arXiv subjects

S. Wagner

Publications and source records attributed to S. Wagner.

97 records · Page 6Linked to original sources

arXiv2006

Absorption of Nuclear Gamma-rays on the Starlight Radiation in FR I Sources: the Case of Centaurus A

Several BL Lac objects are confirmed sources of variable and strongly Doppler-boosted TeV emission produced in the nuclear portions of their relativistic jets. It is more than probable, that also many of the FR I radio galaxies, believed to be the parent population of BL Lacs, are TeV sources, for which Doppler-hidden nuclear gamma-ray radiation may be only too weak to be directly observed. Here we show, however, that about one percent of the total time-averaged TeV radiation produced by the active nuclei of low-power FR I radio sources is inevitably absorbed and re-processed by photon-photon annihilation on the starlight photon field, and the following emission of the created and quickly isotropized electron-positron pairs. In the case of the radio galaxy Centaurus A, we found that the discussed mechanism can give a distinctive observable feature in the form of an isotropic gamma-ray halo. It results from the electron-positron pairs injected to the interstellar medium of the inner parts of the elliptical host by the absorption process, and upscattering starlight radiation via the inverse-Compton process mainly to the GeV-TeV photon energy range. Such a galactic gamma-ray halo is expected to possess a characteristic spectrum peaking at ~0.1 TeV photon energies, and the photon flux strong enough to be detected by modern Cherenkov Telescopes and, in the future, by GLAST. These findings should apply as well to the other nearby FR I sources.

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arXiv2004

Internal kinematics of spiral galaxies in distant clusters. Part II. Observations and data analysis

We have conducted an observing campaign with FORS at the ESO-VLT to explore the kinematical properties of spiral galaxies in distant galaxy clusters. Our main goal is to analyse transformation- and interaction processes of disk galaxies within the special environment of clusters as compared to the hierarchical evolution of galaxies in the field. Spatially resolved MOS-spectra have been obtained for seven galaxy clusters at 0.3 2 hours we reach an S/N>5 in the emission lines appropriate for the deduction of the galaxies' internal rotation velocities from [OII], Hbeta, or [OIII] profiles. Preselection of targets was done on the basis of available redshifts as well as from photometric and morphological information gathered from own observations, archive data, and from the literature. Emphasis was laid on the definition of suitable setups to avoid the typical restrictions of the standard MOS unit for this kind of observations. In total we assembled spectra of 116 objects of which 50 turned out to be cluster members. Position velocity diagrams, finding charts as well as tables with photometric, spectral, and structural parameters of individual galaxies are presented.

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arXiv2002

Caught in the act: a helium-reionizing quasar near the line of sight to Q0302-003

We report the discovery of a quasar at z=3.050+/-0.003, closely coincident in redshift with the isolated low-opacity feature seen near z~3.056 in the otherwise black portion of the HeII Gunn-Peterson absorption trough seen toward the z=3.286 background quasar Q0302-003, located 6.5' away on the sky. We explore plausible models for the HeIII ionization zone created by this neighboring quasar and its interception with the line of sight toward Q0302-003. At its present brightness of V=20.5 and separation of D=3.2 Mpc, the quasar can readily account for the opacity gap in the HeII absorption spectrum of Q0302-003, provided it has been active for t_Q > 10^7 y. This is the first clear detection of the `transverse' proximity effect and imprint of a quasar on the intervening absorption detected along an adjacent line of sight.

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arXiv2000

SNR0540-69 -the Crab twin- revisited: Chandra vs. HST

High resolution images of the Crab-like SNR SNR0540-69 have been recently obtained by the HRC, on board the Chandra X-ray Observatory, and by the FORS1 instrument, attached to ESO/VLT1. A detailed comparison of these images with an archival Planetary Camera one will be presented.

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arXiv2000

Complex Spectral Variability from Intensive Multi-wavelength Monitoring of Mrk421 in 1998

We conducted a multi-frequency campaign for the TeV blazar Mrk~421 in 1998 April. The campaign started from a pronounced high amplitude flare recorded by SAX and Whipple; ASCA observation started three days later. In the X-ray data, we detected multiple flares, occuring on time scales of about one day. ASCA data clearly reveal spectral variability. The comparison of the data from ASCA, EUVE and RXTE indicates that the variability amplitudes in the low energy synchrotron component are larger at higher photon energies. In TeV Gamma-rays, large intra-day variations -- which were correlated with the X-ray flux -- were observed when results from three Cherenkov telescopes are combined. The RMS variability of TeV Gamma--rays was similar to that observed in hard X-rays, above 10 keV. The X-ray light curve reveals flares which are almost symmetric for most of cases, implying the dominant time scale is the light crossing time through the emitting region. The structure function analysis based on the continuous X-ray light curve of seven days indicates that the characteristic time scale is ~0.5 day. The analysis of ASCA light curves in various energy bands appears to show both soft (positive) and hard (negative) lags. These may not be real, as systematic effects could also produce these lags, which are all much smaller than an orbit. If the lags of both signs are real, these imply that the particle acceleration and X-ray cooling time scales are similar.

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arXiv2000

The DENIS Point Source Catalogue towards the Magellanic Clouds

We have compiled the near infrared Point Source Catalogue (PSC) towards the Magellanic Clouds (MCs) extracted from the data obtained with the Deep Near Infrared Survey of the Southern Sky - DENIS (Epchtein et al. 1997). The catalogue covers an area of of 19.87* 16 square degrees centered on (RA, DEC)=(5h27m20s, -69o00'00'') for the Large Magellanic Cloud (LMC) and 14.7* 10 square degrees centered on (RA, DEC)=(h02m40s, -73o00'00'' for the Small Magellanic Cloud (SMC) at the epoch J2000. It contains about 1300000 sources towards the LMC and 300000 sources towards the SMC each detected in at least 2 of the 3 photometric bands involved in the survey (I, J, Ks). 70% of the detected sources are true members of the Magellanic Clouds, respectively and consist mainly of red giants, asymptotic giant branch stars and super-giants. The observations have all been made with the same instrument and the data have been calibrated and reduced uniformly. The catalogue provides a homogeneous set of photometric data.

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arXiv2000

Coronal lines and the warm X-ray absorber in Seyfert 1 Galaxies

The connection between the coronal lines and the warm absorber is examined systematically. In an earlier work it was found that the coronal line emitting plasma and the warm absorber gas share the same density and temperature. If there is a connection between the warm absorber gas and the forbidden high-ionization line (FHIL) plasma, one can use the profiles of coronal lines to derive the kinematics and dynamics of the warm absorber due to the high spectral resolution available in the optical range. Further support for a connection is a correlation between the equivalent width of [Fe X] 6375 and the ROSAT spectral index found for an optically selected sample. For X-ray selected objects with absorption edges observed by ASCA, we looked for a correlation between the coronal lines and the warm absorber gas. A direct correlation cannot be confirmed.

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