Radio observations made by Metsähovi, Finland’s only astronomical radio observatory, and Caltech's Owens Valley Radio Observatory (OVRO) in California have revealed that radiation from the remote PKS 2131-021 quasar is subject to periodic variation.
Quasars are distant active galaxies that radiate brightly because the supermassive black hole in the centre of them helps them to generate energy. This radiation can be detected in several wavelength ranges, including radio waves.
The radio waves originate in a jet that shoots out at near the speed of light from the vicinity of the black hole. If there are two mutually-orbiting supermassive black holes in the centre of the quasar, this can be verified from periodic variations in the radiation emitted.
This is why the periodicity now observed is strong evidence that PKS 2131-021 is a system of two supermassive black holes that orbit each other.
The Finnish participants in the multinational research group were Docent Merja Tornikoski and Professor Anne Lähteenmäki from Aalto University as well as Academy Research Fellow Talvikki Hovatta and Postdoctoral Researcher Yannis Liodakis from the Finnish Centre for Astronomy with ESO, a separate institution coordinated by the University of Turku.
The study made use of data from the ongoing observation programmes of Caltech's (California Institute of Technology) OVRO and Aalto University's Metsähovi Radio Observatory and also archive data from various other telescopes.
Hovatta, who has also worked at the Owens Valley Radio Observatory, explains that OVRO has been making observations since 2008, but the time series were also supplemented with archive data from observatories that have previously made observations at the same radio wavelength.
‘Cases like this are not considered impossible, but proving their existence is very challenging. Quasars are located billions of light years away, and finding periodicities requires high-quality data spanning several decades.’
"The unique, long-term observation results of the Metsähovi Radio Observatory at Aalto University confirmed the same periodicity at a higher radio frequency. The study conducted at Metsähovi is very valuable because we have been making variability observations of some quasars for up to forty years. Long time series are essential for understanding the behaviour of quasars,’ explains Tornikoski, who is responsible for Metsähovi's observation programme.
‘The radio emission of many quasars vary considerably, but it is very rare for them to have this striking periodicity that follows the sine curve. We quickly realized that this was a very special object of study,’ Hovatta continues.
