A pulsar has devoured enough of its stellar companion to grab the title for most massive known neutron star.
It’s the astronomical equivalent of an Olympic athlete winning a silver medal in one sport and a gold one in another. The pulsar PSR J0952-0607, which is some 20,000 light-years away in the constellation Sextans, already holds the title of second-fastest-known rotator, spinning around its axis 707 times per second. Now, it has also shattered the record for most massive neutron star known, weighing in at 2.35 solar masses.
Neutron stars are the city-size, extremely compact leftovers of supernova explosions. Beams of high-energy particles and radiation from their magnetic poles sweep through space as they rotate. Depending on their orientation, we see some of them as pulsars – rapidly pulsating sources of radio waves and/or X-rays.
J0952 was discovered in December 2016 by Dutch radio astronomers Cees Bassa, Ziggy Pleunis, and Jason Hessels (none of whom were involved in the new study), using the International Low-Frequency Array (LOFAR) — a European network of small radio antennas with its core in the Netherlands.
“The design of our search was biased towards finding neutron stars that are bright at low radio frequencies, which were expected to be fast spinning,” says Bassa. Indeed, PSR 0952 has a rotation period of a mere 1.41 milliseconds – just shy of the 1.40 millisecond rate of the current record holder, PSR J1748-2446. The surfaces of these objects whip around at some 20% the speed of light!
These millisecond pulsars spin up as they accrete material from an orbiting companion star. In some cases, the companion is slowly devoured, which is why objects like J0952 are also called black widow pulsars, after the spider that first mates with and then eats her partner.
The accumulating gas gradually beefs up the mass of the neutron star. The companion of J0952 has probably lost at least one solar mass to the pulsar, dwindling down to a sub-stellar object of a few tens of Jupiter masses. “Pulsars that have accreted may be the most massive neutron stars that can be found in nature,” comments Hessels. However, determining their mass isn’t straightforward.
A team led by Roger Romani (Stanford University) has now succeeded in taking spectra of the extremely faint (23rd-magnitude) companion of J0952, using the 10-meter Keck I telescope at Mauna Kea, Hawai’i. In a study to appear in Astrophysical Journal Letters, they report Doppler measurements indicating an orbital velocity of 380 kilometers per second (850,000 mph). Combined with brightness measurements over the orbital period of 6.42 hours, this yields a mass estimate for the neutron star of 2.35 solar masses. The previous record holder (PSR J0740+6620) weighed in at just 2.08 solar masses.
The result is important because no one knows how matter behaves under the most extreme conditions. The interiors of neutron stars may consist of ordinary elementary particles, or of completely new forms of matter. This so-called equation of state determines how massive a neutron star can get before it further collapses into a black hole.
The mass estimate of J0952 is still quite uncertain, with a possible error of ± 0.17 solar masses. “Of course, we would like an even tighter mass measurement of this especially important system,” Romani and his colleagues write, “but improved [radial velocity measurements] likely await the 30-m telescope era.”
According to Victoria Kaspi (McGill University, Canada), who was not involved in the study, the new result provides an important constraint, but it’s still too early to draw definitive conclusions about neutron star matter.
“I think this result sends warning to modelers of ultra-dense matter that neutron stars may be capable of having pretty high masses,” she says. “If I were a theorist wedded to a model that permitted only lower-mass neutron stars, I’d start scratching my head about now, though I wouldn’t yet be in a full panic mode.”