A few years ago we celebrated the first half a century of X-Ray astronomy. According to you what have been the main achievements? What are the goals for the next few decades?

As usual, when we open a new window onto the Universe we are in for a number of discoveries, most of them unexpected. In the case of X-rays, which are generated only at very high temperatures, this led to the discovery of the most extreme objects existing both inside and outside our galaxy. We unveiled the existence of collapsed objects, black holes and neutron stars and realized that the process of accretion onto them is one important aspect of astrophysics.

As our instrumentation becomes more advanced and sensitive, in the future decades we will be able to perform tests of gravity theories, to measure the state of neutron matter inside neutron stars and to understand the complex phenomenon of accretion.

X-ray radiation allows us to probe the innermost regions around compact stars and test theories of gravity. What technology do we need to extract information from these areas?

X-rays cannot penetrate our atmosphere, fortunately for us. This means we must send our instrumentation into space, which is a limitation both for costs and technology. We need sensitive detectors and better X-ray mirrors to concentrate the radiation. At the same time, space missions that can study the variability of X-sources are proving to be essential in many fields of X-ray astronomy: these require different detectors and telescopes and therefore present different challenges. As usual, a global approach is needed.

Can we be certain that General Relativity provides the correct description of gravity?

A theory can be verified only to a certain precision. General Relativity has been tested successfully, to high precision, on and around the Earth (GPS navigation satellites need to consider it in order to work) and in a number of celestial systems (the first was the planet Mercury, now we have pulsars in binary systems).

However, Newtonian gravity works rather well for our daily life, because in our environment the difference between it and GR is very small. In order to provide a very strong test, one needs to go into extreme gravitational fields and this can be achieved only by observing matter close to collapsed objects, which are heavy and small. As this matter is very hot, this can be done only with X-ray astronomy.

How massive can a neutron star become before collapsing into a black hole?

There is a theorem, discovered by Rhoades and Ruffini in 1974, that says that the maximum mass of a neutron star is 3.2 times the mass of our Sun. This is almost independent of the condition of matter inside the neutron star (which we do not yet know well). Pushing parameters one can go down a bit so let us say 3 solar masses. In our galaxy, we measure black-hole masses of around ten solar masses and in the centres of active galaxies the masses go up to one billion solar masses. There is no upper limit to how massive a black hole can be.

Annia Domènech