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.[8]: 2 Because of its optimal location away from the crowded Galactic Center and its pulsar-like[12]: 8 gamma-ray emission peak at 1.4 GeV, it was deemed a prime millisecond pulsar candidate for follow-up.[8]: 2 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.[8]: 3 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.[8]: 3 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.[8]: 3 The discovery was published in The Astrophysical Journal Letters and was announced in a NASA press release in September 2017.[8][6]
Distance and location
The distance of PSR J0952–0607 from Earth is highly uncertain.[2][3][4]
Binary system
The PSR J0952–0607 binary system is composed of a massive pulsar and a substellar-mass (<0.1 M☉)[9]: 127 companion in close orbit around it.[4] 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.[7] 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.[9]: 127 [4]: 1
Companion
Artist's impression of a black widow pulsar system, where a stellar-mass companion is being ablated by the intense radiation of its host pulsar
The companion orbits the pulsar at a distance of 1.6 million km (1 million mi)[b] with an orbital period of 6.42 hours.[4] Because it orbits so closely, the companion is presumably tidally locked, with one hemisphere always facing the pulsar.[3]: 8 The companion does not appear to eclipse the pulsar,[8]: 1 [3]: 12 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.[4]: 4 The companion's orbital motion also does not appear to modulate the pulsar's pulsations, signifying a circular orbit with negligible orbital eccentricity.[3]: 4
The companion was likely a former star that had been reduced to the size of a large gas giantplanet or brown dwarf,[7][3]: 12 with a present-day mass of 0.032±0.002 M☉ or 34±2 MJ according to radial velocity measurements.[4]: 4 Due to intense irradiation and heating by the host pulsar, the companion's radius is bloated up to 80% of its Roche lobe[3]: 8 [4]: 4 and brightly glows with a thermalluminosity of about 10 L☉,[a] thereby accounting for much of the system's optical brightness.[4]: 1, 4 [2]: 1 As a result of bloating, the companion attains a low density likely around 10 g/cm3 (with significant uncertainty due to the system's unknown distance from Earth),[2]: 11 making it susceptible to tidal deformation by the pulsar.[3]: 12
The companion's pulsar-facing irradiated hemisphere is continuously heated up to a temperature of 6,200 K, whereas the companion's unirradiated hemisphere experiences a uniform[2]: 4 temperature of 3,000 K.[4]: 4 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.[8]: 4 [3]: 8 This brightness variability is demonstrated in PSR J0952–0607's optical light curve, which exhibits an amplitude greater than one magnitude.[2]: 4
Mass
PSR J0952–0607 has a mass of 2.35±0.17 M☉, making it the most massive neutron star known as of 2022[update].[4] The pulsar likely acquired most of its mass by accreting up to 1 M☉ of lost material from its companion.[4]: 5
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.[8][6] Assuming a standard neutron star radius of 10 km (6.2 mi),[3]: 11 the equator of PSR J0952–0607 rotates at a tangential velocity over 44,400 km/s (27,600 mi/s)—about 14% the speed of light.[13] 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 4.6×10−21 seconds per second, corresponding to a characteristic age of 4.9 billion years.[3]: 11
Magnetic field
Measurements of PSR J0952–0607's spin-down rate show that the pulsar has a remarkably weak surface magnetic field strength of 6.1×107gauss (6.1×103T), placing it among the 10 weakest pulsar magnetic fields known as of 2022[update].[4]: 1 For context, ordinary pulsar magnetic fields usually lie on the order of teragauss (1×1012 G, 1.0×108 T), over 10,000 times greater than that of PSR J0952–0607.[14][4]: 1 Other millisecond pulsars exhibit similarly weak magnetic fields, hinting at a common albeit unknown mechanism in these types of systems;[3]: 2 possible explanations range from accreted matter burying the pulsar's surface magnetic field to heat-driven evolution of the pulsar's solid crust.[14]: 1–2 [4]: 1
Gamma-ray emissions
PSR J0952–0607 appears very faint in gamma-rays and was not detected in July 2011.[11][3]: 2
^ abLuminosity converted from erg/s to L☉, given 3.81+0.46 −0.43×1034 erg/s from Romani et al. (2022)[4] and the solar luminosityL☉ = 3.826×1033 erg/s.
^ abOrbital semi-major axis calculated with Kepler's Third Law: given primary mass = 2.35 M☉ and orbital period = 6.42 h.[4] Nieder et al. (2019) determined a minimum projected semi-major axis of 0.0626670light seconds (18787 km) from gamma-ray and radio pulsation timing.[3]: 6
^Kumar, R.; Kumar, M.; Thakur, V.; Kumar, S.; Kumar, P.; Sharma, A.; Agrawal, B.K.; Dhiman, S.K.; et al. (May 2023). "Observational constraint from the heaviest pulsar PSR J0952-0607 on the equation of state of dense matter in relativistic mean field model". Physical Review C: 19. arXiv:2306.05097.
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