Wednesday Colloquium


"Beam geometry for pulse modulation in radio pulsars"

Jarosław Dyks (CAMK, Toruń)

Radio pulsations of pulsars are modulated in a variety of ways, and the modulation patterns look truly enigmatic or striking. I will describe a recently proposed radio beam geometry that is capable of producing the main types of the observed behaviour.


"Dancing discs: Lense-Thirring precession and rapid variability > in truncated accretion flows"

Deepika Bollimpalli (Max Planck Institute, Garching)

Many accreting black holes and neutron stars exhibit rapid variability > in their X-ray light curves termed as quasi-periodic oscillations > (QPOs). The most common are the low-frequency (< 10 Hz), type-C QPOs > typically observed during the low/hard state and are thought to > originate from the Lense-Thirring precession of a hot, geometrically > thick accretion flow that is misaligned with respect to the black hole > spin axis. However, it is not yet clear how the coupling between the > outer geometrically thin disc and the inner hot flow may influence the > precession, and this has not been accounted for in any of the > currently existing models. To address this, we perform GRMHD > simulations of a truncated disc with the inner hot flow misaligned > with the spin axis of the black hole. Our results suggest that the > inner hot flow precesses irrespective of the presence of the outer > thin disc. We also noted that the misalignment excites variability in > the inner hot flow, which is otherwise absent in the aligned discs.


"Searching for low frequency gravitational waves with Pulsar Timing Experiments"

Małgorzata Curyło (Astronomical Observatory, Warsaw University)

While LIGO/Virgo collaboration continues on discovering gravitational waves (GWs) from stellar-mass compact binaries it is worth discussing the current status of projects that probe the lower part of the GW spectrum. In the talk I will present the idea of Pulsar Timing Arrays, which are expected to detect signals from supermassive black hole binaries at nanohertz frequencies. The presentation will focus on the latest major data releases, astrophysical implications and future prospects.


"Long term X-ray time-lags studies"

Angelos Karakonstantakis (CAMK, Warsaw)

The X-ray emission from AGN is highly variable, on short (minutes/hours/day) and long (months/years) time scales. A particular characteristic of the observed variations is that the higher energy band variations are delayed with respect to lower energy photons. I will present the long term, frequency dependent, X-ray time-lags of the Seyfert galaxies NGC 4051, MCG-6-30-15, Ark 564 and Mkn 766 in the 2-4, 4-7, and 7-10 keV bands. I will combine these results with the results from the model fitting of the time lags at higher frequencies to investigate if these delays at low frequencies are consisted with the time-lag estimates at higher frequencies.