AGN and TDEs

You can find a simpler description of Active Galactic Nuclei (also known as quasars) and Tidal Disruption Events on the National Schools' Observatory website.

Quasi-simultaneous Follow-up

Active Galactic Nuclei (AGN) are energetic and turbulent centres of galaxies in which material has come too close to the central supermassive black hole. The accreted material is heated by friction and creates a glowing 'active' disk around the galaxy's centre. Through processes not fully understood the accreted material creates powerful, beamed jets of charged particles which travel from the polar regions of the black hole. 

Multi-wavelength variability studies are a key technique for exploring the structure of the material surrounding the central engine in AGN. ‘Disk reverberation mapping’, the study of the time delays between light curve measurements of physically-connected emission regions associated with the central supermassive black hole, is a proven observational test of accretion disc structure (Blandford and McKee 1982, ApJ, 255, 419; Peterson and Horne 2004, AN, 325, 248). Such studies, often quasi-simultaneous with X-ray or UV observations, probe the unresolved accretion structure and the broad line region, allowing the investigation of links between black hole mass, mass accretion rate and disk geometry (see, e.g. Starkey+ 2017, ApJ, 835, 65).

Credit: Aurore Simonnet, Sonoma State University

agn_structure.png

An illustration of an Active Galactic Nucleus showing the central black hole, accretion disk, dusty torus and jet.

When an AGN is orientated on the sky so that the jet is pointed towards the observer, the object is called a blazar. The jets are threaded with magnetic fields which create polarised light when charged particles spiral around the field lines. Polarimetric monitoring of blazars has also been a productive area for the LT; for example the campaign surrounding the 2015 outburst of OJ287 enabled the measurement of the rotation rate of the black hole, and the confirmation of the loss of orbital energy to gravitational waves within two per cent of the prediction from General Relativity; the first indirect evidence for the existence of a massive spinning black hole binary emitting gravitational waves (Valtonen+ 2016, ApJ, 819, L37). This is an important result in the context of future Pulsar Timing Array efforts to directly detect gravitational waves from such systems. A typical observing strategy for AGN monitoring campaigns might involve a daily observing cadence over many months, with each visit to the target only minutes in length. The flexibility and low overheads (rapid target acquisition) from a robotic telescope clearly make it a better option for such campaigns compared to a visitor or service mode observatory. NRT will greatly improve the scope of studies such as these: long term monitoring campaigns are excellent ‘observing queue fillers’ for a telescope focused on target of opportunity science, and the NRT design goal of a world leading acquisition time will greatly improve the efficiency of short visit, long term monitoring programmes, enabling larger samples of objects to be pursued.

tde.jpg

An artist's impression of a tidal disruption event; a star is torn apart by the tidal forces of a black hole.

Tidal disruption events (TDEs) are events in which stars are torn apart by tidal forces near supermassive black holes. Candidate flares need to be classified in order to catch the rising emission and constrain the time of disruption, and then regular monitoring over the decay (lasting tens to hundreds of days) in order to constrain the models of mass accretion rate. Discovery channels for new events are UV/optical or X-ray surveys, but the two populations show differences: many of the UV/optically detected events do not produce X-rays. Explanations include the reprocessing (from soft X-ray to UV/optical) of photons in an optically thick shell of material (e.g. Guillochon+ 2014, ApJ, 783, 23) or that the UV/optical photons are produced by interactions in the debris stream (e.g. Piran+ 2015, ApJ, 806, 164). The number of known events has been small, but the capabilities of modern surveys are rapidly moving this field from single-object to population studies. van Velzen+ (2020, arXiv:2001.01409) for example reported 17 new events recovered from the first 1.5 years of ZTF operations, and the yield from Rubin is anticipated to be of the order of 1000s per year (Bricman & Gomboc, 2020, ApJ, 890, 73). The key to unlocking this population is identification and rapid spectroscopic characterisation of candidates. Spectral line ratios depend on the radii of emitting regions (Roth+ 2016, ApJ, 827, 3) and blueshifts constrain the presence of outflows and winds (Holoien+ 2019, APJ, 880, 120). While these transients are often bright, a high S/N is required since they are located in the brightest regions of their host galaxies, and so the ‘background noise’ is high. Our proposed IFU-fed SPRAT-type classification spectrograph on a 4-metre aperture NRT is ideal for this task.

 [NASA/ESA/D. Player (STScI)]