Electromagnetic channel – From the observational point of view, the electromagnetic channel is essentially the only one efficiently used for the moment. Indeed, X-ray spectroscopy and continuum fitting methods have given some insight on the spin of massive BHs. The spin distribution of BH contains important information about how they accrete matter. Supermassive BH’s spin seems to range from Kerr parameter ~.2 -.998. In fact, these measures are still under controversy, and eLISA (see below) will be more than helpful to settle the issue. Remarkably, pulsar timing arrays may have the potential to detect GW. This technique has been originally introduced to study diffuse stochastic backgrounds but it has been recently applied to detect the early inspiral of massive BH binaries [31]. Such method allows to measure the mass and distance of binary BH and GW signal might even already be present in currently available pulsar timing array data [32]. NS are already observed with space-based telescopes such as Chandra in the X-ray regime and estimations for the radius are derived, though still not reaching consensus. On another hand, over the next decade, upcoming experiments such as the Neutron-Star Interior Composition Explorer (NICER), the Large Observatory For X-ray Timing (LOFT) [33] and the Horizon Telescope will uncover regimes never accessed before and likely lead to invaluable insights in compact object physics. Let us stress the European project ATHENA-X, which has been selected as second Large project (L2) by the Cosmic vision program of the ESA. ATHENA-X, for Advanced Telescope for High ENergy Astrophysics, will explore the X-ray universe with an angular resolution of 5 arcsec and will probe the universe out to a redshift z=10. See [34] for some project web pages.

Gravitational Channel – In parallel, the gravitational wave channel will likely be explored during this century. Gravitational waves allow to probe the most extreme regime of gravity, without requiring luminous electromagnetic sources. The most promising sources for gravitational waves are binary systems. The signal is essentially unperturbed from ambient matter and the signal reaching the earth is clean, contrary to electromagnetic signals which are distorted by their environment. The first generation of ground-based gravitational wave interferometers have reached their design sensibility some years ago (see LIGO, VIRGO). For instance, the European ground based detector VIRGO is composed of two 3 km long arms and is based on the same principle as the Michelson interferometer. The experiment had its first science run in 2007. Improved detectors such as aLIGO, aVIRGO will start operating in the next few years. Third-generation detectors like the Einstein Telescope or KAGRA are currently under study. Additionally, space-based observatories are planned, such as the eLISA mission [35] which is the best candidate for the L3 mission of the Cosmic Vision program. It will be sensitive to low frequency GW and is expected to start operations in the next few decades. While the earth based detectors will be sensitive to the frequency region from few Hz to 10 kHz, the space based antenna will cover the low frequency regime. “Astrophysical” sources emit mainly in higher frequencies while super-massive black holes will be potentially seen by space-based observatories. The present, planed upgrades and upcoming GW observatories will cover almost all the frequency regimes relevant for CO physics. See [36] for some GW detector project web pages.


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