PSR J0348+0432 is a pulsar-white dwarf binary system. It was discovered in 2007 with the National Radio Astronomy Observatory, Green Bank's Robert C. Byrd Green Bank Telescope in a drift-scan survey. In 2013, a mass measurement for this neutron star was announced:. This measurement was done with a combination of radio timing and precise spectroscopy of the white dwarf companion. This is slightly higher than, but statistically indistinguishable from, the mass of PSR J1614−2230, which was measured using the Shapiro delay. This measurement confirmed the existence of such massive neutron stars using a different measuring technique. The notable feature of this binary pulsar is its combination of high neutron star mass and short orbital period: 2 hours and 27 minutes. This allowed a measurement of the orbital decay due to the emission of gravitational waves, as observed for PSR B1913+16 and PSR J0737−3039.
Background
The first radio pulsar was discovered in 1967 by Jocelyn Bell and her adviser, Antony Hewish using the Interplanetary Scintillation Array. Franco Pacini and Thomas Gold quickly put forth the idea that pulsars are highly magnetized rotating neutron stars, which form as a result of a supernova at the end of the life of stars more massive than about 10 times the mass of the Sun. The radiation emitted by pulsars is caused by interaction of the plasma surrounding the neutron star with its rapidly rotating magnetic field. This interaction leads to emission "in the pattern of a rotating beacon," as emission escapes along the magnetic poles of the neutron star. The "rotating beacon" property of pulsars arises from the misalignment of their magnetic poles with their rotational poles. Historically, pulsars have been discovered at radio wavelengths where emission is strong, but space telescopes that operate in the gamma ray wavelengths have also discovered pulsars.
Observations
In 2007, the Green Bank Telescope underwent track repair, and was unable to track for several months. An international team of astronomers was nevertheless able to record the data from the antenna, letting the Earth do the job of moving the beam of the telescopeacross the sky, a process known as a drift scan survey. They found a total of 35 new pulsars, including 7 new millisecond pulsars and PSR J0348+0432. In 2011 the white dwarf companion to the pulsar was observed with the FORS2 spectrograph of the European Southern Observatory's Very Large Telescope, in Chile. These data were combined with radio observations to determine the mass of the white dwarf and the pulsar. Radio timing of the pulsar with the 305-m radio telescope at the Arecibo Observatory and the Effelsberg 100-m Radio Telescope soon also detected the orbital decay of the system due to the emission of gravitational waves. This matched the rate predicted by general relativity.
Significance
The combination of a large neutron star mass, low white dwarf mass and short orbital period allows astronomers to test general relativity in a regime of extreme gravitational fields, where it has never been tested before. The result also has implications for the direct detection of gravitational waves and for understanding of stellar evolution. The measured mass of puts an empirical lower bound on the value of the Tolman–Oppenheimer–Volkoff limit. PSR J0348+0432 is also a candidate for a hyperon star, a massive neutron star containing hyperons.
