Three Dutch-Belgian telescopes have started operating at the ESO La Silla Observatory in Chile. This so-
called BlackGEM array will scan the southern sky to hunt for cosmic events that produce gravitational
waves, such as mergers of neutron stars and black holes. Leiden astronomer Rudolf le Poole is system
engineer of the BlackGEM array.
Some cataclysmic events in the Universe, such as the collision of black holes or neutron stars, create
gravitational waves, ripples in the structure of time and space. Observatories like the Laser
Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo Interferometer are designed to
detect these ripples. But they cannot pinpoint their origin very accurately nor see the fleeting light that
results from the collisions between neutron stars and black holes. BlackGEM is dedicated to quickly
scanning large areas of the sky to precisely hunt down gravitational-wave sources using visible light.
‘With BlackGEM we aim to scale up the study of cosmic events with both gravitational waves and visible
light,’ says Paul Groot of Radboud University in the Netherlands, the project’s Principal Investigator. ‘The combination of the two tells us much more about these events than just one or the other.’
By detecting both gravitational waves and their visible counterparts, astronomers can confirm the nature
of gravitational-wave sources and determine their precise locations. Using visible light also allows for
detailed observations of the processes that occur in these mergers, such as the formation of heavy
elements like gold and platinum.
To date, however, only one visible counterpart to a gravitational-wave source has ever been detected.
Furthermore, even the most advanced gravitational-wave detectors such as LIGO or Virgo cannot
precisely identify their sources; at best, they can narrow the location of a source down to an area of
approximately 400 full moons in the sky. BlackGEM will efficiently scan such large regions at high enough resolution to consistently locate gravitational-wave sources using visible light.
Chili
BlackGEM’s three constituent telescopes were built by a consortium of universities: Radboud University,
the Netherlands Research School for Astronomy, and KU Leuven in Belgium. The telescopes are each 65
centimetres in diameter and can investigate different areas of the sky simultaneously; the collaboration
eventually aims to expand the array to 15 telescopes, improving its scanning coverage even more.
BlackGEM is hosted at ESO’s La Silla Observatory in Chile, making it the first array of its kind in the
southern hemisphere.
‘Despite the modest 65-centimetre primary mirror, we go as deep as some projects with much bigger
mirrors, because we take full advantage of the excellent observing conditions at La Silla,’ says Groot.
Once BlackGEM precisely identifies a source of gravitational waves, larger telescopes such as ESO’s Very
Large Telescope or the future ESO Extremely Large Telescope can carry out detailed follow-up
observations, which will help to shed light on some of the most extreme events in the cosmos.
Southern sky
In addition to its search for the optical counterparts to gravitational waves, BlackGEM will also perform
surveys of the southern sky. Its operations are fully automated, meaning the array can quickly find and
observe ‘transient’ astronomical events, which appear suddenly and quickly fade out of view. This will
give astronomers deeper insight into short-lived astronomical phenomena such as supernovae, the huge
explosions that mark the end of a massive star’s life.
‘Thanks to BlackGEM, La Silla now has the potential to become a major contributor to transient research,’
says Ivo Saviane, site manager at ESO’s La Silla Observatory. ‘We expect to see many outstanding results
contributed by this project, which will expand the reach of the site for both the scientific community and
the public at large.’
Consortium
The BlackGEM consortium comprises: NOVA (Netherlands Research School for Astronomy, the national
Dutch alliance in astronomy between the University of Amsterdam, University of Groningen, Leiden
University, and Radboud University); Radboud University, the Netherlands; KU Leuven, Belgium; the
Weizmann Institute, the Hebrew University of Jerusalem and Tel Aviv University, Israel; the University of Manchester and the Armagh Observatory and Planetarium, UK; Texas Tech University, the University of California at Davis and the Las Cumbres Observatory, USA; the University of Potsdam, Germany; the
Danish Technical University, Denmark; the University of Barcelona, Spain; and the University of
Valparaíso, Chile.