Source: Science

Think of a radio telescope, and you may picture a massive dish, tens or even hundreds of meters across. That’s the classic design, which gathers radio waves from space and focuses them onto an antenna above the dish, creating a field of view that resembles a narrow beam.

The Bustling Universe Radio Survey Telescope in Taiwan (BURSTT), a humble-looking device nestled among the trees at the Fushan Botanical Garden south of Taipei, does away with the dish. Instead, it packs 256 antennas, shaped like miniature pine trees, into a so-called phased array to catch radio waves directly. Computers digitally combine the signals from the antennas, synthesizing multiple beams that can view roughly half of the visible sky at once. The array has no moving parts, but the beams can be steered electronically to view other parts of the sky. “There’s no common nomenclature for this gizmo yet,” says University of California, Berkeley radio astronomer Dan Werthimer. “I call them all-sky, all-the-time telescopes.”

BURSTT, which is expected to begin its sky surveys in the coming months, showcases the hottest new technology in radio astronomy. Three similar arrays are due to come online by the end of the year in China, the Netherlands, and California (see table, below). “I see this kind of array design becoming central to radio astronomy in the coming years and decades,” says Liam Connor, an astrophysicist at the Center for Astrophysics | Harvard & Smithsonian who is co–principal investigator for the Coherent All-Sky Monitor (CASM), an array being deployed at the Owens Valley Radio Observatory in California.

The downside of the approach is that, without a dish, the instrument’s sensitivity drops. The arrays will likely detect only the brightest radio sources in the Milky Way and neighboring galaxies. Researchers are embracing that limitation, however. They are using the all-sky telescopes to hunt for fast radio bursts (FRBs), which are orders of magnitude more luminous than any other radio sources in the sky.

Since FRBs were first discovered in 2007, astronomers have identified a thousand or so of the mysterious, fleeting blasts. Theories suggest they might be generated by black holes, supernovae, or spinning, magnetized, stellar remnants called magnetars. But, with one exception, known FRBs are so distant that even the biggest telescopes can trace them only to their host galaxies and not to specific astrophysical objects. And because of their narrow field of view, those telescopes are rarely pointed in the right direction at the right time to catch events closer to Earth. Devices like BURSTT “will be great for detecting more [Milky Way] bright radio bursts that have been elusive otherwise,” says Victoria Kaspi, an astrophysicist at McGill University who is principal investigator of the Canadian Hydrogen Intensity Mapping Experiment’s FRB project.

Catching nearby bursts could be the key to unraveling their mysteries. “Finding [FRBs] in our Galaxy means we can then point a bunch of other telescopes in that direction and confirm what produces them,” says Jessica Dempsey, director of ASTRON, the Netherlands Institute for Radio Astronomy, which plans to turn on its Arthropod array this month.

A major impetus for the new telescopes came in 2020, when astronomers captured the first and only confirmed Milky Way FRB and connected it to a magnetar called SGR 1935+2154. Astronomers had already been enlarging the fields of view of traditional radio telescopes by replacing the single antennas that lie at the dishes’ foci with phased arrays. The Milky Way event suggested that by deploying the antenna arrays as direct detectors, observers could capture many more. “It was a game changer,” says Ron Ekers, a retired radio astronomer at Australia’s Commonwealth Scientific and Industrial Research Organisation.

Vikram Ravi, a radio astronomer at the California Institute of Technology and a CASM co–principal investigator, says the new telescopes “are very complementary” to existing FRB-hunting radio telescopes that probe deep space. Comparing nearby and distant events could, for example, reveal different FRB rates that depend on the galaxy type, offering clues to the conditions that give rise to FRBs and hints to how they evolve over cosmic time.   

No dish required
One of the hottest trends in radio astronomy is using antennas to capture a wide field of view without a dish. Four facilities are set to come online in the coming months.

A bigger haul of FRBs could enable astronomers to use them as cosmic probes. Electrons in the wispy gases of the intergalactic medium distort radio waves, and the effect is greatest at low frequencies. By measuring those low-wavelength distortions in distant FRBs, astronomers can “weigh” the matter the signals have passed through on their way to Earth. Similarly, catching FRBs from the nearby universe will allow scientists to determine how much ordinary matter the Milky Way contains, which is “a major open question in astrophysics,” Connor says.

The new dishless telescopes could also watch for signals from the colliding black holes and other cosmic cataclysms that generate gravitational waves. At present, gravitational wave observatories send out an alert when they detect an event, triggering a scramble to observe its source with other telescopes. All-sky monitors, in contrast, might capture radio signals from gravitational wave events as they occur. “We may not find anything, or it may be spectacular,” Ekers says.  

The search for extraterrestrial intelligence (SETI) might benefit as well. SETI hunters typically look for a continuous signal, as might be emitted by a transmitter, Werthimer says. But if the signal is like an intermittent lighthouse beam, “you’re never going to find it with a telescope that looks at a little piece of the sky at a time,” he says. “You really want one of these very wide field, all-sky [monitors].”

Tsinghua University’s Li Di, who leads China’s Cosmic Antennae pilot project, is expecting the unexpected. “If you go into a new parameter space, there’s always something weird that pops up,” he says.