X-ray binaries (XRBs) are arguably the best sources for studying accretion and jets, because they traverse various accretion states with greatly differing jet strength, over timescales of the order of months, easily amenable to observation. Multi-wavelength variability data, combined with timing analysis techniques, provides information about the relationship between different physical components in accreting XRBs. It is a powerful tool to study accretion disc-jet coupling, constraining jet launching mechanisms, and the possible interaction between the magnetic field of the neutron star (NS) and the accretion disc. However, with just a handful of observations, we are lacking information on generality of such fast multi-wavelength variability. In this project we aim to undertake a multi wavelength variability study of XRBs using X-rays and multi-band high time resolution instruments such as HiPERFCAM, ULTRACAM and HAWK-I.
(1) We intend to use the full potential of simultaneous fast X-ray/optical/near-infrared timing to build the first sample of jet measurements for black hole (BH-) and NS-XRBs. Rapid sub-second optical/near-IR (O-IR) monitoring of XRBs is opening up a new window into accretion and jet studies.
This is because fast timing is able to probe plasma flows much closer-in to the jet base than is possible at radio wavelengths, and so can constrain with high detail the coupling with the disk. Our discovery of a short time lag (approx. 0.1 s) between the inflow (X-rays) and jet emission (O-IR) in BH-XRBs has led to new stringent, model independent constraints on the size of the jet base and its speed. Moreover, is our recent extension of fast O-IR timing techniques to a single NS-XRB jets, has allowed us to constrain the magnetic field at the jet base. By determine fundamental parameters, such as jet speed and jet power for a sample of BH and NS-XRBs, we will establish if jets from BHs and NSs can be powered by the same mechanism.
It is well known that X-ray quasi-periodic oscillations (QPOs) evolve throughout an XRB outburst, tracking the evolution of the accretion flow with distinct timing and spectral characteristic. Thanks to the increasing number of fast multi-wavelength datasets, we now know that QPOs also have an O-IR counterpart, which follows the evolution of the X-ray QPO.
(2) We intend to perform the first dedicated optical/X-ray timing multi-epoch study of QPOs in the NS XRBs, probing a similar frequency range as is observed in the X-rays. These will allow us to test ideas that the QPOs are due to the precession from the hot corona or from the jet itself.
(3) We also aim to characterize the optical QPO observed at very low luminosities in BH-XRBs. By determining the broad-band variability spectrum of the QPO we will be able to disentangle between the jet and accretion disk origin, with strong implications for accretion flow models.
