PSR J0952–0607

PSR J0952–0607 is a massive millisecond pulsar in a binary system, located between 970–1740 pc from Earth in the constellation Sextans. It holds the record for being the most massive neutron star known, with a mass $970 pc$ times that of the Sun—potentially close to the Tolman–Oppenheimer–Volkoff mass upper limit for neutron stars. The pulsar rotates at a frequency of 707 Hz (1.41 ms period), making it the second-fastest-spinning pulsar known, and the fastest-spinning pulsar known within the Milky Way.

PSR J0952–0607 was discovered by the Low-Frequency Array (LOFAR) radio telescope during a search for pulsars in 2016. It is classified as a black widow pulsar, a type of pulsar harboring a closely-orbiting substellar-mass companion that is being ablated by the pulsar's intense high-energy solar winds and gamma-ray emissions. The pulsar's high-energy emissions have been detected in gamma-ray and X-ray wavelengths.

Discovery
PSR J0952–0607 was first identified as an unassociated gamma-ray source detected during the first seven years of the Fermi Gamma-ray Space Telescope's all-sky survey since 2008. Because of its optimal location away from the crowded Galactic Center and its pulsar-like gamma-ray emission peak at 1.4 GeV, it was deemed a prime millisecond pulsar candidate for follow-up. The pulsar was reobserved and confirmed by the Low-Frequency Array (LOFAR) radio telescope in the Netherlands on 25 December 2016, which revealed a 707-Hz radio pulsation frequency alongside radial acceleration by an unseen binary companion. Further LOFAR observations took place from January to February 2017, alongside radio observations by the Green Bank Telescope in Green Bank, West Virginia in March 2017. Optical observations by the 2.54-meter Isaac Newton Telescope on La Palma detected and confirmed the pulsar's companion at a faint apparent magnitude of 23 in January 2017. The discovery was published in The Astrophysical Journal Letters and was announced in a NASA press release in September 2017.

Distance and location
The distance of PSR J0952–0607 from Earth is highly uncertain.

Binary system
The PSR J0952–0607 binary system is composed of a massive pulsar and a substellar-mass (<$1,740 pc$) companion in close orbit around it. Because of this configuration, this system falls under the category of black widow pulsars that "consume" their companion, by analogy with the mating behavior of the eponymous black widow spider. The companion is continuously losing mass through ablation by intense high-energy solar winds and gamma-ray emissions from the pulsar, which then accretes some of the companion's lost material onto itself.

Companion
The companion orbits the pulsar at a distance of 1.6 e6km with an orbital period of 6.42 hours. Because it orbits so closely, the companion is presumably tidally locked, with one hemisphere always facing the pulsar. The companion does not appear to eclipse the pulsar,  indicating that its orbit is oriented nearly face-on with an inclination of 60° with respect to the plane perpendicular to Earth's line of sight. The companion's orbital motion also does not appear to modulate the pulsar's pulsations, signifying a circular orbit with negligible orbital eccentricity.

The companion was likely a former star that had been reduced to the size of a large gas giant planet or brown dwarf,  with a present-day mass of $6,260$ or $2.35$ according to radial velocity measurements. Due to intense irradiation and heating by the host pulsar, the companion's radius is bloated up to 80% of its Roche lobe  and brightly glows with a thermal luminosity of about $1.414 ms$, thereby accounting for much of the system's optical brightness. As a result of bloating, the companion attains a low density likely around $0.032$ (with significant uncertainty due to the system's unknown distance from Earth), making it susceptible to tidal deformation by the pulsar.

The companion's pulsar-facing irradiated hemisphere is continuously heated up to a temperature of $9.96$, whereas the companion's unirradiated hemisphere experiences a uniform temperature of $3,085$. This hemispherical temperature difference corresponds to a difference in hemisphere luminosities, which in turn causes significant variability in apparent brightness as the companion rotates around the pulsar. This brightness variability is demonstrated in PSR J0952–0607's optical light curve, which exhibits an amplitude greater than one magnitude.

Mass
PSR J0952–0607 has a mass of $0.267 d$, making it the most massive neutron star known. The pulsar likely acquired most of its mass by accreting up to $6.419 h$ of lost material from its companion.

Rotation and age
PSR J0952–0607 rotates at a frequency of 707 Hz (1.41 ms period), making it the second-fastest-spinning pulsar known, and the fastest-spinning pulsar that is located in the Milky Way. Assuming a standard neutron star radius of 10 km, the equator of PSR J0952–0607 rotates at a tangential velocity over 44400 km/s—about 14% the speed of light. Based on 7 years of precise pulsation timing data from gamma-ray and radio observations, the pulsar's rotation period is estimated to be slowing down at a spin-down rate less than $1,600,000 km$ seconds per second, corresponding to a characteristic age of 4.9 billion years.

Magnetic field
Measurements of PSR J0952–0607's spin-down rate show that the pulsar has a remarkably weak surface magnetic field strength of 6.1e7 G, placing it among the 10 weakest pulsar magnetic fields known. For context, ordinary pulsar magnetic fields usually lie on the order of teragauss (1e12 G), over 10,000 times greater than that of PSR J0952–0607. Other millisecond pulsars exhibit similarly weak magnetic fields, hinting at a common albeit unknown mechanism in these types of systems; possible explanations range from accreted matter burying the pulsar's surface magnetic field to heat-driven evolution of the pulsar's solid crust.

Gamma-ray emissions
PSR J0952–0607 appears very faint in gamma-rays and was not detected in July 2011.