The unidentified TeV source (TeVJ2032+4130) and surrounding field: Final HEGRA IACT-System results

The unidentified TeV source in Cygnus is now confirmed by follow-up observations from 2002 with the HEGRA stereoscopic system of Cherenkov Telescopes. Using all data (1999 to 2002) we confirm this new source as steady in flux over the four years of data taking, extended with radius 6.2 arcmin (+-1.2 arcmin (stat) +-0.9 arcmin (sys)) and exhibiting a hard spectrum with photon index -1.9. It is located in the direction of the dense OB stellar association, Cygnus OB2. Its integral flux above energies E>1 TeV amounts to \~5% of the Crab assuming a Gaussian profile for the intrinsic source morphology. There is no obvious counterpart at radio, optical nor X-ray energies, leaving TeVJ2032+4130 presently unidentified. Observational parameters of this source are updated here and some astrophysical discussion is provided. Also included are upper limits for a number of other interesting sources in the FoV, including the famous microquasar Cygnus X-3.Comment: 7 pages, 3 figures. Accepted for publication in Astronomy & Astrophysic

The dynamical Green's function and an exact optical potential for electron-molecule scattering including nuclear dynamics

We derive a rigorous optical potential for electron-molecule scattering including the effects of nuclear dynamics by extending the common many-body Green's function approach to optical potentials beyond the fixed-nuclei limit for molecular targets. Our formalism treats the projectile electron and the nuclear motion of the target molecule on the same footing whereby the dynamical optical potential rigorously accounts for the complex many-body nature of the scattering target. One central result of the present work is that the common fixed-nuclei optical potential is a valid adiabatic approximation to the dynamical optical potential even when projectile and nuclear motion are (nonadiabatically) coupled as long as the scattering energy is well below the electronic excitation thresholds of the target. For extremely low projectile velocities, however, when the cross sections are most sensitive to the scattering potential, we expect the influences of the nuclear dynamics on the optical potential to become relevant. For these cases, a systematic way to improve the adiabatic approximation to the dynamical optical potential is presented that yields non-local operators with respect to the nuclear coordinates.Comment: 22 pages, no figures, accepted for publ., Phys. Rev.

Optical study of orbital excitations in transition-metal oxides

The orbital excitations of a series of transition-metal compounds are studied by means of optical spectroscopy. Our aim was to identify signatures of collective orbital excitations by comparison with experimental and theoretical results for predominantly local crystal-field excitations. To this end, we have studied TiOCl, RTiO3 (R=La, Sm, Y), LaMnO3, Y2BaNiO5, CaCu2O3, and K4Cu4OCl10, ranging from early to late transition-metal ions, from t_2g to e_g systems, and including systems in which the exchange coupling is predominantly three-dimensional, one-dimensional or zero-dimensional. With the exception of LaMnO3, we find orbital excitations in all compounds. We discuss the competition between orbital fluctuations (for dominant exchange coupling) and crystal-field splitting (for dominant coupling to the lattice). Comparison of our experimental results with configuration-interaction cluster calculations in general yield good agreement, demonstrating that the coupling to the lattice is important for a quantitative description of the orbital excitations in these compounds. However, detailed theoretical predictions for the contribution of collective orbital modes to the optical conductivity (e.g., the line shape or the polarization dependence) are required to decide on a possible contribution of orbital fluctuations at low energies, in particular in case of the orbital excitations at about 0.25 eV in RTiO3. Further calculations are called for which take into account the exchange interactions between the orbitals and the coupling to the lattice on an equal footing.Comment: published version, discussion of TiOCl extended to low T, improved calculation of orbital excitation energies in TiOCl, figure 16 improved, references updated, 33 pages, 20 figure

Simultaneous X-Ray and TeV Gamma-Ray Observations of the TeV Blazar Markarian 421 during February and May 2000

In this paper we present the results of simultaneous observations of the TeV blazar Markarian 421 (Mrk 421) at X-ray and TeV Gamma-ray energies with the Rossi X-Ray Timing Explorer (RXTE) and the stereoscopic Cherenkov Telescope system of the HEGRA (High Energy Gamma Ray Astronomy) experiment, respectively. The source was monitored from February 2nd to February 16th and from May 3rd to May 8th, 2000. We discuss in detail the temporal and spectral properties of the source. Remarkably, the TeV observations of February 7th/8th showed statistically significant evidence for substantial TeV flux variability on 30 min time scale. We show the results of modeling the data with a time dependent homogeneous Synchrotron Self-Compton (SSC) model. The X-ray and TeV gamma-ray emission strengths and energy spectra together with the rapid flux variability strongly suggest that the emission volume is approaching the observer with a Doppler factor of 50 or higher. The different flux variability time scales observed at X-rays and TeV Gamma-rays indicate that a more detailed analysis will require inhomogeneous models with several emission zones.Comment: Accepted for Publication in ApJ, 21 Pages, 5 Figure

Is the giant radio galaxy M 87 a TeV gamma-ray emitter?

