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Universität zu Köln
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Mathematisch-Naturwissenschaftliche Fakultät
Fachgruppe Physik

I. Physikalisches Institut

STRONGGRAVITY

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Probing Strong Gravity by Black Holes Across the Range of Masses - StrongGravity

EU FP7-SPACE research project  

Project ID: 312789
Funded underFP7-SPACE

 

StrongGravity is an EU reseach project funded under European Union Seventh Framework Program. See also the project webpage and the consortium webpage.

The goal of this project is to creat tools to study the mechanisms happening close to astrophysical black holes, achieve observational data on binary black holes, super-massive black holes in the nuclei of galaxies and also in center of Milky Way, and better constraining the properties of black holes and their immediate environment by using the developed tools together with the archival and new data.

In this project, information in different wavelengths from X-ray to radio - by current X-ray missions of ESA, together with wide wavelength coverage of ground-based observatories in ESO and elsewhere - is obtained and analyzed in order to study radiation mechanisms close to black holes. The observation of the matter moving in close to the vicinity of black hole, orbiting it, falling down or being ejected from it can reveal the black hole properties since one can not observe the black hole directly. Moreover, different fast timing and multiwavelength observations of different types of black holes, according to their mass, are interpreted.  

The research team includes experts from 7 leading universities and research institutes. StrongGravity project has several work packages that some are led by Universität zu Köln (UCO) in collaboration with Astronomický ústav AV ČR (AsU). In our group we intend to probe strong gravity around Sagittarius A* (Sgr A*), which is associated with the super-massive black hole located at the centre of Milky Way. The dusty S-cluster Object (DSO/G2) fly-by and changes in the S2 orbit are the mechanisms that could help us to test the strong gravity close to Sgr A*. The first case has been heavily monitored and studied by our group members because of its possible influence on the accretion flow of Sgr A* during its fly-by around the super-massive black hole and therefore possible changes of Sgr A* emission. The latter is an important study case because of the S2 passage through its periapse in 2018, for the second time during its near-infrared observations. We can search for the S2 periapse precession and other tests of General Relativity (GR).

 

Variability of Sgr A* in different wavelengths

Our main aim was to develop a tool for modeling of Sgr A* X-ray and IR flares through their polarization signal and their time evolution in various geometries. We used a 3D ray-tracing code, that is developed at AsU, modified and delivered it for the particular needs of near-infrared polarimetry. Also, we produced a library of observed polarized near-infrared light curves of Sgr A* for further use in the community, as well as a library of theoretical light curves, which includes the variability of polarization patterns for different source geometries using the 3D ray tracing code. 

We also intended to exploit the data archive of near-infrared polarized light curves of Sgr A*, its variability at different wavelengths and to model them with the theoretically calculated scenarios of the optically thin emission with the aim of constraining physical parameters of the emitting region. Our first plan was to fit near-infrared light curves but decided instead to model X-ray flares, beacuse of the higher signal to noise ratio. We compared the observed X-ray light curves of Sgr A* with the ones simulated with the novel 3D ray-tracing code and the fitting allowed us to determine the black hole mass. This task was carried out in collaboration between UCO and AsU nodes. Furthermore, we developed further the code for modeling the optically thick emission (i.e. sub-mm/radio) and parallel to that we performed multiwavelength observations of Sgr A*. We compiled a large set of radio and submillimeter data, statistically analyzed them, and found a possbile link between the radio and submm emission and the near-infrared one.

 

  • First statistical analysis of Sgr A* polarization properties 

   In Shahzamanian et al. 2015, we delivered all light curves of Sgr A* observed in polarization mode in K-band using NACO/VLT between 2004 and 2012. We obtained that the linear polarization degree and angle of Sgr A* system are 20% +/- 10% and 13° +/- 15°, respectively. The obtained preferred polarization angle shows very likely the intrinsic orientation of the Sgr A* system, i.e. a disk or jet/wind scenario associated with the super-massive black hole, since the emission is most likely due to optically thin synchrotron radiation. Therefore, the data imply a rather stable accretion process and geometry for the Sgr A* system.

