The Young Supernova Experiment

The Young Supernova Experiment (YSE) is an optical survey using the PanSTARRS telescopes at the Mauna Kea Observatory in Hawaii. YSE currently monitors 1500 square degrees of the Northern sky in down to ~21.5 mag in g, r, i, and filters at a 3 day cadence.

However, what really makes YSE special is its observing strategy. The YSE observations work in synergy with those of the public fields of the Zwicky Transient Facility (ZTF) survey. YSE pointings are carefully chosen to overlap with those of ZTF, and the observations are ‘interweaved’, with YSE observing the same fields ~24 hours prior to ZTF but to approximately 1 magnitude deeper. This one-of-kind strategy creates a survey with an effective cadence of 1.5 days in the g and r bands (common filters to both surveys).

The depth, filter choices, and unique observing strategy of YSE open up whole new discovery spaces for transient science. As the name implies, YSE is poised to easily discover some of the youngest supernovae (within hours to days of explosion), whilst also probing extremely red transient discovery spaces. The high quality, multi-band PanSTARRS imaging for every transient with allow for exotic transients to be studied in depth, alongside providing large samples or canonical transients with high fidelity data sets for population studies.

You can read more about the capabilities of YSE here

The Dark Energy Survey

The Dark Energy Survey (DES) is an optical-near infrared photometric survey in the Southern hemisphere which combines four complimentary sciences (galaxy clusters, weak gravitational lensing tomography, galaxy angular clustering, and supernova distances) to probe the nature of dark energy.

To do this, DES uses the Blanco 4-m telescope at the Cerro Telolo Inter-American Observatory (CTIO) to image 5000 square degrees of the Southern sky down to ~24th magnitude in g, r, i, z and Y bands. The camera used to do this, DECam, has a field of view of 2.7 square degrees and contains 62 red-sensitive science CCDs.

These red-sensitive CCDs make DES ideal for finding very distant supernovae (i.e. at high redshift). I have been involved with the DES supernova program (DES-SN), which scans 10 DECam fields with a cadence of approximately 7 days in g, r, i, z  bands.  This program is designed primarily to search for high redshift SNe-Ia to use for cosmology.

However, DES-SN also finds thousands of other interesting transient events, including Superluminous Supernovae.

Superluminous Supernovae

Superluminous Supernovae (SLSNe) are an exceptionally bright subclass of supernovae. At peak, they’re typically 10 times brighter type Ia supernovae, and more than 100 times brighter than “normal” core collapse ones. They’re also very long lived events, remaining bright at optical wavelengths for ~100’s of days. Such events have the potential to be seen out to very large cosmic distances, which makes them an appealing candidate class of events to use as distance measures in cosmology.

However, before we can do this, we have to really understand how SLSNe are produced. This is a topic is still currently under debate. Their longevity and high luminosities of SLSNe makes the difficult to describe with normal core collapse physics, requiring large amounts of 56Ni to be produced during the explosion in order to produce the main peak of the lightcurve, whilst their declines rates are much swifter than would be expected if driven by radioactive decay.

My research involves the study of these events, particularly the sample found within the Dark Energy Survey.  Through study of their lightcurves (see below for an example ), we may begin to understand the physics behind these explosions. Within SLSNe, we see lots of diversity within their lightcurve behaviours – a wide range of evolutionary timescales and peak brightnesses. Some events even show re-brightening at late times.

Example of the g,r,i,z lightcurve of a SLSN event seen within DES. The lightcurve has been interpolated using Gaussian Processes, which allows models to be fit at every epoch of the SN event. They also help highlight some of the small-scale re-brightening features seen at late times.

Bumpy SLSNe

One feature of SLSNe I’m particularly interested in are pre-peak bumps – these are small peaks which occur a few days prior to the start of the main supernova. It’s thought that these occur due to a process called shock breakout – a burst of high energy radiation which is visible when the shock from the supernova explosion first “breaks out” of the stellar surface. In SLSNe, we think this shock breakout can interact with a dense shell of surrounding material, which we see as a bump!

Some of my work with the DES SLSN sample has involved studying the properties of these bumps, (which are as diverse as the SLSN sample!), and in some cases, we can completely rule out their presence!

Describing SLSNe bumps from the DES sample at the EEU2019 Meeting.
The arms help.

Host Galaxies

A complementary route to understanding the progenitors of different kinds of transients is to look at the environments in which they occur. From the properties of the environment in which a star explodes, we can make general assumptions about the underlying population of stars within the galaxy, and therefore the most likely progenitor type to have produced the transient.