Astronomers find signatures of short-lived, hypermassive neutron stars

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

Just some frames later, nonetheless, and the star can be gone, sucked into itself and changed by a black hole.

Sadly, astronomers have restricted avenues to check such objects, referred to as hypermassive neutron stars (HMNSs). Although neutron stars emit gravitational waves — ripples within the material of space-time — as they spiral in towards one another, present detectors are usually 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 staff led by Cecilia Chirenti of the College of Maryland in Faculty Park reported Jan. 9 in Nature that some HMNSs emit a signature quick burst of gamma-rays throughout their dying throes. When the staff analyzed the gamma-ray bursts (GRBs), they discovered a few circumstances the place they weren’t purely noise. As an alternative, that they had attribute frequencies that have been stronger than others, a signature “in line with a hypermassive neutron star,” Chirenti mentioned Jan. 9 at a press convention through the 241st assembly of the American Astronomical Society in Seattle.

The brand new research offers astronomers hope that they can be taught extra about these objects, that are fastest-rotating stars recognized, and in addition be taught extra concerning the dynamics of neutron star mergers normally.

“There isn’t any escape”

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

Provided 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 could 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 could 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 at present assume that if the ensuing neutron star is extra huge than roughly 2.2 instances that the Solar, it is going to collapse right into a black hole.

If it’s not too huge, then a hypermassive neutron star can survive — however just for a split-second. “There isn’t any escape,” mentioned Chirenti. “It simply hangs in there for this temporary 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 staff famous that pc fashions predict that the gamma-ray brightness of a HMNS might flicker a couple of thousand instances per second. By figuring out the exact charge of flickering, astronomers may achieve perception into the dimensions and spin charge of the HMNS. However thus far, no such gamma-ray oscillations had been recognized.

NASA’s Goddard House 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 House Telescope and the Neil Gehrels Swift Observatory (each operational as we speak), 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 every part is completely in tune, however you may nonetheless make out some tones which are stronger than others.

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

Their evaluation discovered the strongest oscillations have been at a frequency of roughly 2,600 instances per second. In keeping with simulations, this implies that the HMNS itself is spinning not less than 1,300 instances per second.

Nevertheless, that spin charge is barely a decrease restrict — similar to gentle is redshifted by the growth of the universe, the frequency of the quasi-periodic oscillation might have been increased initially. Irrespective of, even when it was very close by, the HMNS is spinning almost twice as quick because the quickest recognized pulsar, a category of quickly spinning neutron stars.

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