Pulsar timing uncovers a massive neutron star

Pulsar timing arrays are designed to spot gravitational waves by clocking the arrival times of regular pulses from millisecond pulsars (see Physics Today, July 2017, page 26 ). Subtle variations in timing from pulsar to pulsar could indicate that en route to Earth, a swell of gravitational radiation generated by, say, the merging of supermassive black holes washed over the stars.

But gravitational waves aren’t the only explanation for a pulsar’s beacon arriving a tick off schedule. Astronomers have identified a handful of binary systems in which a pulsar is gravitationally and closely bound to a white dwarf. If the system is oriented so that the white dwarf crosses directly in front of the pulsar as viewed from Earth, then researchers can detect a relativistic delay in the pulsar signal and use it to calculate the mass of both binary objects. Now, using data from the NANOGrav Pulsar Timing Array, Thankful Cromartie of the University of Virginia and her colleagues have measured such a system and found that its pulsar very likely has the highest mass of any known neutron star.

To make the mass measurement, the researchers relied on a relativistic effect known as the Shapiro delay, which is related to the path of the pulsar beam as it traverses the curved spacetime surrounding its companion (see Physics Today, January 2011, page 12 ). By combining the data from five years of NANOGrav observations with those from a pair of dedicated observing campaigns, Cromartie and colleagues uncovered a precise pattern of timing variations throughout the orbital period, as shown in the graph. From the amplitude and shape of the curve, the researchers determined the white dwarf’s mass and the inclination of the orbit as viewed from Earth; those data in turn enabled them to calculate the pulsar’s mass. Weighing in at about 2.14 solar masses, the pulsar, named J0740+6620, is the first neutron star with a measured mass that exceeds 2 M_⊙ within the entire 68% confidence interval. The inferred value exceeds the Tolman-Oppenheimer-Volkoff limit of 2.1 M_⊙, which was derived in 1939 for nonrotating neutron stars, and it falls just short of the 2.16 M_⊙ limit derived last year for rotating neutron stars.

The newly weighed neutron star is one of several measured in recent years at around 2 M_⊙, a trend that has implications for understanding the objects’ makeup. When theorists model the structure and composition of a neutron star, they derive an equation of state—the star’s mass as a function of radius. By itself, the substantial mass of J0740+6620 is already making some theorists uncomfortable. A big test for models would come if x-ray observations of the pulsar succeed in yielding a radius. (H. T. Cromartie et al., Nat. Astron., in press, doi:10.1038/s41550-019-0880-2 .)