 

 

Left: Distribution of polarization degree and angle from all the polarized K-band light curves of Sgr A* 

Right: Direction of intrinsic polarization angle of Sgr A* in the Galactic Centre

 

See "Polarized light from Sgr A* in the near-infrared Ks-band", B. Shahzamanian, A. Eckart, M., Valencia-S., G. Witzel, M. Zamaninasab, N. Sabha, M. García-Marín, V. Karas, G.  D. Karssen, A. Borkar, M. Dovčiak, D. Kunneriath, M. Bursa, R. Buchholz, J. Moultaka, C. Straubmeier, Astronomy & Astrophysics 576, Art. No. A20 (2015)

 

 

  • VLBA observations triggered by flaring event

   In Rauch et al. 2016, we showed radio VLBA observations triggered by a near-infared flaring event observed with VLT. We found a delay of ~ 4.5 hours between the radio and near-infrared Sgr A* flares, which agrees with the expected time delay of events connected by adiabatic expansion.

 

  

Two-hour Left Circular Polarization maps of Sgr A* observed on May 17 2012. Sgr A* appears as a point-like source in the initial plots which show its quiescent state and then during its flaring state it develops an extended feature of ∼(1.7±0.3) mas toward the southeast.

 

See "Wisps in the Galactic center: Near-infrared triggered observations of the radio source Sgr A* at 43 GHz", C. Rauch, E. Ros, T. P. Krichbaum, A. Eckart, J. A. Zensus, B. Shahzamanian, K. Mužić, Astronomy & Astrophysics 587, Art. No. A37 (2016)

 

 

  • Black hole mass estimation from X-ray light curves

   In Karssen et al. 2017, we tried to understand whether the light curves of X-ray flare events of galactic nuclei can constrain independently the mass of the central super-massive black hole. We analyzed the four brightest flares of Sgr A* in X-ray. We used the sim5lib library that sets up individual geodesics for each photon using the Kerr-metric. We found that the flare properties agrees well with our ray-tracing simulations. We implemented a hot-spot model that simulates a localized brightness excess within an accretion flow. All the brightest X-ray flares show a double peak structure that is expected from the hot-spot simulations. The shape of the light curve depends mostly on the radial position of the emitting region with respect to the black hole and the observer's line of sight, and the size of the emitting region. The obtained mass for each of the flares agrees with the previous mass estimations through the orbits of stars around the super-massive black hole.

We finalized the first detailed version of the library of the simulated light curves. In this version of library, we focused on the hot-spot model (e.g., bright overdensity on the middle plane of accretion flow), and allowed for variations (within reasonable ranges) of the system's inclination with respect to the line of sight, the spot size, and its radial position with respect to the central black hole. The orientation of the magnetic field and optical depth of the accretion disk can also be chosen. For some details related to the library, click here: Library. The theoretical light curves and the source code are under progress and will be available soon.

 

        Presentation of the origin of the double-peak structure in the total flux                                   The effect of the blob's size and the blob's position on the shape of the light curve 

 

 

See "Bright X-ray flares from Sgr A*", G. D. Karssen, M. Bursa, A. Eckart, M. Valencia-S, M. Dovčiak, V. Karas, J. Horák, Mon. Not. R. Astron. Soc. 472, 4422-4433 (2017)

 

We have further developed the ray-tracing code to include the posibility for a plasmoid ejection, as it has been proposed for the case of Sgr A* variability. This version of the code is used to model the region of jet/counter-jet component formation to explore the motion effects of the jet launching mechanism. To achieve this, a jet component is ejected from the accretion disk surrounding a super-massive black hole from different radial positions, with varying radial velocity and velocity along the z-axis. Considering a rotating disk, the components will be launched at an angle and travel outwards, along a spiral path. Other parameters such as the inclination with respect to the observer's line of sight or the size of the spot can be changed.

 

 

 

 

 

 

A single jet component ejected from an accretion disk around a super-massive black hole, with also polarization information. The used parameters are: inclination = 45 degrees (observer’s inclination), initial radial position = 6 Rg (gravitational radii), the radial velocity rv = 0.01, and the velocity along the z-direction vz = 0.02.

 

 

 

 

 

 

 

 

 

 

 

Jet consisting of 13 components, all ejected at the same radial position (= 6 Rg), but with different parameters for radial/z-velocity, varying from: radial: 0.01-0.05 c and z-velocity: 0.1-0.5 c.

 

 

 

 

 

 

 

 

In another ongoing project we added the radiative-transfer code to our ray-tracing code in order to get the polarization information (Stokes parameters) of the emitting region, and therefore to explain the observed variability of Sgr A*. We considered the hot spot model with different initial conditions such as the blob size, density parameter, maximum radial distance of the blob, and so on. We assumed that circular polarization is negligible. 

