Catching a Spider Pulsar In The Act

Pulsars are some of nature’s most extreme objects - they’re the rapidly rotating compact remains of dead stars, have magnetic fields billions to trillions of times stronger than your fridge magnet, and pack in roughly 1.4 times the mass of the Sun into a space about the size of a small town. Even their birth is dramatic - forged in the collapsing heart of a dying massive star, as it detonates in a supernova explosion. 

Like regular stars, pulsars also come in a number of different sub-categories, and one of the most interesting happens to be named after Earth’s arachnids - spiders. And much like a spider that feeds on its mate, these pulsars devour their low-mass orbiting companions with their radiation, evaporating the companion over time.

Spider pulsar systems come in two sub-varieties - the black widows, and their Australian-inspired named cousins, the redbacks. Black widows tend to have millisecond pulsars with very low mass companions (on the order of 0.05 solar masses) in very tight orbits (roughly completing entire orbits in several hours), whereas redbacks can have the bigger companion masses (reaching up to 0.2 solar masses) with orbital periods that can last as long as a day.

Now, Western Sydney University astronomers have used one of the largest and most sensitive radio telescopes in the world, the Five-hundred-meter Aperture Spherical Telescope (FAST) to take a deeper look at the first-ever spider pulsar discovered - PSR B1957+20 - carrying out an in-depth analysis that revealed new features that have never been observed before. 

“Black widow pulsars are in extremely compact binary systems,” said Dr Shi Dai from Western Sydney University, one of the authors of the paper. “The pulsar and its very light companion orbit around each other in only a few hours, and interact with each other very closely.” 

“This provides us rare opportunities to probe a range of physics, such as properties of dense plasma in the system and pulsar radiation.”   

“The superior sensitivity of FAST allows us to probe some of the subtlest features of the pulsar radio emission, especially when we move into the eclipse region where plasma density is extremely high. These regions with dense plasma are where a lot of exciting physics are going on.”

PSR B1957+20, also known as the Black Widow Pulsar, was discovered in 1988 and resides approximately 6,500 light-years away from Earth. Along with its low-mass binary companion, the pair race around the system’s centre of mass every nine hours. The companion is thought to be a brown dwarf, with an approximate mass of 22 Jupiters - which is very much on the lighter side when it comes to stellar objects. 

Spider pulsars orbit at a relatively close radius to their companions, and the energy from the pulsar causes the companion to become bloated and enlarged. As the two objects orbit each other, the pulsar’s ticking radio signals disappear from our view as it swings behind the companion’s expanded atmosphere (which can form a cometary-like tail) - and thus, the pulsar becomes eclipsed. 

The cause of this eclipse is due to the ionising material that makes up the large envelope that the companion sheds - with this material interfering with the transmission of radio waves from the pulsar. However, the properties and nature of this ionsied material remain a mystery to scientists, due to the lack of sensitivity of radio telescopes, especially at the higher frequency ranges. 

Using FAST, the international team - led by researchers from the Guizhou Normal University in China, and Western Sydney University in Australia - have carried out an in-depth study into PSR B1957+20, revealing new features of this system, and in particular, learning more about how the ionised material affects the pulsar’s signal as it traverses towards Earth. 

“B1957+20 is the first black widow ever discovered and also one of the brightest black widows. It is probably the best system to investigate the mechanism of radio eclipsing and has been studied extensively in the last couple of decades,” said Dr Dai. 

“However, B1957+20 gets a lot fainter as we move to higher radio frequencies, and most previous studies have been focused on low frequencies. FAST, for the first time, enabled us to have a closer look at it at higher frequencies.”

They’ve detected weak signals from deep within the eclipsing region and found that the radio wavelength pulses of light from the pulsar become broader and wider, deeper within the eclipse region. The research team has indicated that this can be explained by the strong turbulence that is occurring within the ionised material from the slowly-ablated companion. 

This is the first time evidence of this turbulence has been observed in the eclipsing material of any spider pulsar, suggesting that this type of turbulent activity can be an important mechanism when considering eclipsing radio emissions from pulsars, especially at high frequencies. 

“Our results show that the plasma in the orbit is not only dense but also highly turbulent,” said Dr Dai.

“At a distance of about 10⁸ metres from the star, the electron density and turbulence level are similar to those in the solar corona. Such strong turbulence scatters the radio waves, which is likely the main course of the eclipse of radio emission from the pulsar at above 1 GHz.” 

“This also supports the picture that energetic emission from the pulsar is evaporating its companion and powering a strong wind.”

This new research might also shed some light on one of astrophysics recent mysteries - the elusive Fast Radio Bursts, which also can exhibit this kind of broadening of their pulse signals. The new research has been submitted to the journal the Monthly Notices of the Royal Astronomical Society. 

Video Credit: NASA.