Brown, Gregory C. (2016) A study of exotic nuclear extragalactic transients. PhD thesis, University of Warwick.

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Abstract

The analysis of nuclear flares is unfortunately difficult. Contamination of supernova surveys by active galactic nuclei (AGN) variability, and the difficulty in detecting transients in the high surface brightness nuclei of galaxies, has led to many surveys avoiding nuclear transients entirely. Even in cases where transients are detected, their identification and classification remains complex, with many possible progenitor pathways, overlapping models and wide ranges of observed properties to explain. Here I consider a sample of these events, placing them in the wider context of transient astronomy.

The detection of a class of relativistic tidal disruption flare, thought to be the capture and disruption of Sun-like stars that also powers a moderately relativistic jet, has prompted the search for more of these events. Within this work I analyse the properties of one such candidate, Swift J1112.2-8238, conforming its extragalactic origin and showing it came from a galaxy at a redshift of z = 0.89. Its high energy and optical properties are consistent with the previous candidates and its position, close to the centre of a likely star-forming host, continues to support the tidal disruption flare origin of these events. The rates of these events suggest that only a small fraction of tidal disruption flares launch similar jets.

Prompted by these findings, I proposed and obtained medium resolution spectroscopy and radio observations of the source, and analysed high-resolution HST imaging to further constrain the position of the transient within its host. The HST imaging shows that the host has a complex morphology, perhaps due to an interaction with another galaxy, with the transient loosely consistent with the centre of compact bulge-like component. I confirm the host's redshift and determine its nature as a star-forming galaxy. Radio emission detected coming from the host, that is too luminous to be associated with star-formation, shows evidence of variability, suggesting that it is associated with the transient flare and thus is perhaps confirmation of the jetted nature of the event. In almost all respects, Swift J1112.2-8238 remains an excellent candidate relativistic tidal disruption flare.

Finally, I analyse HST imaging of a number of flares with unusual properties. I greatly improve the astrometric tie of ASASSN14ae and ASASSN14li to the nuclear regions of their hosts and show that their properties are still most consistent with a tidal disruption flare origin. In the case of CSS100217 and ASASSN15lh however, the “accepted" classification of their origins as superluminous supernovae appears to be at odds with their host galaxies, with ASASSN15lh in particular coming from a massive host with minimal star-formation. I show that CSS100217 has undergone a significant drop in apparent quiescent-level emission following the flare, indicating the possibility that the flare may have directly impacted, or been caused by, a change in the accretion of the known AGN. I consider the possibility that both flares could be associated with unusual tidal disruption flares or AGN variability, though the current observations make it difficult to make strong claims about either flare's true origins.

Item Type:	Thesis (PhD)
Subjects:	Q Science > QB Astronomy
Library of Congress Subject Headings (LCSH):	Active galactic nuclei, Black holes (Astronomy) -- Observations, Astronomical spectroscopy
Official Date:	September 2016
Dates:	Date Event September 2016 Submitted
Institution:	University of Warwick
Theses Department:	Department of Physics
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Levan, Andrew J. ; Stanway, Elizabeth R.
Format of File:	pdf
Extent:	ix, 179 pages : illustrations, charts
Language:	eng

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