Bridget Andersen

About Me

I am currently a Physics PhD candidate at McGill University in the Trottier Space Institute. My research interests lie in the study of Fast Radio Bursts (FRBs) and pulsars through the development of new radio telescopes and detection algorithms. I am a member of the CHIME/FRB Collaboration, with Vicky Kaspi as my advisor. For my PhD program, I am building hardware and software to enable milliarcsecond localizations of FRBs using the CHIME/FRB Outrigger telescopes. After the telescopes are commissioned, I will be using these localizations to learn more about the surrounding environments of FRB sources through follow-up of their host galaxies with multiwavelength observations.

In 2021, I completed my Physics MSc thesis at McGill University on FRB flux calibration using the CHIME radio telescope. Before that, I graduated from the University of Virginia (UVA) in May 2018 with bachelors degrees in both Astrophysics and Computer Science. For my senior thesis at UVA, I worked with Scott Ransom to extend the PRESTO binary pulsar search software to account for orbital "jerk".

I love talking about all things related to FRBs, pulsar detection algorithms, and radio interferometry! See my CV here: link

Research

CHIME/FRB Flux Calibration

Every day, several hundred millisecond-duration explosions of radio light, known as a fast radio bursts (FRBs), arrive at Earth from distant galaxies. In just a millisecond, a typical FRB releases as much energy as the Sun does in three days (~10³⁶ - 10⁴² erg or ten quadrillion times global energy consumption since 1990). Since their discovery in 2007, the exact origin of FRBs has remained one of the greatest new mysteries in astrophysics.

The CHIME telescope's novel cylindrical design gives it a 200 square degree field of view and a large collecting area. These design choices make CHIME a particularly good FRB detector, however, it also gives CHIME a truly byzantine sensitivity pattern. As a function of frequency, CHIME's sensitivity exhibits sharp discontinuities from CHIME/FRB's novel beamforming scheme ("FFT beamforming") superimposed on top of a complex ripple from the primary beam response that changes rapidly as a function of sky location. This variable response poses a difficult challenge for calibrating physical fluxes for FRBs detected with CHIME, which are crucial for answering scientific questions about FRB energies. During my MSc, I developed an automated software pipeline that calibrates FRBs in real-time by continuously observing and processing calibrator sources of known flux as they transit the telescope. With this pipeline, I was responsible for calibrating all 535 bursts published in the first CHIME/FRB catalog! See more here: paper link

CHIME/FRB Outriggers

One promising new way to investigate FRB origin scenarios is to study the properties of the galaxies that FRBs come from. However, by itself, CHIME can only confidently pinpoint a burst to within an arcminute, which is insufficient for confidently associating all but the nearest host galaxies in the local Universe. To address this challenge with the CHIME/FRB collaboration, I have developed key software and hardware for operating three other "outrigger" telescopes spread throughout North America to improve CHIME’s localizing capabilities to within 100 milliarcseconds through Very Long Baseline Interferometry (VLBI). With this enhancement, CHIME will soon be positioned to revolutionize the FRB field through the study of many FRB host galaxies. During my PhD, I built the outrigger at Green Bank Observatory (GBO) from the ground up: I installed 128 feeds and 10 km of cabling to form the analog chain, constructed the on-site compute cluster (F- and X-engine), commissioned calibration and beamforming capabilities for the telescope to operate as a stand-alone instrument, and validated VLBI localization accuracy with CHIME by observing radio sources of known position.

Binary Pulsars

Of the more than ~3,000 Galactic pulsars currently known, only ~300 are in binary systems. Binaries are challenging to detect because orbital motion and companion interactions modulate or obscure the pulsar's regular periodic emission. While I was an undergraduate, one avenue of focus for my research was improving search algorithms to enable identification and timing of these binary pulsar systems. For my undergraduate thesis, I worked with NRAO astronomer Scott Ransom to implement a "jerk" search for the PRESTO software package. This algorithm has been used in a wide range of pulsar surveys, from ALMA searches for Galactic center pulsars to globular cluster searches with MeerKAT. See more here: paper link

