PSR J1946+2052 is a short-period binary pulsar system located 11,000–14,000 light-years (3,500–4,200 pc) away from Earth in the constellation Vulpecula. The system consists of a pulsar and a neutron star orbiting around their common center of mass every 1.8837 hours, which is the shortest orbital period among all known double neutron star systems as of 2022. The general theory of relativity predicts their orbits are gradually decaying due to emitting gravitational waves, which will eventually lead to a neutron star merger and a kilonova in 46 million years. The PSR J1946+2052 system was discovered by radio astronomers on 19 July 2017, during a survey for pulsars with the Arecibo Observatory's 305-metre (1,000 ft) radio telescope at Arecibo, Puerto Rico. The primary component of PSR J1946+2052 system, the pulsar, has a rotation period of 16.96 milliseconds and an estimated mass below 1.31 solar masses. The invisible neutron star companion likely has a lower mass of at least 1.18 solar masses, which amounts to a total system mass of approximately 2.50 solar masses, making PSR J1946+2052 potentially the lowest-mass double neutron star system known as of 2022.
Discovery The PSR J1946+2052 system was discovered by radio astronomers on 19 July 2017, during the PALFA Survey for pulsars in the Milky Way's galactic plane with the Arecibo Observatory's 305-metre (1,000 ft) radio telescope at Arecibo, Puerto Rico. A search in archival imagery shows that PSR J1946+2052 was not detected in infrared to gamma-ray wavelengths.
Location and distance PSR J1946+2052 is located in the northern celestial hemisphere in the constellation Vulpecula. Its equatorial coordinates based on the J2000 epoch are RA 19h 46m 14.13s and Dec +20° 52′ 24.6″; these are indicated in its pulsar identifier PSR J1946+2052. In galactic coordinates, it lies in the Milky Way's galactic plane with a galactic latitude 1.98° south and a galactic longitude 57.66° east from the Galactic Center. The time delay between different frequencies of PSR J1946+2052's radio pulses indicates a dispersion measure of 93.965±0.003 pc/cm3, which suggests a distance between 11,000–14,000 light-years (3,500–4,200 pc) from Earth, depending on the electron number density in the interstellar medium between the pulsar system and Earth. It is unlikely in the near future that PSR J1946+2052's distance could be determined more precisely with direct methods such as very-long-baseline interferometry or hydrogen line absorption, as it is too faint and distant.
Origin
Double neutron star systems such as PSR J1946+2052 are thought to have formed from the asynchronous evolution of two high-mass stars in a wide binary system. The higher-mass star first evolves and explodes in a supernova, leaving a neutron star remnant in an eccentric mutual orbit with the surviving companion star. As the companion star evolves into a supergiant and expands beyond its Roche lobe, it begins transferring mass to the neutron star, which energetically accretes the material and spins up to a rotation period of a few milliseconds, becoming a recycled millisecond pulsar and an X-ray binary. The aging companion star eventually engulfs the pulsar in a gaseous common envelope and their mutual orbit begins to circularize and shrink due to drag forces within the envelope. The pulsar continues accreting and strips the companion star of its hydrogen envelope, turning it into a helium star. The helium star eventually explodes in an ultra-stripped supernova with minimal ejecta, resulting in a low momentum kick that leaves the resulting neutron star pair bound in a low-eccentricity orbit around each other. The second-born neutron star from this supernova is expected to pulsate for only a few million years before its rotation slows down sufficiently for its pulsation mechanism to turn off. On the other hand, the first-born pulsar is expected to continue pulsating for billions of years due to the high angular momentum it had acquired from accretion.
Physical characteristics The total mass of the PSR J1946+2052 system is 2.50±0.04 M☉, which is determined from the components' mutual orbital period using Kepler's third law. This is potentially the lowest mass measured for a double neutron star system as of 2022, though it could be tied with PSR J1411+2551 (2.538±0.022 M☉) within uncertainty bounds. Although the individual component masses have not been measured directly, the binary mass function constrains them to be <1.31 M☉ and >1.18 M☉ for the pulsar and companion, respectively. A more detailed analysis of the Einstein delay (gravitational time dilation and Doppler shift effects) in the pulsar's pulsation timing would enable a more precise measurement of both components' masses.
Pulsar The pulsar is the only electromagnetically detectable component of the PSR J1946+2052 system. It pulsates in radio wavelengths 59 times per second, corresponding to a rotation period of 16.96 milliseconds. Due to the generation of electromagnetic radiation by its rotating magnetic field, the pulsar is gradually losing rotational kinetic energy at a spin-down luminosity of 7.5×1033 ergs per second (7.5×1026 W or 2.0 L☉) and its rotation period is increasing at a rate of (9±2)×10−19 seconds per second. This is a relatively low spin-down rate for a neutron star, which suggests the pulsar must have a weakened surface magnetic field strength of 4×109 gauss (4.0×105 T). This weakened magnetic field is thought to be the result of the pulsar having accreted matter from a past companion star, which accumulated onto the pulsar's surface and buried its original surface magnetic field. This indicates that the pulsar is the first-born stellar remnant of the PSR J1946+2052 system. The pulsar is estimated to have a characteristic age of 290 million years, assuming it only experienced constant spin-down to its present rotation period. However, this is likely not accurate to pulsar's true age because it underwent rotational spin-up through accretion in the past.
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