Related papers: Sgr A*: The Optimal Testbed of Strong-Field Gravit…
After exactly a century since the formulation of the general theory of relativity, the phenomenon of gravitational lensing is still an extremely powerful method for investigating in astrophysics and cosmology. Indeed, it is adopted to study…
It has been suggested that there is possibly a class of stellar-mass black holes (BHs) residing near (distance $\le 10^3 M$) the galactic center massive black hole, Sgr A*. Possible formation scenarios include the mass segregation of…
The Higgs field is conventionally treated as a small local perturbation atop a large, constant vacuum value that uniformly permeates the universe. I propose instead that in regions of extreme gravitational intensity -- such as near…
We report on recent near-infrared (NIR) and X-ray observations of Sagittarius A* (Sgr A*), the electromagnetic manifestation of the ~4x10^6 solar masses super-massive black hole (SMBH) at the Galactic Center. The goal of these coordinated…
Understanding gravity is at the heart of some of the biggest questions in modern physics. While General Relativity (GR) is a theoretically unique and experimentally well-tested framework, it remains important to question whether it…
Collapsed objects have definitely been observed: some are stellar-mass objects, the endpoint of massive stars; others, millions of times more massive, have been discovered in the cores of most galaxies. Their formation poses some…
Motivated by inclination of the Earth's orbit that is not located at galactic plane for observing the shadow of Sgr A*, we consider the black hole shadow for arbitrary inclinations and different velocities of observers. A surprising finding…
Astrophysical black hole candidates are thought to be the Kerr black holes of general relativity, but there is not yet direct observational evidence that the spacetime geometry around these objects is described by the Kerr solution. The…
Images from the vicinity of the black hole horizon at the Galactic centre (Sgr A*) could be obtained in the near future with a Very Large Baseline Array of sub-millimetre telescopes. The recently observed short-term infrared and X-ray…
Here we discuss the effects of strong gravity that can be observed in electromagnetic spectra of active galactic nuclei (AGN). According to the unification model of an AGN, there is a supermassive black hole ($10^7 - 10^9 M_\odot$) in its…
The centre of the Milky Way is occupied by a nuclear star cluster, which contains the supermassive black hole Sgr A*. The cluster is embedded in the larger surrounding nuclear stellar disc. These three components dominate the mass budget of…
The realm of strong classical gravity and perhaps even quantum gravity are waiting to be explored. In this letter we consider the recently detected triple system composed of two stars and a non-accreting black hole. Using published…
Einstein's theory of general relativity has been extensively tested in weak gravitational fields, mainly with experiments in the Solar System and observations of radio pulsars, and current data agree well with the theoretical predictions.…
X-ray studies of stellar mass black holes in X-ray binaries and mass-accreting supermassive black holes in Active Galactic Nuclei have achieved a high degree of maturity and have delivered detailed information about the astrophysical…
The spin of the supermassive black hole that resides at the Galactic Centre can in principle be measured by accurate measurements of the orbits of stars that are much closer to SgrA* than S2, the orbit of which recently provided the…
According to the asymptotically safe gravity, black holes may have characteristics different from those described according to general relativity if the running of the gravitational constant coupling happens at low energies. Particularly,…
Whereas considerable effort has been afforded in understanding the properties of galaxies, a full physical picture, connecting their baryonic and dark-matter content, super-massive black holes, and (metric) theories of gravity, is still…
Here we report on recent near-infrared observations of the Sgr A* counterpart associated with the super-massive ~ 4x10^6 M_sun black hole at the Galactic Center. We find that the May 2007 flare shows the highest sub-flare contrast observed…
One of the Holy Grails of observational astronomy is to confirm the prediction that black holes in the Universe are described by the Kerr solution of Einstein's field equations of general relativity. This Topical Collection provides a…
Most X-ray novae (aka soft X-ray transients) contain black hole primaries. In particular, the large mass functions measured for six X-ray novae directly clinch the argument (within general relativity) that they contain black holes. These…