BlackGEM

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Image above: Early design of a single BlackGEM telescope on its mount. © NOVA / RU technocenter

The aim of the BlackGEM project is to detect the optical light emitted by the remnants of a merger of a neutron star and a black hole or of two neutron stars. These events themselves herald their occurrence by the arrival of a burst of gravitational waves, to be detected by the Advanced LIGO & Virgo laser-interferometer systems. The first such direct detection of gravitational waves has been accomplished by LIGO in 2015 in the event GW150914. After the actual merger of the neutron star and black hole, a mass as large as ~1% of a Solar mass is ejected from the event after being heated up to ~5 billion degrees. The radiation from this ejected mass peaks in the optical-infrared regime and is referred to as a ‘kilonova’.

The added value of also detecting these merger events in the optical is that it allows a clear positioning in the sky and identifying the system in which  the event has happened: In a galaxy? Outside a galaxy? In a star-forming region? Or perhaps in a globular cluster? From the time evolution of the optical event (fading and reddening and detailed spectra) it can be deduced which elements are synthesized in the ejecta.

Finding these optical signals is a major challenge for multiple reasons:

1) Gravitational detectors can pinpoint the location of these merger events only with an accuracy of ~400 times the size of the full Moon. To know exactly where it happened the localization accuracy needs to be increased by a factor of a billion! Any optical system must therefore be able to scan this complete area on the sky quickly.

2) The kilonova signals are expected to be faint: 2.5 million times fainter than the faintest object visible to the naked eye.

3) The kilonova signals are expected to fade quickly: modelling shows that they should be visible for hours – days only.

The research into these gravitational wave merger events is a key priority for NOVA. The aim of one of the three NOVA research programmes is to understand the (astro)physics of neutron stars, black holes and the binary star systems and stellar populations in which they occur. No telescope system available to NOVA researchers is able to meet the above three challenges, and therefore the BlackGEM project was initiated to design and build such a system. Once a signal is detected with BlackGEM it will be followed in detail with larger telescopes, primarily those of ESO.

The BlackGEM project will deliver three major science products:

1) The optical counterparts to gravitational wave mergers;

2) A six-band multi-color survey of the complete Southern Sky (‘a southern Sloan Survey’);

3) A fast synoptic survey to characterize the variability of faint sources in the night sky on the time scales of minutes to hours.

In its full configuration, the BlackGEM system will consist of several stations located around the world, each station containing three BlackGEM telescopes. The first of these stations will be constructed at ESO’s observatory at La Silla, Chile. A single BlackGEM telescope will be located in Sutherland, South Africa. This telescope, called MeerLICHT will be used in tandem with the MeerKAT radio telescope to identify optical counterparts to radio transients. MeerLICHT is the first telescope to be built and acts as the prototype for BlackGEM.

The optical system of each BlackGEM telescope consists of a 65 cm primary mirror, a 23 cm secondary mirror, a triplet lens system, the last of which acts as an atmospheric dispersion corrector. After passing through a filter, this creates a focal plane that is flat and achromatic and is imaged by a single 110 Megapixel detector, the largest available single detector in the world. The optical elements are held in place in an optical telescope assembly made of carbon-fiber, co-designed between NOVA and Airborne Composites BV. The telescope is pointed using a mount that is co-designed by the TechnoCenter and Fornax Mounts. Each telescope is housed inside a clam-shell dome and placed on top of a completely open tower structure, to minimize any air turbulence created by the structure itself.

In September 2016, the MeerLICHT telescope achieved first light at the NOVA optical instrumentation lab in Dwingeloo, the Netherlands. It is expected to become operational in South Africa in the spring of 2017. The first BlackGEM station at La Silla will become operational in 2018.

More information can be found on the BlackGEM/MeerLICHT website.

Principal investigator: Prof. dr. Paul Groot (Radboud University, Nijmegen)