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High cadence optical polarimetry for time domain astronomy on the Liverpool Telescope

Arnold, D (2017) High cadence optical polarimetry for time domain astronomy on the Liverpool Telescope. Doctoral thesis, Liverpool John Moores University.

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Gamma-ray bursts are the most violent of known astrophysical events, with up to 10^53 ergs of energy released on the order of seconds. These extreme explosions, first observed in 1960s, form a fast moving field of research within astrophysics which relies on multi-wavelength observations of these transient events to probe the early- time (< 15 minutes) parameter space of these events. In the optical regime, follow-up observations to the prompt emission are ideally suited to the 2.0 metre Liverpool Tele- scope (LT), situated at an altitude of 2363m on the Observatorio Roque de las Mucha- chos (ORM), La Palma, Canary Islands. The LT is fully robotic and able to respond automatically to triggers of new gamma-ray burst (GRB) events, starting within 2-3 minutes of the detection of the prompt emission. The observed radiation from GRBs is released from relativistic jets. Launched from a black hole central engine, the energy within the jets is converted to the observed radi- ation predominantly through a synchrotron process, which can produce highly linearly polarised radiation. Polarimetric observations of this radiation are a key resource to infer the magnetic field structure of the emission region and distinguish between baryonic and magnetic models of jet physics. For this reason, the Liverpool Telescope has hosted the RINGO series of polarimeters which use a novel design to enable early-time polarimetric measurements of these highly variable optical sources. RINGO was mounted on the LT in 2005, and observed GRB 090102A providing a measure of 10.2 ± 1.3% average linear polarisation in the period of 160-220s post burst. RINGO2, mounted in June 2009 improved on the original design utilising a triggered electron multiplying CCD system. Both RINGO and RINGO2 were single band instruments. The development of RINGO3 extended the design of RINGO2 into a simultaneous 3 band polarimeter. This work focuses on the characterisation of RINGO2 and analysing the sample of GRB observations made during its lifetime. The observations of GRB 120308A provide measurements with a high confidence, inferring the existence of stable, or- dered magnetic fields within the jet. Analysis of other GRB afterglows observed with RINGO2 provides confidence in this result and confirm that jets can be highly magne- tised, with the majority of energy being contained in magnetic field recombination and not through kinetic energy of baryonic matter. RINGO3, a multi-band extension to RINGO2, was developed and tested within the Astrophysics Research Institute labs before being commissioned on the Liverpool Telescope in November 2012. Lab tests of instrument throughput with calculations of the signal to noise ratio across the operating wavelength defined the optimal cut-offs of the 3 wavelength bands. This instrument was then characterised using similar methods to RINGO2, and whilst not an ideal instrument was found to be within the required performance for the prime science goal of early-time GRB afterglow observations.

Item Type: Thesis (Doctoral)
Uncontrolled Keywords: Gamma-ray bursts; Time domain astronomy; Optical polarimetry; The Liverpool Telescope
Subjects: Q Science > QB Astronomy
Divisions: Astrophysics Research Institute
Date Deposited: 17 Jul 2017 08:46
Last Modified: 23 Nov 2022 09:51
DOI or ID number: 10.24377/LJMU.t.00006687
Supervisors: Steele, IA
URI: https://researchonline.ljmu.ac.uk/id/eprint/6687
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