Pulsar astronomers scored big with the finding of a triple star system that includes a millisecond pulsar. This unique find has allowed them to precisely measure the physics of this system and has the potential to test Einstein’s general relativity with incredible precision.
The system in question contains a millisecond pulsar, or a neutron star that has been left over from a supernova explosion of a massive star at the end of its lifetime. It is in a 1.6 day binary orbit with a white dwarf, a small hot star left over after a mid-sized star lives out its life. Another white dwarf star completes the triple in a larger orbit with a 327 day orbital period.
Millisecond pulsars spin at incredibly high speed — up to thousands of times per second — and have a “hotspot” that rotates into view, creating a radio pulse. By measuring the exact time at which these pulses arrive at Earth, they can be used as incredibly accurate clocks, allowing for exquisite physical measurements to be made.
Finding a triple system like this is extremely rare, and the stars have to survive several catastrophic events. First, the massive star goes supernova to create the neutron star that becomes the pulsar without too significantly disturbing the nearby mid-sized stellar companion.
The pulsar and the star then continue to interact with each other with the incredibly dense pulsar pulling material off of the star as it expands and cools into a red giant phase. This acquisition of material is what eventually speeds up the rotation of the pulsar. The outer star will also go through a red giant phase, and all three components are still together after this. Scott Ransom, one of the lead investigators of the project, estimates that this alignment is actually “one in a billion.”
The pulsar was discovered in a large survey done with the Green Bank Telescope in West Virginia in the summer of 2007. This survey took advantage of the fact that the entire telescope had ceased normal operations in order for the track supporting the massive telescope to be repaired, as it was beginning to buckle under its weight. But astronomers do not waste time, and the telescope scanned vast swaths of the sky looking for pulsars even while the construction was being done.
This pulsar was discovered by graduate student Jason Boyles in the very last bit of data to be analyzed just two years ago. Follow up observations with the GBT, The Arecibo Radio Telescope in Puerto Rico, and the Westerbork Synthesis Array accurately timed the pulses as they arrived at Earth. The 1.6 day period from the orbit of the closer white dwarf was easily detected, but only after careful analysis and checking was the second companion discovered with a period of 327 days. The companion stars were confirmed to be white dwarfs by observations in the infrared, optical, and ultraviolet.
With a truly unique system, the motions and masses of the stars were measured with exquisite precision, with masses of 1.437 solar masses for the pulsar and 0.1975 and 0.4101 solar masses for the white dwarf stars. Yes, those are FOUR significant figures to those measurements! In fact, the error bars on the plot of the pulsar timing are a million times too small to even see on the chart.
This triple system allows such precise measurements that astronomers are testing a key component on general relativity, the strong equivalence principle. This basically states that mass, and only mass, is the important quantity to determining gravity, even when gravitational fields are incredibly strong, such as around a pulsar. By measuring the precise motions of the two white dwarfs around the pulsar, this principle can be tested. Limits are already being made and general relativity does pass the test so far, but more monitoring will provide an even better answer in the future. general relativity still holds supreme, but the team expects to have more precise measurements worked out over the next year.
Watch a video of the system in action:
This work appeared in the January 5th edition of Nature; and a preprint is available at arXiv.org.