J/AJ/147/143 MOJAVE. XI. Spectral distributions (Hovatta+, 2014)
MOJAVE: monitoring of jets in active galactic nuclei with VLBA experiments.
XI. Spectral distributions.
Hovatta T., Aller M.F., Aller H.D., Clausen-Brown E., Homan D.C.,
Kovalev YU.Y., Lister M.L., Pushkarev A.B., Savolainen T.
<Astron. J., 147, 143 (2014)>
=2014AJ....147..143H 2014AJ....147..143H
ADC_Keywords: QSOs ; Active gal. nuclei ; BL Lac objects ; Galaxies, radio ;
Redshifts
Keywords: BL Lacertae objects: general - galaxies: active - galaxies: jets -
quasars: general - radio continuum: galaxies
Abstract:
We have obtained milliarcsecond-scale spectral index distributions for
a sample of 190 extragalactic radio jets through the Monitoring of
Jets in Active Galactic Nuclei with the VLBA Experiments (MOJAVE)
project. The sources were observed in 2006 at 8.1, 8.4, 12.1, and
15.4GHz, and we have determined spectral index maps between 8.1 and
15.4GHz to study the four-frequency spectrum in individual jet
features. We have performed detailed simulations to study the effects
of image alignment and (u, v)-plane coverage on the spectral index
maps to verify our results. We use the spectral index maps to study
the spectral index evolution along the jet and determine the spectral
distributions in different locations of the jets. The core spectral
indices are on average flat with a mean value of +0.22±0.03 for the
sample, while the jet spectrum is in general steep with a mean index
of -1.04±0.03. A simple power-law fit is often inadequate for the
core regions, as expected if the cores are partially self-absorbed.
The overall jet spectrum steepens at a rate of about -0.001 to -0.004
per deprojected parsec when moving further out from the core with flat
spectrum radio quasars having significantly steeper spectra (mean
-1.09±0.04) than the BL Lac objects (mean -0.80±0.05). However,
the spectrum in both types of objects flattens on average by ∼0.2 at
the locations of the jet components indicating particle acceleration
or density enhancements along the jet. The mean spectral index at the
component locations of -0.81±0.02 corresponds to a power-law index
of ∼2.6 for the electron energy distribution. We find a significant
trend that jet components with linear polarization parallel to the jet
(magnetic field perpendicular to the jet) have flatter spectra, as
expected for transverse shocks. Compared to quasars, BL Lacs have more
jet components with perpendicular magnetic field alignment, which may
explain their generally flatter spectra. The overall steepening of the
spectra with distance can be explained with radiative losses if the
jets are collimating or with the evolution of the high-energy cutoff
in the electron spectrum if the jets are conical. This interpretation
is supported by a significant correlation with the age of the
component and the spectral index, with older components having steeper
spectra.
Description:
MOJAVE (Monitoring of Jets in Active galactic nuclei with VLBA
Experiments) is an observing program to monitor the changes in a large
sample of parsec-scale AGN jets in total intensity and polarization at
15.4GHz with the VLBA (Paper V, Lister et al. 2009, cat.
J/AJ/137/3718). In 2006, the monitoring was expanded to include
multifrequency observations at 8.1, 8.4, 12.1, and 15.4GHz. Altogether
191 sources were observed (twenty of them twice) in batches
distributed over 12 epochs. Our sample includes 133 flat-spectrum
radio quasars (hereafter quasars), 33 BL Lac objects, 21 radio
galaxies, and 4 optically unidentified objects. In addition to
studying the spectral index distribution, these data were used to
study Faraday rotation in these sources (Paper VIII, Hovatta et al.
2012, cat. J/AJ/144/105) and the frequency-dependent shift of the
optically thick base of the jet, the "core-shift" effect (Paper IX,
Pushkarev et al. 2012, cat. J/A+A/545/A113).
The observations were made in dual polarization mode using frequencies
centered at 8.104, 8.424, 12.119, and 15.369GHz. The bandwidths were
16 and 32MHz for the X and U bands, respectively. The observations
were recorded with a bit rate of 128Mbits/s. In the 8GHz bands the
observations consist of 2 sub-bands in both frequencies and 4
sub-bands in the 12 and 15GHz bands. All ten VLBA antennas were
observing except at epoch 2006 August 9 when Pie Town was not
included. The sources and their observing epochs are listed in Table1.
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table1.dat 65 210 Sources and their spectral index properties
table2.dat 72 1080 Fitted gaussian components at 15.4GHz and
their spectral index values
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See also:
VII/258 : Quasars and Active Galactic Nuclei (13th Ed.) (Veron+ 2010)
J/AJ/144/105 : MOJAVE. VIII. Faraday rotation in AGN jets. (Hovatta+, 2012)
J/A+A/545/A113 : MOJAVE IX. Core shift effects (Pushkarev+, 2012)
J/AJ/137/3718 : 15GHz monitoring of AGN jets with VLBA (Lister+, 2009)
Byte-by-byte Description of file: table1.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 8 A8 --- Name Source IAU name (HHMM+DDd, B1950)
10- 23 A14 --- OName Other known name
25- 32 F8.6 --- z [0/3.4]? Redshift
34 A1 --- O [QGBU] Optical classification (1)
36- 39 F4.1 --- beta ? Maximum apparent speed in light speed units (2)
41- 51 A11 "Y:M:D" ObsDate Date of spectral index observation
53- 57 F5.2 --- alpha1 ? Spectral index α in the core component
59- 63 F5.2 --- alpha2 ? Spectral index α in the median jet
ridge line
65 A1 --- A [Y] Y if 2D cross-correlation not possible for
image alignment (see Section 2.2 for details)
--------------------------------------------------------------------------------
Note (1): Optical classification is defined as follows:
Q = quasar;
B = BL Lac object;
G = active galaxy;
U = unidentified.
Note (2): Used in viewing angle calculation of Fig. 9, taken from
Paper X (Lister et al. 2013, cat. J/AJ/146/120).
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table2.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 8 A8 --- Name Source IAU name (HHMM+DDd, B1950)
10- 11 I2 --- m_Name [0/46] Component number (0=core)
13- 23 A11 "Y:M:D" ObsDate Date of spectral index observation
25- 28 F4.1 mas r [0/67] Component distance from phase center of
intensity map
30- 35 F6.1 deg PA [-180/180] Position angle of the component from
phase center
37- 41 F5.2 --- alpha [-2.9-3.6] Spectral index of the component
43- 45 F3.1 --- e_alpha [0.1/0.5] Uncertainty in alpha
47- 50 F4.1 yr Age ? Age of the component (1)
52- 55 F4.1 % Fpol [0/25]? Fractional polarization of the component
57- 59 F3.1 % e_Fpol [0/6]? Uncertainty in FPol
61- 66 F6.1 deg EVPA [-180/180]? Faraday-corrected electric vector
position angle of the component
68- 70 F3.1 deg e_EVPA ? Uncertainty in EVPA
72 A1 --- B [Y] Indicates whether a jet component is at
least one beam size away from the core
--------------------------------------------------------------------------------
Note (1): Calculated as the difference between the observing epoch and the
ejection epoch from Paper X (Lister et al. 2013, cat. J/AJ/146/120).
--------------------------------------------------------------------------------
History:
From electronic version of the journal
References:
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(End) Greg Schwarz [AAS], Sylvain Guehenneux [CDS] 03-Oct-2014