Black holes and neutron stars can accrete matter thanks to the presence of a geometrically thin, optically thick accretion disks. As matter falls into the potential well of the central object, it heats up and therefore the disk glows and emits thermally in the soft X-rays. Studying the direct emission from the disk can tell us a lot about the system, e.g. the temperature of the disk or the internal truncation radius (see also QPOs).
There is another form of X-ray emission that carries information about the physics of the system: reverberation. In both neutron stars and black holes, X-ray emission from outside the disk (coming from the neutron star’s surface or from a glowing corona above the black hole) can be reflected by the disk into our line of sight. By correlating the primary emission with the reflected one we can map the accretion disk: as more distant regions of the accretion disk will be illuminated in subsequent times, photons of different energy will present different time delays.
Accreting black holes are thought to have a compact, optically thin corona right above the black hole, which produces a power-law emission in the hard X-rays. Some of the photons emitted by the corona are reflected back into the line of sight by the disk, generating the reverberation signal. For neutron stars, the emission comes from the surface of the star itself. Accretion onto neutron stars causes repeating thermonuclear reactions that are observed as bright bursts of X-ray emission (Type I X-ray bursts) and that are caused by unstable hydrogen and helium burning. Intermediate bursts and superbursts are rarer and are thought to be produced by thermonuclear flashes in deep layers of helium and carbon respectively. Of much shorter duration and different origin are the so-called Type II X-ray bursts, which are thought to be caused by instabilities in the accretion flow. Similar to the coronal emission for black holes, the X-ray-burst emission from the neutron star surface can be reflected by the accretion disk surrounding the neutron star, and indeed reflection spectra have been observed for neutron stars as well.
The reflection emission presents peculiar features in its spectrum, that includes an iron line at 6.4 keV, formed via fluorescence, and a reflection “bump” that peaks at about 30 keV, formed via inelastic scattering from free electrons. Gravitational redshifts from the central object and relativistic motion of the orbiting plasma in the inner disk distort the spectrum, providing insight on the dynamics of the accretion disk. The rapid variability of the principal emission (coronal emission for black holes and surface emission for neutron stars) can provide a way to map the inner regions of the accretion disk as fluctuations in the continuum emission are reflected in the reverberation spectrum with a light-crossing time delay. Time delays can be of the order of a few hundreds of microseconds. Also, as different parts of the accretion disk will be illuminated in subsequent times, photons of different energy will present different time delays, reflecting the characteristic Doppler shift of the reflection region.
Observations up to date are limited by either the low energy resolution or low timing resolution of current X-ray telescopes. Reverberation lags have only been detected for supermassive black holes, for which the timescales are of the order of milliseconds and not microseconds. The combination of high energy- and high time-resolution of Colibrì will allow detecting these lags in the reverberation spectrum of stellar mass black holes and neutron stars. High resolution spectral fitting of the X-ray emission, especially of the iron line profile, provides information on the strong-field gravity effects on the orbiting plasma and its dynamics, from which radii can be inferred in units of the gravitational radius, other than on the spin of the central object and on the inclination angle of the system. For black holes, the possibility of performing reverberation mapping, which yields distances in absolute units given by the light travel time, simultaneously to spectral fitting would therefore provide a test of the Kerr metric itself, as well as a measurement of the mass of the black hole.