in the "green valley" which is between the bulk of star-forming and quiescent galaxies. The plausible host galaxies identified by our analysis are consistent with the overall population of repeating and non-repeating FRB hosts while increasing the fraction of massive and bright galaxies. Coupled with these previous host associations, we identify a possible excess of FRB repeaters whose host galaxies have M_u - M_r colors redder than the bulk of star-forming galaxies. Additional precise localizations are required to confirm this trend.Comment: 11 pages, submitted to AAS journal