Abstrakti
Aims: We have investigated highly atypical morphological
parsec-scale changes in the flat spectrum extragalactic radio source
2023+335 which are coincident with an extreme scattering event (ESE)
seen at radio wavelengths during the first half of 2009. Methods:
We used (i) 15.4 GHz Very Long Baseline Array (VLBA) observations of the
quasar 2023+335 obtained at 14 epochs between July 2008 and Nov. 2012 as
part of the Monitoring Of Jets in Active galactic nuclei with VLBA
Experiments (MOJAVE) program; (ii) earlier archival VLBA observations of
the source performed at 1.4, 2, 8, 15, 22, and 86 GHz to analyze the
properties of the proposed turbulent screen toward 2023 + 335; and (iii)
data sets from the Owens Valley Radio Observatory (OVRO) and University
of Michigan Radio Astronomy Observatory (UMRAO) single-dish monitoring
programs performed at 15 and 14.5 GHz, respectively, to study integrated
flux density changes. Results: We report on the first detection
of the theoretically-predicted rare phenomenon of multiple parsec-scale
imaging of an active galactic nucleus induced by refractive effects due
to localized foreground electron density enhancements, e.g., in an
AU-scale plasma lens(es) in the ionized component of the Galactic
interstellar medium. We detected multiple imaging in the low galactic
latitude (b = -2.°4) quasar 2023+335 from the 15.4 GHz MOJAVE
observations when the source was undergoing an ESE. While the
parsec-scale jet of the source normally extends along PA ~ -20°, in
the 28 May 2009 and 23 July 2009 images a highly significant
multi-component pattern of secondary images is stretched out nearly
along the constant galactic latitude line with a local PA ≈ 40°,
indicating that the direction of relative motion of the plasma lens is
close to orbital. Weaker but still detectable imaging patterns at
similar position angles are sporadically manifest at several other
epochs. Modeling the ESE that occurred in early 2009 and lasted ~0.14
yr, we determined that the foreground screen has a double-lens
structure, with proper motion (~6.8 mas yr-1), and angular
size (~0.27 mas). We also found that the angular separation between the
two brightest sub-images roughly follows a wavelength-squared dependence
expected from plasma scattering. Furthermore, by analyzing archival
non-simultaneous VLBA observations covering a wide frequency range from
1.4 to 86 GHz, we found that the scattered angular size of the VLBI core
follows a ν-1.89 dependence, implying the presence of a
turbulent, refractive dominated scattering screen that has a confined
structure or is truncated transverse to the line of sight toward
2023+335.
Alkuperäiskieli | Englanti |
---|---|
Artikkeli | A80 |
Sivumäärä | 13 |
Julkaisu | Astronomy & Astrophysics |
Vuosikerta | 555 |
DOI - pysyväislinkit | |
Tila | Julkaistu - 2013 |
OKM-julkaisutyyppi | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä |