Small black hole or giant neutron star?

Astronomers have found an object that is either a new class of black hole or the largest-known neutron star, circling a red-giant star 10,000 light years away in the constellation Auriga.

Previous black-hole discoveries involved objects much larger than the one in Auriga, says Todd Thompson, an astrophysicist at The Ohio State University, US, and lead author of a paper in the journal Science.

Some have been detected via powerful gravitational waves propagating far across the Universe when they are caught in the act of merging, he says. “These are 20 to 30 times the mass of the Sun.”

Others, within our own galaxy, have been found because they are part of binary systems in which they are so close to their companion stars that they siphon gas off from them, producing immense amounts of X-ray radiation as they gobble up the gas.

These tend to have masses five to 15 times that of the Sun. (There are also supermassive black holes in the centres of galaxies, but they are a different category altogether).

The black hole in Auriga is unique, partly because it’s too far from its companion to siphon off gas, and therefore does not produce X-rays. But it also appears to be the smallest black hole yet discovered.

It was found, Thompson says, via an effort to survey the sky for star systems that might host “non-interacting” black holes, too far out from their companions to have X-ray emissions. There might be millions of such systems in the galaxy, he says, with black holes “just hanging around” invisibly.

The hunt began by searching data from a pair of large sky surveys – one containing more than 50 million stars – looking for changes in star motions as the gravity from unseen companions tugs them first toward us then away from us.

Such motions, known as Doppler wobbles, are detectable via alternating red and blue shifts in the star’s spectrum, much like changes in the pitch of a train whistle show whether the train is approaching or receding. They have long been used to detect unseen objects, like exoplanets.

Based on that, Thompson says, “this one immediately popped up” – a large star (about three times the mass of the Sun) being tugged back and forth by an unseen companion once every 83 days.

His team then combined the unseen companion’s orbital period with their best estimate of the star’s mass to calculate that the unseen object was 3.3 times the mass of the Sun.

Though, he notes, that figure still has uncertainties. On the high end, it could be as large as 6.1 times the mass of the Sun, comparable to some of the black holes found via X-ray emissions.

Or, it could be as small as 2.6 times the mass of the Sun, making it either an extremely small black hole or right on the cusp of being the largest-possible neutron star, whose sizes, he says, max out at 2.5 to 2.6 times the mass of the Sun “depending on who you ask”.

But, he says, “our best determination of the mass indicates that it is in the range that hasn’t been seen before.”

Meanwhile, he adds, nobody is really sure how you form a three-solar-mass black hole.

In most models, dying stars either blow up, blasting away most of their mass and leaving a neutron star at their wake, or their cores are large enough to collapse under their own gravity into black holes larger than the one found in Auriga.

To form a black hole of the size his study appears to have found, Thompson says, the best guess is that it would have to have been created by a star that “partially” exploded. That would, initially, have produced a neutron star, but as material from the explosion fell back onto that neutron star’s surface, it would have gained enough mass to eventually collapse into a black hole.

The new discovery is important, says Paul Mason, an astrophysicist at New Mexico State University, US, who wasn’t part of Thompson’s team, because two important questions in astrophysics are what is the lowest possible mass of a black hole, and what is the highest possible mass of a neutron star.

Also, he notes, the discovery of a black hole that isn’t an X-ray source “gives us another laboratory to study black holes that we didn’t have before”.

Thompson agrees. “You try to find black holes in a new way,” he says, “and the first thing you find is in the low mass range where we haven’t seen black holes before.”