Artist’s concept of a neutron star’s pulse warped by a white dwarf.Illustration: BSaxton, NRAO/AUI/NSF
Scientists have discovered what could be the largest neutron star on record, according to new research.
Neutron stars are among the strangest objects in the sky. They’re wildly compact: just a few miles across yet more massive than the Sun. Measurements have shown that a neutron star called MSP J0740+6620 is the first whose mass is unambiguously at least twice the Sun’s, which may be about as big as these objects can possibly get.
“Finding more and more massive neutron stars is exciting because it helps us constrain the neutron star equation of state… that describes how matter acts in their interiors,” Thankful Cromartie, the study’s first author and a Ph.D candidate at the University of Virginia, told Gizmodo.
Calculating the mass of neutron starts is important for understanding how such strange objects can exist in the first place. Though astronomers generally consider the neutron star mass to be around 1.4 solar masses, more recent measurements have revealed increasingly huge examples.
An international collaboration of scientists, called NANOGrav, has been observing dozens of neutron stars for the past 12 years using the Green Bank and Arecibo telescopes. The effort is part of a hunt for low-frequency gravitational waves—ripples in spacetime like those recently discovered by LIGO but with more time between the crests. Specifically, they’re interested in a kind of pulsing neutron star called, appropriately, a pulsar. MSP J0740+6620 is one of those pulsars.
The researchers took dedicated observations of MSP J0740+6620 using the Green Bank telescope to determine its mass using a method called relativistic Shapiro delay. This pulsar forms a binary with another compact object called a white dwarf. The two objects orbit each other, and, from our viewing perspective here on Earth, pass in front of one another. When the white dwarf passes in front of the pulsar, it appears to causes a slight delay in the pulsing, because the light must travel a slightly longer path as the white dwarf’s gravity slightly warps the shape of space. Researchers use this method to determine the white dwarf’s mass, and, in turn, the pulsar’s mass—which they found to be approximately 2.14 times the mass of the Sun, and definitely more than twice the Sun’s mass, according to the paper published in Nature Astronomy.
Finding these kinds of systems to study is rare, since they must be tilted at just the right angle from our perspective on Earth. But when scientists spot them, they’re among the most reliable tools to determine a neutron star’s mass, Cromartie said. She noted that other techniques might have measured more massive neutron stars, but those other techniques produce less precise results.
This pulsar’s mass places it at what might be the upper limit to this kind of object—any more massive, and it could collapse into a black hole, explained Harshal Gupta, NSF program director for the Green Bank Observatory in West Virginia. Gupta was also excited about how NANOGrav would complement the discoveries at the LIGO and Virgo gravitational wave observatories. “It’s a very solid effort in terms of astronomy and the physics of compact objects.”
Cromartie told Gizmodo she was looking forward to seeing more data on this object, such as more frequent observations from the CHIME radio telescope in Canada, and measurements of the object’s radius from the NICER x-ray space telescope. These observations can help physicists further refine our knowledge of these bizarre stars.
Said Cromartie: “I feel super lucky to be involved in such exciting results.”