bluuebunny/crossref_metadata_2025_split huggingface dataset:
High-Energy Continuum Variability in Active Galactic Nuclei
Authors: Edelson Rick 
Published: July 2016

CGRO and IUE observations suggest that the strong, aperiodic variability seen in the Exosat long-look observations of AGN extends over a much wider energy band. Some BL Lac objects (but no Seyfert 1 galaxies) have shown X-ray variations which were so rapid that they violate the assumptions of isotropy inherent in the Eddington limit. In the ultraviolet, Seyfert 1s as a class show an anti-correlation between the variability amplitude and luminosity, while BL Lacs show a positive correlation. Furthermore, Seyfert 1s show strong flux-correlated spectral variability, while BL Lacs show little or none. All of this suggests that the high-energy continua of BL Lacs are beamed towards us, while the ultraviolet continua of Seyfert 1s are emitted isotropically.The November 1991 multi-waveband monitoring of the BL Lac PKS 2155−304 showed strong correlated variability, with the soft X-rays leading the ultraviolet by a few hours, and no measurable lag between the ultraviolet and optical down to a limit of ≲ 1.5 hr. This indicates that the X-rays from this BL Lac are not produced by Compton upscattering, and that the ultraviolet does not come directly from a thermal source such as an accretion disk. This also strongly constrains the relativistic jet model, suggesting that all of the radiation is produced in a flattened region like a shock front.Low temporal resolution ultraviolet/optical monitoring of the Seyfert 1 NGC 5548 in 1989 yielded a strong correlation with no measurable lag to a limit of ≲4 days, casting some doubt on the standard model of thermal emission from an accretion disk in Seyfert 1s. Upcoming X-ray/ultraviolet/optical monitoring of the Seyfert 1 NGC 4151 in December 1993 will have much faster sampling, to permit a strong test of both this model and the competing reprocessing model.
mm-VLBI: Jets in the Vicinity of Galaxy-Cores
Authors: Krichbaum T.P. , Standke K.J. , Graham D.A. , Witzel A. , Schalinski C.J. , Zensus J.A. 
Published: July 2016

Millimeter-VLBI provides an angular resolution of up to a few tens of microarcseconds and allows imaging of compact radio sources, self-absorbed at longer wavelengths, with unsurpassed angular resolution. At 43 GHz the participation of the VLBA and the 30 m-MRT at Pico Veleta (e.g. Krichbaum et al., 1993 a&b), and at 86 GHz the addition of the 100 m-RT at Effelsberg and the 30 m-MRT (Schalinski et al., 1993, and this volume) have improved the imaging capabilities of mm-VLBI observations.Results: The increased sensitivity of mm-VLBI observations allows the investigation of fainter objects, previously not accessible. As one example we show in Fig.1 the first detection of the compact radio source Sgr A∗ in the Galactic Center with VLBI at 43 GHz in May 1992 (Krichbaum et al., 1993d) and at 86 GHz in April 1993 (Krichbaum et al., 1994). In both observations the size of Sgr A∗ appeared to be larger than its expected scattering size, indicative of intrinsic source structure showing up at mm-wavelengths. Future monitoring with mm-VLBI is necessary to search for (not unexpected) structural variability.Monitoring of AGN with mm-VLBI reveals in all cases observed in sufficient detail jet curvatures of increasing amplitude towards the self-absorbed VLBI-cores (e.g. in 18034-784: Krichbaum, 1990, OJ 287: Krichbaum et al., 1993c), and sub- or superluminal motion along ‘quasi-helically’ bent trajectories (e.g. 3C 84: Krichbaum et al., 1993b; 3C 273: Krichbaum et al., 1993c), which differ sometimes for adjacent jet components (e.g. 3C 345: Krichbaum & Witzel, 1992, Krichbaum et al., 1992&1993a). In 3C 84, 3C 273 and 3C 345 the apparent velocity of jet components varies systematically along the jet axis, in 4C 39.25 (Alberdi et al., 1993) a moving component decelerates and brightens, all of this suggesting differential Doppler boosting and motion along three-dimensionally curved trajectories. In 3C 345 the complex kinematics of C4 and C5 (Zensus, this volume) has been geometrically modeled by motion along a helical path on the surface of a conical jet (Qian et al., 1992, Steffen et al., 1993, and this volume; see also Camenzind, this volume). As a new example, the oscillations of the inner jet and its velocity variations βapp(r) are shown for the BL Lac object 1803+784 in Fig. 2 (see the maps in: Krichbaum et al., 1993b). The frequent occurence of ‘quasi-sinusoidal’ bends in the inner jets of very different classes of AGN (QSO's, BL Lac's, Seyfert's) suggests that this effect is common in a large fraction of AGN and that the underlying jet-physical process may be fundamental for the understanding of the creation of jets.
Ground-Based Observations of PKS2155-304 in November 1991
Authors: Blecha A. , Courvoisier T. J.-L , Aller H. D. , Aller M. F. , Bouchet P. , Bratschi P. , Carini M. T. , Donahue M. , Feigelson E. D. , Filippenko A. V. , Glass I. S. , Heidt J. , Hughes P. A. , Kollgaard R. I. , Matheson T. , Miller H. R. , Noble J. C. , Smith P. S. , Wagner S. 
Published: July 2016