 

  • The Milky Way’s Supermassive Black Hole: How good a case is it?

There are a lot of evidences that identify Sgr A* as a super-massive black hole and reject alternative possibilities. We reviewed the observational evidence, theoretical grounds and aspects of Sgr A* being a black hole. We used the framework of the philosophical discussions about “(Anti)Realism and Underdetermination” in order to deal with observations and theory. We discussed the existence of super-massive black holes in general and Sgr A* in particular. The combination of concepts established the results of our investigation.

 

 

 

 

 

 

 

 

 

 

 

 

 

Here the linkage between theory and experiment interpreted by the concept of realism, underdetermination and a “rounded out” version of the Eleatic Principle, is shown regarding the results of our analysis. The case of study here is the Galactic Centre super-massive black hole.

See "The Milky Way's Supermassive Black Hole: How Good a Case Is It?", Eckart, Andreas; Hüttemann, Andreas; Kiefer, Claus; Britzen, Silke; Zajaček, Michal; Lämmerzahl, Claus; Stöckler, Manfred; Valencia-S, Monica; Karas, Vladimir; García-Marín, Macarena, Springer, Foundations of Physics 47, Issue 5, 553-624

 

  • Statistical analysis of radio and submilimeter data

   In Borkar et al. 2016, we observed the flaring events of Sgr A* at 3 mm using the Australia Telescope Compact Array (ATCA) between 2010 and 2014. We detected six significant flux density variations of Sgr A*, with variations between 0.5-1.0 Jy that last for 1.5-3 hours, by performing structure function analysis and the Bayesian blocks representation. In order to explain the short time scale flux density variations, we used the adiabatically expanding plasmon model. We obtained the physical parameters of the modeled flare emission. We did not detect any exceptional flux density variations during the flyby of the DSO/G2.

 

The differential light curves of Sgr A* at 3mm  observed with ATCA for the flare activity days. For comparison with a day with no strong flare activity, 25 May 2011 light curve is shown. 

 

See "Monitoring the Galactic Centre with the Australia Telescope Compact Array", A. Borkar, A. Eckart, C. Straubmeier, D. Kunneriath, B. Jalali, N. Sabha, B. Shahzamanian, M. Garcia-Marin, M. Valencia-S, L. Sjouwerman, S. Britzen, V. Karas, M. Dovciak, A. Donea, A. Zensus, Mon. Not. R. Astron. 458, 2336-2349 (2016) 

 

 

​   In Subroweit et al. 2016, we studied Sgr A* submilimeter light curves at 350 GHZ observed with Large Apex Bolometer Camera (LABOCA) mounted at the APEX telescope and also radio light curves at 100 GHZ taken with ATCA telescope. Former studies of members of our group have shown that the flux density distribution in near-infared K-band can be described by a power-law with a slope of 4. We found that the both radio and submm flux density distributions are also described well with a power-law with the slope of 4.Therefore, the flare activities in all wavelengths originates from the same source components. Also assuming the adabatically expanding clouds as the emission model, our findings could put additional constraints to the model.

 

 

 

 

 

 

 

 

 

Left: a single measurement map of the Galactic Centre (GC) at submm. Center: model of the extended submm emission. Right: remaining map of the measurement map after subtraction of the GC model for the extended emission.

 

                      all available light curves at submm between 2004 and 2014                                                 all ATCA light curves at 100 GHz between 2010 and 2014  

 

See "Submillimeter and radio variability of Sagittarius A*. A statistical analysis", M. Subroweit, M. García-Marín, A. Eckart, A. Borkar, M. Valencia-S., G. Witzel, B. Shahzamanian, C. Straubmeier, Astronomy & Astrophysics 601, Art. No. A80 (2017)

 

 

Dusty S-cluster Object (DSO/G2)

  • Fly-by of DSO/G2

   In Valencia-S. et al. 2015, we analyzed the near-infared observations of the DSO/G2 during its approach to the position of the super-massive black hole that were taken with VLT/SINFONI between February and September 2014. We detected the DSO/G2 to be a compact source on an elliptical orbit with the peribothron time passage in 2014.39 +/- 0.14. The flaring activity of Sgr A* did not show any significant change in the near-infrared regime during the periapse passage. We concluded based on our observations that the DSO/G2 might be a young accreting star and not a coreless dust and gas cloud.

 

Observations of DSO/G2 with ESO’s VLT during different years are shown. It has remained compact after its close encounter with the black hole. The red color shows the recession and the blue presents the approach of the source.

