By Mandeep S. S. Gill

Take a star that weighs about twice as much as our Sun, and compact it down to the size of a medium-sized city, to make a neutron star whose extreme mass warps the spacetime everywhere near it.  Next, put a much smaller companion star in orbit around it at very close range, and let the system evolve: what happens now?

In star systems like this, the neutron stars that have become known as "black widow pulsars" slowly eat their companions 'alive', shredding their outer layers and claiming them as their own, accreting them onto their surface steadily, then afterwards evaporating the companion star to a shadow of its former self over time.  In the process, the black widow spins up until it is rotating at hundreds of times per second, like a ballerina gone mad.
 

In the artist’s rendition above the pulsar is the tiny bright dot at center of the green and purple cones, and the orange companion star at right is being heated and ablated by the powerful energy emitted from the pulsar.  The colored cones are beams of gamma rays and radio waves emitted into two directions that rotate with the pulsar as it spins rapidly, and which are observed on Earth as pulses if the cones happen to point in our direction (image by Cruz deWilde, and courtesy of Goddard Space Flight Center; to see a highly informative and compelling narration of the animated version of this, please follow this link).

The most extreme case of this situation seen so far appears to have been found recently by Professor Roger Romani and his collaborators, by close observation over the last year of one such system of a mad ballerina and her unfortunate companion, using some of the largest telescopes on Earth to peer deeply into the dynamics of their frenzied whirling embrace.

Romani and his team had had their eye on this system ever since it had been first detected in gamma rays by the Fermi Gamma-ray Space Telescope.  As it was only one of the three remaining systems in the set of the 250 brightest Fermi sources not yet identified with any visible light counterpart, they very much wanted to track down exactly what was going on inside of it.  Further, knowing that several other similar candidates had, after follow-up observations with normal visible light telescopes, been found to contain extreme objects of the black widow variety, they definitely had a hunch that they might find a similar situation in this case.  What they didn't know was that they were about to discover the fastest orbiting pulsar binary system that had ever yet been seen.

Tracking down the voracious ballerina

Using telescopes in Chile and Arizona (as well as using archival data to get a longer ‘lever arm’ in time and pin down the orbital parameters better), they pinned down the overall period of the orbit by observing the Doppler shift of the companion star as it whipped around the neutron star in a record-breaking 75 minutes per orbit. They then followed up with a specialized instrument on a telescope on Mauna Kea in Hawaii to measure the spectrum of the emitted light from the companion star in more detail.  This allowed them to determine the star’s outer layers are already devoid of hydrogen, missing the gas that has already been stripped off by the ravenous neutron star (and with more of it having being steadily evaporated away afterwards by the black widow’s blindingly bright energetic glow).

We see in the above plot from Romani et al’s paper the velocity of the companion star as it orbits first towards and then away from us cyclically. One cycle of the “Binary Phase” represents the 75 minute period of the orbit.

Note that thus far nearly all of their information about this system is coming from the observations of the companion star as it is heated up on its ‘day’ side, which faces in the direction of the energetic glow of the neutron star. No pulses have yet been seen from the neutron star, but the difference in temperature between the day and night sides of the companion star tell us approximately how far away from it the neutron star must be - and it’s deadly close, about the same as the distance between the Earth and the Moon.

Insights into long-time puzzles in the microscopic realm

Combining all their results, the team estimate that to hold its companion in such a high speed, tight low radius orbit, the neutron star must be quite massive, and possibly “weigh in” at over 2 solar masses even. This has major implications for something called the "equation of state" (EOS) of nuclear matter: the EOS determines what form the matter takes when neutrons are packed so tightly together as they are in a neutron star, into extreme pressures and densities.  The EOS is something that nuclear physicists have long tried to determine, but these kinds of conditions are simply not accessible on the Earth (although one can start to approach them for very short periods, and hence get some limited knowledge, by smashing heavy lead nuclei together at places like the Large Hadron Collider in Geneva Switzerland, or at the Relativistic Heavy Ion Collider in Brookhaven, New York).

It turns out that the existence of such a high mass neutron star as Romani et al. have found indicates that some of the more exotic equations of state that have been proposed, such as those which describe curious theoretical states of matter such as 'quark-gluon plasma', 'hyperon matter', 'strange matter' etc, are actually not at play inside these dense stars.  Instead, the most simple situation possible, of just neutrons packed tightly together with very little 'elbow room' to make anything else, seems the most likely scenario.  (Essentially, if a neutron star gets to a high enough mass, the internals must be ‘stiff’ enough to hold up the outer shells, meaning ‘hard-packed’ neutrons are all that is allowed, and there is no room for the more hypothetical states of nuclear matter with the exotic particles dancing about inside.)

Ultimately, through research of this kind we may also learn something about the theory of the strong nuclear force (also known as “quantum chromodynamics”) which can further lead to insights about grand unified theories, as well as for early Universe cosmology and the conditions that held within the first few violent and fiery hundreds of microseconds after the Big Bang itself.

And all because we were able to watch, live, as a black widow neutron star in some corner of our galaxy felt the pangs of hunger and started to dine on her companion.

More about black widow pulsars:

Highly compelling 9 min video made by NASA explaining how these systems work (made for Pulsar J1311)

NASA Press Release for Pulsar J1311

Journal paper about this system: 2FGL J1653.6-0159: A New Low in Evaporating Pulsar Binary Periods  (Romani et al. 2014)

Journal paper about the J1311 system: PSR J1311-3430: A Heavyweight Neutron Star with a Flyweight Helium Companion (Romani et al. 2012)

To learn more about the theoretical background for the EOS, see this review by J. Lattimer