Pulsars, transients and relativistic astrophysics

Abstract:

We extend the relativistic time-dependent thin-disc TDE model to describe non-thermal (2-10 keV) X-ray emission produced by the Compton up-scattering of thermal disc photons by a compact electron corona, developing analytical and numerical models of the evolving non-thermal X-ray light curves. In the simplest cases, these X-ray light curves follow power-law profiles in time. We suggest that TDE discs act in many respects as scaled-up versions of XRB discs, and that such discs should undergo state transitions into harder accretion states. XRB state transitions typically occur when the disc luminosity becomes roughly one per cent of its Eddington value. We show that if the same is true for TDE discs then this, in turn, implies that TDEs with non-thermal X-ray spectra should come preferentially from large-mass black holes. The characteristic hard-state transition mass is MHS ≃ 2 × 107M⊙. Hence, subpopulations of thermal and non-thermal X-ray TDEs should come from systematically different black hole masses. We demonstrate that the known populations of thermal and non-thermal X-ray TDEs do indeed come from different distributions of black hole masses. The null-hypothesis of identical black hole mass distributions is rejected by a two-sample Anderson-Darling test with a p-value <0.01. Finally, we present a model for the X-ray rebrightening of TDEs at late times as they transition into the hard state. These models of evolving TDE light curves are the first to join both thermal and non-thermal X-ray components in a unified scenario.

Abstract:

We have measured the scattering time-scale, τ, and the scattering spectral index, α, for 84 single-component pulsars. Observations were carried out with the MeerKAT telescope as part of the Thousand-Pulsar-Array programme in the MeerTime project at frequencies between 0.895 and 1.670 GHz. Our results give a distribution of values for α (defined in terms of τ and frequency ν as τ ∝ ν^−α) for which, upon fitting a Gaussian, we obtain a mean and standard deviation of 〈α〉 = 4.0 ± 0.6. This is due to our identification of possible causes of inaccurate measurement of τ, which, if not filtered out of modelling results, tend to lead to underestimation of α. The pulsars in our sample have large dispersion measures and are therefore likely to be distant. We find that a model using an isotropic scatter broadening function is consistent with the data, likely due to the averaging effect of multiple scattering screens along the line of sight. Our sample of scattering parameters provides a strong data set upon which we can build to test more complex and time-dependent scattering phenomena, such as extreme scattering events.