arXiv subjects

S. J. Wagner

Publications and source records attributed to S. J. Wagner.

51 records · Page 3Linked to original sources

arXiv2001

Gamma-Ray Emission from Active Galactic Nuclei - an Overview

Gamma-ray emission from AGN provides us with unprecedented insights into the physics of extragalactic jets. The emission from these jets fits naturally into the existing theoretical framework of relativistic jets as inferred from parsec scale and kiloparsec scale observations. Models of the $γ$-ray fluxes give us important knowledge of jet parameters, in particular magnetic fields and the size of the emitting region. For example, in the case of Markarian~501, the jet plasma is subequipartiion and the energy flux is particle dominated. Discussion of the physics of jets on the sub-parsec scale leads naturally to questions of jet composition. The dynamics of jets are consistent with conventional electron-proton composition if the minimum Lorentz factors, $γ_{\rm min} > 100$. Jets can be electron-positron if $γ_{\rm min} \approx 100$.

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arXiv2001

A wide field survey at the Northern Ecliptic Pole: II. Number counts and galaxy colours in B_j, R, and K

We present a medium deep survey carried out in the three filters B_j, R and K. The survey covers homogeneously the central square degree around the Northern Ecliptic Pole (NEP) down to a completeness limit of 24.25, 23.0 and 17.5mag in B_j, R and K, respectively. While the near infrared data have been presented in the first paper of this series, here we concentrate on the optical data and the results based on the combined B_jRK-data. The unique combination of area and depth in our survey allows to perform a variety of investigations based on homogeneous material covering more than ten magnitudes in apparent brightness. We analyze the number counts for point-like and extended sources in B_j and R to determine the slopes in dlogN/dm and to test for possible breaks therein. While we can confirm the slopes found in previous works with a higher statistical significance, the largest uncertainty remaining for the amplitudes is galactic extinction. We determine the colour distributions of galaxies in B_j-R and R-K down to B_j=24.0 and K=18.0mag, respectively. The distributions in both colours are modeled using galaxy spectral evolution synthesis. We demonstrate that the standard models of galaxy evolution are unable to reproduce the steady reddening trend in R-K despite flawless fits to the colour distributions in the optical (B_j-R). The B_jRK data collected over a large area provides the opportunity to select rare objects like candidates for high-redshift galaxies and extremely red objects (EROs, R-K>5.0) and to determine their surface density. Our EROs are selected at an intermediate magnitude range and contain contribution from both galactic as well as extragalactic sources. At K<16.5mag, where a morphological classification is possible, the stellar component dominates the sample.

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arXiv2001

The FORS Deep Field

Dedicating a major fraction of its guaranteed time, the FORS consortium established a FORS Deep Field which contains a known QSO at z = 3.36. It was imaged in UBgRIz with FORS at the VLT as well as in J and Ks with the NTT. Covering an area 6-8 times larger as the HDFs but with similar depth in the optical it is one of the largest deep fields up to date to investigate i) galaxy evolution in the field from present up to z $\sim$ 5, ii) the galaxy distribution in the line of sight to the QSO, iii) the high-z QSO environment and iv) the galaxy-galaxy lensing signal in such a large field. In this presentation a status report of the FORS Deep Field project is given. In particular, the field selection, the imaging results (number counts, photometric redshifts etc.) and the first spectroscopic results are presented.

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arXiv2000

Kinematics and morphology of the Narrow-Line Region in the Seyfert galaxy NGC1386

We present a high spatial and spectral resolution 2-D echelle spectrogram of the Narrow-Line Region in the Seyfert 2 galaxy NGC1386. This Seyfert galaxy was observed with CASPEC in the wavelength range 5270-7725 Angstrom which covers the H-alpha and the [N II] lines. With the use of spatially high resolved images taken with the WFPC2 aboard the Hubble Space Telescope we could identify individual components of the Narrow-Line Region in our spectra. A Gaussian decomposition of the spectra revealed 9 distinct emission-line complexes. The brightest component is blue-shifted by -120+-10 km/s with respect to the systemic velocity and shows an offset of -1.6" relative to the nucleus of the galaxy. The true nucleus of NGC1386 has a much lower apparent H-alpha luminosity than this component. The nucleus is probably highly absorbed. Although the majority of the Narrow-Line Region components follows a regular velocity field, we find evidence for a separate kinematic component. The Narrow-Line Region is aligned anti-parallel to the radio-jet which propagates from the center of NGC1386 to the south.

