Pulsars, transients and relativistic astrophysics

Abstract:

Flares produced following the tidal disruption of stars by supermassive black holes can reveal the properties of the otherwise dormant majority of black holes and the physics of accretion. In the past decade, a class of optical-ultraviolet tidal disruption flares has been discovered whose emission properties do not match theoretical predictions. This has led to extensive efforts to model the dynamics and emission mechanisms of optical-ultraviolet tidal disruptions in order to establish them as probes of supermassive black holes. Here we present the optical-ultraviolet tidal disruption event AT 2022dbl, which showed a nearly identical repetition 700 days after the first flare. Ruling out gravitational lensing and two chance unrelated disruptions, we conclude that at least the first flare represents the partial disruption of a star, possibly captured through the Hills mechanism. Since both flares are typical of the optical-ultraviolet class of tidal disruptions in terms of their radiated energy, temperature, luminosity, and spectral features, it follows that either the entire class are partial rather than full stellar disruptions, contrary to the prevalent assumption, or some members of the class are partial disruptions, having nearly the same observational characteristics as full disruptions. Whichever option is true, these findings could require revised models for the emission mechanisms of optical-ultraviolet tidal disruption flares and a reassessment of their expected rates.

Abstract:

We reproduce the luminosity functions of the early-time peak optical, the late-time ultraviolet (UV)-plateau, and the peak X-ray luminosities of tidal disruption events, using an entirely first-principles theoretical approach. We do this by first fitting three free parameters of the tidal disruption event black hole mass distribution using the observed distribution of late-time UV-plateau luminosities, using a time-dependent relativistic accretion model. Using this black hole mass distribution we are then, with no further free parameters of the theory, able to reproduce exactly the peak X-ray luminosity distribution of the tidal disruption event population. This proves that the X-ray luminosity of tidal disruption events are sourced from the same accretion flows which produce the late-time UV plateau. Using an empirical scaling relationship between peak optical luminosities and black hole masses, itself calibrated using the same relativistic accretion theory, we are able to reproduce the observed peak optical luminosity function, again with no additional free parameters. Implications of these results include that there is no tidal disruption event ‘missing energy problem’, that the optical- and X-ray-selected tidal disruption event populations are drawn from the same black hole mass distribution, that the early-time optical luminosity in tidal disruption events is somewhat simple, at least on the population level, and that future Legacy Survey of Space and Time (LSST) observations will be able to constrain the black hole mass function at low masses.

Abstract:

Flat radio spectra of compact jets launched by both supermassive and stellar-mass black holes (BHs) are explained by an interplay of self-absorbed synchrotron emission up to some distance along the jet and optically thin synchrotron at larger distances. Their spatial structure is usually studied using core shifts, in which the position of the peak (core) of the emission depends on the frequency. Here, we propose a novel and powerful method to fit the spatial dependence of the flux density at a given frequency of the jet and counterjet (when observed), using the theoretical spatial dependencies provided as simple analytical formulae. We apply our method to the spatial structure of the jets in the luminous hard spectral state of the BH X-ray binary Swift J1727.8−1613. It was the most resolved continuous jet from an X-ray binary ever observed. We find that the observed approaching jet is significantly intrinsically stronger than the receding one, which we attribute to an increase in the emission of both jets with time (observationally confirmed), together with the light travel effect, causing the receding jet to be observed at an earlier epoch than the approaching one. The jets are relatively slow, with a velocity of ∼(0.3–0.4)c. Our findings imply that the magnetic field strength increased with time. Additionally, the magnetic flux is significantly lower than in jets launched by “magnetically arrested disks.” Our method is general, and we propose that it be applied to jets launched by both stellar-mass and supermassive BHs.