We present ground-based data of the BL Lac object PKS 2155-305 obtained during a large international campaign spanning the electro–magnetic spectrum from the radio waves to X-rays in November 1991. For the complete description of the observations and data analysis we refer to the paper by Courvoisier et al. 1993, and references therein. The ground-based data include radio, infrared JHKL and UBVRI fluxes as well as optical and near IR polarimetry.The broad-band optical and near IR data from U to I exhibit the same behaviour in all bands: the flux nearly doubled over the well-covered period of 23 days. The cross-correlation function does not reveal any significant changes in the light-curves. Though significant variations in 24 hours have been recorded, the cumulated Fourier power spectrum drops to the noise level for periods shorter than 2.5 days. The spectral index remained constant.The polarised flux varied by a larger factor than the total flux and did not follow the same pattern. The degree of polarisation and polarisation angle are nearly independent of the wavelength and are strongly correlated in all filters.In the radio domain the spectral index increased from −0.1 on November 5 to +0.02 on 25-th.The absence of the lag between the optical and infrared bands and the polarisation variations are consistent with a model in which the variability is caused by micro-lensing of the source (Stickel, Fried and Kühr 1988). One would, however, expect in this model that the variation in the polarisation and the total flux are tightly correlated contrary to what is observed.The constant shape of the continuum spectral energy suggests that only the number of electrons whose emission is beamed towards the observer changes, rather than the arrival of fresh electrons that are being accelerated.The variability of the polarisation may be explained by changes in the geometry of the magnetic field (dominant direction). This is consistent with the observed variations of the polarisation angle.
Soft X-ray Emission of Quasars
Authors: Schartel N. , Walter R. , Fink H.H. 
Published: July 2016

From a list of known quasars compiled from various catalogues we selected all sources detected by the PSPC (0.1 – 2.4 keV) aboard ROSAT with more than 80 counts during the all sky survey. A sample of 102 sources resulted. At higher redshifts most of the selected sources are radio-loud. At a redshift smaller than 0.50 we found 54 radio-quiet quasars and 30 radio-loud sources. For this reduced sample the mean spectral index of the radio-quiet sources (< Γ > = 2.53) and that of the radio-loud ones (< Γ > = 2.26) are clearly different with a significance of 3.3 σ.About 2/3 of the bright quasars observed with Einstein also belong to our sample. The spectra observed with ROSAT are sytematically steeper than the ones observed with Einstein yielding a < ΓROSAT – ΓEinstein > of 0.66 ± 0.18 for radio quiet and of 0.68 ± 0.19 for radio-loud sources, respectively.For radio loud quasars, the mean spectral slope decreases from 2.3 to 1.5 when the redshift increases beyond 0.5 (figure 1). The fact that high redshift sources show a photon index of about 1.5, which is similar to the mean index observed with Einstein for radio-loud sources, suggests that this decrease towards higher redshifts can be interpreted by the shift of the soft X-ray excess outside of the ROSAT spectral band when the redshift increases. The solid lines in figure 1 represent theoretical pathes of the photon index as a function of the redshift as derived from simulations assuming a power law plus black body model spectrum for the quasars X-ray emission. In curve No 1 the powerlaw index is fixed to 1.4. To be compatible with the observation the temperature of the blackbody component must range between 50 and 70 eV. Curve No 2 asssumes the same model with a powerlaw index fixed to 1.8 to account for radio quiet sources.
ROSAT Spectra and Lightcurves of Bright BL Lacertae Objects at Low Interstellar Absorption
Authors: Thomas H.-C. , Fink H. H. 
Published: July 2016

