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Sökning: L773:1432 0746 OR L773:0004 6361 > (1990-1994)

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1.
  • Bååth, L.B. 1948-, et al. (författare)
  • 0735+178 : The cosmic conspiracy
  • 1991
  • Ingår i: Astronomy and Astrophysics. - Les Ulis : EDP Sciences. - 0004-6361 .- 1432-0746. ; 250:1, s. 50-56
  • Tidskriftsartikel (refereegranskat)abstract
    • This paper discusses a radio outburst in the BL Lac-object 0735+178 which started in early 1988, ended during 1990 and was succeeded by a new outburst which is still in progress. Our image obtained with a global VLBI network shows the source to mainly consist of a core-jet like structure ending in diffuse emission region of low surface brightness located to the east of the core. We identify two new components as having been produced during the late 1980's. They are moving out with apparent superluminal motion in a viewing angle of greater-than-or-similar-to 10-degrees at a distance of less-than-or-similar-to 2 mas from the core. The core includes another new component, but our resolution is just sufficient to resolve out this component from the core at our epoch of observation. We suggest that the weak emission region is caused by an older component reaching the point at which the Doppler boosting is at its maximum and that the long time scale flux variation are due to successive events when evolved components reach that point.
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2.
  • Bååth, L.B. 1948-, et al. (författare)
  • IMAGES OF THE BL LACERTAE OBJECT 1749+701 AT 5 GHZ
  • 1992
  • Ingår i: Astronomy and Astrophysics. - Les Ulis, France : EDP Sciences. - 0004-6361 .- 1432-0746. ; 262:1, s. 1-7
  • Tidskriftsartikel (refereegranskat)abstract
    • This paper presents images of 1749+701 from data obtained between 1980 and 1983 as a part of our VLBI programme to monitor a subsample of BL Lac objects at 5GHz. Our images show that this source has a complex structure with a jet pointing towards the north-west at a position angle of about -65-degrees. The brightest component is growing in extension along this position angle. We suggest this to be the core and that the growing extension was caused by a shock generated during an outburst and moving out along the jet. From the extension rate, we obtain an angular separation speed of 0.18 mas yr-1, which corresponds to an apparent transverse speed of 5c. The distances and position angles of two outer components seem remarkably constant. The phenomenon can be understood through a model in which a relativistic jet bends such that its axis is very close to the line of sight in two points. We also deduce that the Lorentz factor must be 4.5 less-than-or-equal-to gamma less-than-or-equal-to 6.5 and that the viewing angle must be 5-degrees less-than-or-equal-to theta less-than-or-equal-to 10-degrees. In this paper, we also present two images obtained from 5GHz MERLIN and combined MERLIN-VLBI data respectively. These maps show that there is a weak component located at a distance of 0.42 arcsec from the core at p.a -155-degrees.
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3.
  • Bååth, L.B. 1948-, et al. (författare)
  • The microarcsecond structure of 3C 273 at 3 MM
  • 1991
  • Ingår i: Astronomy and Astrophysics. - Les Ulis : EDP Sciences. - 0004-6361 .- 1432-0746. ; 241:1, s. L1-L4
  • Tidskriftsartikel (refereegranskat)abstract
    • Recent improvements in data analysis and receiver techniques have allowed us to produce a map of the 100GHz emission from the compact radio source 3C273 with the unsurpassed resolution of 50-mu-as (microarcseconds). Our map shows that the structure within 300-mu-as (approximately 1.5.10(18).h-1 cm) has a position angle significantly different from the position angle of the jet observed at lower frequencies. There are also indications in our map that the inner structure has a more pronounced wiggling structure than has been observed on larger scales. The observations were made about 60 days from the start of the outburst of 1988. Most of the flux from the outburst is concentrated in a component which is elongated approximately (56 x 5).10(16).h-1 cm perpendicular to the overall jet-axis. The distance between this component and the core is approximately 128-mu-as, which corresponds to the distance expected from an apparent velocity of approximately 800-mu-as year-1.
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4.
