Observations and modelling of super-luminous supernovae A thesis submitted for the degree of Doctor of Philosophy by Matthew Nicholl, M.Phys. (Oxford University 2012) Faculty of Engineering and Physical Sciences School of Mathematics and Physics Department of Physics and Astronomy Queen’s University Belfast Belfast, Co. Antrim, Northern Ireland September 2015 For Ashley Acknowledgements I would like to start by thanking my supervisor, Professor Stephen Smartt. I can’t imag- ine a better supervisor than Stephen, who gave me space to pursue ideas, but was always available (despite his many other commitments!) to discuss progress and push me in the right direction. It was a real privilege to be able to draw on a wealth of knowledge and academic nous from such a respected researcher. So thank you, Stephen; I don’t think I could have achieved half as much without your help and encouragement. And of course thank you for all the travel money and opportunities to further my career. I have received invaluable assistance from the supernova group here. I’d especially like to thank Drs. Cosimo Inserra, Anders Jerkstrand, Stuart Sim, Morgan Fraser, and my second supervisor, Rubina Kotak, who between them have taught me most of what I know about astronomy. Thanks to Janet (now Dr. Chen) for being a great person to work alongside on SLSNe, and for all the Taiwanese treats! To all the other members of the SN group and ARC, thank you for making Queen’s such a stimulating – and most importantly, fun – place to work. In particular, I’d like to thank my fellow PhD students. The burritos, curries and nerf wars were an essential part of my Queen’s experience! One of the great things about coming back to Northern Ireland for these three years has been the chance to spend more time with my amazing family. Thank you, Mum and Dad, for making home such a thoroughly enjoyable place to spend a weekend – and not just because of the free food! Your encouragement during my PhD studies is just the tip of the iceberg when I think of the love and help and support that you’ve always shown me. And it would be remiss of me not to mention that Mum bravely proofread this entire thesis! I have also found myself in the lucky position to have a brother, James, who I count among my best friends. I’m proud of you and can’t wait to see you fulfil your own ambitions, be it in music, software, or something totally unexpected! To all of the friends I have made in Belfast, Derry and Oxford: thank you for all of the fun times over the years. You’ve helped me to remain a rounded person whenever science has threatened to take over! I know we’ll continue to stay in touch, and hopefully have many more reunion weekends. Special thanks to James and Susie Dickey for being the best possible housemates, and to Ryan Nicholl (no relation) for first introducing me to Stephen Smartt and the QUB physics department back in 2011. Finally, I want to thank Marisa. I met you at the halfway point in my PhD, and I don’t think it’s a coincidence that I’ve enjoyed the second half so much more than the first (not that it was bad before!). Thank you for the love and adventures, and for being as interested in my studies as I am in yours. To the rest of the McVey family: I cannot thank each of you enough for the warmth and hospitality you’ve shown me; it’s been like having a second family here in Belfast. M.N. July 2015, Belfast i Abstract In the last decade, supernova science has undergone a revolution. Technological ad- vances have made it possible to search for transients over the whole sky with no inher- ent galaxy bias. This strategy has revealed a population of extremely bright explosions – in otherwise anonymous galaxies – that are now known as super-luminous super- novae (SLSNe). While this discovery has increased our knowledge of the diverse ways in which massive stars end their lives, it is still unclear what conditions lead to a SLSN (though low metallicity seems to be required). Even more fundamentally, we do not know the power source underlying their luminosity. The nature of these explosions is one of the biggest outstanding questions in stellar astrophysics. If we wish to build a complete picture of massive-star evolution, it is essential to understand the properties of SLSNe. Moreover, their intrinsic brightness and blue colours will offer a new window into the high-redshift Universe with next-generation instrumentation. In this thesis, we present the results of extensive ground-based monitoring cam- paigns for a number of hydrogen-poor (Type Ic) SLSNe at relatively low-redshift (0:1 < z < 0:35), making these some of the best-observed examples of this class in the literature. We use photometric and spectroscopic data, primarily in the optical but also in the UV and NIR, to