Gamma-Ray Bursts

Ultra-rapid 

Follow-up

Gamma-Ray Bursts (GRBs) are the most energetic explosions to be detected in the Universe and offer unique access to regions of extreme physics: ultra-relativistic speeds, strong gravity and intense magnetic fields, as well as acting as luminous stellar beacons that probe conditions in the early Universe. GRBs can be classified into two categories based on the duration of their afterglow emission: long GRBs are thought to be associated with the death of massive stars, while short GRBs are believed to be the merger of two compact objects. The GW170817 gravitational wave event confirmed the association of short GRBs with neutron star mergers and further accelerated interest in these sources.

Credit: NASA/GSFC

The Liverpool Telescope’s autonomous and robotic follow-up has been key for Gamma-ray burst science. The rapidly fading nature of short Gamma-ray bursts means that traditional telescopes struggle to catch the source before it has become too faint. The automated response to NASA Swift triggers, without human intervention, allows the LT to take data within minutes of outburst. For the NRT we aim to be on target, taking observations, within 30 seconds of a trigger. This will allow a new generation of transient objects, discovered with facilities such as SVOM, to be routinely optically observed during the prompt emission phase for the first time.

Robotic Telescopes like the NRT are uniquely suited to the task of transient follow-up since they can flexibly and automatically react to new discoveries in real time. They are quick to react to the most exciting targets: in the era of fast discovery, the follow-up must proceed with a similar alacrity. The LT is currently a world-leading facility for time-domain rapid response, but the sensitivity of the new discovery facilities means the fainter targets will require a larger aperture optical follow-up facility.

The LT's rapid response is crucial for early-time observations of GRBs. The Figures show RINGO2 (polarimeter) and RATCAM (imager) observations of GRB 120308A, ~4 minutes after the object was discovered in Gamma-rays. These early time observations captured the high levels of polarisation associated with the fireball jet physics. Later observations measured the low levels of polarisation from the large scale jet interacting with the ambient medium. Mundell et al. (2013), Nature, 504, 119 and Copperwheat et al. (2014), SPIE, 9145, 914511.

The Liverpool Telescope’s autonomous and robotic follow-up has been key for Gamma-ray burst science. The rapidly fading nature of short Gamma-ray bursts means that traditional telescopes struggle to catch the source before it fades. The automated response to NASA Swift triggers, without human intervention, allows the LT to take data within minutes of outburst. For the NRT, we aim to be on target and taking observations, within 30 seconds of a trigger. This will allow a new generation of transient objects, discovered with facilities such as SVOM, to be routinely optically observed during the prompt emission phase for the first time.

The GRB science case is an important driver for the telescope design: the fast-fading nature of GRB afterglows means target acquisition time is at least as important as aperture for follow-up. Rapid IR imaging or spectroscopy is necessary to estimate the redshift of any event, and so there is a crucial need for ground based follow-up to, for example, identify the rare high-redshift events that can be used as cosmic beacons. With fast polarimetry a particularly unique selling point, MOPTOP has been designed for the LT with the intention of transferring it to the NRT during commissioning (“NR-MOPTOP”), enabling the polarimetric characterisation of many more GRB afterglows. 


The Cherenkov Telescope Array (CTA) will begin science operations in 2021 and as such open up the time-variable sky at very high energies (~TeV). The Northern component of CTA will be located on La Palma, so NRT will be ideally placed to exploit CTA targets of opportunity. The much improved sensitivity and field-of-view of CTA should see it provide a wealth of high energy data on GRBs and other transient sources, which coupled with fast optical polarimetry will provide a deeper insight into the properties of the jet magnetic fields, leading to a greater understanding of initial ejection and collimation processes.


Contact

New Robotic Telescope UK Project Office,

Astrophysics Research Institute,

Liverpool Science Park, IC2,

L3 5RF, UK.

 

NRT@ljmu.ac.uk
 

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© 2020 Helen Jermak, LJMU