Astronomers discover a ‘forbidden’ pulsar fleeing a supernova in a seemingly empty region of the Milky Way

Astronomers have discovered an extraordinary celestial system containing a runaway pulsar fleeing the scene of a massive stellar supernova explosion. What makes this system even more spectacular is the fact that it should be “forbidden” in the empty region of the Milky Way in which it was found.

The system, given the name “Calvera” after the villain in the 1960 Western “The Magnificent Seven,” exists around 6,500 light-years above the densely populated plane of the Milky Way. In this region, stellar populations are sparse, and stars with the necessary mass needed to go supernova and to birth a neutron star at the heart of a pulsar should be vanishingly rare.

That means that the discovery of Calvera, given its name because it exists at the fringes and operates outside the norm like its namesake antagonist, could change our view of massive star formation as well as our picture of the outer region of the Milky Way.

“Massive stars — that is, at least eight times more massive than the sun — form almost exclusively in the galactic plane, where the gas density is highest and favors star birth,” team leader Emanuele Greco of the Istituto Nazionale di Astrofisica (INAF) said in a statement. “Finding their remnants at such distances from the plane is extremely rare. Our analysis has allowed us to more precisely estimate the distance, age, and even the characteristics of the possible progenitor star that gave rise to both the Calvera pulsar and its supernova remnant.”

The explosive story of Calvera

Astronomers first became fascinated with Calvera in 2022 when it was spotted by the Low-Frequency Array (LOFAR) radio telescope, a network of antennas across 8 European countries. Calvera was detected as an extended structure with an almost perfectly circular shape.

This led to it being identified as the wreckage of a supernova, which was curious because these explosive stellar death throes usually occur within the thick disk of stars across the central plane of our galaxy.

Pulsars are neutron stars, stellar remnants created when massive stars collapse at the end of their lives. They can spin as fast as 700 times per second. Astronomers had already identified a pulsar (also called Calvera) in this region thanks to its intense X-ray emission.

Looking at the trajectory of this pulsar, astronomers determined that it appears to be racing away from the center of the supernova explosion. That suggests the supernova wreckage in the form of an expanding shell of gas and dust, and the runaway pulsar are connected, the result of the explosive death of a massive star thousands of years ago.

The team behind this research wanted to get a better picture of the cosmic history of the Calvera system, so they examined X-ray data regarding the system collected by the European Space Agency (ESA) spacecraft XMM-Newton. The researchers combined this with data from other telescopes across the electromagnetic spectrum.

The characteristics of the supernova’s hot gas, combined with the motion of the pulsar, allowed the team to determine the age of the system and its distance more precisely. This revealed that the supernova explosion erupted between 10,000 and 20,000 years ago and that Calvera is between 13,000 and 16,500 light-years away.

This further solidified the connection between the supernova wreckage and the runaway pulsar.

The research is even more interesting because of how different this region of the Milky Way is from the galactic plane, where supernovas usually rage. This is interesting because it is thought that the gamma-ray emissions of supernovas are caused by a high density of particles, particularly protons. However, this investigation of Calvera shows that the mechanism that launches gamma-rays from supernovas can also occur in low-density conditions such as those found at the outskirts of the Milky Way.

“Thanks to space telescopes like XMM-Newton and Fermi/LAT, and ground-based instruments like the Telescopio Nazionale Galileo, we can analyze supernova remnants and pulsars in different bands of the electromagnetic spectrum,” Greco said. “In the case of Calvera, we have shown that even in rarefied environments, plasma emission at millions of degrees can occur if the shock wave from the explosion encounters local clumps. These clumps, in turn, reveal something about the evolutionary history of the star that exploded.”

“Our study shows that even the quietest and seemingly empty regions of the galaxy can harbor extreme processes,” Greco concluded. “Not only have we precisely constrained the physical properties of the Calvera system, but we have also demonstrated that, locally, it is possible to find densities sufficient to generate X-ray and gamma-ray emissions even very far from the galactic plane.

“This discovery invites us to look with new eyes at the peripheries of the Milky Way.”

The team’s research was published on Friday (Aug. 29) in the journal Astronomy & Astrophysics.