constrain their luminosities, velocities and temperatures. We measure the characteristic diffusion time for all objects in the literature with rea- sonable light curve coverage and use this to infer ejected masses, showing that masses are typically larger than in normal-luminosity hydrogen-poor supernovae (including GRB supernovae). High ionisation may also be a factor in broadening the light curves of SLSNe Ic. However, we show that even for the slowest-evolving objects, the light curve rise time and inferred ejecta mass are well below the predictions for hypothetical pair-instability supernovae (PISNe). This is true even for slowly fading SLSNe identi- cal to the well-studied SN 2007bi, an object caught during the decline phase and that was thought to be a strong candidate for a PISN. We therefore argue that no SLSNe observed so far are PISNe. For one SLSN Ic caught soon after explosion, a double- peaked light curve seems to indicate a surprisingly extended progenitor, which may be an important new clue. These properties are modelled using a semi-analytic code that we have adapted and helped to develop as part of this work. Three possible power sources are investigated: ii radioactive 56Ni; a central engine such as magnetar spin-down; and interaction with a massive circumstellar medium. We find that the latter two can reproduce the light curves of all objects reasonably well, but that 56Ni-powering requires unrealistic model parameters, even for slowly fading objects. The observed spectra of SLSNe Ic may be able to break the degeneracy between central engine and interaction models. The lack of narrow emission or absorption lines, the presence of broad lines well before light curve maximum, high velocities with little sign of deceleration, the overall similarity to SNe Ic, and agreement with literature models for magnetar-energised ejecta all suggest that a millisecond magnetar (or possibly black hole accretion) engine is the most probable power source in SLSNe Ic. iii Contents Acknowledgementsi Abstract ii List of Tables viii List of Figures xi Publications xii 1 Introduction1 1.1 Novae and Supernovae..........................2 1.2 Observational properties of supernovae..................2 1.3 Energy generation in supernovae.....................3 1.3.1 Thermonuclear supernovae....................4 1.3.2 Core-collapse supernovae....................6 1.4 Supernovae and gamma-ray bursts....................7 1.5 Super-luminous supernovae........................9 1.5.1 Discovery.............................9 1.5.2 Hydrogen-poor SLSNe...................... 10 1.5.3 Hydrogen-rich SLSNe...................... 12 1.6 Proposed models for SLSNe....................... 12 1.6.1 Central engine.......................... 12 1.6.2 Pair instability.......................... 13 1.6.3 Circumstellar interaction..................... 14 1.7 Thesis structure.............................. 16 2 Methods 17 2.1 Sky surveys................................ 18 2.1.1 Pan-STARRS1.......................... 18 2.1.2 PESSTO............................. 20 2.1.3 La Silla QUEST......................... 22 iv 2.2 Other large programs and data pipelines................. 24 2.2.1 VLT and X-Shooter........................ 24 2.2.2 Las Cumbres Observatory Global Telescope Network...... 24 2.2.3 The Liverpool Telescope..................... 25 2.3 Manual data reduction.......................... 25 2.3.1 Reducing data with iraf ..................... 25 2.3.2 Photometry with SNOoPY.................... 26 2.3.3 Absolute magnitudes and K-corrections............. 28 2.3.4 Bolometric light curves...................... 29 2.4 Models.................................. 30 2.4.1 Nickel-powered models..................... 31 2.4.2 Magnetar-powered models.................... 32 2.4.3 Interaction-powered models................... 33 2.4.4 Exploring the models....................... 37 3 Three SLSNe Ic from PESSTO year one 39 3.1 Introduction................................ 40 3.2 Discovery and classification....................... 40 3.2.1 LSQ12dlf............................. 40 3.2.2 SSS120810............................ 42 3.2.3 SN 2013dg............................ 42 3.3 Spectroscopy............................... 44 3.3.1 Data aquisition and reduction.................. 44 3.3.2 Spectral evolution........................ 44 3.4 Photometry................................ 51 3.4.1 Data aquisition and reduction.................. 51 3.4.2 Light curves........................... 52 3.4.3 Bolometric light curves...................... 55 3.5 Modelling................................. 57 3.5.1 LSQ12dlf............................. 59 3.5.2 SSS120810............................ 61 3.5.3 SN 2013dg............................ 62 3.5.4 CSM configuration........................ 64 3.6