Meet the pulsar that defies the laws of physics

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Feb 24, 2017

Phenomena in space often defy our expectations and challenge everything we thought we knew about the cosmos, but blindingly bright pulsar ULX-1the brightest and most distant pulsar currently known to exist — actually laughs at the laws of physics from 40 million light years away.

Scientists were previously convinced ULX-1 was a supermassive black hole because of its high luminosity. Supermassive black holes found in galactic centers vampirically feeding off unlucky companion stars, and are surrounded by enormous accretion discs that are like graveyards for astral gas, dust and debris that breathe their last before being devoured by extreme gravity. These accretion discs also produce an insane amount of X-ray emissions that were thought to make them outshine most other objects, but there are exceptions.

ULXs (Ultraluminous X-ray Sources) are brilliant celestial bodies unassociated with supermassive black holes. They can be, but are not necessarily, random black holes that do not reside at the center of any galaxy. ULX-1 is a different kind of stellar corpse. It was recently determined to be a spinning neutron star, aka a pulsar. Neutron stars are the densest and smallest stars in existence, remnants of collapsed stellar cores after their violent supernova deaths. Intense would be an understatement for their monstrous magnetic fields. Pulsars take that even further by flashing once per revolution during their rapid spin cycles, kind of like a cosmic disco ball.

ULX-1 not only spins almost unfathomably fast, but accelerates at a rate that would slash five hours off a day on earth in hardly more than a decade. Don’t think too hard on that or your head might explode.

The Knife Edge Galaxy (NGC 5907), where ULX-1 “misbehaves”.

"This object is really challenging our current understanding of the ‘accretion’ process for high-luminosity stars," said Gian Luca Israel, who has been researching the rebel pulsar with his team at the Italian National Institute for Astrophysics. "It is 1,000 times more luminous than the maximum thought possible for an accreting neutron star, so something else is needed in our models in order to account for the enormous amount of energy released by the object."

NASA's NuSTAR (Nuclear Spectroscopic Telescope Array) and the XMM-Newton (X-ray Multi-Mirror Mission) revealed through archival data that this pulsar keeps rotating faster and faster on its axis at such an unusual rate because it accretes material so rapidly. That still leaves the question of how it became an astrophysical rule-breaker.

Physics limits luminosity for stellar-type objects. The pressure exerted by photons should not be able to overpower a star or black hole’s force of gravity, because resulting disruptions in its accretion disc would end up extinguishing X-rays. This is known as the Eddington limit and increases with the size of an object (and vice versa). ULX-1 may not be a supermassive black hole, but somehow exceeds the Eddington limit for not only a typical neutron star but even an average black hole. Try a thousand times. This is obviously problematic — but there is one loophole. Turns out that pulsars only emit radiation from one relatively small area while the Eddington limit assumes that objects radiate from all different directions, or isotropically.

ULX-1 is not an anomaly. There are other ULXs in the universe that scientists have caught breaking the rules, but its sheer size and luminosity, not to mention its incredible speed, make it a standout—and shed light on why it can cheat the laws of physics.

(via Astronomy Magazine)

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