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The EU-funded Multifast project focused on understanding how a black hole ‘takes’ matter that falls into its path, like bath residue spins and settles around the plughole, and then ejects part of it in a powerful beam that penetrates deep into the dark surrounds.
“Multifast set out to track matter as it is ‘taken’ from something called an ‘accretion disk’ which forms around the black hole and is then ejected at great speed,” says project coordinator Luigi Stella of Osservatorio Astronomico di Roma in Italy.
The team, under the guidance of Piergiorgio Casella, a research fellow hired as a staff astronomer at Osservatorio Astronomico di Roma, has developed innovative instruments and software to measure and analyse at what distance from the black hole these jets are launched, how thick they are and perhaps, most importantly, where their enormous power originates.
These sensitive tools, including X-ray detectors in space and infra-red detectors mounted on ground-based telescopes, can detect and compare variations between the incoming and outgoing matter, which has its own magnetic field and emits a type of radiation.
“We were the first team in the world to use a new readout system in these infra-red detectors to study such extreme phenomena; it was instrumental in getting a dedicated state-of-the-art unit set up in Europe,” reveals Casella.
Some illuminating findings
Access to this new tool, together with the software developed to crunch complex and innovative statistical regression methods ultimately led to the discovery of variable infra-red jet emissions from a stellar mass black hole on time scales shorter than one second, “We measured the time delay between the emission from the ‘in-falling’ matter and the outgoing ‘ejecta’, which was enough to estimate the jet speed,” Casella notes.
The researchers also discovered that, in some cases, this jet emission varies in a regular, almost constant way. This is similar to what is observed in the X-ray radiation from the ‘in-falling’ matter, and the scientists think it has something to do with a phenomenon called ‘relativistic precession’.
“In fairly simple terms, that is caused by a spinning black hole dragging around the space time close to it, so the accreted/ejected matter and the jet move around their rotation axis like a spinning top,” Stella explains.
The findings are now being used to compare the behaviour of jets launched by ‘small’ black holes, weighing about 10 times more than the Sun, with much larger and faster jets launched by larger, heavier black holes hosted in the centre of galaxies. These massive jets, which are difficult to study, as they vary on time scales longer than human life, may play a fundamental role in the evolution of our Universe.
Career boost, too
The researchers involved in the project benefited from funding through the EU’s Marie Skłodowska-Curie fellowship programme, which aims to train young researchers and provide a boost to their careers.
The Multifast team, which includes Stella, the principal investigator Casella, a post-doctoral fellow and two undergraduates as assistant researchers, has published the results and more in leading journals during the course of the four-year project.
Both assistants delivered Masters’ dissertations on the basis of the project; one of them has recently entered a PhD programme at the Università dell’Insubria, under Dr Casella’s co-supervision. Several international collaborators were also invited to work with the team on various aspects of the projects.
Multifast provided the principal investigator, research fellow and assistants visibility both within and outside the host country, and boosted Europe’s standing in a fascinating and complex branch of astrophysics.