The
properties
of the
Gamma-Ray
Blazars
in the
CJ-F
VLBI
sample
S.Britzen*,
R.C.
Vermeulen*,
G.B.
Taylor
1
",
R.M. Campbell
+
,
LW.
Browne
0
,
P.
Wilkinson
0
, T.J.
Pearson'
and
A.C.S.
Readhead'
*NFRA, P.O.
Box
2,
NL-7990
AA
Dwingeloo,
The
Netherlands
1
^NRAO,
P.O.
Box O,
Socorro,
NM
87801, USA;
+
JIVE, P.O.
Box 2,
NL-7990
A A
Dwingeloo,
The
Netherlands;
°NRAL,
Jodrell
Bank,
Macclesfield,
Cheshire SK11
9 DL,
England
UK;
1
CIT,
Department
of
Astronomy, 105-24, Pasadena,
CA
91125,
USA
Abstract.
We
present
first
results
from
the
analysis
of
multi-epoch VLBI observations
of
the
EGRET
detected sources
[9] in the
CJ-F sample (Caltech
Jodrell-Flat-spectrum,
[10]).
These objects
form
a
subsample
of 14
sources within
the 293 AGN of the
full
CJ-F sample.
5 GHz
VLBI snapshot observations
of the
CJ-F sources
are
continuously
being performed
in
order
to
create
a
valid
database
for
thorough
statistical
tests
of
pc-scale
jet
motion
in
AGN.
All
gamma-bright CJ-F
AGN
have been observed
at
least
twice with
the
VLB
A,
which enables
us to
investigate
jet
component motions
and
paths.
In
particular,
we
concentrate
on the
analysis
of
those
properties supposed
to be
essential
for
gamma-ray production, i.e., superluminal motion
and
bending.
A
paper discussing
the
possible relation between morphological changes
and
gamma-ray
flaring/production
is in
preparation.
INTRODUCTION
With
66
high-confidence
blazar identifications
of
sources detected
by
EGRET [9],
it
has
become increasingly clear
that
GeV
gamma-ray sources
are
preferentially radio-
bright, compact-core, flat-spectrum sources, many
of
which have been
classified
as
optically violent variables (OVV)
or
blazars.
The
active galaxies
Mrk 421 and Mrk
501,
and — at a
lower
level
of
significance
— a few
other
AGN
(some
are
members
of
the
CJ-F sample), have been detected
by
ground-based Cherenkov telescopes
at the
highest-energy
end of the
electromagnetic spectrum accessible with current
technology
(TeV-regime). Superluminal motion
is
common
to all
7-ray sources
studied
in
sufficient
detail
(e.g.,
[1,2,4,5]).
Many sources also exhibit
a
parsec-scale
jet
that
is
bent
or is
misaligned with respect
to the
kiloparsec-scale
jet
(see, e.g.,
[2]).
As
pointed out,
for
instance
in
[7], these facts suggest
that
strong gamma-ray
emission
and
blazar properties
are
physically related.
It has
also been suggested
that
relativistic beaming
may
explain
the
observation
that
although
all
EGRET-
identified
AGNs
are
radio loud with
flat
spectra,
not all
radio-loud flat-spectrum
AGNs
are
detectable gamma-ray sources.
In
order
to
test
these hypotheses,
we
*)
This
work
was
supported
by the
European Commission,
TMR
Programme, Research Network
Contract ERBFMRXCT97-0034 CERES.
CP558,
High
Energy
Gamma-Ray
Astronomy,
edited
by F. A.
Aharoman
and H. J.
Yolk
©
2001
American
Institute
of
Physics
l-56396-990-4/01/$18.00
721
Downloaded 02 Oct 2007 to 131.215.225.176. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jsp
started
an
analysis
of the
properties
of the
gamma-bright
blazars
in the
CJ-F
sample
— 14
CJ-F
sources
according
to the
third
EGRET
catalog
[9].
