'Transformer' Pulsar is More Than Meets the Eye

Astronomers may have not yet found Cybertron but this "transforming" pulsar definitely has a shape-shifting double personality. Continue reading →

Astronomers may have not yet found Cybertron but this "transforming" pulsar definitely has a shape-shifting double personality.

Using NASA's Fermi Gamma-Ray Space Telescope, an international team of researchers has observed a peculiar type of binary star system named AY Sextantis that consists of a rapidly-spinning millisecond pulsar - that is, a bright radio-beaming neutron star, the compacted corpse of a dead star that's since gone supernova - with a larger, low-mass star.

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The dense neutron star periodically slurps up material from its swollen companion as the two whirl around each other every 4.8 hours, but when too much material from the low-mass star crowds the accretion disk surrounding the neutron star it gets hot enough to glow in x-ray wavelengths.

At this point turbulence in the disk at a mere 50 miles above the surface of the spinning neutron star gets the superheated material caught up in powerful magnetic fields. The radio beacons are snuffed out as jets blast from the star's poles, crackling with gamma rays... AY Sextantis has transformed from a low-mass X-ray binary to a transient, compact, low-mass gamma-ray binary.

"It's almost as if someone flipped a switch, morphing the system from a lower-energy state to a higher-energy one," said Benjamin Stappers, astrophysicist at the University of Manchester, England, and lead on the research team. "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."

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With such close proximity and rapid orbital period, the pulsar will soon completely dismantle its larger companion through its intermittent but energetic feeding periods.

Watch the video for an illustration of how this complex process is thought to occur:

click to play video

Named PSR J1023+0038 (J1023 for short) the pulsar was first discovered in 2007 by Anne Archibald, a postdoctoral researcher at the Netherlands Institute for Radio Astronomy. It's located about 4,400 light-years away in the southern constellation Sextans.

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J1023 is helping astronomers determine how other, more distant pulsars shift from x-ray to gamma ray emissions, as well as how millisecond pulsars develop their incredible rates of rotation.

The findings were published in the July 20 edition of The Astrophysical Journal.

Video credit: NASA Goddard Space Flight Center. Source: NASA

These artist's renderings show one model of pulsar J1023 before (top) and after (bottom) its radio beacon (green) vanished. Normally, the pulsar's wind staves off the companion's gas stream. When the stream surges, an accretion disk forms and gamma-ray particle jets (magenta) obscure the radio beam.

Artist's impression of a spinning supermassive black hole with a surrounding accretion disk and relativistic jets.

How to measure the spin of a black hole: This chart illustrates the basic model for determining the spin rates of black holes. The three artist's concepts represent the different types of spin: retrograde rotation, where the disk of matter falling onto the hole, called an accretion disk, moves in the opposite direction of the black hole; no spin; and prograde rotation, where the disk spins in the same direction as the black hole.

Two models of black hole spin: Scientists measure the spin rates of supermassive black holes by spreading the X-ray light into different colors. The light comes from accretion disks that swirl around black holes, as shown in both of the artist's concepts. They use X-ray space telescopes to study these colors, and, in particular, look for a "fingerprint" of iron -- the peak shown in both graphs, or spectra -- to see how sharp it is. Prior to observations with NASA's Spectroscopic Telescope Array, or NuSTAR, and the European Space Agency's XMM- Newton telescope, there were two competing models to explain why this peak might not appear to be sharp. The "rotation" model shown at top held that the iron feature was being spread out by distorting effects caused by the immense gravity of the black hole. If this model were correct, then the amount of distortion seen in the iron feature should reveal the spin rate of the black hole. The alternate model held that obscuring clouds lying near the black hole were making the iron line appear artificially distorted. If this model were correct, the data could not be used to measure black hole spin.

This chart depicts the electromagnetic spectrum, highlighting the X-ray portion. NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) and the European Space Agency's XMM-Newton telescope complement each other by seeing different colors of X-ray light. XMM-Newton sees X-rays with energies between 0.1 and 10 kiloelectron volts (keV), the "red" part of the spectrum, while NuSTAR sees the highest-energy, or "bluest," X- ray light, with energies between 3 and 70 keV.

This image taken by the ultraviolet-light monitoring camera on the European Space Agency's (ESA's) XMM-Newton telescope shows the beautiful spiral arms of the galaxy NGC1365. Copious high-energy X-ray emission is emitted by the host galaxy, and by many background sources. The large regions observed by previous satellites contain so much of this background emission that the radiation from the central black hole is mixed and diluted into it. NuSTAR, NASA's newest X-ray observatory, is able to isolate the emission from the black hole, allowing a far more precise analysis of its properties.

What XMM-Newton saw: The solid lines show two theoretical models that explain the low-energy X-ray emission seen from the galaxy NGC 1365 by the European Space Agency's XMM-Newton. The red line explains the emission using a model where clouds of dust and gas partially block the X-ray light, and the green line represents a model in which the emission is reflected off the inner edge of the accretion disk, very close to the black hole. The blue circles show the measurements from XMM-Newton, which are explained equally well by both models.

Two X-ray observatories are better than one: NASA's Nuclear Spectroscopic Telescope Array, or NuSTAR, has helped to show, for the first time, that the spin rates of black holes can be measured conclusively. It did this, together with the European Space Agency's XMM-Newton, by ruling out the possibility that obscuring clouds were partially blocking X-ray right coming from black holes. The solid lines show two theoretical models that explain low-energy X-ray emission seen previously from the spiral galaxy NGC 1365 by XMM-Newton. The red line explains the emission using a model where clouds of dust and gas partially block the X-ray light, and the green line represents a model in which the emission is reflected off the inner edge of the accretion disk, very close to the black hole. The blue circles show the latest measurements from XMM-Newton, and the yellow circles show the data from NuSTAR. While both models fit the XMM-Newton data equally well, only the disk reflection model fits the NuSTAR data.