X-ray pulsar eclipse reveals neutron star's secrets

Artist's rendition of the J1749.4-2807 system, where the pulsar is embedded within the disk on the left, and the companion star is on the right.
Artist's rendition of the J1749.4-2807 system, where the pulsar is embedded within the disk on the left, and the companion star is on the right. NASA/GSFC
/ Source: Discovery Channel

What's in a neutron star? Well, neutrons, obviously. However, despite the deceptively simple label, the inner working of neutron stars remain elusive, partly because their basic properties, such as mass, are so difficult to measure.

But they may not remain a mystery for much longer, as a fascinating x-ray pulsar — with the unassuming name J1749.4-2807 — may yet reveal its secrets.

Neutron stars are the extremely dense remnants of massive stellar explosions called supernovae. Imagine the mass of the sun packed into the size of a city; that gives you a density such that a teaspoonful of material from the neutron star would weigh as much as a mountain. Astrophysicists are still working on the model for how matter behaves with such high densities; knowing the range of masses, especially the upper limit of mass, of neutron stars would certainly help.

Neutron stars are difficult to detect, but some make themselves known as pulsars. These rapidly-spinning, highly magnetized neutron stars have "hotspots" that glow in the radio or x-ray. If you are in the path of one of these "lighthouse beams," you see it "pulsing." By accurately measuring the time of the pulses, you can learn a lot about the neutron star itself.

This particular pulsar was found to be locked in an orbital dance with a partner star, probably just 0.7 times the mass of our sun. The pair is lined up just right so that the pulsar and the star eclipse each other from our vantage point. When the pulsar underwent an x-ray outburst in April, the Rossi X-Ray Timing Explorer (RXTE) was there to watch the pulses come in at 518 times per second and three eclipses of the pulsar by the star.

The accurate timing of the pulses, along with eclipse observations (the first of their kind) allowed astronomers Craig Markwardt and Tod Strohmayer to make a model involving the star's mass and radius, the angle at which the companions orbit each other (with respect to our line of sight) and the neutron star mass. Since this neutron star is gobbling up mass from the companion star, it may be at the high end of the mass spectrum. Uncovering a precise mass would be an important measurement.

However, though the companion star's mass and radius are approximately known, an exact measurement of either would complete the model, giving an accurate mass for the pulsar. Or, the RXTE could be used to detect the Shapiro delay, an effect of Einstein's general relativity. The gravity of the companion star will affect the light coming from the neutron star ever so slightly just before and after eclipse, allowing the star's mass to be measured by the delay in the pulses.

Sound complicated? It is, a bit. Weird physics? You bet. The discovery and study of this unique system is probing the edge of our knowledge, and it is really exciting to watch it unfold. As with most scientific stories, there is more work to be done before the universe shares its secrets.

This work was published in Astrophysical Journal Letters, and a preprint is available on arxiv.org.