Mystery of the transformer pulsar: Rapidly spinning star shapeshifts as it sucks gas from its stellar partner
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Astronomers have found a bizarre rapidly spinning star that appears to 'vanish' and transform every so often.
Known as a pulsar, it is part of a binary system with another star one fifth the mass of the sun, and occasionally its radio beacon vanishes while the system brightens considerably.
Scientists had been baffled by the odd behaviour, but now they think it might be caused by the pulsar sucking in material from its companion star and spitting it out again.
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Astronomers have found a pulsar that seems to change its behaviour. In this artist's concept one model of pulsar J1023 is shown before (top) and after (bottom) its radio beacon (green) vanished. When the stream surges from its partner, an accretion disk forms and gamma-ray particle jets (magenta) obscure the beam
WHAT IS A PULSAR?
When a massive star collapses and explodes as a supernova, its crushed core may survive as a compact remnant called a neutron star or pulsar.
This is an object squeezing more mass than the sun's into a sphere no larger than Washington, D.C.
Young isolated neutron stars rotate tens of times each second and generate beams of radio, visible light, X-rays and gamma rays that astronomers observe as pulses whenever the beams sweep past Earth.
Pulsars also generate powerful outflows, or 'winds,' of high-energy particles moving near the speed of light.
The power for all this comes from the pulsar's rapidly spinning magnetic field and over time, as the pulsars wind down, these emissions fade.
More than 30 years ago, astronomers discovered another type of pulsar revolving in 10 milliseconds or less, reaching incredible rotational speeds up to 43,000 revolutions per minute, which became known as millisecond pulsars.
While young pulsars usually appear in isolation, more than half of millisecond pulsars occur in binary systems, which suggests an explanation for their rapid spin.
The two-star system, known as AY Sextantis, is located about 4,400 light-years away in the constellation Sextans.
Measurements were made by Nasa's Earth-orbiting Fermi Gamma-ray Space Telescope, which is used to observe emissions of gamma rays, the most powerful form of light.
This particular observation was made more than a year ago in June 2013, but only now do astronomers think they have an explanation for what's going on.
Known as PSR J1023+0038, the rapidly spinning neutron star is referred to as a 1.7-millisecond pulsar, denoting the time it takes to rotate at an incredible 43,000 revolutions per minute.
The other star - which is a more regular sun-like star by comparison - completes an orbit of the pulsar in just 4.8 hours, which places them so close together that the pulsar will gradually evaporate its companion.
What is happening is that the pulsar is changing from a low-mass X-ray binary (LMXB) to a radio pulsar periodically as it does so.
An LMXB is a binary star that is luminious in X-rays, while a radio pulsar is one that emits a radio beam.
For some reason the pulsar seems to be switching between the two states.
Astronomers now think that it is because the pulsar is sucking material from its companion star.
As the companion star orbits it is continuously losing gas to the pulsar.
However it seems to occasionally undergo surges, for reasons unknown, when it loses more material than normal to the pulsar.
This forms a disk of material around the pulsar, which slowly falls closer to the pulsar up to an altitude of just 50 miles (80 kilometres) above it, where it becomes superheated.
When this happens it obscures the emission of radio waves from the pulsar's poles.
But the disk itself emits X-rays, so the pulsar switches from emitting beams of gamma and radio waves to beams of X-rays.
The pulsar can be obscured for years while the transformation takes place, until it becomes an LMXB-like object and begins emitting X-rays.
Eventually the disk dissipates and the pulsar goes back to just emitting noticeable radio waves.
When a massive star collapses and explodes as a supernova, its crushed core may survive as a compact remnant called a neutron star or pulsar. This is an object squeezing more mass than the sun's into a sphere no larger than Washington, D.C. Pulsars rotate tens of times each second and generate beams
Dr Benjamin Stappers, an astrophysicist at the University of Manchester, who led the international effort to understand this striking transformation, explains to MailOnline that the pulsar changed from being like an LMXB just over a decade ago.
It then became a radio pulsar for about six to seven years before last year, when it changed back into an LMXB-like object.
'What we don't know is how long it will be in this phase for because we haven't seen a complete cycle,' he explains.
'We don't know when it will happen again nor whether it is periodic or even episodic.
'So we are continuing to monitor the source at radio, optical, X-ray and gamma-ray wavelengths to try and catch the exact moment it changes again.'
In a press release Dr Stappers added: 'It's almost as if someone flipped a switch, morphing the system from a lower-energy state to a higher-energy one.
'The change appears to reflect an erratic interaction between the pulsar and its companion, one that allows us an opportunity to explore a rare transitional phase in the life of this binary.'
Anne Archibald, a postdoctoral researcher at the Netherlands Institute for Radio Astronomy (Astron) in Dwingeloo who discovered J1023 in 2007, added: 'Astronomers have long suspected millisecond pulsars were spun up through the transfer and accumulation of matter from their companion stars, so we often refer to them as recycled pulsars.'
The findings were published in the The Astrophysical Journal.
The team reports that J1023 is the first example of a transient, compact, low-mass gamma-ray binary ever seen.
The researchers anticipate that the system will serve as a unique laboratory for understanding how millisecond pulsars form and for studying the details of how accretion takes place on neutron stars.
Nasa's Fermi Gamma-ray Space Telescope was launched on 11 June 2008. It is situated in low Earth orbit and is used to perform gamma-ray astronomy observations with its main instrument, the Large Area Telescope, which can perform an all-sky surveys and is used to study pulsars like J1023
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