A programme to obtain soft X-ray spectra of bright BL Lac's from pointed observations with ROSAT has been started. So far 13 objects have been observed and another 6 have been accepted for observation. Available data for the following sources were reduced: OQ 530, OJ 287, B2 0912+29, GB 1011+496, ON 231, B2 1215+30, B2 1308+32, and Mrk 421.In this sample the most outstanding observation clearly is that of Mrk 421. This source was observed with the ROSAT PSPC for a total of 34 ksec between May 5 and May 7, 1992, at a mean countrate of 159 cts/s (see Fink et al., 1991, A&A246, L6 for ROSAT survey results). At the time of the observations Mrk 421 was also very luminous in the optical bands (see poster by S. Kikuchi, this conference). Within the ROSAT band the spectrum is flatter at the soft end (photon index for broken power law model between −2.2 and −2.0) than at the hard end (photon index between −2.6 and −2.4). The countrate increases during the first 5 orbits with a maximum e-folding time of 1.5 days, reaching a maximum luminosity of 4 × 1045erg/s in the energy range of 0.1 to 2.4 keV (for H0 = 75km/s/Mpc). It is followed by a slow decline until a second rise starts which is much stronger in harder X-rays than at low energies. Plotting the hardness ratio (defined as the difference in the countrates above and below 0.5 keV divided by the total countrate) versus the total countrate the second rise displays a much steeper gradient than the first one (see figure 1a).For most of the other BL Lac's the countrates are too low to detect variability on these timescales of hours to days, except for B2 1215+30 and OQ 530, where the chance probability for the observed variations is 4 × 10−4 and 10−10, respectively. Also no clear correlation between hardness ratio and countrate could be found. Spectral fits of single power law models with absorption result in photon indizes ranging from −3.1 to −1.9 with no detectable absorption above the galactic value for 5 of these sources, and some additional absorption for GB 1011+496 and B2 1215+30 (see figure 1b).
On the Accretion Disk Models by Stationary and Non-Stationary Shock Waves
Authors: Chakrabarti Sandip K. 
Published: July 2016

An important point which emerged from this meeting is that disks in AGNs are not simply thin, Keplerian type; they show more complex behaviour. Chakrabarti (1990a and references therein) has shown that in an inviscid accretion disk with significant angular momentum, the centrifugal barrier is strong enough to produce axisymmetric standing shock wave. Subsequently, this work was extended to include the non-axisymmetric and viscous disks (Chakrabarti, 1990b). Particularly important are the solutions with viscosity, as they show that as the viscosity is increased, the stable becomes weaker and weaker till it disappears completely. This solution has a unifying character that inviscid pressure driven disks have almost constant angular momentum and can have shock discontinuities, but viscous driven disks dissipate angular momentum quick enough not to have centrifugal barrier and therefore no shock waves. Chakrabarti & Molteni (1993), using Smoothed Particle Hydrodynamics have shown that shocks are produced in inviscid disks, exactly where they are predicted.Unlike a Keplerian disk, a disk with a shock has basically two temperature zones. The post shock solution is responsible for the Big Blue Bump and UV excess (Chakrabarti and Wiita, 1992). At the shock location, the disk is ‘bulged’ the hard radiation from this region is intercepted by the cooler pre-shock flow. The shock strength and location are sensitive to input specific energy of the flow. This configuration might be responsible for the ‘zero-lag’ correlated variability of, say, NGC 5548 (Chakrabarti, Haardt, Maraschi & Molendi, AA, submitted) discussed in this meeting. Spiral shocks which may be produced in disks in a binary system can also appear in disks around AGNs; the perturbation may be due to passage of massive objects (Chakrabarti & Wiita, 1993a). They also cause time variations in the double horned pattern from disk line emission (Chakrabarti & Wiita 1993b) as observed in, say ARP 102B. All these observations point that shocks are probably important ingredients in any accretion disk in AGNs
Laboratory data on atomic spectra for extreme ultra-violet solar spectroscopy
Authors: Edlén B. 
Published: July 2016

Expected relative prominence of elements and ionization stages is estimated from existing recordings of the solar XUV spectrum. The following 3 groups of elements, apart from H and He, need be considered : (C, N, O), (Ne, Mg, Si, Fe), (Na, Al, S, Ar, Ca, Ni), the relative abundance descending by approximately an order of magnitude from one group to the next, and all other elements being at least another order of magnitude less abundant. Lines of low and medium stages of ionization are expected only from most abundant elements. The observed presence in the solar spectrum of lines from the neon-like Fe XVII suggests that very high stages of ionization must not be a priori excluded.These considerations indicate that a discussion of laboratory data may be confined to atomic systems containing from 1 to 18 electrons. Observed data on these 18 isoelectronic sequences, and the possibilities of their extrapolation, are reviewed. It is concluded that required data for the first 10 sequences are well in hand, but considerable laboratory work remains to be done on the third-period sequences.A comprehensive paper is being prepared for later publication.
Some preliminary results from the solar monitoring Satellite NRL 1964 1 D obtained at the Arcetri Station
Authors: Landini M. , Piattelli M. , Righini G. , Russo D. , Tagliaferri G. L. 
Published: July 2016

