Seminars in 2020


Michaela Kraus & Rhys Taylor

Joint journal club

Michaela Kraus will present the following paper: "The evolution of red supergiant mass-loss rates" Beasor et al., 2018, MNRAS 475, 55 and Rhys Taylor will talk about this paper on the size of Ultra Diffuse Galaxies possibly being hugely overestimated: "Are ultra-diffuse galaxies Milky Way-sized?" Chamba et al., 2020, A&A 633, L3


Francesca Panessa

Restarting radio activity in a hard X-ray selected sample

Radio activity is episodic in nature and radio galaxies witness such activity. However, the duration and the duty cycle of the jet activity is not known, notwithstanding its importance in the context of radio galaxy evolution and feedback to the environment. Due to their large scale and old age, Giant Radio Galaxies (GRG) are ideal targets to study the duty cycle of radio activity. However, GRG usually constitute only a small fraction of sources in radio surveys and a systematic study of the occurrence of restarting activity is still missing. Cross-correlating the INTEGRAL+Swift AGN population with radio catalogues (NVSS, FIRST, SUMSS), we found that 22% of the sources are GRG (a factor four higher than those selected from radio catalogues). Remarkably, all of the sources in the sample show signs of restarting radio activity. The X-ray properties are consistent with this scenario, the nuclei being in a high-accretion, high-luminosity state. I will discuss in details the multi-frequency evidence of restarting activity in this sample.


Julieta Sanchez & Pavel Jachym

Joint Journal Club

Meeting ID: 96137754167 Password: 7089 Meeting link: Scheduled time: 30/04/2020 10:30 CEST (08:30 UTC) Duration: 150 minutes Julieta will present the paper: "Low-frequency gravity waves in blue supergiants revealed by high-precision space photometry by Bowman et al., Pavel will review the paper: "A Link Between Ram Pressure Stripping and Active Galactic Nuclei" by Ricarte et al.,


Riccardo Arcodia

Do stellar-mass and super-massive black holes have similar dining habits?

We compare the relationship between the luminosity of the accretion disc and corona in AGN and X-ray binaries in their radiatively efficient (and non-jetted) phases. The observed scatter in the log Ldisc - log Lcorona plane of XRBs is high (~0.43 dex) and significantly larger than in AGN (~0.30). On the other hand, we also find that XRBs and AGN show different accretion rate and power-law index distributions, with the latter in particular being broader and softer in XRBs. Remarkably, once similarly broad photon index and accretion rate distributions are selected, the AGN sample overlaps nicely with XRBs observations in the mass-normalised log Ldisc - log Lcorona plane, with a scatter of ~0.30-0.33 dex in both cases. This indicates that a mass-scaling of properties might hold after all, with our results being consistent with the disc-corona systems in AGNs and XRBs exhibiting the same physical processes, albeit under different conditions for instance in terms of temperature, optical depth and/or electron energy distribution in the corona, heating-cooling balance, coronal geometry and/or black hole spin.


Duccio Macconi

Radio Galaxies flavours: how accretion and environment can make the difference

In radio galaxies a correlation between accretion onto supermassive black hole and jet production is expected from both theoretical and obervational works. However, there is a population of radio galaxies that seems to break the classical accretion-ejection scheme. They are defined FRII-LERG and are characterized by strong jets (typical of FRII sources) up to hundreds of kpc, but inefficient accretion engine on pc-scales, testified by their NLR optical spectra. In order to understand their nature, in our work we analyzed all the FRII-LERG sources (19) belonging to the 3CR catalog with X-ray data available (both Chandra and XMM-Newton) with z<0.3. We then analyzed all classical FRII (32) from the same catalog classified both in radio and in optical band as a control sample. We compared X-ray results of FRII-LERG with classical FRII and also FRI, which data are taken from literature. Hence, we matched X-ray analysis with radio and optical data available in literature for all three populations. From our work two scenarios are the most plausible to explain FRII-LERG nature: they could be evolved FRII or they could be a distinct class of sources, inhabiting intermediate environments. Surprisingly, in the latter direction we found a general anti-correlation between accretion rate (measured both from optical and X-ray band) and extra-galactic environment.


Chris Done

AGN spectra and the soft X-ray excess

I will discuss how AGN spectra evolve with L/LEdd, including changing look episodes, and superEddington flows. I will especially focus on the 'soft X-ray excess' component and its connection to the UV downturn in AGN, showing how we can use results from the multiwaveband intensive monitoring campaigns to understand the origin of the UV and soft X-ray emission.


