Science alert features paper on Boson stars

I'm happy to announce that our paper led by NOVA VIA Postdoc Hector Olivares was just featured on science alert.

In the study, we conducted GRMHD simulations of accretion onto condensations of light bosons called Boson stars. While Boson stars show little interaction with ordinary matter except for their gravitational pull, they do bend the light similar to a black hole. We find that, depending on the compactness (the ratio of mass to radius), some Boson stars also give rise to empty interiors, mimicking the shadow observed by the EHT. In our models however, the shadow turned out too small to account for the EHT observations of M87.

See the MNRAS paper for all the details!

Update: for a nice summary, see the following YouTube video by Anton Petrov.

9th Sep 2020; last edit 14th Oct 2020 by Oliver
tags: science

BHAC released

With great please I can finally announce the release of BHAC v1.0 under the GPL3 license! You can find usage instructions and updated documentation at our new website

I'm really looking forward to the science coming out of the public code!

29th Nov 2019; last edit 30th Apr 2020 by Oliver
tags: bhac

Alberto Sanchez Choza

Alberto has stared his master project in September 2019. He investigates formation and properties of mixed-type supernova remnants using numerical simulations of stellar winds and supernovae explosions.

Alberto is jointly supervised with Dr. Jacco Vink.

4th Sep 2019 by Oliver
tags: group, alberto

Gibwa Musoke

Gibwa has started her postdoc in the NOVA Virtual Institute for Accretion (VIA) in July 2019. She focuses on spectral timing properties of black-hole X-ray binaries from GRMHD simulations.

4th Sep 2019 by Oliver
tags: group, gibwa

Pushpita Das

Pushpita has started her PhD in September 2019. She studies dynamics and X-ray emission in accreting Pulsars with the aim to improve mass-radius measurements performed for example with the NICER satellite.

Pushpita is jointly supervised with Prof. Anna Watts

3rd Sep 2019; last edit 1st May 2020 by Oliver
tags: group, pushpita

Sebastiaan Selvi

Sebastiaan has started his PhD project in June 2019. He investigates routes to fast dissipation in relativistic flows leading to rapid particle acceleration and flares. For this purpose he performs simulations of relativistic reconnection and instability development in jets.

3rd Sep 2019; last edit 4th Sep 2019 by Oliver
tags: group, sebastiaan

Colloquiua at API

I am organizing the Anton Pannekoek Institute for Astronomy weekly colloquium series. Suggestions from API staff for interesting and diverse speakers are always welcome!

For the official announcements, see the API website.

3rd Sep 2019; last edit 2nd Sep 2020 by Oliver
tags: uncategorized

Radiative Signatures

We know about relativistic astrophysical sources through the radiation they emit. In cases of jets, pulsars and their nebula, the radiation is emitted by particles in a non-thermal distribution making these objects shine throughout the entire electromagnetic spectrum.

My interest in modeling radiation from such astrophysical sources is two-fold: on the one hand, it lets us extract important source parameters like the speed of the flow or its energy; on the other hand, the mechanisms transferring energy to the radiating particles are rich in physics themselves -- and often poorly understood!

Magnetic Reconnection

Currently, the most promising mechanism to explain rapid flares and plasma heating in astrophysical sources is magnetic reconnection. In this process, the magnetic energy is dissipated by topologically re-arranging the magnetic field via current sheets. The non-linear dynamics and effective dissipation of magnetic reconnection is subject to much speculation as its study requires an understanding of the formation of the current sheet (e.g. through turbulent processes) as well as a description of the microphysical parameters in the current sheet itself.

With collaborators, we have recently shown how plasma turbulence in a black hole accretion flow leads to copious magnetic reconnection along with magnetic islands.

Current density in a black hole accretion simulation showing formation of current sheets and reconnection. From Nathanail et al. (2020).

Sometimes, reconnection occurs in the highly magnetised regions above the black hole. Then plasmoids trapping emitting particles can be flung out as an episodic relativistic outflow. See the paper led by Dr. Antonios Nathanail for details.

Plasma density of a relativistic reconnection simulation with anti-parallel magnetic fields showing a plasmoid chain.

In his first thesis project, Sebastiaan investigates the reconnection process both from the fluid scale and from the microphysical kinetic scales with the aim to understand how reconnection proceeds and energizes particles in astrophysical sources.

17th Jun 2019; last edit 2nd May 2020 by Oliver
tags: website, radiative signatures

Pulsar Wind Nebulae

Multi-frequency image of the Crab Pulsar Wind Nebula by the Hubble, Chandra and Spitzer space telescopes. Credit: NASA, ESA, CXC, JPL-Caltech, J. Hester and A. Loll (Arizona State Univ.), R. Gehrz (Univ. Minn.), and STScI

Once a massive star has had its final showdown in a super-nova explosion, it leaves behind an extremely dense remnant: matter is packed so tight as if it was forming a single atomic nucleus. Given enough mass of the compact remnant (around two solar masses), it can even collapse further to form a black hole.
Neutron stars (the remnants which don't become black holes) are particularly interesting astrophysical objects: they are stabilized against ultimate gravitational collapse by the strong nuclear force. Thus studying their structure, we can learn about the fundamental physics of matter at extreme densities. Many rapidly spinning neutron stars (performing up to a thousand turns every second) are endowed with strong magnetic field and emit pulses of electromagnetic radiation with every one of their turns. These are called Pulsars.

