| 1. |
- Bååth, Lars, 1948-, et al.
(författare)
-
High resolution interferometry of the QSO 1422+202
- 1997
-
Ingår i: Astronomy and Astrophysics. Supplement Series. - Berlin : Springer. - 1286-4846 .- 0365-0138. ; 125:1, s. 453-458
-
Tidskriftsartikel (refereegranskat)abstract
- We present VLA A-array observations at 8.4 and 15GHz and European VLBI Network (EVN) observations at 1.6GHz of the radio source 1422+202. It is suggested that 1422+202 is a Medium-size Object in the evolutionary sequence from Compact Steep-spectrum Sources to larger sized radio sources. The VLBI data were analysed with the phase referencing technique and we show that the EVN can work as a phase stable instrument for separations between the calibrator source and the target source up to ~ 10 degrees. With the VLA and VLBI observations we investigate some of the issues about the nucleus of 1422+202 and we discuss the possible cause for the low frequency variability detected while monitoring the source.
|
|
| 2. |
- Chu, H.S., et al.
(författare)
-
AO 0235+164 - A "Heretic" BL Lac
- 1994
-
Ingår i: VLBI TECHNOLOGY. - Tokyo : Terra Scientific Publishing Company. ; , s. 146-155
-
Bokkapitel (refereegranskat)
|
|
| 3. |
- Chu, H.S., et al.
(författare)
-
VLBI observations of the puzzling BL Lacertae object 0235+164
- 1996
-
Ingår i: Astronomy and Astrophysics. - : Springer. - 0004-6361 .- 1432-0746. ; 307:1, s. 15-20
-
Tidskriftsartikel (refereegranskat)abstract
- For over 20 years, many models have been proposed to explain the variability of AO0235+164. Among them, the most favorable has been micro-lensing. We have made a series of VLBI observations on this source in order to better understand its nature. The resultant maps indicate dramatic changes in the position angle of the jet between observations as well as a correlation between the intensity of the VLBI core and flux outbursts which occurred between the observing sessions. These characteristics, in conjunction with other arguments, suggest that the source variability is intrinsic, i.e. microlensing is of minor importance. We suggest two models: 1) a model in which the jet starts at a very small angle to the line of sight and then curves away to become about 6deg at about 1 mas from the core; and 2) a CME (Coronal Mass Ejection) model that also may explain the violent variability in intensity, polarization position angle, and jet direction in AO0235+164.
|
|
| 4. |
|
|
| 5. |
- Israel, F.P., et al.
(författare)
-
The millimeter wave continuum spectrum of Centaurus A and its nucleus
- 2008
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 483:3, s. 741-748
-
Tidskriftsartikel (refereegranskat)abstract
- Aims. We study the radio emission mechanism of the FR-I AGN NGC 5128 (Centaurus A).Methods. We determine the centimeter and millimeter continuum spectrum of the whole Centaurus A radio source and measure at frequencies between 86 GHz (3.5 mm) and 345 GHz (0.85 mm) the continuum emission from the active radio galaxy nucleus at various times between 1989 and 2005.Results. The integral radio source spectrum becomes steeper at frequencies above 5 GHz, where the spectral index changes from alpha(low) = -0.70 to alpha(high) = -0.82. The SW outer lobe has a steeper spectrum than the NE middle and outer lobes (alpha = -1.0 vs. -0.6). Millimeter emission from the core of Centaurus A is variable, a variability that correlates appreciably better with the 20-200 keV X-ray variability than with 2-10 keV variability.Conclusions. In its quiescent state, the core has a spectral index alpha = -0.3, which steepens when the core brightens. The variability appears to be mostly associated with the inner nuclear jet components that have been detected in VLBI measurements. The densest nuclear components are optically thick below 45-80 GHz.
|
|
| 6. |
- 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.
|
|
| 7. |
- Rantakyrö, F.T., et al.
(författare)
-
50 μas resolution VLBI images of AGN’s at λ3 mm
- 1998
-
Ingår i: Astronomy and Astrophysics. - Berlin : Springer-Verlag. - 0004-6361 .- 1432-0746. ; 131, s. 451-467
-
Tidskriftsartikel (refereegranskat)abstract
- We present 15 images from the global mm-VLBI sessions in 1990 April at 100 GHz and 1993 April at 86 GHz. These observations probe the central engines of the 16 observed AGN's with up to 50 mu as resolution. Among other sources previously observed with lambda 3 mm VLBI we present the first lambda 3 mm maps of 0735+178, 0748+126, 1055+018, 2145+067, and CTA102, in total we have been able to image 13 out of the 16 observed sources. 6 out of the 13 imaged sources observed exhibit curvature and rapid structural changes, although the low dynamic range in two thirds of the maps limits the detection of weak features. Most of the sources have unresolved cores even at this high resolution. There is substantial evidence that the observed sources can be grouped into two general groups: A misaligned population with parsec scale jets in the form of low pitch helices and an aligned population with straight jets with small changes in PA due to intrinsic bends.
|
|
| 8. |
- 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.
|
|
| 9. |
- Rantakyrö, F.T., et al.
(författare)
-
Multifrequency interferometer and radio continuum monitoring observations of CTA 102
- 2003
-
Ingår i: Astronomy and Astrophysics. - Berlin : Springer. - 0004-6361 .- 1432-0746. ; 42:5, s. 473-485
-
Tidskriftsartikel (refereegranskat)abstract
- This paper represents the work of several years of observation of CTA 102 with both single dish telescopes andinterferometric arrays, covering wavelengths from 1.2 cm to 1.3 mm. The resolution of the maps covers an order of magnitudefrom 0.5 mas to 50 μas. The images presented demonstrate the rapid structural changes in this source. These rapid structuralchanges correlates with the outbursts seen in the total flux, turnover frequency and in the spectral index. We have calculated theBrightness Temperatures of the components (TB ) directly from the determined component sizes and also using the logarithmicvariability amplitudes and timescales to calculate the TB s. Both the methods yielded measurements of source frame T Bs nohigher than∼a few × 1012K. We find that the observed average proper motion is0.4 0.07 mas/yr, which corresponds to anapparent transverse velocity of 11± 2 c.
|
|
| 10. |
- Rantakyrö, F. T., et al.
(författare)
-
Structural Changes in CTA102
- 1996
-
Ingår i: Extragalactic radio sources. - Boston : Kluwer. - 079234121X ; , s. 39-40
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)
|
|