  • Bååth, L.B. 1948-, et al. (författare)
  • The radio fine structure of the BL Lacertae object 0735 + 178
  • 1991
  • Ingår i: Astronomy and Astrophysics. - Les Ulis : EDP Sciences. - 0004-6361 .- 1432-0746. ; 243:2, s. 328-332
  • Tidskriftsartikel (refereegranskat)abstract
    • The BL Lacertae object 0735 + 178 has been mapped with global VLBI arrays at four epochs from 1979 to 1983. The hybrid maps presented in this paper show a complex structure with a component separating from the core with an apparent transverse velocity of almost-equal-to 7.9 c. We suggest a model in which new components are born in the core during an outburst, initially move away towards northeast and then at a distance of almost-equal-to 4.2 mas turn towards the southeast. At this point, the component does not show any discernible motion away from the core. We suggest that the jet curls towards the south and towards the observer and at this point has its minimum viewing angle, with maximum boosting and minimum apparent velocity. We associate the outburst of 1989 with the event when a superluminally moving component reaches this point.
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5.
  • Bååth, L.B. 1948-, et al. (författare)
  • VLBI observations of active galactic nuclei at 3 MM
  • 1992
  • Ingår i: Astronomy and Astrophysics. - Les Ulis : EDP Sciences. - 0004-6361 .- 1432-0746. ; 257:1, s. 31-46
  • Tidskriftsartikel (refereegranskat)abstract
    • Recent improvements in data analysis and receiver techniques have allowed us to produce maps of the 100 GHz emission from the compact cores of active galactic nuclei with the unsurpassed resolution of 50-mu-as (microarcseconds). We present here hybrid maps of a set of compact radio sources observed at two epochs with a global VLBI array. The high resolution enables us to show details of active galactic nuclei on size scales of 10(16)-10(17) cm. Jets are shown to be more curved in these inner parts than further out in the areas mapped with VLBI at lower frequencies. Our maps of the quasar 3C345 show that the curvature seen with lower resolution instruments continues very close to the core. New components are seen separating from the cores of 3C84 and BL Lac. We observe a component in 3C84 separating from the core with an apparent speed approximately 21000 km sec-1. The radio source OJ287 is still unresolved with our array, having a core size of less-than-or-similar-to 10(17) cm. There is no indication of any compact component in 3C279 which would be associated with the outburst in integrated flux density which happened some months before our observation. The flux density of the most compact component we observe in 3C279 agrees well with that of the quiet core as extrapolated from its radio spectrum at lower frequencies. The inner part of the radio jet of the giant elliptical galaxy M87 also shows a continuation of the structure on a larger size scale, with a structure we interpret as parts of a helical pattern. No fringes were found for 4C39.25 or Sgr A.
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6.
  • Dietrich, M., et al. (författare)
  • Monitoring of active galactic nuclei. IV. The Seyfert 1 galaxy NGC 4593
  • 1994
  • Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 284:1, s. 33-4343
  • Tidskriftsartikel (refereegranskat)abstract
    • Results of a five-month campaign of optical monitoring the Seyfert 1 galaxy NGC 4593 are presented. High resolution Halpha and Hbeta spectra and direct images in the Johnson U, B, V, R and I bands were obtained between January and June 1990. The emission lines and the continuum showed strong variations on time scales of weeks to days. The Halpha line varied by more than 30% within only 5 days. Cross-correlating the light curves of the Balmer lines with that of the optical continuum gives a lag of around 4 days. Therefore, this low luminosity Seyfert galaxy may have one of the smallest broad line regions known so far
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7.
  • Donner, K.J., et al. (författare)
  • Structure and evolution of long-lived spiral patterns in galaxies
  • 1994
  • Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 290, s. 785-795
  • Tidskriftsartikel (refereegranskat)abstract
    • We use N-body simulations to study the non-linear evolution of spontaneous and tidally induced spiral patterns in galaxies on time-scales of several rotation periods of the disc. We find that the patterns can sometimes survive with an almost constant amplitude for five revolutions or more, and tend to be regenerated after disappearing temporarily. The pattern velocity is such that the corotation radius is where the self-gravity as measured by the swing amplification of the m=2 component is strongest, and the amplitude of the pattern is larger when swing amplification is stronger. The shape of the spirals is independent of the origin of the pattern, and corresponds to the critical wave number kappa^2^/(2piGmu), where mu is the disc surface density. The pattern survives longer if the self-gravity of the disc is strong. For a given disc model the life-time of the pattern depends on its amplitude, with stronger patterns having shorter life-times. If self-gravity is weak, the dominant tidal effect is a kinematic spiral at the inner Lindblad resonance.
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8.
