Scientists suspect that a rapidly spinning, highly magnetic neutron star, or “pulsar,” dwells at the heart of the Milky Way. The discovery could change our understanding of how many of these extreme dead stars dwell close to our galaxy’s central supermassive black hole, Sagittarius A* (Sgr A*).
Like all neutron stars, pulsars are born when stars with masses around that of the sun reach the end of their fuel for nuclear fusion and can no longer support themselves against gravitational collapse. Though the region of the Milky Way called the Galactic Center is expected to be replete with pulsars, spotting them is challenging because of how extreme, turbulent, and densely packed the heart of our galaxy is. However, radio waves aren’t obscured by this region to the same extent that visible light and other forms of electromagnetic radiation are.
Scientists behind the study say there were surprised by how few pulsars were found. “Our survey is one of the most sensitive ever conducted toward the Galactic Center,” team leader Karen Perez of the Search for Extraterrestrial Intelligence (SETI) Institute said in a statement. “We should have been sensitive to approximately 10% of millisecond pulsars and 50% of canonical, slow pulsars, assuming the pulsar population in the Galactic Center resembles that of the broader Milky Way.
“Despite this sensitivity, we detected only a single candidate – dubbed the Breakthrough Listen Pulsar (BLPSR) – which remains under active investigation.”
Putting Einstein to the test with cosmic lighthouses
The gravitational collapse of a massive stellar core to create a neutron star results in a body with between one and two times the mass of the sun crammed into a width of 12 miles (20 kilometers). Not only does this create the densest material in the known universe (a teaspoon of neutron star “stuff” would weigh 10 million tons, about the same as 85,000 adult blue whales, if brought to Earth) but like an ice skater at the Winter Olympics drawing in their arms to speed up their spin, the rapid contraction of a stellar core that births a neutron star can create an object that can spin a staggering 700 times per second.
If this didn’t make neutron stars extreme enough, in the case of pulsars, these dead stars blast out twin parallel beams of radio wave radiation from their poles. As the pulsar spins, these beams sweep across the cosmos like the beams of light from a lighthouse. Hence, pulsars are often referred to as “cosmic lighthouses.”
The precision of pulsars means that the periodicity of their beams can be used as cosmic clocks, which can be used to investigate physics in extreme conditions, such as in the vicinity of bodies with immense masses. That includes Einstein’s 1915 magnum opus theory of gravity, general relativity, which suggests that objects with mass warp the very fabric of space and time, unified as a four-dimensional entity called “spacetime.” Gravity arises from the warping of space, and an impact on time can be detected with precise enough clocks. Clocks like pulsars.
“Any external influence on a pulsar, such as the gravitational pull of a massive object, would introduce anomalies in this steady arrival of pulses, which can be measured and modelled,’ team member Slavko Bogdanov of the Columbia Astrophysics Laboratory said. “In addition, when the pulses travel near a very massive object, they may be deflected and experience time delays due to the warping of space-time, as predicted by Einstein’s theory of general relativity.”
Sgr A*, with a mass equivalent to over 4 million suns, has a radical impact on spacetime in its vicinity, and as such, it certainly provides a suitable laboratory to study such physics. If pulsars exist in close proximity to Sgr A*, they can serve as the right lab equipment for these experiments.
The result would be an unprecedented test of general relativity around a supermassive black hole. In the meantime, the fact that BLPSR was the only pulsar potentially detected by the researchers in the Galactic Center raises serious questions about the predicted population sizes of these extreme dead stars at the heart of the Milky Way.
These are questions that may be answerable with future astronomy projects such as the next-generation Very Large Array (ngVLA) and the Square Kilometer Array (SKA), which should have the sensitivity and resolution needed to truly determine the population density of pulsars at the center of our galaxy.
“We’re looking forward to what follow-up observations might reveal about this pulsar candidate,” Perez said. “If confirmed, it could help us better understand both our own galaxy and general relativity as a whole.”
The team’s results were published on Feb. 9 in The Astrophysical Journal.