OBSERVATIONS
AND
RESULTS
CJ-F,
is a
complete
flux-limited
sample
of 293
flat-spectrum
radio
sources
drawn
from
the 6 cm and 20 cm
Green
Bank
Surveys
[8,12]
with
selection
criteria
as
fol-
lows:
5(6
cm)>350
mJy,
a!|
0
>-0.5,
5(1950)>35°,
and
|&
n
|>10°.
A
series
of
global
VLBI
and
VLBA
observations
(5
GHz)
of the
CJ-F
sources
have
been
performed
since
1990.
We
believe
that
for
unambiguous
determination
of the jet
component
position
and
motion
parameters,
it is
necessary
to
have
at
least
three
observing
epochs,
spread
over
roughly
4
years.
These
observations
are now 96%
complete;
for
a
subsample
of 34
sources,
the
last
epoch
will
be
obtained
this
fall.
This
then
com-
pletes
the
observational
part
of the
survey.
The
data
reduction
is 92%
complete.
For
241
objects,
three
epochs
of
observations
have
now
been
obtained
and
reduced.
For 32
sources,
either
no
redshift
information
is
available
or the
source
reveals
no
structure.
For the
remaining
209
objects,
jet
component
motion
can be
studied.
In
Table
1 we
list
the
sources
of the
gamma-bright
CJ-F
subsample
by
EGRET
name
[9],
the
radio-source
identification
[9],
the
redshift,
and an
indication
whether
the
results
for
this
source
are
already
included
in
Fig.
1.
TABLE
1.
7-bright
sources
in the
CJ-F
sample
EGRET
name
3EG
J0222+4253
3EG
J0721+7120
3EG
J0808+4844
3EG
J0845+7049
3EG
J0917+4427
3EG
J0952+5501
3EG
J0958+6533
3EG
J1104+3809
3EG
J1635+3813
3EG
J1738+5203
3EG
J2202+4217
3EG
J2352+3752
3EG
J2358+4604
identified
radio
source
0219+428
0716+714
0804+499?
0836+710
0917+449?
0954+556
0954+658
MRK
421
1633+382
MRK
501
1739+522
BL
Lacertae
2346+385?
2351+456
Comment
redshift
0.444
~0.3
poss.
Id.
1.43
2.172
poss.
Id.
2.180
0.901
0.368
0.031
1.814
0.033663
1.375
0.069
poss.
Id.
1.032
1.992
reduced
y
y
y
n
y
n
y
y
y
y
n
y
y
y
The
apparent
velocities
of
components
show
a
wide
range
of
behaviour:
differ-
ent
components
in the
same
source
can
show
different
velocities;
components
can
722
Downloaded 02 Oct 2007 to 131.215.225.176. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jsp
accelerate,
decelerate,
merge,
or
split;
and in
some
cases
a
stationary
component
can
co-exist
with
moving
components.
In
Fig.
1
(top)
we
show
the
/?
app
-
redshift
relation
for all the 209
objects.
Jet
component
speeds
have
been
averaged
for
each
source
and
only
the
absolute
values
are
shown
(calculations
use
HQ
=
65 km
s"
1
Mpc"
1
,
q
Q
—
0.5).
The
tendency
of
higher
/3
app
at
larger
redshift,
already
visible
in
the
former
analysis
(when
most
sources
had
only
two
epochs,
[3]),
can be
confirmed;
it
even
seems
to be
more
pronounced
with
the
additional
data
epoch
here.
We
also
confirm
the
lower
/3
app
for BL Lac
objects
as
compared
to
those
for
quasars
or
galaxies
[3].
A
preliminary
analysis
of the
average
velocities
of the
(small)
gamma-
bright
subsample
indicates
similar
values
as
obtained
for the
gamma-quiet
objects.
One
of the
important
results
of the
analysis
of the
present
survey
is
that
curved
jet
structures
seem
to be a
very
common
phenomenon
in
AGN.
