Unveiling the secrets of black holes and neutron stars with high-throughput, high-energy resolution X-ray spectroscopy

Neutron stars and black holes (compact objects) are among the most fascinating and puzzling objects in the Universe.  They uniquely provide an environment to test the laws of physics at their extremes, as density in a neutron star reaches values several times higher than nuclear density, magnetic fields are billions of times higher than the Sun’s, and gravity around black holes is so strong as to trap light itself. Compact objects, however, do not like to reveal their secrets all at once. Fifty years after their discovery, we still do not know what neutron stars are made of, and the question of how black holes modify space and time around them is still open. The X-ray emission of compact objects presents a rich phenomenology that can lead us to a better understanding of their nature and to address more general physics questions, like:

• Does general relativity (GR) apply in the strong gravity regime? Is spacetime around black holes described by the Kerr metric?
• What are the masses, radii and composition of neutron stars and their atmospheres?

Recent advancements in transition-edge sensors (TES) present a unique opportunity to open a new window on compact objects: high energy-resolution spectroscopy combined with high-precision timing and high sensitivity.