I am a post-doctoral fellow in the Astrophysics group at Warwick, starting in January 2014 having obtained my PhD at Liverpool John Moores University.
My research is concerned primarily with observing and characterising astrophysical transients. I have particular interest in constraining the progenitors of core-collapse supernovae through observations of their explosions and the environments they inhabit within their hosts galaxies. Alongside this I study unusual or peculiar subclasses of supernovae, the progenitors of which are poorly understood.
I am also involved in the followup of electromagnetic counterparts to gravitational wave events, including taking some of the earliest observations GW170817/AT 2017gfo - the first multi-messenger gravitational wave event.
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GW170817 was the first multi-messenger gravitational wave event. It was caused by the merger of two neutron stars about 130 million light years away and in August 2017 astronomers detected its presence in both gravitational waves and photons (two 'messengers' of information). The first of these photons was a burst of extremely high energy gamma-rays known as a GRB, occuring about 1.7 seconds after the neutron stars merged, as extremely fast-moving material (almose the speed of light) is launched in a jet from the merger. This was followed over the next week or so in optical and infrared light by a kilonova - the site where heavy elements like gold and platinum are made. GW170817 then went behind the Sun and most telescopes were unable to continue monitoring its evolution.
Hubble Space Telescope observations
At 110 days after the merger first occured, as soon as the Hubble Space Telescope was again able to point at its location, we immediately took very deep images to see what was going on. Remarkably we still saw an optical signature from the source (see figure). This signature was far too bright to be due to the kilonova. Instead, we interpreted this as a result of the GRB. When the fast-moving material launched in the merger collides with the surrounding gas and dust in the circumstellar environment, it gives rise to an afterglow. This afterglow is visible across the electromagnetic spectrum, from X-rays, through optical, to radio waves.
Jet or cocoon?
When we looked more closely at how this afterglow was evolving, however, it did not seem to match what we have seen in previous GRBs. Although very faint, it continued to slowly get brighter for months after the merger. The GRB itself we saw 1.7 seconds after GW170817 was itself quite faint. (The fact that is was remarkably nearby (in GRB terms) meant we were still able to detect and characterise it.) With a rising afterglow, and a faint GRB, it was generally accepted that the jet launched by the merger was not pointed at us. There are currently two competing scenarions for marking the afterglow. One scenarion suggests the jet began punching through the slower moving material of the merger and was 'choked' causing it to trasnfer its energy to the material creating a cocoon of material accelerated to mildly relativistic velocities. Another suggests that the jet indeed sucessfully punched through this slower material and instead we are viewing this event in the wings of this jet.
We will continue to monitor GW170817 all through 2018 in order to determine exactly what caused the unusual behaviour we have seen thus far, and to answer the question of whether this is related to the higher-energy GRBs we have seen before.
The results are contained in the paper The optical afterglow of the short gamma-ray burst associated with GW170817
On the 17th August 2017, the LIGO-Virgo consortium announced the detection of a binary neutron star merger. Within two seconds high-energy gamma ray emission was seen coming from the same region of the sky. What followed was one of the largest coordinated observing campaigns in astronomy.
Almost every telescope able to search the southern sky for the eletromagnetic counterpart set about this task. Within a day a candidate was found in the galaxy NGC 4993, around 40 Mpc (130 million lightyears) away. I was lucky enough to be at the European Southern Observatory at this time, observing explosive transients for ePESSTO. At the telescope, with David Homan (U. Edinburgh), we took one of the first spectra of this candidate (named AT 2017gfo) and were the first to classify it as a kilonova and alert the astronomical community to its uniqueness. Because it looked like nothing we'd seen before, we were condfident that the gravitational wave counterpart had been found.
Our followup campaign withing ePESSTO revealed a somewhat blue, fastly-evolving transient (Smartt et al. 2017). This suggested to us that the merger was not producing so-called 'lanthanide' elements, which include gold and platinum. If the merger had produced these lanthanides we would expect the transient to have been much redder (appearing mainly in the near infrared) and evolve slower, taking perhaps a week to reach its maximum brightness. This is because lanthanides have extremely high opacities. Instead it appeared to show evidence for relatively lighter (but still heavier than iron) elements such as caesium and tellurium.