 

Read the ESO press release about our work on the DSO/G2 passing the black hole in the Galactic Centre:  http://www.eso.org/public/news/eso1512/

See "Monitoring the Dusty S-cluster Object (DSO/G2) on its Orbit toward the Galactic Center Black Hole", M. Valencia-S., A. Eckart, M. Zajacek, F. Peissker, M. Parsa, N. Grosso, E. Mossoux, D. Porquet, B. Jalali, V. Karas, S. Yazici, B. Shahzamanian, N. Sabha, R. Saalfeld, S. Smajic, R. Grellmann, L. Moser, M. Horrobin, A. Borkar, M. Garcia-Marin, M. Dovciak, D. Kunneriath, G. D. Karssen, M. Bursa, C. Straubmeier, H. Bushouse,  Astrophys. J. 800, Art. No. 125 (2015)​

 

  • Polarimetry of DSO/G2

   In Shahzamanian et al. 2016, we detected the DSO/G2 for the first time in near-infrared polarimetry data of different epochs (2008-2012). We obtained the intrinsic polarization degree of about 30% and a varying polarization angle for this source as it approaches the position of the super-massive black hole and could constrain its geometrical properties. These results combined with our former results show that the DSO/G2 might be a dust-enshrouded star forming a bow shock as it approaches Sgr A*. The significant high polarization degree shows that this source has a non-symmetric geometry. We reproduce the observed properties of DSO/G2 by using a 3D radiative-transfer model.

 

 

Ks-band polarimetry images of the central arcsecond of Galactic Centre in different years showing DSO before its periapse passage

 

See "Polarized observations of a Dusty S-cluster Object (DSO/G2) at the Galactic Centre", B. Shahzamanian, A. Eckart, M. Zajaček,  M. Valencia-S., N. Sabha, L. Moser, M. Parsa, F. Peissker, C. Straubmeier, Astronomy & Astrophysics 593, Art. No. A131 (2016)

 

 

   In Zajacek et al. 2017, we compared the 3D radiative transfer models of the DSO/G2  with the near-infared polarized data.We constrained the geometry and the nature of this source. We modeled the DSO/G2 as a dust-enshrouded star consisting of a star, optically thick dusty envelope, bipolar cavities, and a bow shock. This composite model can fit well with the near-infrared total and polarized properties of the observed spectral energy distribution (SED). Therefore the DSO/G2 is most probably a young stellar object enshrouded in a non-spherical dusty envelope. On the other hand, the continuum emission can have a non-thermal origin produced by a young neutron star and its wind nebula.

 

The DSO/G2 compsite model and its components as a pre-main-sequence star

 

See "Nature of the Galactic centre NIR-excess sources. I. What can we learn from the continuum observations of the DSO/G2 source?", M. Zajaček, S. Britzen, A. Eckart, B. Shahzamanian, G. Busch, V. Karas, M. Parsa, F. Peissker, M. Dovčiak, M. Subroweit, F. Dinnbier, J. A. Zensus, Astronomy & Astrophysics 602, Art. No. A121 (2017)

 

 

GR effects on the orbit of S2

   In Parsa et al. 2017, we estimated the mass of and the distance to Sgr A* to be MBH = 4.15 ± 0.13 ± 0.57 × 10^6 solar masses and R0 = 8.19±0.11±0.34 kpc by using three shortest period stars (S2, S38, and S55/S0-102) and Newtonian models. These results are in good agreement with the recently published values. We developed a new and practical method to investigate the GR effects on the proper motion of the stars closest to the Sgr A* position. We generated stellar orbits using a first-order post-newtonian approximation with a broad range of periapse distance. Then, we applied the results of the study on S2 star. S2 has a periapse velocity of 0.02 in speed of light units and such a high velocity is a good motivation to search for the periapse precession and other tests of GR. In 2018 this source goes through periapse for the second time during its near-infrared observations. Among the cumulative relativistic effects due to a Schwarzschild black hole up to the first order, the periapse shift is the strongest effect. We obtained the changes in the argument of the periapse of S2 and calculated a relativistic parameter based on the changes in the orbital elements of this source.

 

 

The orbit of S2 is showing the effects predicted by the theory of GR. This is the first time that the general relativistic effects have been achieved for stars orbiting a SMBH.

