On the different flavours of Lense–Thirring precession around accreting stellar mass black holes

Type-C quasi-periodic oscillations (QPOs) in X-ray binaries have been often interpreted as a consequence of relativistic Lense-Thirring precession around a spinning black hole and they potentially offer a way to measure black hole spins and masses. The connection between relativistic precession and the resulting QPOs has been made either in terms of a simplified model involving a single test particle producing the QPO, or in terms of a global model where a geometrically thick accretion flow precesses coherently as a rigid body. In this paper, we analyse similarities and differences between these two models, sometimes considered as in opposition to each other. We demonstrate that the former is the limiting case of the latter when the radial extent of the precessing flow is very small, and that solid lower limits to the black hole spin can be obtained by considering the test particle model alone. We also show that the global precession model naturally accounts for the range of frequencies observed for type-C QPOs without the need to invoke a truncation of the inner accretion flow before it reaches the innermost stable circular orbit. Finally, we show that, in order to maintain rigid precession, the thick accretion flow should be radially narrow, and that if it extends beyond 10-10(2) gravitational radii, it aligns with the black hole spin too fast to produce a coherent QPO

Revealing the spectral state transition of the Clocked Burster, GS 1826-238 with NuSTAR StrayCats

We present the long term analysis of GS 1826-238, a neutron star X-ray binary known as the "Clocked Burster", using data from NuSTAR StrayCats. StrayCats, a catalogue of NuSTAR stray light data, contains data from bright, off-axis X-ray sources that have not been focused by the NuSTAR optics. We obtained stray light observations of the source from 2014-2021, reduced and analyzed the data using nustar-gen-utils Python tools, demonstrating the transition of source from the "island" atoll state to a "banana" branch. We also present the lightcurve analysis of Type I X-Ray bursts from the Clocked Burster and show that the bursts from the banana/soft state are systematically shorter in durations than those from the island/hard state and have a higher burst fluence. From our analysis, we note an increase in mass accretion rate of the source, and a decrease in burst frequency with the transition

Modelling correlated variability in accreting black holes:the effect of high density and variable ionization on reverberation lags

We present a new release of the RELTRANS model to fit the complex cross-spectrum of accreting black holes as a function of energy. The model accounts for continuum lags and reverberation lags self-consistently in order to consider the widest possible range of X-ray variability timescales. We introduce a more self-consistent treatment of the reverberation lags, accounting for how the time variations of the illuminating flux change the ionisation level of the accretion disc. This process varies the shape of the reflection spectrum in time causing an additional source of lags besides the light crossing delay. We also consider electron densities in the accretion disc up to $10^{20}$ cm$^{-3}$, which are found in most of the stellar mass black holes and in some AGN. These high densities increase the amplitude of the reverberation lags below $1$ keV since the reflection flux enhances in the same energy range. In addition, we investigate the properties of hard lags produced by variations in the power-law index of the continuum spectrum, which can be interpreted as due to roughly $3\%$ variability in the corona's optical depth and temperature. As a test case, we simultaneously fit the lag energy spectra in a wide range of Fourier frequency for the black hole candidate MAXI J1820+070 observed with NICER. The best fit shows how the reverberation lags contribute even at the longer timescales where the hard lags are important. This proves the importance of modelling these two lags together and self-consistently in order to constrain the parameters of the system.Comment: Accepted for publication in MNRA