Detecting the presence of black holes is a fairly complex job, especially if they are inside a binary system (a hole and a star). That is why a group of researchers from the Astrophysics Institute of the Canary Islands has developed a new technique to detect these mysterious space objects. The results have been published in Monthly Notices of the Royal Astronomical Society.
A black hole is an object that compacts a lot of mass in a small space. It is so dense and has a force of attraction so intense that not even light can escape from it. That is why they can not be “seen” if it is not through the effects they produce around them.
Usually, the fall of matter over the black hole occurs in an orderly and “silent” way through an accretion disk. However, sometimes, this takes place in a violent and compulsive way, generating a strong eruption of brightness in X-rays.
On the other hand, binary systems composed of a star that donates mass to a black hole are laboratories in real life that help us to understand the physical phenomena behind the formation of these violent phenomena.
To date, about 60 black hole candidates of this type have been discovered in our galaxy, thanks to the detection of transient X-ray eruptions, but only 17 have been confirmed. This is due to the difficulties of studying the movement of the companion star around the black hole.
The knowledge that researchers have about the formation and evolution of this type of objects is limited by the small number of binary systems with black holes confirmed to date. For this reason, we have been looking for new strategies to discover black holes that “hibernate” in the inactive phase, that is, they do not emit X-rays.
To solve this problem IAC researchers Jorge Casares and Manuel A. Pérez Torres have tested a novel technique by measuring the brightness of these systems with a combination of filters centered on the H-alpha hydrogen line. The measurements provide information about the intensity and width of this line, which is formed in the accretion disk around the black hole.
In particular, the width of the H-alpha line can be used as an indicator of the gravitational field and, therefore, as a diagnosis of the presence of a black hole. This technique could reveal very efficiently new binary systems with black holes in the inactive phase.
Put into practice
To demonstrate this, the researchers observed 4 systems that were known to have a black hole with the help of a series of special filters with the ACAM instrument, installed in the William Herschel Telescope (WHT) at the Roque de los Muchachos Observatory (Garafía, La Palm). The results of these observations were compared with direct measurements of the width of the Halfa line, obtained with the OSIRIS instrument, in the Gran Canarias Telescopio (GTC).
The final result showed that, indeed, the width of the H-alpha line can be recovered by photometric techniques, which opens the door to a more efficient selection of inactive black holes in binary systems.
It is estimated that an analysis of around 1000 square degrees (10%) of the galactic plane with this strategy could reveal at least 50 new objects of this type. That is, three times more than the current known population.
Likewise, this search could also provide a detailed census of other galactic populations such as variable cataclysmic short-period stars, X-ray binaries with neutron stars or ultra-compact binaries, with orbital periods of less than one hour.