We present 12 epochs of optical spectroscopy taken across the discovery outburst of the black hole (BH) candidate MAXI J1803-298 with the Gran Telescopio Canarias and Very Large Telescope. The source followed a standard outburst evolution. This means it passed through the so-called "hard" and "soft" states, defined in terms of the relative contribution of high to low energy X-rays. The system displays a "triangular" shape in the hardness intensity diagram, consistent with that seen in high-inclination BH transients and the previously reported detection of X-ray dips. The two epochs observed

The confirmation of the existence of black holes is one of the most basic results in astrophysics. There is a wide range of masses of black holes, from those with stellar mass, which are the result of the catastrophic final phase of very massive stars, to the supermassive black holes at the centres of most galaxies. The mass of a black hole is up to now the only parameter which scientists are able to measure. In this work, we present an original method for measuring the masses of black holes, from those of stellar mass to the supermassive variety, based on a simple measurement of the

All disc-accreting astrophysical objects produce powerful disc winds. In compact binaries containing neutron stars or black holes, accretion often takes place during violent outbursts. The main disc wind signatures during these eruptions are blue-shifted X-ray absorption lines, which are preferentially seen in disc-dominated ‘soft states’. By contrast, optical wind-formed lines have recently been detected in ‘hard states’, when a hot corona dominates the luminosity. The relationship between these signatures is unknown, and no erupting system has as yet revealed wind-formed lines between the