Brankica Kubátová and Rhys Taylor

Joint Journal Club

Brankica Kubátová will present the following paper: Björklund, R.; Sundqvist, J. O.; Puls, J.; Najarro, F., 2020, A&A, accepted "New predictions for radiation-driven, steady-state mass-loss and wind-momentum from hot, massive stars II. A grid of O-type stars in the Galaxy and the Magellanic Clouds" and Rhys Taylor will present: Sungsoon Lim et al., 2020, ApJ, accepted "The Next Generation Virgo Cluster Survey (NGVS). XXX. Ultra-Diffuse Galaxies and their Globular Cluster Systems"


Johannes Buchner

How granular is the obscurer of Active Galactic Nuclei?

Most active galactic nuclei are seen through thick circum-nuclear gas and dust. We would like to understand the origin and geometry of this obscurer. Is this the gas feeding the super-massive black hole? How is it related to the host galaxy gas? I will discuss new approaches to constrain the location, sub-structure and physics of the obscurer, including the search for occultation events with the new X-ray telescope eROSITA and contrasting X-ray reflection spectroscopy from NuSTAR with the open-source XARS modelling framework.


Shifu Zhu

The Lx–Luv–Lradio relation and corona–disc–jet connection in optically selected radio-loud quasars

Radio-loud quasars (RLQs) are more X-ray luminous than predicted by the Lx-Luv relation for radio-quiet quasars (RQQs). The excess X-ray emission depends on the radio-loudness parameter (R) and radio spectral slope (alpha_r). We construct a uniform sample of 729 optically selected RLQs with high fractions of X-ray detections and alpha_r measurements. We find that steep-spectrum radio quasars (SSRQs) follow a quantitatively similar Lx-Luv relation as that for RQQs, suggesting a common coronal origin for the X-ray emission of both SSRQs and RQQs. However, the corresponding intercept of SSRQs is larger than that for RQQs and increases with R, suggesting a connection between the radio jets and the configuration of the accretion flow. Flat-spectrum radio quasars (FSRQs) are generally more X-ray luminous than SSRQs at given Luv and R, likely involving more physical processes. The emergent picture is different from that commonly assumed where the excess X-ray emission of RLQs is attributed to the jets. We thus perform model selection to compare critically these different interpretations, which prefers the coronal scenario with a corona-jet connection. A distinct jet component is likely important for only a small portion of FSRQs. We suggest that RLQs and RQQs are in different accretion states, similar to those of microquasars when their ballistic jets are either active or quenched. The corona-jet, disk-corona, and disk-jet connections of RLQs are likely driven by independent physical processes. Furthermore, the corona-jet connection implies that small-scale processes in the vicinity of SMBHs, probably associated with the magnetic flux/topology instead of black-hole spin, are controlling the radio-loudness of quasars.


Pratik Dabhade

Project SAGAN- Search and analysis of giant radio galaxies with associated nuclei

We present the first results of a project called SAGAN, which is dedicated solely to the studies of relatively rare megaparsec-scale radio galaxies in the Universe, called giant radio galaxies (GRGs). We have identified 162 new GRGs primarily from the NVSS with sizes ranging from ~0.71 Mpc to 2.82 Mpc in the redshift range of ~0.03 - 0.95, of which 23 are hosted by quasars (giant radio quasars, GRQs). As part of the project SAGAN, we have created a database of all known GRGs, the GRG catalogue, from the literature (including our new sample); it includes 820 sources. For the first time, we present the multi-wavelength properties of the largest sample of GRGs. Our results establish that the distributions of the radio spectral index and the black hole mass of GRGs do not differ from the corresponding distributions of normal-sized radio galaxies (RGs). However, GRGs have a lower Eddington ratio (ER) than RGs. Using the mid-infrared data, we classified GRGs in terms of their accretion mode: either a high-power radiatively efficient high-excitation state, or a radiatively inefficient low-excitation state. We find that GRGs in high-excitation state statistically have larger sizes, stronger radio power, jet kinetic power, and higher ER than those in the low-excitation state. Our analysis reveals a strong correlation between the ER and the scaled jet kinetic power, which suggests a disc-jet coupling. Our environmental study reveals that ~10% of all GRGs may reside at the centres of galaxy clusters, in a denser galactic environment, while the majority appears to reside in a sparse environment. We find that the probability of BCG as a GRG is quite low. We present new results for GRGs that range from black hole mass to large-scale environment properties. We discuss their formation and growth scenarios, highlighting the key physical factors that cause them to reach their gigantic size.