This paper contains the following subjects; a) a description of the telemetry set operating in Arcetri (Florence, Italy) in order to record the measurements of solar X and UV radiation, via satellite:b) the values of solar flux at 44–60 Å during the period March-April 1964, as deduced by telemetry of NRL 1964–01 D satellite:c) a typical example of atmospheric absorption at optical and X-ray wavelengths during the sunrise on the satellite:d) a particular feature of recording when the satellite passes through the sub-auroral belt.
Rayonnement visible et infrarouge : Soleil, Étoiles, planètes, comètes
Authors: Swings Par P. 
Published: July 2016

1)Visible region.When carried on board of space vehicles, telescopes can be used with their maximum theoretical resolving power, by avoiding the effects of atmosphere turbulence. As the atmospheric scattering is suppressed, observations of very low contrast phenomena are made easier. Closer observations of the solar system objects are made possible.2)Infra red region.Observations taken from 30 km high are freed of most of the atmospheric absorption. But some infra red observations require the use of satellites (for example scanning of the whole sky within spectral regions). Needed observations are reviewed.
Spaceborne observations of radio noise from 0.7 to 7.0 MHz and their dependence on the terrestrial environment
Authors: Huguenin G. R. , Papagiannis M. D. 
Published: July 2016

Observations of long-wavelength (λ > 30 m) radio radiation in space are reported and analyzed in the framework of the terrestrial environnement.Measurements at low altitudes (~ 300 km) at the frequencies of 4.0 and 7.0 MHz have revealed the existence of very high radio noise levels during the night and on certain occasions during the day. These strong signals were at least 50 db higher than the expected cosmic background and are attributed to man-made and atmospheric radio noise.Measurements at high altitudes (3000–11000 km) at the frequencies of 2.2 and 0.7 MHz have produced a flux value at 2.2 MHz in agreement with the expected cosmic radio background, but the flux value obtained at 0.7 MHz is approximately 15 db higher than the anticipated flux from cosmic sources. Harmonic gyroradiation from the artificial and the outer Van Allen belts might be the cause of the high radio flux observed at 0.7 MHz.
Solar noise observations from the Alouette satellite
Authors: Hartz T. R. 
Published: July 2016

Solar radio noise in the frequency range 1.5 to 10 MHz appears sporadically in the Alouette sweep-frequency recordings above the galactic noise level. The type III bursts can be readily identified, but other spectral types are much more difficult to identify from only the satellite records. Using a plausible model for the coronal electron densities, the Type III frequency drift curves have been interpreted as corresponding to a source velocity in the range 0.1 to 0.15 times the velocity of light. Preliminary estimates have also been made of the coronal temperatures from some of the burst durations.Other solar noise events of longer duration have not been classified other than as enhanced solar noise at this stage. These enhancements frequently show an intensity structure, extending down to the 1.5 MHz lower frequency limit of the equipment. These events are discussed briefly.
The anomalous A-type supergiants in the Magellanic clouds - evidence for post-red supergiant evolution
Authors: Humphreys R. M. , Kudritzki R. P. , Groth H. 
Published: July 2016

A group of A-type stars in the field of the Small Magellanic Cloud (SMC) were noticed by Humphreys (1983) to have anomalously strong hydrogen (H) lines and a Balmer jump that are too strong for their luminosities. Their visual luminosities, based on membership in the SMC, are about two magnitudes brighter than one would expect from the appearance of the Balmer series. Humphreys also found that their colours are too red in U-B by 0.2 to 0.3 mag, but show little or no change in B-V relative to normal A-type supergiants; this is consistent with the strong Balmer jump in these stars' spectra. She also concluded that the objects do belong to the SMC.Earlier, Sanduleak (1972) had identified a group of stars with similar properties in the LMC, but, because of their strong hydrogen lines, rejected them as members. However, Fehrenbach and Duflot (1972) recognized about 30 stars with A and F type spectra with unusually strong H lines. They concluded that they were members of the Large Magellanic Cloud (LMC) because of their large negative radial velocities.
Depth structure in the gas in Shapley III along three lines of sight
Authors: Bomans Dominik J. , De Boer Klaas S. , Koornneef Jan 
Published: July 2016

