This episode's object is the star system PSR J1518+4904 located at a distance of 2635 light years (808 pc) in the constellation Boötes [1]. The PSR in this star system's name indicates that it contains a pulsar, which is a rapidly rotating neutron star that was once the core of a very large hydrogen-burning star that exploded as a supernova but that is now just a ball of neutrons, although as it rotates, it emits electromagnetic radiation from its magnetic poles. The J and the numbers afterwards indicate the coordinates of the star system. The pulsar was discovered in a survey with the Green Bank 140 foot radio telescope in 1994 [2]. However, this star system does not contain just a pulsar. Based on measurements of the periodic Doppler shifting of the electromagnetic radiation emitted by the pulsar, astronomers have determined that the system also contains a second neutron star (which, to be clear, is not a pulsar but is still interesting nonetheless) [1,2,4,5]. In fact, PSR J1518+4904 is one of the very few binary neutron star systems known (although, for reasons I don't quite understand, these things are commonly called "double neutron star systems" and not "binary neutron star systems").
In general, star systems containing a pulsar and a second neutron star are rather interesting objects for people to study in part because they provide astronomers with opportunities to precisely determine the masses of the neutron stars. Without seeing neutron stars in orbit around each other or around other objects, we just would not be able to determine their masses. However, double neutron star systems can also provide other uniquely useful information, which I will explain later.
So, let's run through some numbers for this star system, which hopefully won't get overwhelming or sound too much like one of the Numberwang sketches by Mitchell and Webb. The pulsar (or the neutron star that we can see is rotating) has a rotation period of 40.93 milliseconds and a mass of 1.47 times the mass of the Sun [5]. The second neutron star (which we cannot see rotating) has a mass of 1.25 times the mass of the Sun [5]. Unfortunately, I don't have any good numbers for the diameters of these two stars, but based on the measurements of other neutron stars, the two neutron stars in PSR J1518+490 should have diameters between 20 and 30 km [6].
These two neutron stars orbit each other once every 8.63 days [5]. While the orbits of the stars are not close to circular but actually slightly elongated like the orbit of Pluto, the approximate distance between the two stars is commonly reported as 20 light seconds [5], which is the distance that light travels in 20 seconds. Alternately, this separation can be described as 0.04 astronomical units (or 0.04 times the distance from the Earth to the Sun). Additionally, the distance can be described as about 15 times the distance from the Earth to the Moon or 8.6 times the radius of the Sun.
As you could imagine, placing two objects with such large masses and small diameters so close together could cause some uniquely weird physics effects. Rather remarkably, though, this system isn't extreme enough to produce the most exotic relativistic effects that people are interested in. The stars are orbiting each other at less than 1% the speed of light, which means that the stars are only "mildy relativistic" [2], sort of like how someone who really likes spicy Mexican food might describe a habanero chili pepper as only "mildly hot". In any case, PSR J1518+4904 cannot be used to test some of the more extreme aspects of general relativity, but it has been possible to observe one rather interesting relativistic effect in the system, and that is something called the Shapiro effect.
The theory behind this effect was developed by Irwin Shapiro in 1962, who suggested that light travelling near a massive object like the Sun might be slowed down a little bit because of the effects of the increased gravitation field on the speed of light [7]. Shapiro tested his theory using radio waves transmitted to and reflected off of the surface of Venus when it was almost but not quite on the other side of the Sun [7]. However, PSR J1518+4904 contains two objects more massive than the Sun but much, much smaller and therefore denser than the Sun. The gravity near the surface of one of these neutron stars will be a few billion times stronger than the gravity near the surface of the Sun, so the Shapiro effect observed in the double neutron star system will be much stronger. In fact, even though initial mass estimates for the two neutron stars came from the orbits of the stars around each other, these mass estimates were refined by watching the change in the periodicity of the pulsar's signal as it passed behind the second neutron star in this system and as the second neutron star's gravitational field slowed down the electromagnetic radiation from the pulsar [5].
So this is not the end of the research that can be done with PSR J1518+4904. Quite a few questions remain about how double neutron star systems form. In these systems, both neutron stars are expected to have been created through supernova explosions that take place at different times, and the second star to explode could boot the first neutron star out of the system. In fact, one hypothesis is that, in PSR J1518+4904 specifically at least, the second supernova explosion was symmetric, which avoided kicking the first neutron star out of the system [2]. Also, while it is generally expected that rapidly rotating pulsars like the one in PSR J1518+4904 could be sped up by absorbing material from their companion stars before they explode, this theory still needs some refinement. Additionally, double neutron stars are expected to merge over time, and while the merger of the two neutron stars in PSR J1518+4904 will take place far in the future, it could form both a gamma ray burst and some very strong gravitational waves. So for people who really like to study the formation and evolution of very rare double neutron star systems, PSR J1518+4904 is quite an interesting object to look at.