Lightweight Double Neutron Star Found

More than forty years after the first discovery of a double neutron star, we still haven’t found many others — but a new survey is working to change that.

The Hunt for Pairs

In 1974, Russell Hulse and Joseph Taylor discovered the first double neutron star: two compact objects locked in a close orbit about each other. Hulse and Taylor’s measurements of this binary’s decaying orbit over subsequent years led to a Nobel prize — and the first clear evidence of gravitational waves carrying energy and angular momentum away from massive binaries.

Forty years later, we have since confirmed the existence of gravitational waves directly with the Laser Interferometer Gravitational-Wave Observatory (LIGO). Nonetheless, finding and studying pre-merger neutron-star binaries remains a top priority. Observing such systems before they merge reveals crucial information about late-stage stellar evolution, binary interactions, and the types of gravitational-wave signals we expect to find with current and future observatories.

Since the Hulse-Taylor binary, we’ve found a total of 16 additional double neutron-star systems — which represents only a tiny fraction of the more than 2,600 pulsars currently known. Recently, however, a large number of pulsar surveys are turning their eyes toward the sky, with a focus on finding more double neutron stars — and at least one of them has had success.

A Low-Mass Double

Conducted with the 1,000-foot Arecibo radio telescope in Puerto Rico, the Arecibo 327 MHz Drift Pulsar Survey has enabled the recent discovery of dozens of pulsars and transients. Among them, as reported by Jose Martinez (Max Planck Institute for Radio Astronomy) and coauthors in a recent publication, is PSR J1411+2551: a new double neutron star with one of the lowest masses ever measured for such a system.

Through meticulous observations over the span of 2.5 years, Martinez and collaborators were able to obtain a number of useful measurements for the system, including the pulsar’s period (62 ms), the period of the binary (2.62 days), and the system’s eccentricity (e = 0.17).

In addition, the team measured the rate of advance of periastron of the system, allowing them to estimate the total mass of the system: M = ~2.54 solar masses. This mass, combined with the eccentricity of the orbit, demonstrate that the companion of the pulsar in PSR J1411+2551 is almost certainly a neutron star — and the system is one of the lightest known to date, even including the double neutron-star merger that was observed by LIGO in August this past year.

Constraining Stellar Physics

The intriguing orbital properties and low mass of PSR J1411+2551 have already allowed the authors to explore a number of constraints to stellar evolution models, including narrowing the possible equations of state for neutron stars that could produce such a system. These constraints will be interesting to compare to constraints from LIGO and Virgo in the future, as more merging neutron-star systems are observed.

Meanwhile, our best bet for obtaining further constraints is to continue searching for more pre-merger double neutron-star systems like the Hulse-Taylor binary and PSR J1411+2551. Let the hunt continue!

Citation

J. G. Martinez et al 2017 ApJL 851 L29. doi:10.3847/2041-8213/aa9d87