 

Read the ESO press release about our work on the relativistic motion of stars close to the super-massive black hole in the Galactic Centre:  http://www.eso.org/public/announcements/ann17051/

See "Investigating the Relativistic Motion of the Stars Near the Supermassive Black Hole in the Galactic Center", M. Parsa, A. Eckart, B. Shahzamanian, V. Karas, M. Zajaček, J. A. Zensus, C. Straubmeier, Astrophys. J. 845, Art. No. 22 (2017)

 

 

 

 

Related publications of FP7 project

 

"Variable and Polarised Near-infrared Emission from the Galactic Centre", B. Shahzamanian, A. Eckart, M. Valencia-S., G. Witzel, M. Zamaninasab, M. Zajaček, N. Sabha, M. García-Marín, V. Karas, F. Peissker, G. D. Karssen, M. Parsa, N. Grosso, E. Mossoux, D. Porquet, B. Jalali, M. Horrobin, R. Buchholz, M. Dovčiak, D. Kunneriath, M. Bursa, A. Zensus, R. Schödel, J. Moultaka, C. Straubmeier, The ESO's Messenger 159 (2015)

"Polarized light from Sgr A* in the near-infrared Ks-band", B. Shahzamanian, A. Eckart, M., Valencia-S., G. Witzel, M. Zamaninasab, N. Sabha, M. García-Marín, V. Karas, G.  D. Karssen, A. Borkar, M. Dovčiak, D. Kunneriath, M. Bursa, R. Buchholz, J. Moultaka, C. Straubmeier, Astronomy & Astrophysics 576, Art. No. A20 (2015)

"Wisps in the Galactic center: Near-infrared triggered observations of the radio source Sgr A* at 43 GHz", C. Rauch, E. Ros, T. P. Krichbaum, A. Eckart, J. A. Zensus, B. Shahzamanian, K. Mužić, Astronomy & Astrophysics 587, Art. No. A37 (2016)

"The Galactic Center Black Hole Laboratory", Eckart, A.; Britzen, S.; Valencia-S., M.; Straubmeier, C.; Zensus, J. A.; Karas, V.; Kunneriath, D.; Alberdi, A.; Sabha, N.; Schödel, R.; Puetzfeld, D., Springer International Publishing 179, 759-781 (2015)

See "The Milky Way's Supermassive Black Hole: How Good a Case Is It?", Eckart, Andreas; Hüttemann, Andreas; Kiefer, Claus; Britzen, Silke; Zajaček, Michal; Lämmerzahl, Claus; Stöckler, Manfred; Valencia-S, Monica; Karas, Vladimir; García-Marín, Macarena, Springer, Foundations of Physics, Volume 47, Issue 5, pp.553-624

"Polarized observations of a Dusty S-cluster Object (DSO/G2) at the Galactic Centre", B. Shahzamanian, A. Eckart, M. Zajaček,  M. Valencia-S., N. Sabha, L. Moser, M. Parsa, F. Peissker, C. Straubmeier, Astronomy & Astrophysics 593, Art. No. A131 (2016)

"Nature of the Galactic centre NIR-excess sources. I. What can we learn from the continuum observations of the DSO/G2 source?", M. Zajaček, S. Britzen, A. Eckart, B. Shahzamanian, G. Busch, V. Karas, M. Parsa, F. Peissker, M. Dovčiak, M. Subroweit, F. Dinnbier, J. A. Zensus, Astronomy & Astrophysics 602, Art. No. A121 (2017)

"Polarimetry narrows down the possibilities for the Dusty S-cluster Object (DSO/G2) in the Galactic centre", M. Zajaček, A. Eckart, B. Shahzamanian, The Observatory (A Review of Astronomy) 137, June issue (2017)

"Effect of an isotropic outflow from the Galactic Centre on the bow-shock evolution along the orbit", M. Zajaček, A. Eckart, V. Karas, D. Kunneriath, B. Shahzamanian, N. Sabha, K. Mužić, M. Valencia-S., Mon. Not. R. Astron. Soc. 455, 1257-1274 (2016)

"Submillimeter and radio variability of Sagittarius A*. A statistical analysis", M. Subroweit, M. García-Marín, A. Eckart, A. Borkar, M. Valencia-S., G. Witzel, B. Shahzamanian, C. Straubmeier, Astronomy & Astrophysics 601, Art. No. A80 (2017)

"Monitoring the Galactic Centre with the Australia Telescope Compact Array", A. Borkar, A. Eckart, C. Straubmeier, D. Kunneriath, B. Jalali, N. Sabha, B. Shahzamanian, M. Garcia-Marin, M. Valencia-S, L. Sjouwerman, S. Britzen, V. Karas, M. Dovciak, A. Donea, A. Zensus, Mon. Not. R. Astron. 458, 2336-2349 (2016)

