Location: Park 2.01

S3 - Tuesday 14:00-15:40 (Martin Hardcastle)

Echo Tomography of Black Hole Accretion Flows in Active Galactic Nuclei


Keith Horne (SUPA Physics & Astronomy, University of St Andrews)

Echo tomography uses light travel time delays to resolve micro-arcsecond structure of black hole accretion flows in active galactic nuclei. By observing the spectral variations, using ground-based optical and space-based UV and X-ray telescopes, correlated variations at different wavelengths have time delay distributions that resolve the accretion flow on iso-delay paraboloids. By fitting the reverberating continuum at different wavelengths, we map the radial temperature profile of the accretion disc, thereby testing the standard accretion disc theory predicting T ~ r^{-3/4}. By fitting the reverberating doppler profiles of emission lines, we recover 2-dimensional velocity-delay maps of the photo-ionised emission-line regions, revealing structure inside a virial envelope that determines the black hole mass, disc-like kinematics, inclination, and radial ionisation structure. I will outline the tomography methods and present results from a series of recent intensive monitoring campaigns with the Hubble and Swift space telescopes and the Las Cumbres Observatory global robotic telescope network.

“Failed” tidal disruption events


Andrea Sacchi (Fisica e Astronomia, Università degli Studi di Firenze)

The process of tidal disruption of stars by a supermassive black hole (SMBH) provides luminous UV and soft X-ray flares with peak luminosities of $approx 10^{46}$ ergs/sec and duration of a few months. As part of a wider exploration of the effects of stellar rotation on the outcome of a TDE, we have performed hydrodynamical simulations of the disruption of a rotating star whose spin axis is opposite to the orbital axis. Such a retrograde rotation makes the star more resilient to tidal disruption, so that, even if its orbit reaches the formal tidal radius, it actually stays intact after the tidal encounter. However, the outer layers of the star are initially stripped away from the core, but then fall back onto the star itself, producing a newly formed accretion disc around the star. We estimate that the accretion rate onto the star would be strongly super-Eddington (for the star) and would result in an X-ray flare with luminosity of the order of $approx 10^{40}$ ergs/sec and duration of a few months. We speculate that such events might be responsible for the known X-ray flares from Sgr A* in the recent past.

Testing accretion disc theory with high-cadence reverberation mapping


Juan Hernandez Santisteban (University of St Andrews)

Reverberation mapping of Active Galactic Nuclei has proven to be a fantastic technique to probe to the geometry of the accretion flow around the super massive black hole (SMBH) and its impact on the accretion due to the ever-changing inner X-ray source. This is performed by correlating the variability between energy bands, as the X-rays are reprocessed at larger radii of the accretion disc in timescales of days.
Similar studies have already raised problems with this simple picture as discrepancies between the measured size of the accretion disc differs from its theoretical expectations by factors of ~3 and a puzzling disconnectedness with the X-rays. Here we present the first results of a multi-mission 1.7 yr observational campaign using Swift and Las Cumbres Observatory (LCO) to study Fairall 9 —a Seyfert 1 AGN— in 13 photometric bands, spanning from X-rays to optical wavebands, in an unprecedented sub-daily cadence. The combination of high cadence photometry and a low-extinction AGN such as Fairall 9 allows us to test the standard accretion disc model via the delay spectrum and the variable component SED.

S4 - Tuesday 16:10-17:50 (Martin Hardcastle)

10,000 solar mass black hole in a nearby dwarf galaxy NGC 4395


Jong-Hak Woo (Physics and Astronomy Division, Seoul National University )

We present our reverberation mapping measurements of the lowest-luminosity Seyfert 1 galaxy NGC 4395.
The time delay of the H alpha line emission with respect to the V-band continuum is measured as 83+-14 min.,
which is shortest among the light echo measurements based on the Balmer emission lines.
Combining with the H alpha velocity dispersion 426+-1 km/s, we determine the mass of the central black hole
as 10,000 solar mass, which is the smallest reverberation-based mass, and comparable to the low mass
end of heavy seed primordial black holes. NGC 4395 follows the extension of the M- sigma relation defined
by supermassive black holes and massive galaxies, suggesting that the relation requires no hierarchical galaxy assembly
or AGN feedback. The BLR size – luminosity relation may extend down to L5100
= 10^40 erg/s. However, the clear offset of NGC 4395 indicates
a large intrinsic scatter of the relation, suggesting that indirect black hole mass estimates based on the
size-luminosity relation are much more uncertain than previously considered.