In
Fig.
1
(bottom)
we
show
the
curvature
(in
degrees)
observed
on
pc-scales
by
VLBI
plotted
against
red-
shift.
Most
sources
reveal
jets
curved
by
less
than
50
degrees,
but in
some
sources
curvature
up to 100
degrees
has
been
observed.
In
some
cases
quasi-oscillatory
trajectories
or
ridge
lines
have
been
observed.
Curvature
is
important
with
regard
to the
emission
mechanisms
that
play
a
role
in AGN
(see
below).
It
definitely
has
to be
taken
into
account
when
calculating
velocities,
and
might
significantly
change
the
motion
statistics
(not
yet
taken
into
account
in
Fig.
1) for the
fast,
strongly
curved
objects.
A
final
discussion
of the
results
has to
wait
until
the
analysis
has
been
completed.
However,
some
pecularities
might
turn
out to be
significant:
-significant
jet
curvature
is
visible
in all the
7-bright
sources
— the
curvature
for
these
seems
to be
higher
than
for the
rest
of the
CJ-F
sample
(see
Fig.
1,
bottom).
-multiple
jet
components
in
some
sources
follow
a
single
smoothly
bent
path
(e.g.,
0219+428
and
0954+658).
Also
for the
gamma-bright
blazar
PKS
0420-014
[5],
which
is not in the
CJ-F,
a
smoothly
bent
jet
path
has
been
observed.
In
this
source,
the
jet-components
also
all
follow
the
same
path.
A
binary
black
hole
model
for PKS
0420-014
[6] is
capable
of
explaining
the
motion
of
the
VLBI
components
as
well
as the
observed
optical
flaring
(see
also
[11]
for
MRK
501).
From
the
analysis
of the
paths
and
velocities,
we
conclude
that
the
orientation
of
the jet
with
respect
to our
line
of
sight
is the
most
important
factor
in
understand-
ing
the
properties
of
AGN,
because
it
controls
which
part
of the jet we
observe
and how the jet
components
appear
to
move.
A
careful
analysis
of the
whole
CJ-F
could
potentially
point
towards
the
property(ies)
that
discriminate
gamma-loud
from
gamma-quiet
AGN.
REFERENCES
1.
Barthel,
P.D.,
Highlights
Astron.
10,
551
(1995).
2.
Bower,
G.C.,
Backer,
B.C.,
Wright,
M., et
al.,
ApJ
484,
118
(1997).
3.
Britzen,
S.,
Vermeulen,
R.C.,
Taylor,
G., et
al.,
1999a.
In
L.O.
Takalo
& A. Si
lanpaa
(eds.)
BL Lac
Phenomenon,
ASP
Conf.
Ser.
159,
431.
723
Downloaded 02 Oct 2007 to 131.215.225.176. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jsp
4.
Britzen,
S.,
Witzel,
A.,
Krichbaum,
T.P.,
et
al.,
A&A
341,
418
(1999b).
5.
Britzen,
S.,
Witzel,
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Krichbaum,
T.P.,
et
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360,
65
(2000a).
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Britzen,
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Fichtel,
C.E.,
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Gregory,
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Hartman,
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79
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Taylor,
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37
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Villata,
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347,
30i
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White,
R.L.,
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Becker,
R.H.,
ApJS
79, 331
(1992).
2.0
redshift
FIGURE
1.
The
figures
shown
above
present
the
following
relations:
beta
app
(averaged
over
each
jet)
versus
redshift
for the 209
sources
with
at
least
three
epochs
of
observations,
and the
curvature
observed
on
VLBI
scales
versus
redshift
for
both
types
of
objects
(white
filled
squares:
gamma-quiet;
red
stars:
gamma-bright
CJ-F
sources).
The
legends
introduce
the
symbols
used
for
different
classes
of
objects.
724
Downloaded 02 Oct 2007 to 131.215.225.176. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jsp