SAX J1808.4--3658 and the origin of X-ray variability in X-ray binaries and active galactic nuclei

The aperiodic X-ray variability in neutron star and black hole X-ray binaries (XRBs) and active galactic nuclei (AGN) shows a characteristic linear relationship between rms amplitude and flux, implying a multiplying together or ‘coupling’ of variability on different time-scales. Such a coupling may result from avalanches of flares, due to magnetic reconnection in an X-ray-emitting corona. Alternatively this coupling may arise directly from the coupling of perturbations in the accretion flow, which propagate to the inner emitting regions and so modulate the X-ray emission. Here, we demonstrate explicitly that the component of aperiodic variability that carries the rms–flux relation in the accreting millisecond pulsar SAX J1808.4−3658 is also coupled to the 401-Hz pulsation in this source. This result implies that the rms–flux relation in SAX J1808.4−3658 is produced in the accretion flow on to the magnetic caps of the neutron star, and not in a corona. By extension we infer that propagating perturbations in the accretion flow, and not coronal flares, are the source of the rms–flux relations and hence the aperiodic variability in other XRBs and AGN.