Of the ~300 pulsars in binary systems, only 6 are in orbit with a main sequence companion. Using the CHIME radio telescope, I discovered and timed the sixth such pulsar, J2108+45 (affectionately known as the "Snowbird"). This kind of binary emerges in the early stages of the evolution of two high-mass stars, where the initially more massive star undergoes a supernova to become a neutron star. I used CHIME and Very Large Array (VLA) observations to identify J2108+45's companion as EM^* UHA 138, an O/B/Be-type star. J2108+45 undergoes periods of substantial eclipse and significant changes to its pulse profile due to dispersive delay and scattering, indicating interactions with a circumstellar disk or very dense stellar wind surrounding EM^* UHA 138. Notably, J2108+45's orbital eccentricity is unusually low, e = 0.09, whereas the other five main sequence binaries have e > 0.6. This poses a mystery for our current understanding of binary star evolution, as such binaries are expected to have large eccentricities from the "kick" imparted to the neutron star as it formed from a core-collapse supernova. See more here: paper link

Publications

First Author

1. Flux Calibration of CHIME/FRB Intensity Data, Bridget C. Andersen, Chitrang Patel, Charanjot Brar, [and 11 others]. Published: AJ 166, 138. (2023) [arXiv:2305.11302, ADS:2023AJ....166..138A]
2. CHIME Discovery of a Binary Pulsar with a Massive Non-Degenerate Companion, Bridget C. Andersen, E. Fonseca, J. W. McKee [and 25 others]. Published: ApJ 943, 57. (2023) [arXiv:2209.06895, ADS:2023ApJ...943...57A]
3. The Mass Evolution of Protostellar Disks and Envelopes in the Perseus Molecular Cloud, Bridget C. Andersen, Ian W. Stephens, Michael M. Dunham [and 8 others]. Published: ApJ 873, 54. (2019) [arXiv:1902.05956, ADS:2019ApJ...873...54A]
4. A Fourier Domain "Jerk" Search for Binary Pulsars, Bridget C. Andersen, Scott M. Ransom. Published: ApJL 863L, 13. (2018) [arXiv:1807.07900, ADS:2018ApJ...863L..13A]

Select Supporting Author

Here I highlight a select subset of publications that I significantly contributed to as a supporting author. For a full list of publications, see my ADS record.

1. Sub-second periodicity in a fast radio burst, CHIME/FRB Collaboration. Published: Nature 607, 256. (2022) [arXiv:2107.08463, ADS:2022Natur.607..256C]
2. A Sudden Period of High Activity from Repeating Fast Radio Burst 20201124A, Adam E. Lanman, Bridget C. Andersen, Pragya Chawla. Published: ApJL 927, 59L. (2022) [arXiv:2109.09254, ADS:2022ApJ...927...59L]
3. The First CHIME/FRB Fast Radio Burst Catalog, CHIME/FRB Collaboration. Published: ApJS 257, 59. (2021) [arXiv:2106.04352, ADS:2021ApJS..257...59C]
4. A bright millisecond-duration radio burst from a Galactic magnetar, CHIME/FRB Collaboration. Published: Nature 587, 54--58. (2020) [arXiv:2005.10324, ADS:2020Natur.587...54C]
5. Periodic activity from a fast radio burst source, CHIME/FRB Collaboration. Published: Nature 582, 351–-355. (2020) [arXiv:2001.10275, ADS:2020Natur.582..351C]
6. Detection of Repeating FRB 180916.J0158+65 Down to Frequencies of 300 MHz, P. Chawla, Bridget C. Andersen, M. Bhardwaj. Published: ApJL 896, L41. (2020) [arXiv:2004.02862, ADS:2020ApJ...896L..41C]
7. Nine New Repeating Fast Radio Burst Sources from CHIME/FRB, E. Fonseca, Bridget C. Andersen, M. Bhardwaj. Published: ApJL 891, L6. (2020) [arXiv:2001.03595, ADS:2020ApJ...891L...6F]
8. CHIME/FRB Discovery of Eight New Repeating Fast Radio Burst Sources, CHIME/FRB Collaboration. Published: ApJL 885, L24. (2019) [arXiv:1908.03507, ADS:2019ApJ...885L..24C]