Seminars take place in the lecture room of the Spořilov building (see contacts) unless noted otherwise.

Scheduled seminars

Detection of polarized X-ray emission from the magnetar 4U 0142+61

Roberto Taverna on behalf of the IXPE team and its Magnetars Topical Working Group

Soft gamma repeaters and anomalous X-ray pulsars are believed to host a magnetar, an ultra-magnetized neutron star with surface magnetic field B ~ 10^14-10^15 G. We report on the first detection of linearly polarized X-ray emission from a magnetar with the Imaging X-ray Polarimetry Explorer (IXPE). The IXPE observation of the anomalous X-ray pulsar 4U 0142+61 reveals a linear polarization degree of (12±1)% throughout the instrumental working energy band 2-8 keV at 15σ confidence level. The IXPE measurement demonstrates the power of X-ray polarimetry, giving us completely new information about the properties of the neutron star surface and magnetosphere and lending further support for the presence of the quantum mechanical effect of vacuum birefringence.

Location: Seminar room 101

X-raying the winds of massive stars using high mass X-ray binaries

Victoria Grinberg

We are made of stardust—or, at least in significant parts, of material processed in stars. Hot, massive giant stars can drive the chemical evolution of galaxies and trigger and quench star formation through their strong winds and their final demise as supernovae. Yet optical and X-ray measurements of the wind mass loss strongly disagree and can only be reconciled if the winds are highly structured, with colder, dense clumps embedded in a tenuous hot gas. In (quasi-)single stars, however, wind properties are inferred for the whole wind ensemble only; no measurements of individual clumps or clump groups are possible, limiting our understanding of wind properties. Luckily, nature provides us with perfect laboratories to study clumpy winds: high mass X-ray binaries. The radiation from close to the compact object is quasi-point like and effectively X-rays the wind, in particular the clumps crossing our line of sight. In this talk, I will show how we can use a variety of observations of some of the brightest X-ray binaries to constrain wind properties. Low resolution, high cadence observations combined with simulations reveal the dynamics of clump movements and the large-scale wind structure. Time- and absorption-resolved high resolution X-ray spectroscopy reveals the composition of the multicomponent wind plasma, the layered temperature profile and comet-like structure of clumps. Future X-ray telescopes such as XRISM and Athena will revolutionise the field, allowing us to observe individual clumps in bright sources and, for the first time, make faint sources accessible for high resolution spectroscopy. This will provide us with a sample of HMXBs that will allow us to compare wind properties in massive stars of different stellar (sub-)types and at different radii, thereby directly testing theories of clumpy wind formation and evolution.

Location: Online (via Zoom)

Online connection link:


Sumanta Chakraborty


Location: Sporilov room 101

Accretion and outflows in stellar-mass black holes

Teo Muñoz-Darias

Black-hole (BH) transients are a type of X-ray binary in which a stellar-mass BH accretes material from a low-mass star via an accretion disc. They spend most part of their lives in a dim, quiescent state, but display powerful outbursts when their luminosity increases by up to seven orders of magnitude in all wavelengths. X-ray and radio observations performed during the last couple of decades have provided a rich data base on BH transients. A strong coupling between the properties of the accretion flow and the presence of outflows, such as radio-jets and hot X-ray winds, has been found to be a fundamental characteristic of these systems, and, to a great extend, of X-ray binaries in general. In addition to this, and particularly since the spectacular case of the 2015 outburst of the BH transient V404 Cygni, cold (optical/infrared) accretion disc winds have been discovered in several systems, with observables indicating that they also have a significant impact on the entire BH accretion process. I will review the state-of-the-art of this field, with emphasis on the studies that we are currently carrying out on these novel cold winds with a suite of the largest telescopes.

Location: Online via Zoom

Zoom connection link:

X-ray binary accretion states in AGN? Sensing the accretion disc of supermassive black holes with mid-IR nebular lines

Juan Antonio Fernández-Ontiveros

Accretion states, which are universally observed in stellar-mass black holes in X-ray binaries, are also anticipated in active galactic nuclei (AGN). This is the case at low luminosities, when the jet-corona coupling dominates the energy output in both populations. Previous attempts to extend this framework to a wider AGN population have been extremely challenging due to heavy hydrogen absorption of the accretion disc continuum and starlight contamination from the host galaxies. The luminosity-excitation diagram (LED), based on the [OIV]25.9µm and [NeII]12.8µm mid-IR nebular line fluxes, enables to probe the accretion disc contribution to the ionising continuum. When applied to a sample of 167 nearby AGN, the LED recovers the characteristic q-shaped morphology outlined by individual X-ray binaries during a typical accretion episode, allowing us to tentatively identify the main accretion states in supermassive black holes. The soft state would include broad-line Seyferts and about half of the Seyfert 2 population, showing highly excited gas and radio-quiet cores consistent with disc-dominated nuclei. The hard state mostly includes low-luminosity AGN (<10^-3 Ledd) characterised by low-excitation radio-loud nuclei and a negligible disc contribution. The remaining half of Seyfert 2 nuclei and the bright LINERs show low excitation at high accretion luminosities, and could be identified with the bright-hard and intermediate states. The hosts of hard-state AGN are mostly passive galaxies, whereas intermediate-state AGN exhibit substantial star formation activity in their central kiloparsecs. I will discuss the above scenario, its potential links with the galaxy evolution picture, and the possible presence of accretion state transitions in AGN, as suggested by the growing population of changing-look quasars.

Location: Online via Zoom

Zoom connection link:

If you would like to give a seminar, please contact Georgios Loukes-Gerakopoulos or Vladimír Karas.