High-Energy Emission of Compact Objects

The most compact and extreme objects in the Universe, e.g.,  black holes and neutron stars, give rise to some of the most violent and energetic phenomena known to mankind. Research in the High-Energy Group of the MPE specifically focusses on a few key aspects, such as accretion, strong gravity, and gamma-ray bursts.
Accretion onto compact objects

After direct matter annihilation, accretion onto compact objects (e.g. black holes, neutron stars and white dwarfs) is the most efficient energy source in the Universe. Black holes are regions of space-time where gravity prevents anything, including light, from escaping. The enormous gravitational pull that they exert, attracts surrounding matter that tends to be swallowed into the black hole's event horizon. Spiralling inwards, matter is generally stressed and heated up, shining brightly with its last burst of light before disappearing completely from our observable Universe. Therefore, whilst black holes themselves are the darkest objects in the Universe, accretion normally makes the regions directly outside the black hole event horizon, some of the most luminous stable compact sources known. 

It is now widely accepted that most galaxies host a supermassive black hole at their centres. When these supermassive black holes are in the process of accreting matter we call them Active Galactic Nuclei (AGN). In these AGN, the accretion of matter onto the supermassive black hole is capable of producing a luminosity high enough to out-shine the light produced by all the stars in the host galaxy, emitted from a compact region generally smaller than the size of our own Solar System. Not all material is accreted into the black hole. Part of it can sometimes escape its gravitational pull and be ejected at relativistic velocities as narrow jets. Accretion can also be associated with powerful winds which are believed to have a strong impact on the surrounding material and could possibly affect the entire evolution of the host galaxy. 

Accretion onto stellar mass black holes is also important, and actively studied phenomenon. These objects are thought to behave in many ways like scaled down versions of AGN. They are typically bright, and so allow us to study accretion processes in fine detail.

Researchers of the high energy group at MPE study many aspects of the accretion phenomenon (discs, jets, winds) in all varieties of accreting compact objects from supermassive black holes to stellar mass black holes, neutron stars and white dwarfs. From highly accreting objects (Active Galactic Nuclei, Narrow Line Seyfert 1, X-ray binaries during outburst, etc.) to low accretion rate ones (such as the supermassive black holes found at the Milky Way centre and at the centres of nearby galaxies). 

Strong Gravity

As described by Einstein's theory of General Relativity, the presence of a massive body bends the surrounding space-time. Up to now, the law of physics have only been tested in the weak gravitational field limit. One of the major challenges of modern Astronomy is to test the physical laws in the strong gravitational field regime, i.e. in close proximity to the event horizon of a black hole. 

Hard X-ray emission from accreting black holes is thought to originate in a hot plasma corona through Compton up-scatterings of the accretion disc photons (see Wikipedia). These X-rays in turn illuminate the underlying accretion disc and are in part reflected toward the observer, generating a distinct reflection component. The spectrum of the reflected component is composed of continuum plus emission lines. Line features in the reflection spectrum are shaped by the movement of the material in the disc and by the presence of the strong gravitational field of the compact object (broadened by Doppler shifts, special and relativistic effects, gravitational redshift and light bending). Therefore, the shape of the reflection component as well as its time variability carries information about the geometry and dynamics of the accretion disc and the black hole spin.

Researchers of the high energy group at MPE use X-ray and gamma-ray telescopes to observe the emission produced in the core of AGN and X-ray binaries and model the reflected emission to map the inner regions around black holes, just outside the black hole event horizon, determine black hole properties (such as black hole spins) and to test the laws of physics very close to black holes, in the still untested regime of strong gravitational field.

Gamma-ray Bursts

Gamma-ray bursts  (GRBs) are flashes of high-energy  photons that mark the  most luminous explosions  known to  Mankind.  During  their brief moments of  existence, the equivalent of  the mass of a  star like the sun is turned  into pure energy, allowing GRBs  to outshine the entire Universe.  GRBs fall  into two groups, one with  short (<2s) gamma-ray emission  likely associated  with the  merger of  two  compact stellar remnants (two neutron stars, or a  black hole and a neutron star), and the other with  longer (>2s up to tens  of minutes) gamma-ray emission demonstrated to  be the signposts  of the deaths  of some of  the most massive stars.

Scientists of the MPE high-energy  group study GRBs to address a broad range of questions:

There is a general understanding that the prompt gamma-ray emission is produced in shocks  resulting from the collisions of  shells within an ultra-relativistic  jet. What  is less  clear, however,  is  the exact physical mechanism  (e.g., non-thermal or  thermal). High-energy group scientists study this with data from the Fermi satellite.

Following  the  prompt gamma-ray  emission,  in  nearly  all cases  an optical  afterglow   can  be  found.    This  is  produced   when  the ultra-relativistic  outflow  interacts  with the  interstellar  medium surrounding   the  GRB  progenitor.    Observations  at   optical  and near-infrared wavelengths  (e.g., with GROND at the  2.2m telescope in La Silla, or with spectrographs at  the 8m VLT telescopes) are used to study  the conditions  in  the jet  (geometry,  acceleration) and  the structure and composition of the interstellar medium. This can tell us where and how GRBs and their progenitors are formed.

Their staggering luminosity allows us  to detect GRBs from the edge of the Universe,  or in other  terms, from a  time when our  Universe was infant. Indeed GRBs are in a tight race with deep and expensive galaxy surveys  for  finding the  most  distant  objects  known. Given  their association with the  death of massive stars, GRBs  are natural probes of the  conditions in the first  galaxies and can tell  us under which conditions and where  the first stars were formed.   With this goal in mind,  high-energy group scientist  use observations  of the  GRB host galaxies  (e.g., from  GROND, VLT,  or  in the  mid-infrared from  the Herschel  satellite) in  order to  understand what  GRBs can  teach us about the star formation evolution in the Universe.

text: Gabriele Ponti, Arne Rau

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