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arXiv1999

A wide field survey at the Northern Ecliptic Pole I: Number counts and angular correlation functions in K

We present the results from a multi colour survey performed at the Northern Ecliptic Pole (NEP). The survey is designed to identify the counterparts of faint sources from the ROSAT All Sky Survey and the IRAS survey and to study their optical/near-infrared properties. We observed the central square degree around the NEP in the optical bands B_J and R and in the near-infrared band K. A shallower survey was carried out in the optical I band. Here we present the results of the K-band survey. We discuss the source counts in the magnitude range 7mag < K < 17.5mag and the angular correlation function of galaxies with K < 17.0mag. The galaxy counts at the NEP display a subeuklidean slope in dlog(N)/dm. Our shallower slope does not require the large effects of galaxy evolution or density evolution suggested to explain the steeper slopes found in earlier surveys. The angular correlation function of galaxies follows a power law w(θ) = Aθ^-δwith A=5.7+-0.8 x 10^-3 and δ=0.98+-0.15. This is in accordance with the expected values for stable clustering.

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arXiv1999

A search for broad infrared recombination lines in NGC 1068

We report infrared spectroscopy of the prototypical Seyfert 2 galaxy NGC 1068, aiming at detection of broad components of hydrogen recombination lines that originate in the obscured broad-line region. Using the Short Wavelength Spectrometer on board the Infrared Space Observatory, we have observed for the first time the regions of Brackett beta 2.626um and Pfund alpha 7.460um, and present improved data for Brackett alpha 4.052um. No significant broad components are detected, implying an equivalent visual extinction to the broad-line region of at least 50 magnitudes and an obscuring column density of at least 10^23 cm^-2. While consistent with a highly obscured broad-line region, as required by the classical unified scenario, these limits are not yet significant enough to discriminate strongly between different torus models or to constrain properties of the gas causing the very large X-ray obscuration. We discuss the systematic limitations of infrared broad-line region searches and suggest that Brackett alpha may often be the most favorable transition for future searches.

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arXiv1999

Unusual radio variability in the BL Lac object 0235+164

We present radio observations at three frequencies and contemporaneous optical monitoring of the peculiar BL Lac object AO 0235+164. During a three-week campaign with the VLA we observed intraday variability in this source and found a distinct peak which can be identified throughout the radio frequencies and tentatively connected to the R-band variations. This event is characterized by unusual properties: its strength increases, and its duration decreases with wavelength, and it peaks earlier at 20cm than at 3.6 and 6cm. We discuss several generic models (a `standard' shock-in-jet model, a precessing beam, free-free-absorption in a foreground screen, interstellar scattering, and gravitational microlensing), and explore whether they can account for our observations. Most attempts at explaining the data on 0235+164 require an extremely small source size, which can be reconciled with the $10^{12}$ K inverse Compton limit only when the Doppler factor of the bulk flow is of order 100. However, none of the models is completely satisfactory, and we suggest that the observed variability is due to a superposition of intrinsic and propagation effects.

