Transverse X-ray Spectral Structure in Cen A’s
inner jet, and implications for particle
acceleration
Diana Worrall
University of Bristol
& the Cen-A Chandra VLP Team
• Where are knots located in Cen A’s jet?
• Through probing the X-ray spectrum with projected
location, what inferences can be made about the knot
structures and particle acceleration?
Cen A radio emission
Location
of X-ray
jet
Morganti+
Burns+
Junkes/Klamer
Intermediate power
jets
3C 346 VLA on Chandra
3C 15
VLA on
Chandra
Dulwich+ 2009
M87
Dulwich+ 2007
HST
Perlman+ 1999
High Power
Low Power
Quasar 4C19.44 z=0.72
0.’’45 beam
FRI 3C 296 z=0.024
0.’’25 beam
Radio jets
commonly
are knotty
Jorstad 2006
Laing+ 2006
FRIs slow down and
have transverse velocity
structure.
3C 31
VLA
In straight 2-sided jets the
kinematics can be inferred
from the radio alone via
sidedness and opening-angle
arguments
e.g., 3C 31
Laing & Bridle 2002 MNRAS
Slowing
Core
Flaring, spine
~0.8c
Fast so dim
where sheath is on average
about 0.7x speed of the
spine
Counterjet
Knots in low-power jets are
• Individually less wide than the jet
envelope
• Where the jet flow is still
relativistic
Deceleration and shear flow
downstream suggests shear flow in
in the knotty region
0.’’25 beam
Bridle+ 2006
NGC 315
Worrall, Birkinshaw, Laing, Cotton, Bridle 2007 MNRAS
VLA ridge line, Chandra color
VLA
flare
~0.9 c
v. fast
Filament breaks up beyond relativistic flaring region.
•  superposition of K-H-instability surface modes?
•  B-field or particles injected into flow in shear layer?
NGC 315
Filament is enhanced emissivity in both radio and X-ray  Particle
acceleration since X-ray emission synchrotron (X-rays too bright
and spectrum wrong for inverse Compton).
Much fine tuning of modes if caused by K-H instability
• Likely to lie in shear layer
• Magnetic field vectors along filament
Roughly 2 wavelengths of oscillation
may imply rotating injection creating a
‘magnetic strand’
Chandra 600 ks (7 days)
March-May 2007
First papers:
Hardcastle+ 2007 – jet/counterjet radial structure
Worrall+ 2008 – jet transverse/velocity structure
Jordan+ 2008 – LMXBs/globular clusters
Sivakoff+ 2008 – LMXB transient
Kraft+ 2008 – Gas discontinuity
Croston+ 2009 – SW lobe
Voss+ 2009 - LMXBs
Chandra 0.8-4 keV
Cen A moved to 70 Mpc
(but 10x observing time)
NGC 315 at 70 Mpc
Cen A reveals its jet structure with the highest spatial resolution
Chandra VLP data show X-ray jet with unprecedented sensitivity
Rotated jet. 0.8-3 keV to de-emphasize core and absorption
X-ray weak
Worrall+ 2008 ApJL
Predominantly
diffuse
Predominantly
knotty
Core
~540 pc
Over full jet, spine has flatter spectrum than sheath
But result is diluted by the diffuse
emission that has the
same (steeper) spectrum
in both regions.
Spine
knots
Result even more significant if
consider inner jet (less variation in
NH) and brighter knot emission
Sheath
knots
Diffuse
Inner knots
spine
sheath
Blue region knots
Knots are not in the shear layer, else
expect same spectra for those
projected onto spine and sheath
spine
sheath
Transverse profile across inner jet knots. It is not the brighter
knots that have the flatter spectra. Depends on position.
Inner knots
spine
sheath
In the X-ray, energy loss (and hence acceleration) local  different conditions
for particle acceleration across jet. More kinetic energy available for release in
the spine  harder spectra.
Dynamical situation:
Results point to transverse flow structure.
Knots might be due to intruders (gas lumps?) entering flow at velocities of stellar
velocity dispersion (explains radio/X-ray offsets and VLBI components seen in
some knots)
Knots might be internal flow irregularities originating in the spine and circulating
to shear layer. Will transverse knot motion be seen?
Summary
• Spectral intricacies in Cen A impossible to see in other jets
• Knots not confined to shear layer, but shear key to the results.
• In the X-ray, since energy loss (and hence acceleration) local 
different X-ray spectra  different electron injection
spectra results imply different conditions for particle
acceleration across jet.
• More kinetic energy available for release in the spine  harder
spectra. Shear most important closer to the nucleus, and
differences most detectable here.
Dynamical situation:
• Some knots consistent with intruders (gas lumps or stars)
entering the flow at velocities of stellar velocity dispersion
• Knots might be internal flow irregularities originating in the
spine and slowly circulating to shear layer
Can transverse knot motion be seen?
low-power jets: uncontroversial that X-rays are synchrotron.
X-rays too bright and spectrum wrong for inverse Compton
M87
3C 66B
NGC 2484
PKS 0521
Böhringer+ 2001, Hardcastle+ 2001, Worrall+ 2001, Birkinshaw+ 2002.
Bme~ 100µG (10 nT). Electron lifetime tens of years
 in situ acceleration. How?
• Strong shocks?
• Population of shocks in a messy structure (e.g., supersonic
MHD turbulence)?
NGC 315
X-rays detected in flare & (lesser relative to radio) in outer
region. Radio/X-ray transverse widths comparable in both regions.
VLA contours on
Chandra color
Chandra points, VLA line
Particle
acceleration
throughout
diffuse jet
out to 10 kpc
Flare region
Outer region
Worrall+ 2007
MNRAS
|flare region|outer region