  • Elmegreen, B.G., et al. (författare)
  • Grand design and flocculent spiral structure in computer simulations with star formation and gas heating
  • 1993
  • Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 272, s. 37-58
  • Tidskriftsartikel (refereegranskat)abstract
    • An algorithm for star formation and gas heating is included in numerical simulations of galaxy disks. With a high disk mass and an inner Q-barrier, the simulations spontaneously generate and then maintain for several revolutions a long-lived two-arm spiral wave mode that resembles a grand design galaxy. Eventually a multiple arm pattern appears because of a growing m = 3 component; multiple arm patterns also form right away if there is no Q barrier. When the stellar Q-value exceeds ~2.5 because of a low disk mass or a large velocity dispersion, stellar spirals do not form at all; if the relative gas mass is also small in this case (about 10 % of the total galaxy mass or less), then the spiral structure is purely flocculent, i.e., composed of numerous short arms in only the gas and star formation component. The star formation algorithm is made as realistic as possible, with young stars forming in virialized cloud complexes, moving kinematically as tracer particles, and heating their environments at the observed average rate. The results illustrate the importance for spiral structure of the stellar Q and the star formation thermostat in the interstellar gas.
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9.
  • Jackson, N., et al. (författare)
  • Monitoring of active galactic nuclei. I. The quasars 1302-102 and 1217+023
  • 1992
  • Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 262:1, s. 17-2525
  • Tidskriftsartikel (refereegranskat)abstract
    • Presents the results of a five-month spectroscopic monitoring campaign of two radio-loud quasars PKS 1217+023 and PKS 1302-102. There is a pronounced lack of variability in 1302-102 apart from a possible small change in the continuum level near the end of the monitoring period, but 1217+023 underwent a 20% continuum decrease in the middle of the campaign. No line variability at all was observed. The co-added data represent two of the best spectra yet obtained of any radio-loud quasar other than 3C 273, and the shape of the Hbeta line and the presence underneath it of other contaminating lines are well determined. The Hbeta broad line in 1302-102 is redshifted by 3 Aring with respect to the narrow line region and there is evidence for a separate broad Hbeta feature on top of the main Hbeta emission in both quasars. This feature is stronger in 1217+023 than in 1302-102
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10.
  • Lerner, M.S., et al. (författare)
  • A 100 GHZ map of 3C 446
  • 1993
  • Ingår i: Astronomy and Astrophysics. - Les Ulis : EDP Sciences. - 0004-6361 .- 1432-0746. ; 280:1, s. 117-120
  • Tidskriftsartikel (refereegranskat)abstract
    • The first map made from 100 GHz VLBI observations of the quasar/BL Lac object 3C446 is presented. This map represents a 25-fold increase in resolution compared to earlier maps. Our main conclusions are that the core of 3C 446 is still almost unresolved (less than or similar to 30 muas) at this frequency and that a jet extends several hundred microarcseconds at position angle almost-equal-to -142-degrees. A comparison is also made with observations at other size scales.
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11.
  • Penston, M. V., et al. (författare)
  • The extended narrow line region of NGC 4151. I. Emission line ratios and their implications
  • 1990
  • Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 236:1, s. 53-6262
  • Tidskriftsartikel (refereegranskat)abstract
    • The paper presents the first results from long-slit spectra of the Seyfert galaxy NGC 4151 which give average diagnostic ratios of weak lines in the extended narrow line region (ENLR) of that galaxy and the first direct density measurement in an ENLR. These data confirm that the ENLR is kinematically undisturbed gas in the disc of the galaxy which is illuminated by an ionizing continuum stronger by a factor of 13 than a power law interpolated between observed ultraviolet and X-ray fluxes. Explanations of this apparent excess include a hot thermal continuum, time variations and an anisotropic radiation field. The authors give reasons for favouring anisotropy which might be caused by shadowing by a thick accretion disc or by relativistic beaming. Shadowing by a molecular torus is unlikely, given the absence of an infrared signal from the reradiated flux absorbed by any torus. Anisotropy would have important implications for the bolometric luminosity and nature of active galactic nuclei
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12.
