New progress in the study of "Black Widow" pulsars: a beacon in the universe? Or a bloodthirsty killer?

You may know the beautiful and cold "Black Widow" in the movie, or you may know the "Black Widow" that lives in the jungle and is one of the world-famous poisonous spiders. But do you know that there are also "Black Widows" in the starry sky above our heads? They are not constellations, nor are they alien monsters, but a special type of celestial body discovered by astronomers in the universe.

In 1988, Fruchter and his colleagues used the famous Arecibo radio telescope to discover a strange millisecond pulsar PSR B1957+20. The radiation and high-energy particle winds ejected from its poles were heating and evaporating its companion star, constantly stripping the companion star's matter from its surface, causing their mass to gradually decrease. Imaginative astronomers found that this behavior was like a black widow spider dismembering its partner after mating, so they named this pulsar "black widow".

Schematic diagram of the Black Widow pulsar

(Image credit: NASA)

How "black" are the "black widow" pulsars? Taking PSR J1719-1438 as an example, the mass of its companion star is only one thousandth of the mass of the sun, which is equivalent to Jupiter in the solar system. This shows that this "black widow" has blown its companion star from a star to a planet, directly downgrading a level!

Who created such a ferocious and bloodthirsty "Black Widow"? Let's look through the genealogy of stars and find out its history.

I would like to add that there are cousins ​​of black widow pulsars in the universe - red back pulsars. Red back spiders are also highly venomous spiders. The orbital periods of these two types of pulsars are both less than 1 day, and the difference lies in the mass of the companion star. The companion star of the black widow pulsar has a mass less than 5 percent of the sun, while the companion star of the red back pulsar has a mass of one tenth to one times the sun.

Where were you born?

We know that if a star has a mass more than 10 times that of the sun, it will experience a supernova explosion in the final stage of its evolution, forming a neutron star with a radius of 15 kilometers and a super strong magnetic field. Generally speaking, at this time, the star is about to end its brilliant life, because it no longer has nuclear fuel to emit light. What awaits it is gradually returning to silence in the universe.

Supernova diagram

(Image credit: NASA)

However, the neutron star in the binary star system knows how to "take what is useful" and can change its fate. It relies on its strong gravity to strip matter from its companion star to increase its own mass (called accretion process). When the stripped matter falls to the surface of the neutron star, it will radiate X-rays. In this way, the neutron star will be reborn and continue its stage in the universe as an X-ray source. During this period, the neutron star obtains angular momentum by accreting the companion star's matter to increase its own rotation speed, and eventually becomes a millisecond pulsar with a rotation period of only a few milliseconds. Under the combined effect of the pulsar's high-speed rotation and strong magnetic field, the charged particles on the surface will be accelerated, and electromagnetic radiation and high-energy particle winds will be generated. If its companion star is unlucky and is unfortunately swept by these radiations, then a black widow binary system will be formed.

Here I would also like to add that a pulsar is a type of rapidly rotating neutron star. When the radiation beam of a pulsar sweeps across the Earth, we can use a telescope to record the pulse signal of this pulsar. Due to the stable rotation of the pulsar, the pulse signal will have a cycle similar to that of a human "pulse", sometimes bright and sometimes dim, like a lighthouse pointing the way for people in the dark, so pulsars are also known as "cosmic lighthouses."

Schematic diagram of a pulsar

(Image source: https://outreach.atnf.csiro.au/)

Where to go?

So, will the companion star of the black widow pulsar disappear completely after being evaporated by the pulse radiation for a long time, leaving only an isolated millisecond pulsar in the binary system? It is very likely! In fact, astronomers have discovered hundreds of isolated millisecond pulsars, accounting for about one-third of the total number of millisecond pulsars.
Previous studies have shown that it takes a long time for a black widow pulsar to completely destroy its companion star, a time that even exceeds the age of the universe. So it seems that the traditional black widow model cannot form isolated millisecond pulsars.

Recently, researchers from Yunnan Observatory constructed a type of black widow pulsar system using the model of pulsar evaporation helium star. Their calculation results show that the companion star can be reduced to an extremely low mass in a short period of time, that is, one thousandth or even one hundred thousandth of the mass of the sun. At this time, the black widow's companion star will be torn apart by the gravity of the pulsar. The black widow pulsar will eventually end up alone and become an isolated millisecond pulsar. However, will all black widow pulsars completely destroy their companion stars? This question needs further research.

Conclusion

This is the black widow pulsar. Its companion gradually weakened under its destruction, and even disappeared from this world. But from our distant perspective, it is a beacon in the dark universe. If humans plan to move away from the earth in the future and look for a new habitable zone, they must keep a considerable distance from these "lighthouses" because they implement the strategy of "making friends with distant ones and attacking those who are close". If you get close, you will find that these "lighthouses" are actually played by black widow spiders, who want to tear apart all the "bedmates".

China's Sky Eye

(Photo source: Chinese government website)

Since 2016, the FAST telescope in Guizhou, my country, has discovered more than 660 pulsars, including some black widow pulsars. Perhaps we should also look up at the sky from time to time like the FAST cauldron, and maybe we will discover some new "species"!

References:

[1] Bhattacharyya S., Papitto A., Bhattacharya D., 2022, ASSL, 465, 319

[2] Guo Y., Wang B., Han Z., 2022, MNRAS, 515, 2725

Produced by: Science Popularization China

Author: Guo Yunlang (Yunnan Astronomical Observatory, Chinese Academy of Sciences)

Producer: China Science Expo