Production and evolution path of dileptons at energies accessible to the HADES detector
Dilepton production in intermediate energy nucleus-nucleus collisions as well as in elementary proton-proton reactions is analyzed within the ultrarelativistic quantum molecular dynamics transport model. For C+C collisions at 1A GeV and 2A GeV the resulting invariant mass spectra are compared to recent HADES data. We find that the experimental spectrum for C+C at 2A GeV is slightly overestimated by the theoretical calculations in the region around the vector meson peak but fairly described in the low mass region, where the data are satisfactorily saturated by the Dalitz decay of the {eta} meson and the {delta} resonance. At 1A GeV an underestimation of the experimental data is found, pointing that at lower energies the low-mass region is not fully saturated by standardly parametrized {delta} Dalitz decays alone. Furthermore, predictions for dilepton spectra for pp reactions at 1.25, 2.2, and 3.5 GeV and Ar+KCl reactions at 1.75A GeV are presented. The study is complemented by a detailed investigation of the role of absorption of the parent particles on the corresponding dilepton yields in the regime that has so far been probed by HADES.

title: How sensitive are di-leptons from rho mesons to the high baryon density region?
abstract: We show that the measurement of dileptons might provide only a restricted view into the most dense stages of heavy-ion reactions. Thus, possible studies of meson and baryon properties at high baryon densities, as, e.g., done at the GSI High Acceptance DiElectron Spectrometer (HADES) and envisioned for the Facility for Antiproton and Ion Research (FAIR) compressed baryonic matter experiments, might observe weaker effects than currently expected in certain approaches. We argue that the strong absorption of resonances in the high-baryon-density region of the heavy-ion collision masks information from the early hot and dense phase due to a strong increase of the total decay width because of collisional broadening. To obtain additional information, we also compare the currently used approaches to extract dileptons from transport simulations, i.e., shining, only vector mesons from final baryon resonance decays and instant emission of dileptons and find a strong sensitivity on the method employed in particular at FAIR and the CERN Super Proton Synchrotron energies. It is shown explicitly that a restriction to \ensuremath{\rho} meson (and therefore dilepton) production only in final-state baryon resonance decays provide a strong bias toward rather low baryon densities. The results presented are obtained from ultrarelativistic quantum molecular dynamics v2.3 calculations using the standard setup.

title: Kolmogorov-Smirnov test and its use for the identification of fireball fragmentation
abstract: We propose an application of the Kolmogorov-Smirnov test for rapidity distributions of individual events in ultrarelativistic heavy-ion collisions. The test is particularly suited to recognizing nonstatistical differences between the events. Thus when applied to a narrow centrality class it could indicate differences between events that would not be expected if all events evolved according to the same scenario. In particular, as an example we assume here a possible fragmentation of the fireball into smaller pieces at the quark/hadron phase transition. Quantitative studies are performed with a Monte Carlo model capable of simulating such a distribution of hadrons. We conclude that the Kolmogorov-Smirnov test is a very powerful tool for the identification of the fragmentation process.

title: Elliptic flow fluctuations in heavy ion collisions and the perfect fluid hypothesis
abstract: We analyze the recently measured v2 fluctuation in the context of establishing the degree of fluidity of the matter produced in heavy ion collisions. We argue that flow observables within systems with a non-negligible mean free path should acquire a “dynamical” fluctuation, due to the random nature of each collision between the system’s degrees of freedom. Because of this, v2 fluctuations can be used to estimate the Knudsen number of the system produced at the relativistic heavy ion collider (RHIC). To illustrate this quantitatively, we apply the UrQMD model, with scaled cross sections, to show that collisions at RHIC have a Knudsen number at least one order of magnitude below the expected value for an interacting hadron gas. Furthermore, we argue that the Knudsen number is also bound from below by the v2 fluctuation data, because too small a Knudsen number would break the observed scaling of v2 fluctuations due to the onset of turbulent flow. We propose, therefore that v2 fluctuation measurements, together with an understanding of the turbulent regime for relativistic hydrodynamics, will provide an upper as well as a lower limit for the Knudsen number. We also argue that an energy scan of v2 fluctuations could shed light on the onset of the fluid regime.

title: Dynamic canonical suppression of strangeness in transport models
abstract: It is investigated whether canonical suppression associated with the exact conservation of an U(1)-charge can be reproduced correctly by current transport models. Therefore a pion-gas having a volume-limited cross section for kaon production and annihilation is simulated within two different transport prescriptions for realizing the inelastic collisions. It is found that both models can indeed dynamically account for the canonical suppression in the yields of rare strange particles.

title: Resonance Multiplicities in a Hadron Transport Approach
abstract: In this article we focus on the multiplicities of resonances, ratios of resonant over non-resonant states and rescattering processes in heavy ion collisions. Therefore we utilize a hadron transport model (UrQMD vl.3). We find that rescattering of decay particles is of great importance when studying resonances in a hadronic medium.

title: Coordinated multi-wavelength observations of Sgr A*
abstract: We report on recent near-infrared (NIR) and X-ray observations of Sagittarius A* (Sgr A*), the electromagnetic manifestation of the !4" 10 6 M! super-massive black hole (SMBH) at the Galactic Center. The goal of these coordinated multi-wavelength observations is to investigate the variable emission from Sgr A* in order to obtain a better understanding of the underlying physical processes in the accretion flow/outflow. The observations have been carried out using the NACO adaptive optics (AO) instrument at the European Southern Observatory's Very Large Telescope (July 2005, May 2007) and the ACIS-I instrument aboard the Chandra X- ray Observatory (July 2005). We report on a polarized NIR flare synchronous to a 8" 10 33 erg/s X-ray flare in July 2005, and a further flare in May 2007 that shows the highest sub-flare to flare contrast observed until now. The observations can be interpreted in the framework of a model involving a temporary disk with a short jet. In the disk component flux density variations can be explained due to hot spots on relativistic orbits around the central SMBH. The variations of the sub-structures of the May 2007 flare are interpreted as a variation of the hot spot structure due to di! erential rotation within the disk.

title: Radio Observations of AGN in Low Surface Brightness Galaxies
abstract: We present preliminary results of a study of the low frequency radio continuum emission from the nuclei of Giant Low Surface Brightness (LSB) galaxies. We have mapped the emission and searched for extended features such as radio lobes/jets associated with AGN activity. LSB galaxies are poor in star formation and generally less evolved compared to nearby bright spirals. This paper presents low frequency observations of 3 galaxies; PGC 045080 at 1.4 GHz, 610 MHz, 325MHz, UGC 1922 at 610 MHz and UGC 6614 at 610 MHz. The observations were done with the GMRT. Radio cores as well as extended structures were detected and mapped in all three galaxies; the extended emission may be assocated with jets/lobes associated with AGN activity. Our results indicate that although these galaxies are optically dim, their nuclei can host AGN that are bright in the radio domain.

title: Coordinated mm/sub-mm observations of Sagittarius A* in May 2007
abstract: At the center of the Milky Way, with a distance of ~8 kpc, the compact source Sagittarius A* (SgrA*) can be associated with a super massive black hole of ~4x10^6 solar masses. SgrA* shows strong variability from the radio to the X-ray wavelength domains. Here we report on simultaneous NIR/sub-millimeter/X-ray observations from May 2007 that involved the NACO adaptive optics (AO) instrument at the European Southern Observatory's Very Large Telescope, the Australian Telescope Compact Array (ATCA), the US mm-array CARMA, the IRAM 30m mm-telescope, and other telescopes. We concentrate on the time series of mm/sub-mm data from CARMA, ATCA, and the MAMBO bolometer at the IRAM 30m telescope.