  • Rantakyrö, F. T., et al. (författare)
  • High dynamic-range VLBI observations of 3C 345 at 18 cm wavelength
  • 1992
  • Ingår i: Astronomy and Astrophysics. - Les Ulis : EDP Sciences. - 0004-6361 .- 1432-0746. ; 259:1, s. 8-16
  • Tidskriftsartikel (refereegranskat)abstract
    • This paper presents very high dynamic range VLBI observations of the quasar 3C345 at 18 cm wavelength at two epochs, 1981.11 and 1985.27. We find that the direction of the jet changes drastically and the surface brightness decreases with increasing distance from the core. We show that a true three-dimensional curvature of the jet in space is needed to obtain the observed decrease of intensity with distance from the core, both in an adiabatic expansion model as well as in a model where the comoving flux is constant. Our observations confirm the detection of a previously reported low surface brightness component, C0. In addition, there is a large component extended perpendicular to the jet axis which we associate with a stationary shock at a distance consistent with the expected position of the narrow line region. Models for the intensity of the jet fitted to the observed dependence of intensity with distance from the core suggests this shocked region to be the place where the jet is expanding adiabatically after having had a constant width closer to the core. A limit on the allowed viewing angle theta less-than-or-similar-to 40-degrees is derived from the lack of any detectable counterjet.
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13.
  • Reipurth, Bo, et al. (författare)
  • Star formation in BOK globules and low-mass clouds. V - H-alpha emission stars near SA 101, CG13 and CG22
  • 1993
  • Ingår i: Astronomy and Astrophysics. - : Springer. - 0004-6361 .- 1432-0746. ; 267:2, s. 439-446
  • Tidskriftsartikel (refereegranskat)abstract
    • We have surveyed five fields in the Gum Nebula containing cometary globules, in a search for new regions of low-mass star formation. Seven H-alpha emission stars were found in association with the CG4/CG6/Sa101 cloud complex, and one in association with the CG13 globule. All eight stars are late-type emission line stars, and appear to be low-mass pre-main sequence stars. Low dispersion spectra and optical and IR photometry are presented for the objects, as well as for PH-alpha 92, an already known young star in CG22. Most of the globules surveyed are not associated with H-alpha emission stars. We demonstrate that star formation has occurred in the Gum Nebula over several million years, right up to very recently. 
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14.
  • Romeo, Alessandro, 1961 (författare)
  • How faithful are N-body simulations of disc galaxies?
  • 1994
  • Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 286, s. 799-806
  • Tidskriftsartikel (refereegranskat)abstract
    • High-softening two-dimensional models, frequently employed in N-body experiments, do not provide faithful simulations of real galactic discs. A prescription [Eqs. (17) and (18)] is given for choosing meaningful values of the softening length in standard regimes of astrophysical interest, when both the stellar and gaseous components are present. In addition, a stability criterion [Eq. (10); see also Eq. (11)] is given for choosing meaningful input values of the Toomre parameter for a given softening length. Such a criterion should also provide a key to the correct interpretation of computational results in terms of real phenomena.
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15.
  • Salamanca, I., et al. (författare)
  • Spectroscopic monitoring of active galactic nuclei. III. Size of the broad line region in NGC 3227
  • 1994
  • Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 282:3, s. 742-752752
  • Tidskriftsartikel (refereegranskat)abstract
    • For pt.II see ibid., vol.269, no.1-2, p.39-53 (1993). The authors present the results of a five-month monitoring campaign of the active galactic nucleus in NGC 3227. The Hbeta and Halpha emission lines and the optical continuum have been analysed. The optical AGN continuum and the broad-line region (BLR) emission in NGC 3227 appear to vary by about 40% on a timescale of 1.5 month. The levels of the broad-line emission and the 675.0 nm continuum are well correlated, a fact consistent with the assumption that the BLR material is photoionised by the central continuum source. Cross-correlation analyses between the light curves of the 675.0 nm AGN continuum and the BLR H emission indicate that the lag of the BLR emission with respect to the continuum variations is 17plusmn7 days. This result suggests that the BLR clouds that are affected by the changes in the flux of the central ionising source lie about 17 light days away from it
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16.
  • Stirpe, G. M., et al. (författare)
  • Monitoring of active galactic nuclei. V. The Seyfert 1 galaxy Markarian 279
  • 1994
  • Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 285:3, s. 857-867867
  • Tidskriftsartikel (refereegranskat)abstract
    • For pt. IV see ibid., vol.284, no.1, p.33-43 (1994). Reports on the Lovers of Active Galaxies' (LAG) monitoring of the Seyfert 1 galaxy Markarian 279 from January to June 1990. The source, which was in a very bright state, gradually weakened after the first month of monitoring: the Halpha and Hbeta flux decreased by 20% and 35% respectively, and the continuum under Halpha by 30%. The luminosity-weighted radius of the broad line region (BLR), as derived from the cross-correlation function, is of the order of 10 light days. This result is very uncertain because the features in the light curves are very shallow, but it is unlikely that the radius of the BLR is more than 1 light month. The profile variations of Halpha confirm that the prevailing motions are not radial. The data of the present campaign and those obtained in previous years, when the source was in a much weaker state, show that the red asymmetry of the Balmer lines correlates positively with the broad line flux. This new effect is briefly discussed
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17.