Xiaolong Yang

Exploring the state transition in AGNs

It was long believed that active galactic nuclei (AGNs) will experience accretion state transition similarly with X-ray binaries (XRBs), but with substantially longer timescales, generally lasting millions of years. It is thus hard (actually impossible) to observe a whole burst cycle in AGNs. Alternatively, the statistical similarity of XRGs and AGNs has been explored in recent years. In this colloquium, I will talk about the unification scheme of the accretion process from stellar-mass black holes to supermassive black holes. In recent works, we have studied a sample of super-Eddington accreting AGNs and found an overall inverse correlation between radio loudness and Eddington ratio from the sub- to the super-Eddington regime, indicating the similar radio properties in XRBs and AGNs. Another work from our group introduced a merge triggered accretion state transition model, suggesting that LERG and HERG connected with FRI and FRII radio galaxies, respectively, and the X-shaped radio galaxies act as the transition type. Finally, I will briefly introduce our ongoing projects to understand accretion state transition in AGNs, including milli-arcsec scale VLBA observations of super-Eddington AGNs, a joint X-ray and radio (VLBI) observational campaign of a changing-look AGNs when it is exactly in process of changing, and the possible explanations by invoking the state transition scheme.


Emily Kosmaczewski

Properties of a Sample of the Most Compact Radio Galaxies

We present an overview of the properties of a set of the most compact radio galaxies, specifically 29 sources identified as Gigahertz Peaked Spectrum (GPS) and/or Compact Symmetric Objects (CSOs). We investigate their multi-wavelength properties to determine the regularity of these sources in galactic systems. In our sample of 29 objects, we find that the host galaxies are predominantly red/yellow ellipticals, with some of them displaying distorted morphology. We find a variety of MIR colors, and observe that the sources in which the MIR emission is dominated by the ISM component, uniformly populate the region occupied by galaxies with a wide range of pronounced star formation activity. We compare the MIR color distribution in our sample to several other AGN samples, and conclude that the triggering of radio jets in AGN does not differentiate between elliptical hosts with substantially different fractions of young stars. Instead, there appears to be a relationship between the jet duty cycle and ongoing star formation. We comment on the two gamma-ray detected sources in our sample, 1146+596 & 1718--649 and their star formation rates. Finally, we discuss next steps for this work, and the data required to form more definite conclusions.


Dr. Raneri Baldi

The multi-band properties of FR0 radio galaxies

Radio galaxies (RGs) are active galactic nuclei (AGN) able to launch relativistic jets, the most energetic phenomena in the Universe. They are typically associated with giant elliptical galaxies hosting the most massive black holes (> 10^8 solar masses). In the local Universe (z<0.3) the optical classification of RGs reflects a clearer separation in nuclear and host properties than that based on the radio morphology. However, the results from my mutli-band studies of radio-loud AGN demonstrates that the current picture of the RG population is incomplete and biased. Indeed, moving to low-luminosity AGN, the bulk of this population is dominated by objects which differ from powerful RGs, by showing compact radio structures and more heterogeneous host properties: the FR0s, which lack of large scale (>10 kpc) radio structures and represent the most abundant population of RGs in the local Universe. Considering their properties, I will speculate about the possible origins of FR0s and the possible cosmological scenarios they imply.


Kathryn Ross

The Mystery of Spectral Variability at Low Frequencies

Spectral variability of radio sources encodes information about the conditions of intervening media, source structure and emission processes. With new low-frequency radio interferometers observing over wide fractional bandwidths, studies of spectral variability for a large population of extragalactic radio sources are now possible. Using two epochs of observations from the GaLactic and Extragalactic All-sky MWA (GLEAM) survey that were taken a year apart, we search for spectral variability across 100-230 MHz for roughly 21,000 sources. In this talk, I present a new methodology for detecting variability in the spectrum between epochs and classifying the type of variability. We focus on peaked-spectrum sources in particular, analogues to gigahertz-peaked spectrum and compact steep-spectrum sources, and find they do not follow the same distribution of variability as the population of typical radio galaxies with power-law spectra. This talk will discuss the viability of several potential explanations of the observed spectral variability, such as interstellar scintillation and jet evolution.


