A holy grail of pulsar astronomy lurking within a globular cluster
Mario Cadelano (DIFA)
Tuesday 19/12/2023 @ 14:00, Sala Antonio Sollima (IV piano Battiferro)
Millisecond pulsars (MSPs) are rapidly spinning neutron stars formed in binary systems through mass accretion from an evolving companion star. Although these systems are ubiquitously formed throughout the entire Galaxy, a large fraction is found in globular clusters (GCs). Indeed, GCs serve as excellent MSP factories due to their collisional environment, which promotes 2- and 3-body gravitational interactions between stars. These interactions lead to the formation of new binaries or the hardening of existing ones, eventually producing a large population of MSPs. The same environment is also capable of further perturbing MSP binaries through dynamical interactions which result in a wealth of exotic systems not predicted by standard evolution models. In other words, GCs can produce binaries that are impossible (or highly unlikely) to be found in the Galactic field. Among these, the long-sought MSP orbiting a black hole is considered one of the holy grails of pulsar astronomy, as its discovery would open the door to unprecedented tests of fundamental physics and general relativity. Here, I will present the discovery of the first MSP orbiting a companion in the mass gap between neutron stars and black holes. By exploiting the synergy between radio and optical/near-UV observations, we discovered within the inner regions of the globular cluster NGC 1851 a binary system composed of a neutron star unambiguously orbiting either a low-mass black hole or a high-mass neutron star. I will discuss the possible formation mechanisms of such an exotic binary, the determinant role of the cluster environment, and follow-up developments.