
The story of this paper starts in early 2019, around the inception of the ambitious CHIME/FRB Project: a wild gamble to build the world’s most powerful search engine for finding enigmatic fast radio bursts: millisecond duration pulses of unknown origin, coming from far beyond our Galaxy. From its inception, CHIME/FRB looked well-positioned: with its excellent sensitivity and huge field of view, CHIME was ready to provide the best glimpse into the origin of these exotic bursts.
While CHIME was detecting bursts at a rate 10x its competitors, it had one seemingly-insurmountable Achilles’ heel. The telescope’s design made its burst search engine extremely efficient, but at the expense of directional information. Despite the firehose of ~5000 bursts detected in the last few years, their sky positions could not be precisely determined. To overcome this fundamental issue, the CHIME/FRB team came up with the futuristic idea of expanding CHIME into a network of telescopes. Much like how our two ears allow us to quickly pinpoint the sources of sounds, the idea was to observe FRBs with CHIME and a network of “outrigger” stations, which would allow the signal to be pinpointed with breathtaking accuracy.
In late 2019, the lead authors of this paper were new graduate students. Spread across North America, they kicked off the first chapter of the adventure to pinpoint fast radio bursts using CHIME. Our collective efforts gave CHIME its first set of glasses. Amid the pandemic, we built telescopes, debugged hardware and software, and toasted on Zoom to commemorate our successes and failures, without ever having all sat in the same room at once.
From the University of Toronto, a small team led by Tomás embarked on a multi-year project to the Algonquin provincial park near Toronto to bring back to life an old, decommissioned 10-m dish from the 1970s. First scouting the site, available electronics, and mechanical system, the Toronto team deployed new frontends and digital backends on a telescope dating back to the glorious beginnings of radio astronomy. Tomás and team worked in an isolated part of Canada to commission an operational and autonomous radio telescope at a challenging site with limited internet bandwidth, frequent power outages, and limited site access during the winter. Complications arose from the pandemic; nevertheless the robust 10-m dish operated mostly autonomously for years, setting the stage for early long baseline observations of FRBs.
From West Virginia University, a team led by Pranav built an entirely new interferometer, called TONE. Built and commissioned during a global pandemic, TONE is an array of 8 antennas of 6-m built by the lead authors from the ground up – literally. Working at Green Bank’s Radio Quiet Zone meant forgoing electronic technology, and like an ancient Polynesian explorer, Pranav would use the shadows of the dishes and the time of day, to point TONE’s seven dishes towards CHIME’s survey footprint as windstorms and blizzards threatened to disrupt our quest to localize FRBs.
Meanwhile at MIT, the analysis team led by Calvin wrote the real-time readout software and the sophisticated signal processing algorithms needed to bring together terabytes of data captured by all three stations. Calvin led the development of the software correlator: the heart of the array, which digitally aligns the signals in post-processing to within a nanosecond, over a 3000 kilometer baseline. Today, the calibration tools and software correlator used for this paper lives on in CHIME outriggers array, a continuation of our original effort.
With the stations prepared we began observing in early 2021. We drilled our observations and analysis procedure using the Crab pulsar, a known source which emits FRB-like pulses. Then finally, on the unusually hot morning of June 3, 2021, the universe sent us a fast radio burst, which was designated FRB 20210603A. Our months of practice observing sessions paid off when we saw the FRB signal – booming bright – in our preliminary data analysis. Victory was in sight, but couldn’t celebrate until we had collected calibration data at 3 pm on the afternoon of June 3. That day coincided with the peak of an extraordinary “heat dome” in the Okanagan Valley, which threatened to overwhelm the telescope’s cooling system as outside temperatures approached 40 Celsius. To collect our final set of calibration data, we practically begged the operations team to leave CHIME on in the heat – just this once, and never again. As the minutes ticked by and the calibration data trickled in, we heaved a collective sigh of relief. Our analysis eventually pinpointed the FRB to within the disk of its host galaxy, which had an unusually edge-on geometry. We were able to infer not only the distance to the FRB using its host’s optically-measured redshift. Even more remarkably, the edge-on geometry of the host galaxy allowed us to use the FRB like a natural radar “ping” to study the ionized gas between its stars. The properties of FRB 20210603A revealed the gaseous “atmosphere” of a galaxy almost a billion light years away–full of star formation and ionized gas much like our own Milky Way, suggesting that unremarkable galaxies such as our own could produce fast radio bursts.
Since 2021, CHIME outriggers has been busy expanding into a network of cylindrical telescopes with more collecting area monitoring the entire field of view of CHIME. The outriggers–a set of three cylindrical telescopes similar to CHIME–will pinpoint hundreds of sources with an accuracy of ~50 milliarcseconds: the size of a coin as viewed from 40 kilometers away. Tens of postdocs and students now follow in our footsteps in the CHIME/FRB Outriggers project, a major player in the quest to uncover the origin of FRBs.
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