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arXiv1998

A Study of High Energy Emission from the TeV blazar Mrk 501 during Multiwavelength Observations in 1996

We present the results of a multiwavelength campaign for Mrk 501 performed in March 1996 with ASCA, EGRET, Whipple, and optical telescopes. In the X-ray band, a spectral break was observed around 2 keV. We report here for the first time the detection of high-energy gamma-ray flux from Mrk 501 with EGRET with 3.5 sigma significance (E>100 MeV). Higher flux was also observed in April/May 1996, with 4.0 sigma significance for E>100 MeV, and 5.2 sigma significance for E>500 MeV. The gamma-ray spectrum was measured to be flatter than most of the gamma-ray blazars. We find that the multiband spectrum in 1996 is consistent with that calculated from a one-zone SSC model in a homogeneous region. In the context of this model, we investigate the values of the magnetic field strength and the beaming factor allowed by the observational results. We compare the March 1996 multiwavelength spectrum with that in the flare state in April 1997. Between these two epochs, the TeV flux increase is well correlated with that observed in keV range. The keV and TeV amplitudes during the April 1997 flare are accurately reproduced, assuming that the population of synchrotron photons in 1996 are scattered by the newly injected relativistic electrons, having maximum energies of G_max = 6.0e6. However, the TeV spectrum observed during March 1996 campaign is flatter than predicted by our models. We find that this cannot be explained by either higher order Comptonization or the contribution of the `seed' IR photons from the host galaxy for the first-order external radiation Comptonization.

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arXiv1998

Gamma-ray to radio activity and ejection of a VLBI component in the jet of the S5-quasar 0836+710

Broad-band (gamma to radio) variations of the flux density were observed in the first half of 1992 in the luminous high redshift (z = 2.172) quasar S5 0836+710. VLBI monitoring observations during 1993 -- 1996 performed at 86 GHz, 22 GHz, 15 GHz, and 8 GHz show the ejection of a new jet component, which most probably is directly related to a quasi simultaneous gamma-, X-ray, optical flaring activity which was observed in February 1992. During the period 1992 -- 1993 the flaring propagated through the radio spectrum. From several quasi-simultaneous radio spectra taken during this phase of activity, we determine the time evolution of the spectral turnover of the radio spectrum in the S_m-ν_m diagram. The data indicate a correlation of the jet activity with the variability of the broad-band electromagnetic spectrum of the source. The observational findings are discussed in the framework of relativistic shock models.

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arXiv1998

Intensive monitoring of the strongly variable BL Lac S5 0716+714

The BL Lac object S5 0716+714 was monitored during a multifrequency campaign in 1996. Preliminary analysis of the optical, ROSAT and RXTE data are presented. Strong variability on short time scales was observed. The data suggest an interpretation within a multi-component model.

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arXiv1998

Markarian 501 in X-ray bright state - RXTE observations

Mrk 501 has been in a state of very high flux in X-rays and VHE gamma-rays during 1997. In July 1997 near its hitherto maximum X-ray brightness intense multifrequency observations of Mrk 501 have been performed at radio, near infrared, optical, X-ray, and VHE gamma-ray frequencies. Here we report on Rossi X-ray Timing Explorer (RXTE) observations carried out in 1997 between July 11 and July 16. The X-ray spectrum has been well detected up to 100 keV and is best described by a broken power law with spectral indices alpha1=0.70 and alpha2=0.94 below and above the break energy 5.8 keV. The X-ray flux from Mrk 501 declined and flared by about 30% within about 3 days each, showing an unusual anti-correlation between flux and spectral hardening of both power law components. The break energy remained constant. The observed broad band and X-ray spectra confirm recent observations, that the synchrotron component in Mrk 501 can extend up to energies of 100 keV with the maximum power being emitted at hard X-rays. On longer time-scales the cut-off frequency of the synchrotron spectrum changes by more than two orders of magnitude.

