Old Telescope, New Tricks: Chasing Gravitational Waves Across the Sky

With the Neil Gehrels Swift Observatory nearing the 20th anniversary of its launch, astronomers have demonstrated a new way to use the seasoned telescope. The technique, which involves rapidly slewing to the location of a gravitational wave signal, could open a window onto the critical first minutes after a neutron-star merger.

When Gravitational Waves and Radiation Meet

In 2017, researchers detected gravitational waves and electromagnetic radiation from the collision of two neutron stars for the first time. Just 1.7 seconds after the gravitational wave signal was detected, two all-sky gamma-ray observatories — the Fermi Gamma-ray Space Telescope and the INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) — happened to detect a burst of gamma rays from the collision.

As fortuitous as this detection was, Fermi and INTEGRAL lacked the localization ability needed to pinpoint the burst’s position. The delay in localization cost astronomers precious time, and it wasn’t until another 12 hours had passed that optical and ultraviolet emission from the collision was identified — wavelengths that are critical to collect as soon as possible to understand the physics of the merger and the cascade of element formation that follows. Is there a way to find a neutron-star merger faster?

Swift Follow-Up

In a recent article, Aaron Tohuvavohu (University of Toronto; California Institute of Technology) and collaborators showed how we can rapidly locate neutron-star mergers on the sky with an existing tool: the 20-year-old Swift Observatory. Gamma rays, which are produced just seconds after a neutron-star collision, are ideal for quickly spotting the radiation from a merger — and Swift has the most sensitive gamma-ray detector currently in operation. Once a gamma-ray source falls within Swift’s field of view, which spans 17% of the sky, the telescope has the ability to localize the source to within 1–3 arcminutes, giving follow-up telescopes sensitive to other wavelengths a small area to search. But unlike all-sky gamma-ray telescopes, Swift can’t look everywhere at once, raising the question of how to get the telescope to point toward the right place at the right time.

Tohuvavohu and collaborators outlined a way to use Swift to rapidly follow up on early-warning events — gravitational waves detected from inspiraling neutron stars before they merge. Essentially, after receiving notice of an impending neutron-star merger and a rough map of the merger’s location on the sky, Swift reorients to the likely location of the event, aiming to catch the gamma rays from the collision and pinpoint its location. But when is the best time for Swift to start turning toward the source? Current gravitational wave instruments can give up to 70 seconds of lead time before a neutron-star merger, but at that early time, they can only localize the event to an area spanning thousands of square degrees. As the merger draws nearer, the map of potential on-sky locations gets more precise, but waiting for better location information leaves Swift less time to move — and once the observatory begins slewing, it can’t be redirected until it’s finished moving.

Doubling the Odds

After being alerted to an imminent neutron-star merger, should Swift immediately race to the likeliest location, or should it wait to get a better idea of where the event is happening? Given the time required to wend toward the signal’s origin, curving to avoid forbidden zones too close to Earth or the Sun, Tohuvavohu’s team finds that it’s best for Swift to act on the earliest possible knowledge of where the merger is likely to happen, even though later maps are more accurate.

This method isn’t foolproof — Swift will sometimes rush to a location, only for it to become clear that the merger happened elsewhere — but the team estimated that it could more than double the rate at which neutron-star mergers are precisely localized. Quick localization, in turn, enables the rapid follow-up at optical and ultraviolet wavelengths that is key to distinguishing between competing merger models.

Future, more sensitive gravitational wave observatories will be able to detect the faint hum of spiraling neutron stars even longer before impact, helping Swift get into position more frequently. In the meantime, Tohuvavohu and coauthors recommend working to improve the speed at which notice of an impending neutron-star merger can be transmitted to Swift.

Citation

“Swiftly Chasing Gravitational Waves Across the Sky in Real Time,” Aaron Tohuvavohu et al 2024 ApJL 975 L19. doi:10.3847/2041-8213/ad87ce