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Träfflista för sökning "L773:0004 6361 OR L773:1432 0746 srt2:(1990-1994)"

Sökning: L773:0004 6361 OR L773:1432 0746 > (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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