  • Thomasson, Magnus, 1961, et al. (författare)
  • Simulations of the effect of spiral arms on the cloud-ensemble velocity dispersion
  • 1991
  • Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 250:2, s. 316-323
  • Tidskriftsartikel (refereegranskat)abstract
    • N-body simulations are used to study the effect of spiral heating, viscous heating, and cooling by inelastic collisions on the molecular cloud ensemble velocity dispersion. The simulations include inelastic collisions between clouds as well as long-range gravitational forces. The theory for heating and cooling of the cloud population is described, and the numerical code and the galaxy model are examined. Spiral arms not only heat the cloud population, they also cause increased cooling due to the large number of cloud-cloud collisions in the arms. It is found that the heating dominates, in that a stronger spiral potential leads to a higher velocity dispersion. The spiral potential is stronger for smaller values of Q(star). The steady-state cloud-ensemble velocity dispersion is found to be constant with the radius. Its value for a typical simulation of a multiple-arm spiral is slightly smaller than the value observed in the Galaxy. The stellar velocity dispersion corresponds to a radius-independent Q(star), for multiple-arm galaxies regardless of the initial radial dependence.
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18.
  • van Groningen, E. (författare)
  • An analysis of the spectra of 3 Seyfert-1 galaxies with strong Ca II emission
  • 1993
  • Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 272:1, s. 25-3636
  • Tidskriftsartikel (refereegranskat)abstract
    • High resolution spectroscopy is presented of three Seyfert-1 galaxies-I Zw 1, Akn 564 and Mkn 231. The spectra cover the optical wavelength region and the regions with the Ca II triplet and the [Ca II] lines. The broad lines in I Zw 1 and Akn 564 are relatively narrow (FWHM<1000 km s -1); and in both objects the Ca II triplet lines are considerably broader than the Balmer lines. These lines are formed at large column densities in the line emitting region; electron scattering by thermal electrons in the broad line clouds may explain the larger widths of the Ca II lines. I Zw 1 has three emission line regions. In addition to the broad line region there are two kinematically separated narrow line regions: a high excitation region with relatively broad lines (FWHM~1100 km s -1), blue shifted with regard to the systemic velocity by ~700 km s -1; and a region with very low excitation at the systemic velocity and much narrower lines (FWHM~400 km s -1)
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19.
  • Wanders, I., et al. (författare)
  • Seeing and aperture effects on [O III]-based flux calibration of AGN spectra. NGC 3516 : a case study
  • 1992
  • Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 266:1, s. 72-7676
  • Tidskriftsartikel (refereegranskat)abstract
    • In variability studies of the broad emission lines in the spectra of active galactic nuclei, the narrow emission lines are generally used to flux calibrate spectra taken at different epochs to each other. In objects where the narrow-line region is resolved, seeing affects the ratio of narrow-line flux to broad-line and continuum flux entering the aperture since the broad-line region and the continuum source are unresolved. Thus observing under different seeing conditions introduces artificial correlated variations of the continuum and broad-line fluxes if one assumes the narrow lines to be constant in flux. The authors show that one can correct for this effect by simulating seeing variations using a narrow-band [O III] image, taken in good seeing conditions, of the nearby Seyfert 1 galaxy NGC 3516, in which the narrow-line region is clearly resolved and extends over more than 20". They also show that centring the aperture on the object consistently is very important and miscentring is also a source of artificial variability of the broad-line and continuum fluxes
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20.
  • Wanders, I., et al. (författare)
  • Spectroscopic monitoring of active galactic nuclei. II. The Seyfert-1 galaxy NGC 3516
  • 1993
  • Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 269:1-2, s. 39-5353
  • Tidskriftsartikel (refereegranskat)abstract
    • The authors present the results of a five month spectroscopic monitoring campaign of the Seyfert-1 galaxy NGC 3516. Using a new calibration method and applying a correction for seeing differences during the various observations they can scale the spectra to each other with a much higher accuracy than hitherto achieved. NGC 3516 shows large amplitude variations on a time scale of several weeks. Asymmetric profile variations occur on the same time scale as the continuum variations. The authors show the presence of a varying dip on the blue wing of the Hbeta profile, which is not present on the Halpha profile, and which corresponds to the absorption features previously seen in the UV emission lines of NGC 3516. From cross-correlation analysis they find a time lag of 14plusmn2 days for the Halpha and 7plusmn3 days for the Hbeta emission-line response to continuum variations
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21.