What triggers AGN – Prevalence of neutral gas in centres of galaxy mergers


Rajeshwari Dutta (European Southern Observatory)

“What role do galaxy mergers play in funneling large quantities of gas to the central regions of galaxies? What is the connection between AGN (its trigger and feedback) and gas properties in the circumnuclear region?” These are some of the important questions to be answered in order to understand galaxy formation and associated feedback in the Universe. We attempt to address these questions using HI 21-cm absorption line search in a sample of z<=0.2 galaxy mergers hosting radio-loud AGN. We find statistical evidence for the presence of large amount of neutral gas in the centres of such mergers compared to control sample of non-mergers. In addition, using multi-wavelength data, we investigate the connection of the prevalence and kinematics of cold HI gas in the central regions with the galaxy properties, in order to check if merger-induced gas inflows can trigger AGN activity.

Phenomenology of Axion-Like Particles Coupling with Photons in the Jets of Active Galactic Nuclei


Ahmed Ayad Mohamed Ali (Physics, University of the Witwatersrand)

Axions or more generally axion-like particles (ALPs) are pseudo-scalar particles predicted by many extensions of the Standard Model of particle physics (SM) and considered as highly viable candidates for dark matter (DM) in the universe. If they exist in nature, they are expected to couple with photons in the presence of an external electric or magnetic field through a form of the Primakoff effect. In addition, many sting theory models of the early universe motivate the existence of a homogeneous Cosmic ALP Background (CAB) arising via the decay of string theory moduli in the very early universe. Oscillations between photons and CAB ALPs traveling in intergalactic magnetic fields have been invoked to explain a number of astrophysical phenomena, or used to constrain ALP properties using observations. In this work, we examine the detectability of signals produced by ALP-photon coupling in the highly magnetized environment of the relativistic jets produced by active galactic nuclei (AGNs). Furthermore, we test the scenario that the Coma cluster soft X-ray excess can be explained due to CAB ALPs conversion into photons in the cluster magnetic field in the environment of the M87 AGN jet. Then, we demonstrate the potential of the AGN jet environment to probe low-mass ALP models, and to potentially constraint the model proposed to better explain the Coma cluster X-ray excess.

Multi-wavelength study of large-scale outflows from the Circinus galaxy


Rozeena Ebrahim (Physics, University of the Witwatersrand)

The Circinus galaxy is a composite starburst/Seyfert galaxy which features kpc scale radio lobe outflows along its minor axis. It is located 4 Mpc away, which makes it a unique target to study the physical nature of these outflows. Our task will be to investigate whether this high energy phenomenon originates from nuclear star formation activity or jets from an active galactic core. The MeerKAT array performed 5 arcsecond resolution radio observations, which is in the observed range of the arcminute lobes of the Circinus galaxy. In this work, the result of the MeerKAT radio observations of the lobes will be presented. This will be followed by comparisons with gamma-ray studies of the lobes from Fermi-LAT data. The results of this multi-wavelength study can later be compared to the star-formation driven Fermi bubbles in the Milky Way, which have also been observed in both the gamma-ray and the radio bands, to determine any possible connections to these structures.

A definitive merger-AGN connection at z∼0 with CFIS: mergers have an excess of AGN and AGN hosts are more frequently disturbed.


Akshara Viswanathan (Physics and Astronomy, University of Glasgow)

The question of whether galaxy mergers are linked to the triggering of active galactic nuclei (AGN)continues to be a topic of considerable debate. The issue can be broken down into two distinct questions: 1) Can galaxy mergers trigger AGN? 2) Are galaxy mergers the dominant AGN triggering mechanism? A complete picture of the AGN-merger connection requires that both of these questions are addressed with the same dataset. In previous work, we have shown that galaxy mergers selected from the Sloan Digital Sky Survey (SDSS) show an excess of both optically-selected, and mid-IR colour-selected AGN, demonstrating that the answer to the first of the above questions is affirmative. Here, we use the same optical and mid-IR AGN selection to address the second question, by quantifying the frequency of morphological disturbances in low surface brightness r-band images from the Canada France Imaging Survey (CFIS). Only ∼30 per cent of optical AGN host galaxies are morphologically disturbed, indicating that recent interactions are not the dominant trigger. However, almost 60 per cent of mid-IR AGN hosts show signs of visual disturbance, indicating that interactions play a more significant role in nuclear feeding. Both mid-IR and optically selected AGN have interacting fractions that are a factor of two greater than a mass and redshift matched non-AGN control sample, an excess that increases with both AGN luminosity and host galaxy stellar mass.

