A pulsar and a supernova remnant far from the Galactic plane. The study: “Multi-wavelength study of the high Galactic latitude supernova remnant candidate G118.4+37.0 associated with the Calvera pulsar” of E. Greco (INAF-OAPA) appeared on A&A

The Caldera: a pulsar and a supernova remnant at the remarkable distance of 6500 light years from the Galactic plane, observed in the X-rays by XMM/Newton

In recent years, a few supernova remnants — rapidly expanding nebulae generated by stellar explosions — have been discovered at large distances from the Galactic plane. These objects have drawn significant interest for two main reasons. First of all, their origin is unclear. The two main hypotheses suggest:

High-velocity progenitor stars, which moved rapidly away from the Galactic plane before exploding (core-collapse supernovae originate from massive stars, which have very short lifespans and therefore usually explode close to their birthplace, typically in the Galactic plane);
Type Ia supernovae, that is, explosions of white dwarfs in binary systems that accrete mass from a companion star.

Another point of interest is that these remnants are located in an environment very different from that of the Galactic plane — less rich in nebulae and dense interstellar material — and are therefore likely characterized by weaker interactions with pre-existing matter concentrations.

In 2022, thanks to LOFAR (a network of about 50 antennas designed to observe low-frequency radio waves), a compact and nearly circular supernova remnant — now known as the Calvera supernova remnant — was identified at an angular separation of about 37 degrees from the Galactic plane. Later, Calvera was also recognized as a bright gamma-ray source. About five arcminutes from this remnant, a pulsar (also named “Calvera”) was discovered — a rapidly rotating neutron star. The pulsar is moving away from the remnant at a high velocity (with a proper motion of 78 milliarcseconds per year), suggesting a common origin with the supernova remnant.

A team of researchers led by astrophysicist E. Greco (INAF – OAPA) analyzed new X-ray observations of the Calvera region (including both the remnant and the pulsar), obtained with the XMM-Newton satellite of the European Space Agency. The study provides several pieces of evidence supporting a common origin for the pulsar and the supernova remnant. In particular, two independent diagnostics — an estimate of the Sedov–Taylor timescale, which indicates how long the remnant has been undergoing adiabatic expansion (i.e., without significant energy losses through radiation), and the ionization timescale, which measures the time elapsed since the shock wave impacted the interstellar medium and began ionizing it — allowed the team to estimate the remnant distance to be between 13000 and 16500 light-years, and its age between 10000 and 20000 years. These values are consistent, within uncertainties, with those estimated for the pulsar. Based on these distances, Calvera would be located at the impressive distance of about 6500 light-years from the Galactic plane.

Furthermore, the researchers presented evidence that the supernova remnant is expanding into a relatively dense interstellar medium, despite its high Galactic latitude. This includes:

the presence of filaments emitting strong hydrogen ionization lines, observed with the Telescopio Nazionale Galileo,
and a filamentary structure in gamma-ray emission, consistent with shock–medium interaction.

The figure (click here to view the full image) shows the X-ray image from XMM-Newton of the region, with the positions of the pulsar, and a portion of the supernova remnant where X-rays are emitted by shocked circumstellar material, and the area where hydrogen-rich filaments were identified (marked with a green box).