PhD Colloquia

This week's PhD colloquia are highlighted.

Date	Time	Title/Abstract	Speaker	Affil.
06/11 Wednesday BW.0.32	14:15	On a quest to discover where stellar-mass black holes merge: Testing the AGN binary formation channel with spatial correlation analyses The physical origin of the dozens of stellar-mass binaries, the mergers of which have been detected by the LIGO-Virgo-KAGRA collaboration via gravitational waves, is still unknown. The accretion discs that fuel Active Galactic Nuclei (AGN) represent potential sites of efficient binary formation, due to the interaction between the compact objects and the gaseous environment. I will present the results contained in my thesis, which represent the first observational constraints on the fractional contribution of the so-called "AGN channel" to the total merger rate. These results were obtained by investigating whether there is or not a spatial correlation between the localisation volumes of gravitational-wave detections and the positions of potential hosts. We find that currently available data enable methods like the one discussed in my thesis to put constraints as stringent as the ones that come from analyses that require assumptions on the physical model of the binary formation scenario. We conclude, at a 95 per cent credibility level, that no more than one every five detected gravitational wave event has originated in an un-obscured AGN brighter than 10^44.5 erg/s.	Nicco Veronesi	Leiden Observatory
08/11 Friday BW.0.32	09:30	From Atoms to the Cosmos: Exploring the cosmic web beyond collisional ionisation equilibrium In this talk I will give an overview (with focus on X-ray wavelengths) of the cosmic web filaments, more specifically I will talk about the warm-hot intergalactic medium (WHIM), which has typically temperatures between 10^5 - 10^7 K, and densities between n_e ~ 10^{-6} - 10^{-4} cm^{-3}. The detection of the cosmic web filaments still remains a big challenge with current X-ray missions. The most common techniques how to observe the faint and extremely tenuous plasma in these filaments are either in emission, or in absorption against very bright, point-like sources. Many of these observations are, however, of a low statistical significance or they are biased towards only the densest parts of WHIM. With the upcoming era of the X-ray micro-calorimeters, kick-started by Hitomi, which offer an unprecedented spectral resolution for even the diffuse sources, many of the second order effects that could have not yet been studied, will become feasible for the detection. During this talk I will present two of such second order effects which belong to the findings I reported during my PhD studies. First example is how the galaxy cluster photons alter the ionization state of nearby WHIM and cause an extra photoionisation which can affect the WHIM column densities of e.g. O VI, O VII, C V, or Ne VIII. Secondly, I will discuss a new observational method for the feasibility studies of detecting the circum- and intergalactic medium in X-ray absorption using the extended intracluster medium as a backlight. In this paper I worked with the cosmological hydrodynamical simulations called Hydrangea and I reported the feasibility of the WHIM detection in OVII and in OVIII by creating the mock observations with Athena’s X-IFU (X-ray Integral Field Unit) and LEM’s IFU micro-calorimeters. I will also introduce a “clus” model, which has been newly implemented to the plasma code SPEX, and I will report on the findings of my paper where I suggest a potential explanation of the phenomenon called “lack of metals” which is observed in some (not all!!) galaxy clusters, galaxy groups, or massive elliptical galaxies. Last but not least, I will discuss the importance of updating different plasma codes and I will talk about my contribution to the radiative loss curve and the updates to the cooling curve in the photoionisation model of SPEX and show the comparison to other plasma codes (MEKAL, APEC, Cloudy) and other atomic databases (CHIANTI, ADAS).	Lydia Stofanova	Leiden Observatory/SRON

For questions and/or suggestions concerning the colloquium series. Please contact Andrew Sellek (e-mail ).