Time domain astrophysics

Active Galactic Nuclei (AGN), the energetic centres of galaxies, are powered by supermassive black holes and provide a wealth of data and information for exploring the extreme physics of magnetic fields and relativistic jets. Material close to the black hole creates an accretion disk due to the black hole’s gravitational influence and friction which results in hot, bright optical and infrared emission. This accretion disk feeds relativistic jets which extend from the polar regions of the supermassive black hole through processes not yet fully understood.

To explore how these jets are created and collimated we study AGN sources at different orientations on the sky; of particular interest are those AGN with small jet viewing angles (for example, below 20 degrees). These sources are called blazars and have highly energetic, rapidly variable emission across the electromagnetic spectrum, along with possible detections of neutrino emission. Their orientation on the sky means that spatial resolution of their extensive jets is not possible, so we explore their structure using optical polarimetry. Polarimetry is the study of the angle of waves of light.

We use the MOPTOP polarimeter on the Liverpool Telescope to measure the angle and degree of polarisation of light from blazars in different optical filters to explore emission regions, separate disk and jet emission and thus explore different emission processes and characterise behaviours of these sources. We conduct long-term photo-polarimetric monitoring of a sample of blazars using the LT and carry out intensive observations of their intranight optical variable behaviour using other facilities such as the Carlos Sánchez Telescope and the Gran Telescopio Canarias on Tenerife and La Palma respectively.

Gamma-ray bursts (GRBs) are instantaneously the most luminous objects in the universe, produced by the deceleration of relativistic jets. The core-collapses of massive stars or the mergers of binary compact stellar objects are their progenitors, and the latter are the primary targets for the ground-based gravitational wave (GW) observatories (LIGO, Virgo and KAGRA). We work with leading surveys such as the Zwicky Transient Facility to quickly identify and classify potential optical/infrared counterparts to these events, using the Liverpool Telescope. In both cases of the progenitors, accretion onto a black hole is likely to power the jets. Understanding the nature of the relativistic jets, especially the acceleration, collimation, and energy content (baryonic or Poynting-flux dominated jets), is a major focus of international research efforts, and it has implications to other relativistic jets (AGN, tidal disruption events). We have been studying magnetic fields in jets using polarimeters (RINGO and MOPTOP) on the Liverpool Telescope, and the jet structures using the electromagnetic (EM) counterparts of GW sources.

Thermonuclear novae are massive eruptions that occur on the surface of white dwarfs – the extinguished stellar cores of stars once like our own Sun. However, novae only occur in binary star systems where the separation between the white dwarf and a less massive and less evolved main sequence star is only a few Solar radii. In these extreme binaries, matter is transferred from the main sequence donor star to the white dwarf. That ‘accreted’ matter is compressed and heated until explosive nuclear reactions begin – the nova eruption. Novae, particularly those occurring on high mass white dwarfs, provide a key progenitor pathway toward type Ia supernovae.

The ARI’s Nova Group has existed for three decades, the current focus of its work is exploration of the link between novae and type Ia supernovae. While mainly observational in nature, including extensive use of the Liverpool Telescope and the Neil Gehrels Swift Observatory, the group’s work also involves hydrodynamical modelling of nova ejecta. Highlights of the ARI’s Nova Group’s work includes: extensive panchromatic studies of the recurrent novae RS Ophiuchi and M31N 2008-12a; robust measurement of the nova rate in the Andromeda Galaxy; discovery and modelling of the first (and second and third) nova super-remnants; recovery of the first extragalactic quiescent nova; the first extragalactic survey for quiescent novae; the first ultraviolet nova spectroscopy beyond the Milky Way and Magellanic Clouds; and development of a quiescent nova classification scheme.

Cataclysmic variables (CVs) are compact binary star systems consisting of a white dwarf accreting material from a donor star. The population of CVs is large and relatively easily observed, making them excellent laboratories for the study of accretion physics and binary evolution. The evolution of the white dwarfs in these systems is of particular interest: mass accretion onto white dwarfs leads to thermonuclear novae and is believed also to lead to type Ia supernovae: the critical events which are used as standard candles in cosmology. Many CVs also show transient phenomena known as ‘dwarf novae’, which are sudden brightenings caused by thermal instabilities in the accretion discs in these systems.

Time resolved observations of these systems provided a powerful insight into their underlying physics. Wide field surveys such as the Zwicky Transient Facility and Gaia Alerts have not only been a fantastic source of new discoveries, but also have provided detailed light curves of huge numbers of objects over long baselines. In recent years we have been collaborating with our colleagues in LJMU’s School of Computer Science and Mathematics in order to use machine learning techniques in order to characterise and classify systems according to the photometric variability that is revealed by these data.

Virtually all stars more than 8 times the mass of the Sun end their lives as a supernova (of Type II, Ib, or Ic), and some white dwarfs in a binary system can explode as well (as a Type Ia supernova) if they accrete sufficient matter from a companion to exceed the Chandresekhar mass. Or at least, this is the textbook picture - in fact, much is unknown about how both “types” of explosion actually occur.  Why do some stars lose their envelopes and explode as Ib or Ic supernovae? Do some stars collapse to black holes with little explosion at all? And are Type Ia supernovae single-degenerate, double-degenerate, or a mixture? Why do some supernovae produce powerful jets? To answer these questions, ARI scientists are deeply involved with leading surveys including the Zwicky Transient Facility and the European Public Spectroscopic Survey for Transient Objects to characterise the nature of these explosions in detail. The Liverpool Telescope is used extensively by ARI staff and students as part of their ongoing research.