  • Zhang, F. J., et al. (författare)
  • 1404+286 : A non-ideal calibrator for VLBI observations
  • 1994
  • Ingår i: Astronomy and Astrophysics. - Les Ulis : EDP Sciences. - 0004-6361 .- 1432-0746. ; 281:3, s. 649-652
  • Tidskriftsartikel (refereegranskat)abstract
    • The radio source 1404+286 was observed with global VLBI arrays at 5 and 1.67 GHz in 1990.88 and 1991.45 respectively. High quality maps are presented in this paper, and show the source to be resolved at both frequencies with a core-jet like structure extending approximately 6 mas in position angle approximately -145-degrees. The jet direction is similar in both maps. 1404+286 has often been used in the past as a flux density calibrator or a phase referencing source. Our results show that this source is not an ideal calibrator on VLBI scales.
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22.
  • Zhang, F.J., et al. (författare)
  • Are there two engines at the center of 3C 286?
  • 1994
  • Ingår i: Astronomy and Astrophysics. - Les Ulis : EDP Sciences. - 0004-6361 .- 1432-0746. ; 287:1, s. 32-37
  • Tidskriftsartikel (refereegranskat)abstract
    • This paper presents images of the compact steep-spectrum quasar 3C 286 obtained with global VLBI arrays at 4.99 and 1.66 GHz. The morphology appears to be intermediate between the core-jet types and grossly distorted objects. The central portion is resolved into two components of almost equal peak brightnesses and spectral indices. On the 1.66 GHz map, the jet extends in position angle approximately -136-degrees, up to a knot (W1) (approximately 75 mas from the component C2) from which it curves rapidly towards the west to form another knot (W2). It is possible that the VLBI jet bends further towards the secondary component separated from the core by approximately 2.6'' in p.a. -115-degrees seen in high dynamic range VLA or Merlin maps. The jet has a sinuous feature shown clearly on the 5 GHz map. There is a low brightness extension at the North (designated N1) shown on both the 1.66 and 5 GHz maps, which is connected to the component C1 and bends towards the east.
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23.
  • Zhang, F.J., et al. (författare)
  • The radio fine structure of the BL Lacertae object Mk 421
  • 1990
  • Ingår i: Astronomy and Astrophysics. - Les Ulis : EDP Sciences. - 0004-6361 .- 1432-0746. ; 236:1, s. 47-52
  • Tidskriftsartikel (refereegranskat)abstract
    • The BL Lac object Markarian 421 (Mk 421) has been mapped using VLBI arrays at a wavelength of 6 cm on four epochs from 1980.41 to 1983.43. It has a core-jet like structure with the jet pointing to the direction of NW with a stable position angle of about -45 deg. Almost all of the flux variation is accounted for by the change of the peak flux density of a compact component in the core. The results show that Mk 421 may exhibit apparent superluminal motion with apparent transverse speeds of components of (1.92 + or - 0.03)c (from 1981.25 to 1981.94) and (1.79 + or - 0.23)c (from 1981.94 to 1983.43) for C1; and (1.36 + or - 0.22)c for C2. The position angles of both the components C1 and C2 are nearly the same as the position angle of the jet, and there is no significant change of position angle as they move away from the core.
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24.
  • Bengtsson, G. J, et al. (författare)
  • Determination of Radiative Lifetimes In Neutral Arsenic Using Time-resolved Laser Spectroscopy In the Vuv Region
  • 1992
  • Ingår i: Astronomy & Astrophysics. - 0004-6361. ; 263:1-2, s. 440-442
  • Tidskriftsartikel (refereegranskat)abstract
    • Inspired by the recent Hubble Space Telescope first observations of arsenic in an astrophysical object we have determined the natural radiative lifetime of the upper states of two of the three resonance lines of As I. Short pulse laser excitation at 193.7 and 197.2 nm was achieved using different non-linear optical mixing schemes. Using time-resolved spectroscopy we obtain tau(4p(2)5s P-4(3/2))=4.3(5) ns and T(4p(2)5s P-4(1/2)=4.5(5)ns yielding the absorption oscillator strengths f(abs)(193.7 nm) =0.123 (17) and f(abs)(1 97.2 nm)=0.059(8).
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25.
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