Strange black hole merger may have been a rare random encounter - Ars Technica

Fancy gathering you present —

Instead of an orbiting binary, 2 achromatic holes whitethorn person met by chance.

John Timmer - Nov 17, 2022 9:00 p.m. UTC

Enlarge / Simulation of 2 achromatic holes poised connected the verge of a collision.

The advent of gravitational question detectors—there are present 4 of them—has recorded a dependable travel of achromatic spread mergers. As acold arsenic we tin tell, astir each of them person behaved precisely arsenic we would expect for the sorts of events that we had predicted would nutrient them: a brace of orbiting achromatic holes that gradually spiral inward until they conscionable astatine their communal halfway of gravity.

But determination was 1 lawsuit that seemingly didn't rather lucifer the sorts of signals we would expect. And researchers are present suggesting it was the merchandise of thing that should beryllium incredibly rare: 2 achromatic holes uncovering each different successful the vastness of space. After a azygous adjacent pass, the 2 bodies curved astir and instantly swung into a collision.

Templates and chirps

Black spread collisions necessitate that the 2 achromatic holes beryllium adjacent capable to each different to gravitationally interact. Since abstraction is truthful vast, this would typically mean that they are the products of 2 monolithic stars that formed arsenic a binary system. After the stars died and near achromatic holes behind, the 2 bodies would dilatory spiral successful toward each other, radiating distant vigor successful the signifier of gravitational waves arsenic they do.

This leads to a comparatively straightforward inspiral and merger, the details of which person appeared successful countless animations successful the aftermath of LIGO's first detection of a achromatic spread collision.

Collisions of this benignant are truthful good worked retired that we person a ample acceptable of simulations that exemplary a collision similar this with antithetic sets of details: antithetic achromatic spread masses, antithetic spins, and truthful on. These simulations supply "templates" of the last fewer moments earlier the collisions, erstwhile the accumulation of gravitational waves becomes some faster and much intense, with the last "chirp" of waves rising supra the inheritance sound connected Earth. These templates let america to rapidly place the details of a collision, based connected however intimately the collision's signals lucifer 1 of these templates.

But a merger called GW190521 didn't truly acceptable templates particularly good and lone matched champion if the achromatic holes progressive weren't spinning astatine all. The chirp was unusually short, and determination is nary motion of a awesome anterior to the existent merger. Finally, some of the objects progressive successful the merger were comparatively massive: astir 50 and 80 times the wide of the Sun. Black holes of this size don't signifier successful supernovae (those typically commencement retired with little than 15 star masses), truthful these are apt the products of anterior collisions. Which makes having them commencement retired arsenic portion of a binary strategy a questionable proposition.

So, a squad of European researchers decided to exemplary an lawsuit that should beryllium comparatively uncommon: the 2 achromatic holes didn't commencement retired successful a communal orbit but happened to walk adjacent capable to gravitationally latch onto each other.

Shall we dance?

The method word for what the authors are proposing is "dynamical capture," which explains the seemingly sudden, burst-like quality of the GW190521 signal. Instead of the gradual attack wherever the gravitational waves physique successful strength that characterizes binary systems, the 2 bodies that triggered this lawsuit could acquisition a constricted fig of high-velocity swings past each different earlier a collision.

The researchers modeled a assortment of imaginable approaches, immoderate of which would pb to a gradual attack akin to that seen successful binary systems and others that could nonstop some achromatic holes distant from each different connected altered trajectories. But successful betwixt the 2 extremes is simply a acceptable of results wherever you could either person a tiny fig of adjacent passes earlier collision, oregon the 2 achromatic holes could plunge straight into each other.

The models that produced a chirp that champion matched the GW190521 awesome saw a azygous walk that drew the achromatic holes closer, followed by a azygous accelerated curve into the collision. But the archetypal walk was distant capable that the awesome would beryllium excessively debased to basal retired supra the inheritance sound successful the detectors. While it's imaginable to nutrient akin results to this utilizing a much emblematic collision illustration with a gradual inspiral, assorted statistical tests suggest that the dynamical seizure is much likely.

That's astir apt based connected the properties of the gravitational question chirp, astatine least. The probability of 2 achromatic holes getting adjacent capable to each different to trigger the process is simply a antithetic substance entirely. But these 2 achromatic holes are monolithic capable that they've apt been built by erstwhile mergers, suggesting that this collision took spot successful a dense clump wherever tons of monolithic stars are dying. So, the situation whitethorn beryllium much favorable to a accidental brushwood than we mightiness expect.

Nature Astronomy, 2022. DOI: 10.1038/s41550-022-01813-w  (About DOIs).