Ilaria Caiazzo (University of British Columbia)
Title: Shining Black Holes: what gravitational waves won’t tell you
Abstract: The LIGO experiment’s detections of gravitational radiation from in-spiraling black holes (BHs) and neutron stars represent the most spectacular verification of the unification of gravity with special relativity. Gravitational wave wavelengths, however, are necessarily comparable to the size of the BHs; therefore, without extremely high signal-to-noise measurements, the study of gravitational waves cannot provide a detailed probe of the spacetime surrounding BHs, and electromagnetic signals provide a crucial complementary window into strong gravity. In particular, accreting BHs like to shine in the X-rays, and the X-ray emission from BH accretion disks carries an incredible wealth of information.
Many X-ray observatories are currently under development whose science goals include studying the structure of BH accretion disks and of the spacetime around BHs. In my talk, I will introduce Colibrì, a new concept mission for an X-ray telescope with high spectral resolution and high timing resolution, which will allow the study of the X-ray variability of the emission from BHs with unprecedented precision. This variability, which includes quasi-periodic variations in the light curve, spectrum, and polarization, called QPOs, and reverberation features between the disk and the corona, can provide strong constraints on the structure of the spacetime around BHs. Other missions, including IXPE and eXTP, plan to have an X-ray polarimeter on board. Polarization could provide a powerful diagnostic as well, and I will show that polarization in the X-rays, if properly modelled including the quantum electrodynamics effect of vacuum birefringence, can help us probe the magnetic field strength and structure close to the inner regions of the accretion disk.