Shapley Constellation III in the Large Magellanic Cloud (LMC) is surrounded by a ring of HII regions, called Supershell LMC 4 (Meaburn 1980). In the direction of the centre of this spatial strucutre very little neutral interstellar gas is found (McGee & Milton 1964). The spatial arrangement suggests that a hole exists in the gas layer of the LMC, one which may be blown out by the effect of the winds of the stars of Constellation III or by supernova blasts from the same area. Shock waves moving into surrounding neutral gas triggered star formation in the outer shell. Left inside would be a volume of diluted, and probably hot, ionized gas.To probe the spatial structure of the gas we have collected high dispersion IUE spectra of some stars inside LMC 4. We have analysed the interstellar lines from the various elements and ions available and are able to derive the velocity structure of the lines of sight observed (Bomans & de Boer 1989). We see a well-defined absorption component near 230 km s−1 and a broader structure around 280 km s−1. The column density of neutral gas is small, as confirmed from the HI 21-cm profiles of Rohlfs et al. (1984). We have redetermined the spectral type of the stars from the IUE spectra in order to estimate the strength of the stellar Lyman-Alpha (Lα) line. Two stars are of early enough type to allow the determination of N(H) from Lα.
Variability of 22 GHz H2O masers in circumstellar shells
Authors: Engels D. , Winnberg A. , Brand J. , Walmsley C.M. 
Published: July 2016

We have monitored different types of late-type stars in the 22 GHz maser line of water vapor with single-dish telescopes and the Very Large Array over several years. We find that the emission pattern in the circumstellar shells of Semi-Regular (SR) and Mira variables mapped by the VLA remain stable on the scale of several years while the maser profiles from the single-dish telescopes vary strongly. Thus the strong variability is mainly due to incoherent intensity fluctuations of the individual maser lines, which have lifetimes less than a year.In contrast to this, the variability in OH/IR stars and M-Supergiants is more regular and is mainly a response to the long-period variations of the central star. H2O maser in many OH/IR stars are “extinguished” most of the time, as the excitation temperature is only high enough during the phases close to the maximum of the variability.The strength of the profile variability is decreasing as a function of the radial distance of the maser shell from the star. This gives a natural explanation for the increasing regularity of the maser variations along the sequence SR-, Mira variables, OH/IR stars, M-Supergiants.
The final 105 years of stellar AGB evolution in the presence of a pulsating, dust-induced “superwind”
Authors: Schröder K.-P. , Winters J.M. , Sedlmayr E. 
Published: July 2016

We have computed mass-loss histories and tip-AGB stellar evolution models in the presence of a dust-induced, carbon-rich “superwind”, in the initial mass-range of 1.1 to about 2.5 solar masses and for nearly solar composition (X=0.28, Y=0.70, Z=0.02). Consistent, actual mass-loss rates are used in each time-step, based on pulsating and “dust-driven” stellar wind models for carbon-rich stars (Fleischer et al. 1992) which include a detailed treatment of dust-formation, radiative transfer and wind acceleration. Our tip-AGB mass-loss rates reach about 4 · 10−5M⊙yr−1 and become an influencial factor of stellar evolution.Heavy outflows of 0.3 to 0.6 M⊙ within only 2 to 3·104 yrs, exactly as required for PN-formation, occur with tip-AGB models of an initial stellar mass Mi ≳ 1.3M⊙. The mass-loss of our “superwind” varies strongly with effective temperature (Ṁ ∝ T−8eff, see Arndt et al. 1997), reflecting the temperature-sensitive micro-physics and chemistry of dust-formation and radiative transfer on a macroscopic scale. Furthermore, a thermal pulse leads to a very short (100 to 200 yrs) interruption of the “superwind” of these models.For Mi ≲ 1.1M⊙, our evolution models fail to reach the (Eddington-like) critical luminosity Lc required by the radiatively driven wind models, while for the (initial) mass-range in-between, with the tip-AGB luminosity LtAGB near Lc, thermal pulses drive bursts of “superwind”, which could explain the outer shells found with some PN's. In particular, a burst with a duration of only 800 yrs and a mass-loss of about 0.03 M⊙, occurs right after the last AGB thermal pulse of a model with Mi ≈ 1.1M⊙. There is excellent agreement with the thin CO shells found by Olofsson et al. (e.g., 1990, 1998) around some Mira stars.