S5 - Wednesday 14:00-15:40 (Martin Hardcastle)

Unraveling the Physics of Quasar Jets


Eric Perlman (Aerospace, Physics & Space Sciences, Florida Institute of Technology)

Since the launch of Chandra twenty years ago, one of the greatest mysteries surrounding Quasar Jets is the production mechanism for their extremely high X-ray luminosity. Two mechanisms have been proposed. In the first view, the X-ray emission is inverse-Comptonized CMB photons. This view requires a jet that is highly relativistic (bulk Lorentz factor >20-40) on scales of hundreds of kiloparsecs, and a jet power comparable to or higher than the black hole’s Eddington luminosity. The second possibility is synchrotron emission from a high-energy population of electrons. This requires a much less powerful jet that does not need to be relativistically beamed, but it imposes other extreme requirements, namely the need to accelerate particles to >100 TeV energies at distances of hundreds of kiloparsecs from the active nucleus. We are exploring these questions using a suite of observations from a diverse group of telescopes, including the Hubble Space Telescope, Chandra X-ray Observatory, Fermi Gamma-ray Space Telescope and various radio telescope arrays. Our results strongly favor the origin of the X-ray emission in synchrotron radiation from a second, high-energy electron population. We discuss the observations, results and new questions brought up by these surprising results. We investigate the physical processes and magnetic field structure that may help to accelerate particles to such extreme energies, and reassess the expected emission from active galaxies at TeV energies. We also reassess the kinetic energy flux from these most powerful, relativistic outflows, which has important implications for feedback onto the most massive clusters of galaxies.

Anisotropic Inverse Compton model for the gamma-ray emission from large scale jet in radio galaxy


Wlodek Bednarek (Department of Astrophysics, University of Lodz)

TeV gamma-ray emission is observed from a few nearby radio galaxies, inclined at a relatively large angle in respect to the direction towards the observer. In fact, Cen A has been recently shown to emit TeV gamma-rays at the distance of a few kpc scale from the jet base. We consider an anisotropic inverse Compton model in which relativistic electrons in the mildly relativistic intermediate scale jet comptonize collimated soft radiation produced in the inner regions of the relativistic jet. Due to the geometry of the scattering process, such scenario predicts strong TeV gamma-ray emission at large angle to the jet axis. We investigate the distribution of the TeV emission along the jet for models for the jet structure. The clear predictions of such natural scenario should be tested by the future observations with the next generation Cherenkov telescopes allowing to constrain the high energy content of kpc scale jet in Cen A.

Quasi-periodic behaviour in the optical and gamma-ray light curves of blazars 3C 66A and B2 1633+382


Jorge Otero Santos (Instituto de Astrofisica de Canarias (IAC))

Blazars are radio-loud Active Galactic Nuclei whose relativistic jets point close to our line of sight. This peculiar orientation leads to the observation of the Doppler boosted emission. This type of objects emit through the entire electromagnetic spectrum from radio to gamma rays and display strong variability at different timescales. Variability studies have identified in the past hints of quasi-periodic emission for different sources, although strong evidences are still lacking. The understanding of whether this emission is truly periodic or not can help to explain the structure of blazars and the emission mechanisms taking place in these objects.
In this talk, I will present the results of the periodicity analysis in optical band for the blazars 3C 66A (z=0.444) and B2 1633+382 (z=1.814). Three well-known tools commonly applied in periodicity studies have been used: the Lomb-Scargle periodogram, the z-transform Discrete Correlation Function and the Weighted Wavelet z-transform. We have found significant evidences of long-term periodicity for both sources, with periods of 2-3 years. The study was complemented with the periodicity analysis of the gamma-ray light curves and their correlation with the optical wavelengths. No significant periodic emission in the gamma ray band was found. The results will be interpreted within the framework blazar emission models.



Emmanuel UWITONZE (Physics, Mbarara University of Science and Technology)

Blazars are radio-loud AGNs with relativistic jets oriented towards the observer’s line of sight. Based on their optical spectra, blazars may be classified as flat-spectrum radio quasars (FSRQs) or
BL Lacs. Recent studies show that blazars dominate (~93%) the already-identified EGRET sources, implying that among the unidentified EGRET sources there could still be faint blazars. The aim of this study was to determine the K4000 break for a selected sample among the potential blazar candidates from unidentified EGRET sources to confirm their blazar nature. This study used three blazar candidates, 3EG J1800-0146, 3EG J1709-0817 and 3EG J0159-3603 associated with radio counterparts, J1802-0207, J1713-0817 and J0156-3616, respectively. Their optical counterparts were obtained through spectroscopic observations using Robert Stobie spectrograph (RSS) at the Southern African Large Telescope (SALT) in South Africa. The observed Ca II H & K lines depression at 4000Å, in spectra of 3EG J1800-0146 and 3EG J1709-0817 show a shallow depression K4000=0.35 ± 0.04 and 0.24 ± 0.01, respectively and a deep depression K4000 = 0.54 ± 0.03 for 3EG J0159-3603. The spectra of 3EG J1800-0146 and 3EG J1709-0817 resemble that of BL Lacs with redshifts z = 0.165 and 0.26, respectively. This suggests the presence of a strong non-thermal optical contribution in these sources. On the other hand, the measured redshift for 3EGJ0159-3603 was found to be slightly small, i.e., z = 0.09. Although this confirms its extragalactic nature, the depression observed in the spectrum of 3EG J0159-3603 is not consistent with what is expected for a non-thermal source (blazar), implying that this source may not be a blazar.

