Abstract

This thesis studies type Ia supernovae (SNe Ia) discovered by the Palomar Transient Factory (PTF). We use this large, high--quality imaging and spectroscopic dataset to analyse the very earliest part of the SN Ia light curve, probing the ejecta structure, the post–maximum light curve, the $^{56}Ni~$distribution, and the utility of SNe Ia as standard candles at different wavelengths.

The high cadence of PTF makes it capable of studying SNe Ia at very early times, as they rise just after explosion. In this thesis we use 18 SNe Ia from PTF and the La Silla-QUEST variability survey (LSQ) to measure the time between first light, $t_0$, and maximum light ($t_{rise}$), and rise index ($n$), where $f= (t-t_0)^{n}$. We find that $n$ shows significant variation ($1.48\leq n \leq3.70$), and has a mean value $\bar{n}=2.44\pm0.13$, inconsistent with a simple fireball model ($n=2$) at $3\sigma$. We calculate an average \( t_{rise} \) of our sample $t_{rise}=18.98\pm0.54$~days, and find that the variation seen in $t_{rise}$ and $n$ is principally driven at the very earliest epochs.

We use a further sample of 31 PTF SNe Ia to explore the diversity of the post-maximum light curve in $R$--band, as the emission at these phases is linked to the same driver of variation -- the distribution of \nickel{}~ through the ejecta. By modelling the shoulder on the light curve we quantify the strength and timing of this feature. We find that the timing of the additional emission is correlated with light curve width, but that the duration is not. We also find that the prominence of the gaussian, relative to its peak, is inversely correlated with the light--curve width. We interpret this as the impact of intermediate mass element spectral features dominating over those of iron peak elements.

Having explored the physics that governs these objects, we turn to the primary motivation for studying SNe Ia -- their use as standardisable candles. We consider 60 SNe Ia in three filters, $B$, $R$ and $i$, constructing hubble diagrams for each. We find that the magnitude of the corrections for light curve width and SN colour decreases as a function of wavelength, and that SNe Ia in $i$--band do not need correction for light curve width. We also find that the colour law observed for our sample is not consistent with the standard Milky--Way extinction law, with $R_V = 3.1$. Finally, we find that SNe Ia in $i$ are superior standardisable candles to $B$--band, and that they are overall the best in $R$. This sample is well placed to anchor future high--redshift rest--frame $i$--band cosmological studies.

We present an analysis of the early, rising light curves of 18 Type Ia supernovae (SNe Ia) discovered by the Palomar Transient Factory and the La Silla-QUEST variability survey. We fit these early data flux using a simple power law (f(t) = α × tn) to determine the time of first light (t0), and hence the rise time (trise) from first light to peak luminosity, and the exponent of the power-law rise (n). We find a mean uncorrected rise time of 18.98 ± 0.54 d, with individual supernova (SN) rise times ranging from 15.98 to 24.7 d. The exponent n shows significant departures from the simple `fireball model' of n = 2 (or f(t) ∝ t2) usually assumed in the literature. With a mean value of n = 2.44 ± 0.13, our data also show significant diversity from event to event. This deviation has implications for the distribution of 56Ni throughout the SN ejecta, with a higher index suggesting a lesser degree of 56Ni mixing. The range of n found also confirms that the 56Ni distribution is not standard throughout the population of SNe Ia, in agreement with earlier work measuring such abundances through spectral modelling. We also show that the duration of the very early light curve, before the luminosity has reached half of its maximal value, does not correlate with the light-curve shape or stretch used to standardize SNe Ia in cosmological applications. This has implications for the cosmological fitting of SN Ia light curves.

We present photometry and time-series spectroscopy of the nearby type Ia supernova (SN Ia) SN 2015F over −16 days to +80 days relative to maximum light, obtained as part of the Public ESO Spectroscopic Survey of Transient Objects (PESSTO). SN 2015F is a slightly sub-luminous SN Ia with a decline rate of Δm15(B)=1.35±0.03 mag, placing it in the region between normal and SN 1991bg-like events. Our densely-sampled photometric data place tight constraints on the epoch of first light and form of the early-time light curve. The spectra exhibit photospheric C II λ6580 absorption until −4 days, and high-velocity Ca II is particularly strong at <−10 days at expansion velocities of ≃23000\kms. At early times, our spectral modelling with syn++ shows strong evidence for iron-peak elements (Fe II, Cr II, Ti II, and V II) expanding at velocities >14000 km s−1, suggesting mixing in the outermost layers of the SN ejecta. Although unusual in SN Ia spectra, including V II in the modelling significantly improves the spectral fits. Intriguingly, we detect an absorption feature at ∼6800 \AA\ that persists until maximum light. Our favoured explanation for this line is photospheric Al II, which has never been claimed before in SNe Ia, although detached high-velocity C II material could also be responsible. In both cases the absorbing material seems to be confined to a relatively narrow region in velocity space. The nucleosynthesis of detectable amounts of Al II would argue against a low-metallicity white dwarf progenitor. We also show that this 6800 \AA\ feature is weakly present in other normal SN Ia events, and common in the SN 1991bg-like sub-class.

We present a search for an electromagnetic counterpart of the gravitational wave source GW151226. Using the Pan-STARRS1 telescope we mapped out 290 square degrees in the optical i_ps filter starting 11.5hr after the LIGO information release and lasting for a further 28 days. The first observations started 49.5hr after the time of the GW151226 detection. We typically reached sensitivity limits of i_ps = 20.3-20.8 and covered 26.5% of the LIGO probability skymap. We supplemented this with ATLAS survey data, reaching 31% of the probability region to shallower depths of m~19. We found 49 extragalactic transients (that are not obviously AGN), including a faint transient in a galaxy at 7Mpc (a luminous blue variable outburst) plus a rapidly decaying M-dwarf flare. Spectral classification of 20 other transient events showed them all to be supernovae. We found an unusual transient, PS15dpn, with an explosion date temporally coincident with GW151226 which evolved into a type Ibn supernova. The redshift of the transient is secure at z=0.1747 +/- 0.0001 and we find it unlikely to be linked, since the luminosity distance has a negligible probability of being consistent with that of GW151226. In the 290 square degrees surveyed we therefore do not find a likely counterpart. However we show that our survey strategy would be sensitive to NS-NS mergers producing kilonovae at D < 100 Mpc which is promising for future LIGO/Virgo searches.