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arXiv1997

Intraday variability in x-ray selected BL Lacertae objects

We present a study of the intraday variability behaviour of two samples of x-ray selected BL Lac objects, the EMSS and EXOSAT samples consisting of 22 and 11 sources, respectively. In both samples we were able to detect intraday variability in less than 40% of the sources only. The duty cycle (the fraction of time, when a BL Lac object is variable) in x-ray selected BL Lac objects is 0.4 or less. The typical peak-to-peak amplitudes of the variability are 10%. Typical time-scales and an activity parameter for our variable BL Lac objects were inferred from structure function and autocorrelation function analyses. In only 4 BL Lac objects we were able to measure a characteristic time-scale, which was in the range between 1.3 and 2.7 days. Comparison with our previous study of a complete sample of radio-selected BL Lac objects from the 1 Jy catalogue shows that x-ray and radio-selected BL Lac objects differ in their duty cycle by a factor of 2 and the typical peak-to-peak amplitudes by a factor of 3. The observed time-scales are similar. We also found that the same mechanism may be responsible for the observed variability in the x-ray selected and radio-selected BL Lac objects. The expectations of the various schemes linking x-ray selected and radio-selected BL Lac objects have been compared to our observations. Consistency is found for a scenario, where x-ray selected BL Lac objects have on average stronger magnetic fields and are seen under relatively larger viewing angles than the radio-selected BL Lac objects. However, the suggestion that x-ray selected BL Lac objects have decelerating jets and radio-selected BL Lac objects accelerating jets can also not be ruled out.

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arXiv1996

An Intense Gamma-Ray Flare of PKS1622-297

We report the observation by the Compton Gamma Ray Observatory of a spectacular flare of radio source PKS 1622-297. A peak flux of 17E-6 cm^-2 s^-1 (E > 100 MeV) was observed. The corresponding isotropic luminosity is 2.9E49 erg/s. We find that PKS 1622-297 exhibits gamma-ray intra-day variability. A flux increase by a factor of at least 3.6 was observed to occur in less than 7.1 hours (with 99% confidence). Assuming an exponential rise, the corresponding doubling time is less than 3.8 hours. A significant flux decrease by a factor of ~2 in 9.7 hours was also observed. Without beaming, the rapid flux change and large isotropic luminosity are inconsistent with the Elliot-Shapiro condition (assuming that gas accretion is the immediate source of power for the gamma-rays). This inconsistency suggests that the gamma-ray emission is beamed. A minimum Doppler factor of 8.1 is implied by the observed lack of pair-production opacity (assuming x-rays are emitted co-spatially with the gamma-rays). Simultaneous observation by EGRET and OSSE finds a spectrum adequately fit by a power law with photon index of -1.9. Although the significance is not sufficient to establish this beyond doubt, the high-energy gamma-ray spectrum appears to evolve from hard to soft as a flare progresses.

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arXiv1996

CGRO, Radio and Optical Observations of the Quasar NRAO 140

We report on Compton Gamma Ray Observatory (CGRO), radio and optical observations of the radio-loud, superluminal quasar NRAO 140. The source is not detected (at a significance of greater than 3 sigma) by any of the CGRO instruments OSSE, Comptel and EGRET. Radio observations simultaneous to the OSSE observation and optical monitoring over three-monthly intervals show no signs of extraordinary behaviour when compared to previous observations. We demonstrate that the CGRO non-detections do not require a spectral break at hard X-rays, but can be explained by a steep spectrum in the MeV range comparable to that of 3C273.

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arXiv1996

ROSAT HRI observations of Centaurus A

We present results from a sensitive high-resolution X-ray observation of the nearby active galaxy Centaurus A (NGC 5128) with the ROSAT HRI. The 65~ksec X-ray image clearly distinguishes different components of the X-ray emission from Cen A: the nucleus and the jet, the diffuse galaxy halo, and a number of individual sources associated with the galaxy. The luminosity of the nucleus increased by a factor of two compared to an earlier ROSAT observation in 1990. The high spatial resolution of the ROSAT HRI shows that most of the knots in the jet are extended both along and perpendicular to the jet axis. We report the detection of a new X-ray feature, at the opposite side of the X-ray jet which is probably due to compression of hot interstellar gas by the expanding southwestern inner radio lobe.

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