S6 - Wednesday 16:10-17:50 (Martin Hardcastle)

A comprehensive power spectral density analysis of astronomical time series: the gamma-ray light curves of selected Fermi blazars


Natalia Zywucka-Hejzner (North West University)

We present results of Fermi-Large Area Ttelescope (LAT) light curve (LC) modelling of eight selected Fermi blazars. All blazars possess densely sampled and long-term (~10 years) gamma-ray data. For each blazar we generated three LCs with 7, 10, and 14 days binning, using the newest Fermi-LAT 8-year Source Catalog and binned analysis provided within the fermipy package (which follows the Fermi Science Tools).

The LCs were modelled with several tools: the Fourier transformation and the Lomb-Scargle periodogram (LSP) to construct the power spectral densities (PSDs) and to fit the power law model with the Poisson noise level, the autoregressive moving average (ARMA), the fractional autoregressive integrated moving average, and the continuous-time autoregressive moving average (CARMA) processes, the Hurst exponents and the A-T plane to classify the time series, and the wavelet scalogram.

The power law PSD is indicative of a self-affine stochastic process characterised by the Hurst exponent H, underlying the observed variability. An estimation of the H values was performed with a wavelet lifting transform. We find that all blazars have H > 0.5, indicating long-term memory. The power law spectral indices calculated from the Fourier and the LSP modelling are consistent for the majority of blazars but Mrk 501 and B2 1520+31. In case of the former object all spectra modelled with the LSP are softer, while for the latter the situation is opposite, i.e. all LSP spectra are harder. The ARMA and CARMA results are consistent with each other, reaching higher orders for 7 days binned LCs and lower orders for 10 and 14 days binned LCs for the majority of blazars. The higher-order ARMA/CARMA fits suggest the additional variations present in blazar jets and/or accretion discs that affect the overall shape of the PSD.

Shadow of rotating Black hole


Farruh Atamurotov (INHA University in Tashkent)

The investigation of the optical properties and the spacetime structure of the black hole and construction of new tests of general relativity and other gravity theories are now one of the most important tasks in modern relativistic astrophysics. Theoretical study of the shadow of the black hole in the various gravity theories and study the gravitational lensing contribute an important tool to build the tests of the gravity models using the information events on the central object in our galaxy and M87 within the Black Hole Cam (BHC) and Event Horizon Telescope (EHT) International projects and understand the fundamental properties of the gravity.
It was shown that with increasing deformation parameter of non-Kerr black hole the shadow of the shape changes. This phenomena also related to the fact that the increase of deformation parameter forces light rays orbits to come closer which corresponds to the decrease of gravitational force acting on light rays. It was obtained that the deformed rotating black hole’s shadow is also going to be deformed independently on sign of deformation parameter. A comparison of the obtained theoretical results on the shadow of black hole size with the observational data (for Rs=6M) in the EHT project provides the upper limit for the dimensionless deformation parameter as |ε|<80 and gravitomagnetic charge l/M<0.85.

Simulations of electromagnetic cascades in Kerr black hole magnetospheres


Benjamin Crinquand (IPAG , Université Grenoble Alpes)

A variety of astrophysical phenomena can only be explained as being powered by black holes. In particular, accreting supermassive black holes are responsible for launching relativistic plasma jets and for accelerating ultra energetic particles. The mechanism that channels energy from the black hole to the particles remains a mystery.

Observations have come to help lately. The Event Horizon Telescope collaboration has been able to image the shadow of the supermassive black hole M87*, suggesting that it might be spinning. The GRAVITY collaboration detected a hot spot in infrared orbiting Sgr A*, indicating the presence of a large scale poloidal magnetic field. These observations give new clues to constrain theoretical models that are able to explain jets and particle acceleration.

This problem involves complex interactions between collisionless plasma dynamics in high gravitational field, and pair creation due to the interaction of energetic particles with surrounding photons from the accretion flow. Only kinetic simulations can capture all these effects.

In this talk, I will present General Relativistic Radiative Particle-in-Cell (GRRPIC) simulations of an axisymmetric black hole magnetosphere possessing a monopolar magnetic field, embedded in a soft background radiation field. These simulations use newly developed radiative transfer techniques. They show for the first time the development of a time-dependent electromagnetic cascade, which could explain the high variability of high-energy emission from Sgr A* and M87*, and plasma loading at the base of relativistic jets.