Astronomers have made important discoveries about the properties of the millisecond pulsar PSR J2055+3829. The results — obtained by conducting timing observations at 1.4 GHz — indicate the astronomical object is a ‘black widow’ pulsar — a rapidly spinning neutron star and a low-mass companion star.

The new findings were made by a team of astronomers led by Lucas Guillemot of the University of Orléans in France. Using the Nançay Radio Telescope (NRT) allowed them to learn more about the nature and class of this object. Their findings are published in a paper found on arXiv.org.

The team’s timing observations have identified eclipses of the pulsar’s radio signal. In particular, at 1.4 GHz, the pulsar was found to be eclipsing for about 10% of the orbit, for a few minutes around superior conjunction of the system. The entire pulsar had a combined mass of 1.4 times that of the sun, with the objects separated by a distance about 1.2 times that of the radius of the sun.

These eclipses, asymmetric and variable, are most likely caused by the outflow of material from the companion star, the mass of which the team calculated to be between 0.023 and 0.053 solar masses.

Pulse profile for PSR J2055+3829 at 1.4 GHz, formed by integrating 48.8 hr of coherently dedispersed observations with the Nançay Radio Telescope and the NUPPI backend. Credit: Guillemot et al., 2019.

Thus, taking into account the companion’s estimated mass, and the detection of time variations of the orbital period, the researchers were able to identify PSR J2055+3829 as an eclipsing ‘black widow’ pulsar.

As the name suggests, one of the key-features of black widow binaries is the close proximity to a neutron star has disastrous consequences for the low-mass companion star.

Deadlier than the male — How spider pulsars ‘devour’ their companions

The most rapidly rotating pulsars — those with rotation periods below 30 milliseconds — are known as millisecond pulsars (MSPs). It is assumed that they are formed in binary systems when the initially more massive component turns into a neutron star that has its angular momentum increased — or ‘spun-up’ — due to the accretion of matter from the secondary star.

Spinning 390 times a second, PSR J1311−3430 periodically swings its radio (green) and gamma-ray (magenta) beams past Earth in this artist’s concept. The pulsar heats the facing side of its stellar partner to temperatures twice as hot as the sun’s surface and slowly evaporates it. (NASA Goddard)

Black widow pulsars are an extreme class of binary pulsars with semi-degenerate companion stars, known as ‘spider pulsars’. In the case of black widow pulsars, this companion star has an extremely low-mass — less than 0.1 that of the sun. Spider pulsars exist in which the companion star is more massive than this — these are referred to as ‘redbacks’ — Australian cousin of the black widow — thus, sticking to the arachnid monikers.

As the name suggests, in black widows and redback pulsars the larger star ‘devours’ its companion by accreting material from the degenerate, smaller star to its surface.

Astronomers have found at least 18 black widows and nine redbacks within the Milky Way, thus far. Also, additional members of each class have been discovered within the dense globular star clusters that orbit our galaxy.

How astronomers discovered PSR J1311–3430 a record-breaking black widow binary. (NASA Goddard)

One black widow system — PSR J1311–3430 discovered in 2012 — sets the record for the tightest orbit of its class and contains one of the heaviest neutron stars ever discovered.

This pulsar’s companion star — a dozen or so times the mass of Jupiter and just 60% of its size — completes an orbit in just 93 minutes.

In PSR J1311–3430 the side of the star facing the pulsar is heated to more than 12,000 C — twice as hot as the sun’s surface. Recent studies allow a range of values extending down to 2 solar masses for the pulsar, making it one of the most massive neutron stars known.

Weaving a web — what connects Pulsar Binaries?

The new study shows PSR J2055+3829 — a MSP discovered in 2017 — has a rotational period of approximately 2.09 milliseconds and a dispersion measure of about 91.9 parsecs/cm³. Its low-mass companion exists in a tight, approximately 3-hour orbit.

By comparing the results with other studies, the researchers concluded that eclipsing ‘black widows’ have in general higher mass functions than their non-eclipsing counterparts.

They also try to explain what could be responsible for such a trend: “Larger inclinations could explain the higher mass functions of eclipsing black widows.

“On the other hand, the mass function distributions of Galactic disk and globular cluster BWs appear to be consistent, suggesting, despite the very different environments, the existence of common mechanisms taking place in the last stages of evolution of BWs.”