Free Astronomy Magazine November-December 2018

37 NOVEMBER-DECEMBER 2018 SPACE CHRONICLES transient eruptions of X-rays, but only 17 of these have been con- firmed. This is because of the diffi- culties in the way of studying the motion of the companion star around the black hole, which would allow us to infer its mass, and con- firm the type of object. Researchers have only a limited knowledge of the formation and the evolution of this type of objects, because of the small number of known binaries containing a black hole. That is why it is important to develop new strategies which will let us discover the “hidden” popula- tion of the Galaxy, that is those ob- jects which are “hibernating”, not in an active phase, and so are not emit- ting X-rays. The IAC researchers Jorge Casares, and Miguel A. Pérez Torres have tested a novel technique measuring the brightness of these binary pairs with a combination of filters centred on the line of hydrogen H-alpha. The measurements give information about the intensity and the width of this line, which forms in the accre- tion disc around the black hole. In particular, the width of H-alpha can be used as an indicator of the strength of the gravitational field, and so can be used as a diagnostic of the presence of a black hole. This technique could reveal, very effi- ciently, new black hole binaries in an inactive phase. To show this they ob- served 4 systems with confirmed black holes using a set of special filters on ACAM, an instrument on the 4.2m William Herschel Telescope (WHT) of the Isaac Newton Group of Telescopes at the Roque de los Muchachos Observatory (Garafía, La Palma). The results were then compared with direct measurements of the width of the H-alpha line obtained with the ISIS spectrograph on the Gran Telescopio de Canarias (GTC). The result showed that it is practical to measure the width of the H- alpha line using photomet- ric techniques, which opens the door to a more efficient detection of inactive black holes in binary systems. They estimate that an analy- sis of some 1000 square de- grees (10%) of the zone of the Galactic plane with this strategy should detect at least 50 new objects of this type, which is three times the currently known popula- tion. This search could also yield a detailed census of other Galactic populations, such as short period cata- clysmic variables, X-ray bi- naries containing neutron stars, and ultra-compact bi- naries with a period shorter than one hour. !