"Bright X-ray flares from Sgr A*", G. D. Karssen, M. Bursa, A. Eckart, M. Valencia-S, M. Dovčiak, V. Karas, J. Horák, Mon. Not. R. Astron. Soc. 472, 4422-4433 (2017)

"Approaching hell's kitchen: Molecular daredevil clouds in the vicinity of Sagittarius A*", L. Moser, Á. Sánchez-Monge, A. Eckart, M. A. Requena-Torres, M. García-Marin, D. Kunneriath, A. Zensus, S. Britzen, N. Sabha, B. Shahzamanian, A. Borkar, S. Fischer, Astronomy & Astrophysics 603, Art. No. A68 (2017)

"Monitoring the Dusty S-cluster Object (DSO/G2) on its Orbit toward the Galactic Center Black Hole", M. Valencia-S., A. Eckart, M. Zajacek, F. Peissker, M. Parsa, N. Grosso, E. Mossoux, D. Porquet, B. Jalali, V. Karas, S. Yazici, B. Shahzamanian, N. Sabha, R. Saalfeld, S. Smajic, R. Grellmann, L. Moser, M. Horrobin, A. Borkar, M. Garcia-Marin, M. Dovciak, D. Kunneriath, G. D. Karssen, M. Bursa, C. Straubmeier, H. Bushouse,  Astrophys. J. 800, Art. No. 125 (2015)

"Investigating the Relativistic Motion of the Stars Near the Supermassive Black Hole in the Galactic Center", M. Parsa, A. Eckart, B. Shahzamanian, V. Karas, M. Zajaček, J. A. Zensus, C. Straubmeier, Astrophys. J. 845, Art. No. 22 (2017)

"The extreme luminosity states of Sagittarius A*"  Sabha, N.; Witzel, G.; Eckart, A.; Buchholz, R. M.; Bremer, M.; Gießübel, R.; García-Marín, M.; Kunneriath, D.; Muzic, K.; Schödel, R.; Straubmeier, C.; Zamaninasab, M.; Zernickel, A., Astronomy & Astrophysics, 512, Art. No. A2 (2010)

"The S-star cluster at the center of the Milky Way. On the nature of diffuse NIR emission in the inner tenth of a parsec", Sabha, N.; Eckart, A.; Merritt, D.; Zamaninasab, M.; Witzel, G.; García-Marín, M.; Jalali, B.; Valencia- ., M.; Yazici, S.; Buchholz, R.; Shahzamanian, B.; Rauch, C.; Horrobin, M.; Straubmeier, C., Astronomy & Astrophysics 545, Art. No. A70 (2012)

"Source-intrinsic Near-infrared Properties of Sgr A*: Total Intensity Measurements", Witzel, G.; Eckart, A.; Bremer, M.; Zamaninasab, M.; Shahzamanian, B.; Valencia-S., M.; Schödel, R.; Karas, V.; Lenzen, R.; Marchili, N.; Sabha, N.; Garcia-Marin, M.; Buchholz, R. M.; Kunneriath, D.; Straubmeier, C., Astrophys. J. 203, Art. No. 36 (2012)

"Near infrared flares of Sagittarius A*. Importance of near infrared polarimetry", Zamaninasab, M.; Eckart, A.; Witzel, G.; Dovciak, M.; Karas, V.; Schödel, R.; Gießübel, R.; Bremer, M.; García-Marín, M.; Kunneriath, D.; Mužić, K.; Nishiyama, S.; Sabha, N.; Straubmeier, C.; Zensus, A., Astronomy and Astrophysics 510, id.A3 (2010)

"Near-infrared polarimetry as a tool for testing properties of accreting supermassive black holes", Zamaninasab, M.; Eckart, A.; Dovčiak, M.; Karas, V.; Schödel, R.; Witzel, G.; Sabha, N.; García-Marín, M.; Kunneriath, D.; Mužić, K.; Straubmeier, C.; Valencia-S, M.; Zensus, J. A., Monthly Notices of the Royal Astronomical Society 413, Issue 1 (2011)

 

Andreas Eckart cutting the cake (provided by Vladimir Karas, ASTRONOMICKY USTAV AVCR VVI)
to celebrate the 100 year anniversary of General Relativity during an FP7 meeting in Prague.