Issue |
A&A
Volume 456, Number 1, September II 2006
|
|
---|---|---|
Page(s) | 97 - 104 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361:20054678 | |
Published online | 23 August 2006 |
Multi-frequency VLBI observations of NRAO 530
1
Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030, PR China e-mail: winfeng@shao.ac.cn
2
Graduate School of Chinese Academy of Sciences, Beijing 100012, PR China
3
Joint Institute for Galaxy and Cosmology, SHAO and USTC, PR China
Received:
12
December
2005
Accepted:
2
May
2006
We report on VLBA observations of a γ-ray bright
blazar NRAO 530 at multiple frequencies (5, 8, 15, 22, 39, 43, and
45 GHz) in 1997 and 1999. These multi-epoch multi-frequency
high-resolution VLBI images exhibit a consistent core-dominated
morphology with a bending jet to the north of the core. The
quasi-simultaneous data observed at five frequencies (5, 8, 15, 22,
and 43 GHz) in February 1997 enabled us to estimate the spectra of
compact VLBI components in this highly variable source. Flat spectra
are seen in the two central components (A and B), and the most
compact component A that has the flattest spectral index at the
south end is identified as the core. Based on the synchrotron
cooling timescale argument, it is suggested that the observed
inverted spectrum of component C is caused by the free-free
absorption (FFA), although the synchrotron self-absorption (SSA)
model cannot be ruled out definitely. While the SSA probably exists
in component B, it is likely that the same FFA would produce the
spectral turnover toward component B since the fitted FFA
coefficients in both B and C components are almost the same. If so,
the projected size of such an absorbing medium is at least 25 pc.
By adding our new measurements to previous data, we obtain apparent
velocities of two components, B of 10.2 c and E of 14.5 c. These are
consistent with the emergence of VLBI component associated with the
flux density outburst; i.e. components B and E are related to strong
γ-ray flares in 1994.2–1994.6 and 1995.4–1995.5,
respectively. We investigate the spectral variability further by
making use of the single-dish measurements covering a complete
outburst profile from mid-1994 to mid-1998. It shows a continuous
increase in the turnover frequency during the rising phase, and a
gradual decrease after passing the peak of the flare. Finally, we
discuss the equipartition Doppler-factor () based on
analysis of the magnetic field and obtain
s of 3.7,
7.2, and 0.8 for components A, B, and C, respectively, which are all
consistent with a larger flux density in component B, the
non-detection of proper motion in component C, and a bent jet.
Key words: galaxies: quasars: individual: NRAO 530 / galaxies: jets / radio continuum: galaxies
© ESO, 2006
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