Accreting Black Holes and Neutron Stars

I am involved in studying the probably most extreme objects: black holes and neutron stars that attract matter from their surroundings. The complexity and extent of this field requires us to collaborate amongst different research institutes and to connect different astrophysical disciplines in a strongly interdisciplinary manner. My research has been funded by all sub-divisions of the "Netherlands Research School for Astronomy" (NOVA) and the DLR. The bottom part of this page is rather technical. Here, I want to bridge to the general public and let everyone access the fascinating science we are pushing forward. With these information on hand, it will be easier to understand the condensed information presented further below.

Black holes and neutron stars gather a lot of mass at very little space. Both can result from the collapse of a massive star at the end of its lifetime. During such a collapse, several processes that would keep matter in shape are overcome and a super-dense configuration of matter is formed. Neutron stars can be said to be the last stable (and most dense) configuration of matter known. They are about as massive as our sun but measure only tens of kilometers in diameter. In other cases, massive stars collapse all the way to black holes.

In black holes, the entire mass is found in one "singularity". The "size" of a black hole is pure definition and not the size of any structure - it is the (gravitational) radius at which nothing, not even light can escape the steep trough of space-time formed by the super-dense black hole. We call both neutron stars and black holes "compact objects".

When we neglect time, an analogy of space is a stretched sheet. Now, let's compare two objects of the same mass: a heavy marble (the black hole) and porous Dutch bread (our sun). The marble will curve the sheet deeper than the bread. Still, both will curve the sheet the same way towards its outer edge. A lighter marble thrown onto the sheet will therefore draw circles the same way around both central objects. Curvature of space is exactly what causes gravitational attraction. Likewise, Earth will orbit our sun the same way it would orbit a black hole of the same mass. The story changes only when an observer approaches the black hole very closely and experiences the localized and much deeper (and steeper) curvature of space-time (our sheet plus time as fourth dimension). Black holes therefore only have a very small area of influence. Here, all the fascinating effects come to play. Light, for example, has to follow space-time just as a marble on a curved sheet. Close to a black hole, light from your flashlight may come back to you and you may also see yourself from behind. But be aware of the extreme tidal forces that will stretch everything like a spaghetti. The steeper the sheet (or space) is curved, the stronger the tidal force.

Only the environment of black holes (and neutron stars) makes them the most efficient and energetic power plants in the universe. Everything that formed anywhere in the Universe has angular momentum and follows trajectories around a center of mass. If there is matter close to a black hole with angular momentum, it will thus spiral around the heavy central mass just like planets around their central star(s). This is true for black holes at all sizes and masses. Matter around the largest black holes of millions to billions of solar masses forms the centers of huge galaxies like our Milky Way. Matter around stellar-sized black holes may have originated from gas remaining after the explosion of the foregoing massive star.

Very close to the black hole, gas is assumed to be very dense. Friction can lead to an outward transport of angular momentum and therefore the formation of a disk. This disk can continuously feed the inner object with mass. Einstein's famous equation predicts that this mass is proportional to energy. During the "accretion of matter", this gravitational energy is released via radiation across the electromagnetic spectrum and strong outflows of matter. The efficiency at which radiation is produced exceeds nuclear fusion by a factor of more than 14 and we can certainly call it the most efficient process to produce energy across the Universe. On Earth, however, we have limited possibilities to squeeze the energy contained in pure mass. For example, we burn it (one of our biggest problems), throw it somewhere, try to extract the kinetic energy carried by winds, or let reactions do their thing.

Accretion onto compact objects is an ubiquitous process in the Universe at all mass scales. It is a process of gradual in-spiral and not the consequence of black holes sucking in their environment in free-fall. Galaxies, where their central black hole happens to accrete matter and thus releases heaps of energy, are called "Active Galaxies". Our Milky Way is not an Active Galaxy. Note that, on smaller scales, neutron stars can also attract matter from their surroundings. For them, this process can not last forever. After accumulating a certain mass, an explosion is triggered that may lead to the formation of a black hole. Black holes, however, can merge and accrete matter over cosmological time scales and reach extreme masses. Merging black holes (and neutron stars) were just recently found by detecting gravitational wave signals from these mergers with Earth itself as detector material.

My research focuses on a better understanding of compact objects as cosmic power plants. To do so, me and my colleagues try to picture the invisible. The size of a supermassive black hole inside a distant galaxy is comparable to the size of a bacterium on the moon, which is much smaller than the best telescope could resolve. We therefore use some tricks. We study all forms of the released energy to infer the energizing processes very close to the black hole. We observe the light being emitted from these systems from the lowest to the highest energies (from the radio to the gamma-rays). The energy dependency of the light encodes valuable information on the processes we try to understand. Matter that is thrown out of these systems (the "exhaust"), however, can be observed and imaged in many cases. The most extreme of these outflows are "jets". They emerge from the centers of Active Galaxies and the environment of stellar-sized black holes inside galaxies. Jets are remaining focused at extremely large distances. They accelerate matter to nearly the speed of light, which also produces radiation. We can for example observe their strong radio light with large radio antennas on Earth.

Credit: Kent Biggs

Active Galactic Nuclei

Me and my colleagues have been observing the innermost structures of Active Galactic Nuclei. We push for exploiting the vast information contained in the entire electromagnetic spectrum and observe these systems with numerous facilities. Those are radio telescope (networks), ground-based and space-borne observatories sensitive to the optical, UV, and X-rays, up to the highest energies in the gamma-rays. We performed time-resolved X-ray spectroscopy of a number of sources. By monitoring changes in the emitted X-ray spectrum, we can infer not only the physics intrinsic to these nuclei, but also their highly dynamical morphology. We have to use this approach, as the resolution of the best telescopes is by far not enough to image these compact environments around distant black holes. We compare our observations with state-of-the-art spectral models that describe the photoabsorbing effect of neutral and ionized material in and around these nuclei, general relativistic effects, and many more. We published two dedicated case studies of the Active Galaxies NGC 3227 (Beuchert et al., 2015) and NGC 4151 (Beuchert et al., 2017a).

Credit: NASA/CXC/SAO

X-ray binaries

In our working group at the Anton Pannekoek Institute for Astronomy, we are particularly interested in stellar-mass accreting compact objects. We are investigating accretion and feedback as two processes that seem to be ubiquitous across a large range of masses. In X-ray binaries, a compact object and a star are orbiting each other. The star is loosing mass onto the compact object, which released large amounts of energy. I am particularly interested in systems, where the compact object accretes matter at extreme rates, yet leading to the most luminous but less well understood objects known for that class, so-called "Ultraluminous X-ray Sources". We are interfacing observations and cutting-edge theoretical work to get to the bottom of these mysterious objects.

Credit: ESO/M. Kornmesser

Outflows, Winds, and Jets

Accretion and feedback are linked processes in accreting compact objects. As opposed to matter being attracted, "feedback" describes everything being expelled from these systems. At all mass scales of the central compact object, we frequently observe wide-angle winds of moderate to relativistic speeds and/or magnetically collimated streams ("jets") that accelerate matter to nearly the speed of light. In Beuchert et al. 2018, we studied the feedback of the young radio galaxy PKS 1718-649 in order to find an explanation for extended X-ray-emitting gas that has been observed in this galaxy.
We are currently investigating host galaxy properties of several active galaxies to explain differing jet morphologies with the (G)RMHD code H-AMR developed in our group.
In an ongoing project, we investigate stellar-mass accreting compact objects at their extremes. Ultra Luminous X-ray sources are suspected to accrete at the highest rates but are still not well understood. We use currently developed physical models to describe the UV/X-ray electromagnetic spectrum of ULXs and their powerful feedback. We seek to tighten our constraints by consulting the response of the environment, i.e., the (ionization) properties of gas surrounding these systems.
During past research, me and my colleagues investigated the morphology and physics of a jet stream close to its origin. With the MOJAVE radio interferometer we could spatially resolve gas being accelerated into the jet of 3C 111 (Beuchert et al., 2017b). The data also allowed us to study the radio-polarization properties of shocked gas while traveling downstream.

Credit: HEAG/UCSD

Instrumentation

Our atmosphere is not transparent to all wavelengths. In order to observe for example the UV or X-ray light emitted from astrophysical sources, we have to place our instruments in orbit around Earth.
I have been involved in design studies of future X-ray instruments. Using the end-to-end simulator SIXTE at the Dr. Remeis Observatory Bamberg, I studied the scientific telemetry limits of the planned Brazilian hard X-ray instrument MIRAX. The same simulator is also used for the upcoming ESA X-ray mission ATHENA. Follow this link for our proceedings providing more background information.

“Because it's there.” (George Mallory)
Image credit: DESY, Science Communication Lab