For the first time an excess of photons above an energy threshold of 730 GeV from the giant radio galaxy M 87 has been measured at a significance level above 4 σ. The data have been taken during the years 1998 and 1999 with the HEGRA stereoscopic system of 5 imaging atmospheric Cherenkov telescopes. The excess of 107.4 ± 26.8 events above 730 GeV corresponds to an integral flux of 3.3% of the Crab flux or Nγ (E > 730 GeV) = (0.96 ± 0.23) × 10-12 phot cm-2 s-1. M 87 is located at the center of the Virgo cluster of galaxies at a relatively small redshift of z = 0.00436 and is a promising candidate among the class of giant radio galaxies for the emission of TeV γ-radiation. The detection of TeV γ-rays from M 87 - if confirmed - would establish a new class of extragalactic source in this energy regime since all other AGN detected to date at TeV energies are BL Lac type objects.F. A. Aharonian ...G. P. Rowell...et al

Observations of H1426+428 with HEGRA -- Observations in 2002 and reanalysis of 1999&2000 data

The HEGRA system of imaging air Cherenkov telescopes has been used to observe the BL Lac object H1426+428 ($z=0.129$) for 217.5 hours in 2002. In this data set alone, the source is detected at a confidence level of $5.3~\sigma$, confirming this object as a TeV source. The overall flux level during the observations in 2002 is found to be a factor of $\approx 2.5$ lower than during the previous observations by HEGRA in 1999&2000. A new spectral analysis has been carried out, improving the signal-to-noise ratio at the expense of a slightly increased systematic uncertainty and reducing the relative energy resolution to $\Delta E/E\le 12$ over a wide range of energies. The new method has also been applied to the previously published data set taken in 1999 and 2000, confirming the earlier claim of a flattening of the energy spectrum between 1 and 5 TeV. The data set taken in 2002 shows again a signal at energies above 1 TeV. We combine the energy spectra as determined by the CAT and VERITAS groups with our reanalyzed result of the 1999&2000 data set and apply a correction to account for effects of absorption of high energy photons on extragalactic background light in the optical to mid infrared band. The shape of the inferred source spectrum is mostly sensitive to the characteristics of the extragalactic background light between wavelengths of 1 and 15~$\mu$mComment: 12 pages, 4 Figures, submitted to A&

Rejection of the hypothesis that Markarian 501 TeV photons are pure Bose-Einstein condensates

The energy spectrum of the Blazar type galaxy Markarian 501 (Mrk 501) as measured by the High-Energy-Gamma-Ray Astronomy (HEGRA) air Cerenkov telescopes extends beyond 16 TeV and constitutes the most energetic photons observed from an extragalactic object. A fraction of the emitted spectrum is possibly absorbed in interactions with low energy photons of the diffuse extragalactic infrared radiation, which in turn offers the unique possibility to measure the diffuse infrared radiation density by TeV spectroscopy. The upper limit on the density of the extragalactic infrared radiation derived from the TeV observations imposes constraints on models of galaxy formation and stellar evolution. One of the recently published ideas to overcome severe absorption of TeV photons is based upon the assumption that sources like Mrk 501 could produce Bose-Einstein condensates of coherent photons. The condensates would have a higher survival probability during the transport in the diffuse radiation field and could mimic TeV air shower events. The powerful stereoscopic technique of the HEGRA air Cerenkov telescopes allows to test this hypothesis by reconstructing the penetration depths of TeV air shower events: Air showers initiated by Bose-Einstein condensates are expected to reach the maximum of the shower development in the atmosphere earlier than single photon events. By comparing the energy-dependent penetration depths of TeV photons from Mrk 501 with those from the TeV standard-candle Crab Nebula and simulated air shower events, we can reject the hypothesis that TeV photons from Mrk 501 are pure Bose-Einstein condensates.Comment: 9 pages, 2 figures, published by ApJ Letters, revised version (simulation results added