Tonima Ananna

Using the Cosmic X-ray background to constrain AGN population synthesis model and X-ray spectra

As matter accretes onto the central supermassive black holes in active galactic nuclei (AGNs), X-rays are emitted. We present a population synthesis model that accounts for the summed X-ray emission from growing black holes; modulo the efficiency of converting mass to X-rays, this is effectively a record of the accreted mass. We need this population synthesis model to reproduce observed constraints from X-ray surveys: the X-ray number counts, the observed fraction of Compton-thick AGNs [log (N H/cm-2) > 24], and the spectrum of the cosmic X-ray background (CXB), after accounting for selection biases. Over the past decade, X-ray surveys by XMM-Newton, Chandra, NuSTAR, and Swift-BAT have provided greatly improved observational constraints. We find that no existing X-ray luminosity function (XLF) consistently reproduces all these observations. We take the uncertainty in AGN spectra into account and use a neural network to compute an XLF that fits all observed constraints, including observed Compton-thick number counts and fractions. This new population synthesis model suggests that, intrinsically, 50% ± 9% (56% ± 7%) of all AGNs within z ≃ 0.1 (1.0) are Compton-thick. As all population synthesis models are limited by the X-ray parameters assumed while producing these models, we further use the CXB to conclusively rule out regions of the X-ray parameter space that cannot produce the CXB for any population synthesis model, and present a web tool which can be used to interactively explore the AGN X-ray spectral parameter space.


Alberto Masini

The Chandra Deep-Wide Field Survey of the Bootes Field

In the past years, important insights in the field of AGN and galaxy co-evolution have come from deep and wide X-ray surveys of the sky, which provided highly reliable and complete samples to statistically investigate the cosmological evolution of the AGN population. The Chandra X-ray telescope has played a major role in driving such discoveries, with a huge time investment in some areas of the sky. However, the widest installment of the Chandra surveys — the Bootes field — has lagged behind in sensitivity. Recently, thanks to an ambitious large program, it was finally possible to update the Bootes field depth to the standard of other Chandra surveys. I will present the Chandra Deep-Wide Field Survey, which combined its 9.3 deg^2 area with a relatively deep exposure to robustly detect ~6900 X-ray point sources. Thanks to the rich multi-wavelength coverage of the field, counterparts for almost all the X-ray sources have been assigned. The high reliability of the X-ray catalog holds the promise to unveil new exciting discoveries in the near future, complementing what eRosita will accomplish at brighter fluxes.


Nicole Thomas




Michalis Kourniotis and Alex C. Gormaz-Matamala

Joint Journal Club

Michalis Kourniotis will present Matsukoba et al. (2020), MNRAS, accepted: Disk fragmentation and intermittent accretion onto supermassive stars and Alex C. Gormaz-Matamala will present Dessart & Hillier (2020), A&A Letter accepted: Radiative-transfer modeling of supernovae in the nebular-phase. A novel treatment of chemical mixing in spherical symmetry. Zoom link: Meeting ID: 82303403641 Password: 9055 Scheduled time: 12/11/2020 10:30 CET (09:30 UTC) Duration: 150 minutes Meeting link:


Claudio Ricci

The destruction and recreation of the X-ray corona in a accreting supermassive black hole

Accreting supermassive black holes (SMBHs) are known to show variable optical, ultraviolet and X-ray emission. One of the most intriguing aspects of this behaviour is associated with “changing- look’ sources, in which the optical/ultraviolet broad emission lines, produced by rapidly-moving material surrounding the SMBH, appear or disappear. In my talk I will discuss the drastic transformation of the X-ray properties of a nearby active galactic nucleus (AGN), following a changing-look event. After the optical/UV outburst the power-law component, produced in the X-ray corona, completely disappeared, and the spectrum instead became dominated by blackbody-like emission. This implies that the X-ray corona, ubiquitously found in AGN, was destroyed in the event. In my talk I will discuss in detail the results of our 450 days monitoring campaign on this source, and provide possible explanations for this phenomenon.


Nicole Thomas

The Radio Galaxy Population in the Simba Simulations

Essentially all massive galaxies host a supermassive black hole (SMBH). During strong periods of accretion, these SMBHs are identified as active galactic nuclei (AGN) and a small fraction of them emit jets which can be observed at radio wavelengths via synchrotron emission. These AGN, also called radio galaxies, are key contributors to the quenching of star formation in massive galaxies. However, the mechanisms driving the accretion and feedback processes of these objects are still poorly understood and defined. Simba is a suite of cosmological hydrodynamic simulations with a primary focus on galaxy evolution. Simba employs a two-mode sub-resolution prescription for accretion, namely, gravitational torque limited accretion from cold gas, and Bondi accretion from hot gas as well as feedback that physically corresponds to observed AGN. Using Simba we identify a population of jet-mode AGN and study their global properties in a comparison with observations. We show that Simba reproduces populations of high- and low excitation radio galaxies (HERGs and LERGs) and provide a state-of-the-art cosmological context for understanding radio galaxies in the era of the Square Kilometer Array.

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