Determining the inner geometry of AGN - i.e. the size and location of the central X-ray emitting corona relative to the accretion disc, the shape, size and structure of the disc, the location of the broad line region and its possible connection with disc winds, the location and structure of obscuring material - remains one of the main challenges of astrophysical research. Apart from M87 and...
AGN variability carries information about the geometry of the accretion flow which is usually unaccessible to direct imaging methods. In particular, the reverberation signals at optical wavelengths of reprocessed high-energy photons provide insight into the size of the disc itself as well as inclination, mass accretion rate and the temperature profile of the disc itself. Over the last decade,...
Over the last decade reverberation mapping (RM) campaigns of active galactic nuclei (AGN) have enabled us to probe their inner regions in unprecedented detail. Whilst observations have broadly confirmed that the short-term variability of the accretion disc is driven by variations in the X-ray corona a number of puzzles have also emerged, including: the contribution of the broad line region...
Quasars are a class of objects in the Universe with very apparent flux variation. UV/optical variability of such sources has attracted particular attention. The radiation in this band is thought to come from the accretion disk. As the central dynamical region of a quasar, the accretion disk is theoretically believed to be related to structures such as the corona and emission line region....
Reverberation mapping (RM) is a powerful tool to explore the unresolved central region of active galactic nucleus (AGN), e.g., accretion disk. Determining the structure of accretion disks in AGN is fundamental to understanding the growth of supermassive black holes, confirming the standard thin disk theory, and examining the X-ray reprocessing variability model. However, recent continuum RM...
I will discuss the use of longer timescale “negative” lags, where the variability in high frequency bands lags the corresponding variability at low frequency, as a probe of accretion disk structure. Traditional reverberation mapping uses lags of variations in AGN photometry from high frequency to low frequency wavebands on the light-crossing timescale which come from the reprocessing of light...
Recent multi-wavelength surveys of a few AGN have given us the opportunity to constrain well the cross-correlation between the X-rays and the UV/optical variations in these objects. The variations in the UV lead the variations detected at longer wavelengths in almost all cases where good quality light curves, in many wavebands, exist. However, there have been indications that the optical...
The variability of Active Galactic Nuclei (AGNs) has been studied for decades, with the UV/optical continuum observed to stochastically fluctuate at the 10% level over timescales of weeks to months. Fundamentally, this variability should be driven by temperature fluctuations in the accretion disc surrounding the central black hole. Where multiband lightcurves are available, the variability...
From optical to X-ray, the variable continuum emissions of AGN are generally found to be correlated with variations at longer wavelengths lagging the shorter ones. Both the correlation and the lag-wavelength relation are usually understood within the widespread X-ray reprocessing scenario. However, both of them do not always preserve and challenge the reprocessing scenario. In recent years, we...
Several active galactic nuclei (AGN) show UV/optical variability lagging behind the X-ray emission by a few days. The simplest and most straightforward interpretation is that the variable X-ray flux from the corona illuminates the accretion disc below where it is partially reflected and observed as fast X-ray reverberation signal, and partially absorbed and thermalised in the disc, which...
We measured the dust torus size of 86 quasars with bolometric AGN luminosity in the range 10^43.4 to 10^46.4 erg/s by determining the lag between the optical continuum emission obtained from ground-based optical surveys, i.e., CRTS, ASAS-SN, PTF and ZTF, and the mid-infrared continuum observed with the W1 and W2 bands from the Wide-field Infrared Survey Explorer (WISE) survey. By combining...
NGC 6814 is a nearby ($z = 0.005$) Seyfert 1.5 galaxy that we recently showed had undergone a rapid X-ray occultation event during an XMM-Newton observation from 2016. The X-ray eclipse of high column ($N_{\mathrm{H}} \approx 10^{23}~\mathrm{cm}^{-2}$), mildly ionised ($\log\xi \approx 1~\mathrm{erg~cm~s^{-1}}$) matter lasted ~45ks, with ingress and egress each lasting ~14ks, revealing a...
High frequency radio emission may originate from scales as small as the
innermost accretion disk, and can thus probe directly the relativistic electrons
and the magnetic fields in the coronal gas of radio quiet AGN.
I will present simulations of the time evolution of the distribution functions of
relativistic electrons following their injection due to a coronal reconnection event.
The...
