Wednesday Colloquium


"Magnetically elevated accretion disks in 3D global MHD simulations"

Bhupendra Mishra (JILA, Boulder, CO)

The physics of accretion around compact objects has been widely studied in last few decades. However, it has been only recently possible to perform fully global magnetohydrodynamical simulations using supercomputing models. I shall be talking about the role of strong large scale seed magnetic fields in the time evolution of geometrically thin accretion disks around black holes. Geometrically thin disks are challenging to simulate due to requirement of exceptionally high resolution (about 24 cells per disk scale height). I used cutting edge static mesh refinement (SMR) to resolve such disks with an effective resolution of 2048x256x1024 and maintain a reasonable time step close to polar regions. In this numerical model, I initialized thin disks with purely vertical magnetic field in spherical polar coordinates. Due to differential rotation, the model builds up strong toroidal magnetic field over the dynamical time scale. The toroidal field causes disk 'levitation' and accretion at higher altitudes (z ~ 0.3 in cylindrical coordinates) and outflows in the disk midplane. Such disks are called magnetically 'elevated' disks. There are various astrophysical problems where such strong magnetic field can play an essential role in shaping the accretion flow in the vicinity of black holes. I shall mainly talk about following relevant astrophysical aspects: 1. Angular momentum transportation in accretion disks ? 2. Spectral state transitions in black hole X-ray binaries 3. Magnetically elevated accretion in AGNs and their evolution


"OGLE-ing the Magellanic System: Three-Dimensional Structure Using Classical Pulsators"

Anna Jacyszyn-Dobrzeniecka (Astronomical Observatory, Warsaw University)

I will present a three-dimensional structure of the Magellanic System using over 9000 Classical Cepheids (CCs) and almost 23000 RR Lyrae (RRL) stars from the OGLE Collection of Variable Stars. Given the vast coverage of the OGLE-IV data and very high completeness of the sample we were able to study the Magellanic System in great details. Our analysis of CCs distribution shows that contrary to previous results the LMC bar shows no offset from the galaxy plane. Moreover, we found that the northern arm, along with its smaller component, is located closer to us than the entire sample. The CCs in the SMC have a non-planar distribution that can be described as an ellipsoid extended almost along the line of sight. RRL stars revealed a very regular distribution in both Magellanic Clouds. Lately, we have very carefully studied once again the distribution of both types of pulsators in the Magellanic Bridge area. We showed that there is no evidence of an actual physical connection between the Clouds in RRL stars distribution. We only see the two halos overlapping. Moreover, our study showed that there are several CCs in the Magellanic Bridge area that seem to form a genuine connection between the Clouds. Their on-sky locations match very well young stars and neutral hydrogen distribution.


"The puzzle of Cygnus X-3"

Andrzej Zdziarski (NCAC, Warsaw)

Cyg X-3 is the only known X-ray binary in the Galaxy with a Wolf-Rayet donor. The compact object is probably a black hole. It is a very bright and variable multiwavelength source, but its emission shows features substantially different from those in accreting black-hole binaries. I will review the properties of this system and present new results on its radio, X-ray and gamma-ray emission.


"X-ray variability of Low Frequency Quasi-periodic Oscillation in X-ray binaries"

Bei You (School of Physics and Technology, Wuhan University, Wuhan, China)

One interesting observational appearance in X-ray binaries is the broadband X-ray variability, covering timescales from 0.01 s to thousands of seconds. Fast Fourier analysis of variability reveals Quasi-Periodic Oscillations (QPOs) in some X-ray binaries. However, the physical origin of QPOs is still under debate. As for low-frequency (1∼10 Hz) QPO (LFQPOs), the possible mechanism involves the so-called Lense–Thirring precession, which is due to the frame-dragging effect in the strong gravity field. We developed a Monte Carlo code to compute the Compton-scattered X-ray flux arising from a hot inner flow that undergoes Lense–Thirring precession. In this talk, I will review our modeling results on X-ray variability of LFQPO, and discuss its application on understanding the observed variability, including recent results from the Hard X-ray Modulation Telescope (HXMT, also named "Insight") which is China’s first X-ray astronomy satellite.