INAF researcher involved in the discovery of a millisecond pulsar
A pulsar is a neutron star and is probably formed from the left-overs of a collapsed star. This is a class of neutron stars that in the early stages of formation rotate very fast, emitting electromagnetic impulses at extremely regular intervals that can also be detected on Earth.
Neutron stars are often visible for several million years as a radio pulsar: they emit a bundle of radio waves that, with star rotation, invests us periodically as a cosmic lighthouse. Hence, their characteristic pulsed signal is produced. However, over time, these stars slow down until the mechanism that produces radio beams is interrupted, and the pulsars “turn off”.
In 2015, a team from the National Institute of Astrophysics (INAF) led by Paolo Esposito, by using the observations made with space telescope XMM-Newton of the European Space Agency (ESA), discovered a pulsar in the Andromeda galaxy, our twin galaxy, within the EXTraS (Exploring the X-ray Transient and variable Sky) project that is funded by the European Union, coordinated by INAF. However, the pulsar identified in the Andromeda galaxy proved to be neutron star with peculiar characteristics.
An international team of astronomers led by Ivan Zolotukhin of the Institut de Recherche en Astrophysique et Planétologie (IRAP) of Toulouse, also including Matteo Bachetti, a researcher of the INAF in Cagliari, has discovered that this pulsar is at the beginning of the recycle phase, i.e. the process that causes slow and "turned-off” neutron stars to turn on again as “millisecond pulsar”.
"Millisecond pulsars" are a particular pulsar class of mysterious origin, with a rotational period of between 1 and 10 milliseconds that can be only seen in the microwave spectrum or X-ray portion. The pulsar named XB091D is returning vigorous and bright in the X-ray band, while increasing its rotational speed thanks to the acquisition of material taken from a small companion star.
This process, which occurs when a neutron star encounters a common star to which it binds through gravity to form a binary system, can turn on pulsars that are no longer active. The pulsar captures the outer stars of the common star, forming a hot material disk around it, the growth disk, capable of accelerating its rotation.
The material that accumulates preferably on the star's magnetic poles forms two extremely hot spots (called hotspots) that emit a strong X-ray signal and the neutron star becomes an X-pulsar, creating the incredible cosmic clocks known as millisecond pulsars. This study helps us better understand the mechanisms that lead to the formation of these cosmic clocks.
“The observed phenomenon is rare: only about twenty pulsars are known to be in the recycle phase, all with very high periods” said Bachetti, co-author of the article describing the discovery, published in The Astrophysical Journal. “The nature of the globular cluster B091D, which hosts XB091D, has probably a role. It is a 12-billion-year old cluster and particularly dense, and this has made the meeting between the pulsar and the companion star more likely.”
According to the researchers, the X-ray pulsar XB091D was discovered just in the early stages of its “rejuvenation”. Based on a total of 38 observations by the European Space Agency (ESA) XMM-Newton, astronomers have thoroughly studied the XB091D system. According to their estimates, the pulsar took in less than a million years to turn on, the companion star is a little smaller than our Sun, and the two stars orbit in about 30 and a half hours. Continuing with the same “diet”, within fifty thousand years, the pulsar will accelerate to a few hundred rotations per second, becoming a millisecond pulsar.