Not only do Pulsars emit regular bursts of electromagnetic radiation, their rotating magnetosphere also drives a relativistic outflow of electrons and positrons, called a Pulsar Wind. Similar to the solar wind, the Pulsar Wind is accelerated by the electromagnetic field, but since its density is so low, it reaches highly relativistic speeds (some models predict Lorentz factors of 105, much faster than any other outflow in the Universe). Once this outflow has expanded for around nine orders of magnitude, its dynamic pressure drops below the pressure of the supernova ejecta and a shock forms -- the wind termination shock. At this shock, the particles of the wind are accelerated and non-thermal radiation illuminates the stellar ejecta from the inside out: a Pulsar Wind Nebula is born.

The most celebrated Pulsar Wind Nebula is the Crab Nebula at a distance of 2000 pc. High resolution optical and X-ray observations have revealed the intricate dynamics of the termination shock: the emergence of a 'jet and torus' morphology, emission of wisp-like perturbations in the flow and the appearance of a 'knot' of emission attached to the shock are only a few of the variable features.

3D simulation of the flow in the PWN showing magnetic field strength (left) and the total pressure in the nebula (right), formation of current sheets, turbulence and dissipation in the nebula dramatically alter the dynamics of the flow. See Three-dimensional magnetohydrodynamic simulations of the Crab nebula for further info.

Pulsar Wind Nebulae are ideal laboratories for relativistic plasma dynamics and their modeling is connected to many questions of astro- and fundamental- physics, for example:

  • How does non-thermal particle acceleration take place? How does the particle spectrum evolve in the nebula?
  • How is magnetic energy transported to relativistic particles? How does turbulent magnetic dissipation operate?
  • What is the origin of gamma-ray flares within the Crab nebula?
  • What is the origin of radio-emitting electrons? And how many particles can be supplied by the neutron star magnetosphere?
  • How many leptons escape into the ISM and find their way to Earth? Can PWN explain the observed positron excess compared to cosmological predictions?
A more in-depth discussion can be found in our review
Porth, Oliver, Buehler, Rolf, Olmi, Barbara, Komissarov, Serguei, Lamberts, Astrid, Amato, Elena, Yuan, Yajie, Rudy, Alexander; Modelling Jets, Tori and Flares in Pulsar Wind Nebulae, Space Science Reviews, Volume 207, Issue 1-4, pp. 137-174
and in a number of works I have carried out over the years, see also Publications.

17th Jun 2019; last edit 6th Jul 2019 by Oliver
tags: website, pwn


By studying accretion and outflows from compact objects, we learn about the most efficient energy source in the universe. I'm particularly interested in the formation, collimation and ultimately dissipation of relativistic jets which emanate from the direct vicinity of black holes across all mass ranges.

One of the most exciting observations in this field was the recent imaging of the black hole in the galaxy M87. Using Very Long Baseline Interferometry (VLBI) of high-frequency radio telescopes scattered across the globe, the Event Horizon Telescope Collaboration took this image.

Image credit: EHTC, CC BY-ND 4.0

Numerical simulations of turbulent accretion (with bhac among other codes) were combined with general relativistic ray-tracing to interpret the structure seen on the image: we witness the light emitted from hot plasma on its final orbits before plunging into the black hole. The dark region in the center is the so-called black hole shadow, a gravitationally lensed projection of the unstable photon orbit. Within the shadow area all light vanishes within the event horizon.

The galaxy M87 is also famous for its remarkable relativistic jet which is one of the best studied in astrophysics. Using high resolution 3D adaptive mesh refinement simulations with bhac, we have modeled the launching of the jet with unprecedented accuracy. Modeling the electrons within a kappa-distribution, we find that non-thermal particles are crucial to extend the spectrum beyond the radio band.

Relativistic ray-traced images of the simulated jet at three different radio frequencies: 43GHz (left), 86GHz (middle) and the EHT frequency of 230GHz (right). See the paper led by Jordy Davelaar for details.

17th Jun 2019; last edit 1st Sep 2019 by Oliver
tags: website, accretion

Making the website

It will be glorious, stay tuned!

Ugh, since I'm working with some old setup, have to use html5 with some basic php, so no fancy content management here. Trying to make it look fancy anyways...

7th Jun 2019; last edit 17th Jun 2019 by Oliver
tags: website


A reasonably up-to-date CV can be found here as pdf.

7th Jun 2019; last edit 17th Jun 2019 by Oliver
tags: career

List of Publications

5th Jun 2019; last edit 8th Nov 2021 by Oliver
tags: career

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5th Jun 2019; last edit 10th Jun 2019 by Oliver
tags: website