Astronomers find signatures of short-lived, hypermassive neutron stars

If you happen to might freeze a movie of two neutron stars crashing into one another, simply after they collide, you’ll witness the formation of an object so large and dense it shouldn’t exist: The colliding stars would merge right into a single neutron star that’s spinning so quick it may momentarily maintain itself up towards collapse, defying gravity like Wile E. Coyote after he’s run off a cliff.

Only a few frames later, nonetheless, and the star could be gone, sucked into itself and changed by a black hole.

Sadly, astronomers have restricted avenues to check such objects, known as hypermassive neutron stars (HMNSs). Although neutron stars emit gravitational waves — ripples within the cloth of space-time — as they spiral in towards one another, present detectors usually are not delicate to the frequencies at which the HMNS itself emits.

However now, astronomers might have discovered one other path to understanding hypermassive neutron stars.

A group led by Cecilia Chirenti of the College of Maryland in School Park reported Jan. 9 in Nature that some HMNSs emit a signature brief burst of gamma-rays throughout their dying throes. When the group analyzed the gamma-ray bursts (GRBs), they discovered a few instances the place they weren’t purely noise. As an alternative, that they had attribute frequencies that had been stronger than others, a signature “in step with a hypermassive neutron star,” Chirenti mentioned Jan. 9 at a press convention throughout the 241st assembly of the American Astronomical Society in Seattle.

The brand new examine provides astronomers hope that they can be taught extra about these objects, that are fastest-rotating stars identified, and in addition be taught extra in regards to the dynamics of neutron star mergers basically.

“There isn’t a escape”

Neutron stars are the densest objects that may exist, wanting black holes. They’re the remnants of stars so large that they explode on the finish of their lives as supernovae, however not so large that they instantly collapse into black holes.

On condition that many of the stars within the universe are in binary or a number of star methods, not occasionally, a pair of binary stars can each finish their lives as neutron stars. And over time, they might spiral in towards one another and collide.

When these catastrophic collisions happen, they blast out gamma-rays that may be detected by telescopes after touring for billions of years. The stellar mashups additionally produce gravitational waves, a few of which may be detected by services just like the Laser Interferometer Gravitational-wave Observatory (LIGO) within the U.S. and Virgo in Europe. Based mostly on these observations, scientists presently assume that if the ensuing neutron star is extra large than roughly 2.2 occasions that the Solar, it can collapse right into a black hole.

If it isn’t too large, then a hypermassive neutron star can survive — however just for a split-second. “There isn’t a escape,” mentioned Chirenti. “It simply hangs in there for this transient time as a result of it’s spinning simply so quick.” (Astronomers have seen one that survived for nearly a day, however that one is outstanding.)

Understanding hypermassive neutron stars

To attempt to glean details about HMNSs themselves, Chirenti and her group famous that pc fashions predict that the gamma-ray brightness of a HMNS might flicker a number of thousand occasions per second. By figuring out the exact fee of flickering, astronomers might achieve perception into the dimensions and spin fee of the HMNS. However to this point, no such gamma-ray oscillations had been recognized.

NASA’s Goddard Area Flight Middle and STAG Analysis Centre/Peter Hammond

So, Chirenti and her colleagues scoured archival knowledge from three NASA space-based gamma-ray observatories: the Fermi Gamma-Ray Area Telescope and the Neil Gehrels Swift Observatory (each operational right now), in addition to the Compton Gamma Ray Observatory.

As a result of the HMNS remains to be “shaking and jiggling,” as Chirenti put it, a hypermassive neutron star produces quasi-periodic oscillations (QPOs). Which means, as a substitute of uniformly flickering at a single frequency, there are a wash of frequencies centered round peak frequencies. Chirenti compares it to listening to a tuning fork emit a single clear frequency versus listening to an orchestra tune its devices earlier than a live performance: Not all the things is completely in tune, however you possibly can nonetheless make out some tones which are stronger than others.

Out of the greater than 700 occasions analyzed, the group discovered QPOs in two of them, designated GRB 910711 and GRB 931101B. Each of them had been detected by Compton, which NASA operated throughout the Nineties and deorbited in 2000. Regardless of Compton’s age, for this examine, it “was a tremendous instrument due to its giant detector space and nice timing capabilities,” Chirenti mentioned.

Their evaluation discovered the strongest oscillations had been at a frequency of roughly 2,600 occasions per second. In accordance with simulations, this implies that the HMNS itself is spinning at the very least 1,300 occasions per second.

Nevertheless, that spin fee is barely a decrease restrict — identical to mild is redshifted by the enlargement of the universe, the frequency of the quasi-periodic oscillation might have been greater initially. Regardless of, even when it was very close by, the HMNS is spinning practically twice as quick because the quickest identified pulsar, a category of quickly spinning neutron stars.

Chirenti hopes that by the 2030s, extra superior gravitational-wave detectors might be able to finding out the space-time ripples produced by hypermassive neutron stars. “Within the meantime, we’ll hold in search of them in gamma rays,” says Chirenti.