Transcript An XMM-Newton Study of the Centaurus A

An XMM-Newton Study of the
Centaurus A Northern Middle
Radio Lobe
X-ray Universe 2008
R. P. Kraft, W. R. Forman, M. J.
Hardcastle, M. Birkinshaw, J. H.
Croston, C. Jones, P. E. J. Nulsen, S.
S. Murray, D. W. Worrall
Outline of Talk
• Introduction – What is the Cen A Northern
Middle Radio Lobe and Why is it
Interesting?
• Observations, Data Analysis, Results
• Interpretation
• Summary and Conclusion
Centaurus A - Overview
• Nearest galaxy with
bright active nucleus
(3.7 Mpc – 1”=17.9
pc, 1’=1.076 kpc)
• Classified as an FR I
radio galaxy
• Composite multi-band
image on right taken
from CXC website
Centaurus A (Chandra/ACIS-I – 570 ks )
Centaurus A Radio Montage
(Morganti et al. 1999)
Cen A Northern Middle Lobe (NML)
• Cen A NML – buoyant bubble from previous epoch of
nuclear activity (Saxton et al. 2003) or has the NE inner
lobe burst (Morganti et al. 1999)?
• An X-ray filament associated with the NML was first
reported by Feigelson et al. (1981) based on an Einstein
IPC observation. They argued for a thermal origin for
the emission.
• This filament was detected in several other observations
(ROSAT, ASCA, and EXOSAT) – nature and origin of this
X-ray emission (and the NML more generally) remained
enigmatic.
• Radio depolarization supported thermal interpretation
(Morganti et al. 1999).
XMM-Newton Observation of the
Cen A NML
• We observed the X-ray filament of the Cen A NML with
XMM-Newton (40 ks) to constrain the emission
mechanism of the filament which will give us a better
understanding of the dynamics of the NML more
generally.
• This was a C category observation that was observed!
• The bottom of the filament was also contained within the
FOV of a 100 ks Chandra observation (albeit far off axis
– spatial resolution similar to XMM-Newton).
• Feedback between AGN and the ambient gas may play
a critical role in the suppression of cluster cooling flows
and the formation of stars (and galaxies) at high redshift.
Multi-wavelength Overview
MOS1+2 0.5-2.0 keV smoothed (Gaussian) exposure
corrected image of Cen A NML – all unrelated point sources
removed
Chandra observation of Cen A NML
X-ray Contours on Radio Map (radio data taken from
Morganti et al. 1999) – X-ray features appear to be anticoincident with radio features
Results from Spectral Analysis
• Fit absorbed (Galactic) APEC (single temperature) models to all
knots (PN+MOS1+MOS2 simultaneously).
• Thermal models provide acceptable fits in all cases, nonthermal models are rejected at high confidence (except
for N5) -> X-ray knots are thermal.
• Temperature ranges from 0.4-1.0 keV for knots N1-N4, somewhat
higher for N5 (few keV). Elemental abundance is low (typically <0.2
Solar). Chandra confirms these values for N3, N4, and N5.
• Knots are enormously overpressurized (factor of 10 or
more) relative to ambient ISM and the equipartition
pressure of NML.
• Total mass of knots is about 107 Solar masses, thermal energy is
about 1056 ergs. Lifetime (sound crossing time=diameter/sound
speed) is a few Myrs.
• Diffuse X-ray emission along SE boundary of lobe – perhaps gas
pushing the NML to the NW?
Possible Interpretations
• Synchrotron or IC/CMB
• Super-bubble(s) from jet-induced star
formation
• Photo-ionization from beamed nuclear flux
• Entrainment/buoyant bubble (thermal gas
trunk – Saxton et al. 2003)
• Shock-heating from supersonic inflation of
NML
• Direct interaction with active jet
Disfavored Models
• Synchrotron or IC/CMB – rejected because of thermal
spectra.
• Jet-induced star formation – rejected because thermal
energy and total mass of gas too large (104-5 supernovae
required to create knots) and lack of evidence of star
formation around knots
• Entrainment of gas by buoyantly rising bubble – rejected
as the equipartition pressure of the lobe is too low and
buoyant rise time (about 170 Myrs) too long
• Supersonic inflation of NML – Knots are then interior to
the lobe -> requires pressure of lobe to be roughly equal
to knot pressure. The NML would then be enormously
overpressurized relative to ISM and total energy of radio
lobe would be large (1058 ergs) compared to the inner
lobes.
Feasible Model 1 – Photo-ionization
• Beamed emission from
nucleus could ionize a chain of
dense clouds.
• VLBI jet is roughly aligned with
the filament (at least in
projection).
• NML is from a previous epoch
of nuclear activity and has
perhaps stripped the HI cloud.
• Filamentary X-ray morphology
could represent distribution of
cold gas.
• Naturally explains X-ray/radio
anti-correlation: the knots are
compressing the lobe.
• The NML is currently
unpowered and buoyant.
• May account for `sidedness’ of
NML – large scale gas motions
• Observed X-ray flux (5x1041
ergs s-1) from nucleus is far too
low to ionizes these clouds at
these distances (15-30 kpc
from nucleus)
• Requires (unseen) blazar type
fluxes toward NML
• For ionization parameter
x=L/nd2=100 (typical value for
Tgas=700 eV), Lbeamed=1046
ergs s-1 scaled over 4p
(Kallman and McCray 1982,
Kallman 1992).
• Alternatively, AGN could have
been (much) more luminous in
the past.
Multi-wavelength Overview
Feasible Model 2 - Direct Interaction with Jet
• Unseen jet shock-heating
dense clouds to X-ray
temperatures (De Young 1991,
De Young et al. 2002, Higgins
et al. 1999, Wang et al. 2000).
• NE inner lobe `burst’ (Morganti
et al. 1999)
• The NE inner lobe is a channel
for collimated energy transfer
to NML
• Filamentary structure is the
result of ablation of clouds
• Age/lifetime of X-ray knots of
NML and SW inner lobe
roughly consistent (3x106 yrs).
• Some analogy to large-scale
radio features of M87 (Owen et
al. 2000).
• Jet must bend at least twice
without disruption
• What caused NE lobe to
`POP’? What caused jet to
bend twice? External gas
motions (Kraft et al. 2008)?
• Thermal energy of the knots is
a significant fraction (20-30%)
required to inflate the NML (i.e.
shock heating of clouds must
be efficient)
• Why do the NE and SW inner
lobes appear to be so similar
at GHz radio frequencies?
• Material did not originate from
HI cloud – jet would have to be
proton dominated to provide
sufficient momentum
Multi-wavelength Overview
Summary and Conclusions
• The X-ray emission from the filament along the SE
boundary of the NML is thermal.
• The knots are greatly overpressurized relative to the
NML unless the lobe is far from equipartition (in which
case the NML is enormously overpressurized relative to
the ambient gas)
• Model 1 – hot gas clouds created by photoionization
from beamed nuclear flux – requires blazar-type
(beamed) flux from nucleus.
• Model 2 – cold clouds were shock-heated by direct
interaction with jet - the NML is still being powered by a
collimated outflow from the active nucleus in this
scenario.
• Con-X could distinguish between the two scenarios.