Pulsars, transients and relativistic astrophysics

Abstract:

Galactic nuclei, the densest stellar environments in the Universe, exhibit a complex geometrical structure. The stars orbiting the central supermassive black hole follow a mass segregated distribution both in the radial distance from the center and in the inclination angle of the orbital planes. The latter distribution may represent the equilibrium state of vector resonant relaxation. In this paper, we build simple models to understand the equilibrium distribution found previously in numerical simulations. Using the method of maximizing the total entropy and the quadrupole mean-field approximation, we determine the equilibrium distribution of axisymmetric two-component gravitating systems with two distinct masses, semimajor axes, and eccentricities. We also examine the limiting case when one of the components dominates over the total energy and angular momentum, approximately acting as a heat bath, which may represent the surrounding astrophysical environment such as the tidal perturbation from the galaxy, a massive perturber, a gas torus, or a nearby stellar system. Remarkably, the bodies above a critical mass in the subdominant component condense into a disk in a ubiquitous way. We identify the system parameters where the transition is smooth and where it is discontinuous. The latter cases exhibit a phase transition between an ordered disklike state and a disordered nearly spherical distribution both in the canonical and in the microcanonical ensembles for these long-range interacting systems.

Abstract:

In recent years, certain luminous extragalactic optical transients have been observed to last only a few days¹. Their short observed duration implies a different powering mechanism from the most common luminous extragalactic transients (supernovae), whose timescale is weeks². Some short-duration transients, most notably AT2018cow (ref. ³), show blue optical colours and bright radio and X-ray emission⁴. Several AT2018cow-like transients have shown hints of a long-lived embedded energy source⁵, such as X-ray variability^6,7, prolonged ultraviolet emission⁸, a tentative X-ray quasiperiodic oscillation^9,10 and large energies coupled to fast (but subrelativistic) radio-emitting ejecta^11,12. Here we report observations of minutes-duration optical flares in the aftermath of an AT2018cow-like transient, AT2022tsd (the 'Tasmanian Devil'). The flares occur over a period of months, are highly energetic and are probably nonthermal, implying that they arise from a near-relativistic outflow or jet. Our observations confirm that, in some AT2018cow-like transients, the embedded energy source is a compact object, either a magnetar or an accreting black hole.