At late times however the near infrared stayed brighter than expected for a lanthanide-free merger. This was further investigated in Tanvir et al. (2017) where deep near infrared observations were taken from both the ground, with ESO, and space, with the Hubble Space Telescope (HST). Our observations here showed that the merger also had a lanthanide-rich component that emerged after several days. Spectra in the near infrared showed good agreement with models of lanthanide-rich mergers.
In Levan et al. (2017) we set about investigating where this event had occured. Studying the host galaxy, NGC 4993, and the merger's local environment within that galaxy can provide additional information about the merger. (This is much like investigating what cars were involved in a crash based on where the crash occured - if they crashed turning the corner in Monaco, you would probably think differently to a crash on a local High Street.) We found that the galaxy has recently (in cosmological terms) undergone a merger event - NGC 4993 is the result of two galaxies colliding. This may have happened around a billion years ago as we find a significant population of stars of that age in the galaxy. Because we find no younger stars, we can say that the merger system was at least a billion years old, and perhaps older. From studying the light of the transient compared to the light of the galaxy in the vicinity (specifically looking for absorption features associated with the light having travelled though gas and dust), we find tentative evidence that the merger occured on the near-side of the galaxy. This may indicate that the 'kick' that occured during the birth of the neutron stars was directed towards us.
This first object has confirmed that binary neutron stars create gravitational waves, cause short-duration gamma-ray bursts, produce an electromagnetic signature well described by a kilonova and are a dominant source of heavy element production in the universe.
The results are presented in the following papers (plus many, many others):
A kilonova as the electromagnetic counterpart to a gravitational-wave source
The emergence of a lanthanide-rich kilonova following the merger of two neutron stars
The environment of the binary neutron star merger GW170817
Multi-messenger observations of a binary neutron star merger
The Distance to NGC 4993: The Host Galaxy of the Gravitational-wave Event GW170817
The Diversity of Kilonova Emission in Short Gamma-Ray Bursts
Supernovae type Ia (SNe Ia) have been instrumental in our current understanding of cosmology through their use as standard(isable) candles. There are, however, imitators. In the process of searching for SNe Ia, many events have been found that share characteristics of SNe Ia, but have subleties in the explosions that make them distinct. SNe Ia are thought to arise from the thermonuclear detonation of a massive white dwarf, but it is likely these peculiar subtypes are hinting to us the many different ways there are to blow up a star. Of these peculiar types the most numerous are the supernovae type Iax (SNe Iax).
Although SNe Iax look similar to SNe Ia, clues from their spectra (which show lower-velocity, narrower lines) and light curves (which have fainter peaks and evolve faster) suggest they are fainter, weaker cousins to the SNe Ia. So much so that it is difficult to explain their observational properties with models of SNe Ia explosions. Instead, 'failed' explosions may occur, in which the whole of the white dwarf is not detonated and instead a remnant ('zombie star'?) is left behind post-explosion. Alternatively, there have been suggestions instead that SNe Iax may be due to more massive stars (around 7-9 times the mass of the sun) that undergo an electron-capture supernova. Even very massive stars (more than 25 times the mass of the sun) have been suggests as progenitors - these would explode and produce a blackhole upon collapse. If a large fraction of the star that is exploding falls into this black hole then we would see a weak energy supernova with little material being thrown off, the traits of a SN Iax.
We set about investigating where these weird supernovae are exploding using spectroscopic data of their galaxy environments from the Very Large Telescope (VLT) and Nordic Optical Telescope (NOT), in order to test predictions of these different progenitor models.
The results are presented in the following paper:
Investigating the diversity of supernovae type Iax: A MUSE and NOT spectroscopic study of their environments
Supernovae are seen to explode in or around the bright discs and bulges of their host galaxies. This is expected since these bright regions constain the vast majority of the stars in a galaxy. Calcium-rich supernovae, a peculiar subclass of supernovae, however, have a strong tendancy to explode in the remote out-reaches of their host galaxies where there are very few stars. Quite why they prefer these solitary locations has been a puzzle. Suggestions have been made that they could be formed in very faint (and thus difficult to detect) systems at these remote locations.
Using observations taken with the Hubble Space Telescope (HST) and the Very Large Telescope (VLT), I have investigated the explosion sites of these unusual explosions to search for signs of any potential birth places, such as faint dwarf galaxies, that may have been undetected by other observations.
