Some stars never really die. Pulsars are the undead magnetized cores of massive stars that have met their end in a supernova. They rotate furiously, spewing jets of electromagnetic radiation from their magnetic poles, which makes them appear to flash regularly when observed from Earth.

As if these zombies weren’t already bizarre enough, the behavior of one of them, pulsar PSR J1023+0038, has remained a mystery until now. PSR J1023 does have the usual compact jet of radiation at its poles. But it’s in a close binary system with another star, and, as it orbits this star, it has been observed blazing intensely before quickly dimming again. An international team of astronomers has finally made a breakthrough in understanding what causes the pulsar to switch from intensely bright “high mode” to dimmer “low mode” as it strips material from its companion star. Where that material goes has finally explained why it acts so erratically.

Extreme highs…

PSR J1023 is no ordinary pulsar, but a millisecond pulsar, meaning that it rotates hundreds of times per second. Even before its 2002 discovery, it was thought that millisecond pulsars get their speed from being in binary systems. Their speed comes from stripping material off their companion stars and drawing it in, which keeps feeding the neutron star more energy.

The research team identified PSR J1023 as one of a small number of transitional millisecond pulsar because it keeps shifting from being intensely luminous to relatively dim and then back. What makes it keep switching between its high and low modes?

The material that PSR J1023 strips from its companion star forms an accretion disk around the pulsar. This material, full of highly energetic charged particles, is pulled inward by the pulsar’s gravity. Sometimes, this results in the accretion of an especially large amount of star stuff. When a larger glob of material spirals closer and closer to the pulsar, charged particles from the companion star can collide with charged particles in the pulsar’s powerful winds, which heat up the incoming matter further and push it outward.

On occasion, the pulsar may also flare simultaneously. If it does, the gobs of plasma that erupt from these flares will also blast the hot, charged material into space. X-rays, ultraviolet, and visible light are emitted by the pulsar in an explosive flash during high mode.

“We assume that during the high mode the accretion flow is kept [high] by the radiation pressure of the particle wind from an active rotation-powered pulsar,” the researchers said in a study recently published in Astronomy & Astrophysics.

…and low lows

After so much material is blown off the pulsar in high mode, its X-ray, UV, and visible light emissions plummet. The pulsar wind also dies down because there is not much of an influx of material to feed it anymore. While the weaker wind still blows through the accretion disk and is capable of colliding with some matter flowing in from the companion star, causing some emissions, these are nowhere near those produced while in high mode.

Even outflows from the compact jet that causes the pulsar’s consistent pulsing temporarily cease right after high mode ends. This is because the jet runs on synchrotron emissions generated when pulsar winds collide with blobs of accreting material closest to the pulsar itself. After that material is blasted into space, the wind’s shock runs into material too far away from the pulsar to keep the jets going for a while.

The much dimmer pulsar does still give off radio waves. The researchers think that these mostly come from the compact jet, which keeps ejecting material throughout high and low modes. When the jet operates in low mode, some leftovers from the high mode ejecta can be observed above its usual visible light and X-ray emissions.

Predictably, low mode doesn’t last. The cycle starts again when a new influx of material from the disk approaches the star. It fills in the spaces left by previously ejected plasma and, when it comes head to head with the pulsar winds again, restarts the synchrotron emission that powers the compact jet.

Keeping a finger on the pulse

Observing PSR J1023 carefully enough to figure out what was causing its mysterious mode switches took 12 ground-based and space telescopes, including the Atacama Large Millimeter/submillimeter Array (ALMA), ESO’s New Technology telescope, ESO’s Very Large Telescope, and ESA’s XMM-Newton X-ray telescope. This was the largest multi-wavelength study ever done on PSR J1023, finally revealing how inflowing material affects its winds and overall pulses, which previous studies of the pulsar were not able to observe. However, there are further mysteries to be sorted out. The researchers want to find out possible similarities between transitional millisecond pulsars and black holes.

“Black hole systems and [transitional millisecond pulsars] share intriguing similarities in their phenomenological properties, which emphasizes the need for further research to deepen our understanding of accretion physics in compact objects,” they said in the same study.

The team predicts that there may be similar things going on in their accretion disks, and there have definitely been cases where black holes show bursts of activity. At least neither of them eats human brains.

Astronomy & Astrophysics, 2023.  DOI: 10.1051/0004-6361/202346418