SN2005E and it's explosion site
In the first panel, SN2005E, a member of the Ca-rich supernova class, is seen in the bottom right. Returning to observe this location with HST long after the supernova has faded (second panel), we find no evidence for any birth site (last panel - a zoom in of the square on the middle panel).
Ca-rich supernovae are truly lonely
Thanks to the extreme depth of the observations taken with HST and VLT, the fact that we see no sources at the explosion sites of any of the Ca-rich supernovae we have looked at allows us to rule out their formation in faint underlying systems (such as globular clusters or dwarf galaxies).
These supernovae really are exploding where they have no business to be. Since there are no obvious birth sites for the supernovae, and the fact that the number of stars in these remote locations is so small, we can consider if these explosions are the result of high-velocity pairs of stars that have been flung from their galaxies at hundreds of kilometres per second.
These systems could comprise of a neutron star and a white dwarf. The neutron star is formed when a very massive star collapses under its own gravity and makes a supernova of its own. When it is formed it undergoes a 'kick', accelerating it to large velocities. Since most stars are in binary systems, a companion star can be dragged along for the ride. The supernovae would then be a result of the companion being eventually ripped apart by the neutron star, after the pair have travelled a significant distance to these remote out-reaches of their host galaxy. To the right is an artist's impression of such a system having being ejected from its galaxy.
The results are contained in two papers: The progenitors of calcium-rich transients are not formed in situ and Hubble Space Telescope observations of the host galaxies and environments of calcium-rich supernovae.
Core-collapse supernovae (CCSNe) are thought to arise from the deaths of stars at least 8 times more massive than the Sun. Beyond this cutoff, there is debate as to exactly how different kinds of massive stars die and specifically how their varied deaths produce the varied types of supernova we see.
I have used literature data of CCSN to produce a method of creating the bolometric light curve of a CCSN (which have traditionally been observationally expensive to create) from relatively little data. This method was then applied to a large sample of CCSN in order to determine the properties of their explosions, and thus inform on their progenitor stars.
Bolometric corrections for CCSNe
With just an optical colour, one can estimate the bolometric light curve of a CCSN over a wide range of epochs with a typical rms of the scatter about the relations mag. The case for is shown above.
Bolometric lightcurves of CCSNe
Using the bolometric correction method, a large catalogue of bolometric light curves has been created for stripped-envelope CCSNe. These are types of CCSN that show little or no hydrogen in their specta - it is thought that the massive hydrogen envelopes of the progenitor stars has been largely lost before exploding, resulting in an absence of hydrogen.
There are two (main) mechanisms that can strip the hydrogen envelope from a progenitor star. In this first case the progenitor star is simply that massive and luminous that it sheds these outer layers itself, through very strong stellar winds, over the course of its life.This requires very massive stars, typically at least 20 to 30 times as massive as the Sun. In the second case the star can be more modestly massive and it is the presence of a binary companion that strips this envelope through its gravitational influence. This second mechanism can work for lower mass progenitors (8-20 times the mass of the Sun). Thus if we can determine the masses of the proenitor stars we can distinguish between these mechanisms.
Modelling the bolometric light curves of CCSNe
By employing a simple model to the catalogue of bolometric light curves, one can extract estimates for the explosion parameters of the supernovae. One such parameter is the mass of material that is ejected during the supernova, . This is intrinsically linked to the mass of the star when it was born and we can thus use it, by comparing to results from models of stellar evolution, to constrain the mass range of stripped-envelope CCSN progenitors.
Above is a plot of the distributions of for different types of stripped-envelope CCSNe (IIb, Ib, Ic, Ic-BL). The ranges for from stellar evolution modelling are indicated by gray bars. As can be seen, the distributions are best decribed by binary stars that are between 8 and 20 times the mass of the Sun when formed (). There is very little contribution from more massive stars, either single or binary.
This indicates that the progenitors of stripped-envelope CCSNe are less massive than previously thought, with the binary interaction mechanism being mainly responsible for stripping the hydrogen envelopes of the progenitor stars. A lack of large values for , which would be expected from more massive stars, has dramatic consequences for the fates of these more massive stars. It may indeed be the case that these stars do not produce a luminous supernova, instead directly collapsing to a black hole.
The results are presented in the following papers:
Bolometric corrections for optical light curves of core-collapse supernovae and Bolometric light curves and explosion parameters of 38 stripped-envelope core-collapse supernovae
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