X-ray emission and reprocessing in AGN

1
X-ray emission and reprocessing in AGN
Giorgio Matt
(Dipartimento di Fisica, Università
Roma Tre, Italy)
2
Plan of the talk

• Innermost regions of the accretion disc
  emission and reprocessing
• X-ray reprocessing from the pc-scale torus
• Beyond the nucleus absorption in Compton-thin
  AGN

3
X-ray emission
The standard explanation for the X-ray emission
is Comptoni-zation of soft (disc?) photons by
hot (T100-200 keV) electrons in a corona (Haardt
Maraschi 1991). The resulting spectrum is, in
the first approximation, a power low with a high
energy cutoff, as observed (e.g. Perola et al.
2002, Petrucci et al. 2001)
Unfortunately, not very much is known on the
origin and nature of the corona. Magnetic
flares? Clumpy discs? Aborted jets? We do not
have the answer yet
UV
4
Magnetic flares
Magnetic flares above the accretion disc is a
popular explanation for the heating of the
electrons, in analogy with the solar corona.
Even if the physical details are rather unclear,
MFs are a reasonable working hypothesis to model
X-ray spectra and variability (Haardt et al.
1994, Czerny et al. 2004, Goosmann et al. 2006,
see also poster 174 by Goosmann). The possible
evidence for hot spots (Dovciak et al. 2004) may
support this scenario
SOHO
5
Clumpy discs
An old idea (Gilbert Rees 1988), pursued by
many groups over the years (e.g. Celotti et
al.1992, Collin et al. 1996, Krolik 1998, Malzac
Celotti 1992, Merloni et al. 2006). Disc
instabilities may lead to an inhomogeneous
two-phase structure, one hot and optically thin,
the other cold and optically thick.
Merloni et al. (2006)
6
Aborted jets
First proposed by Henry Petrucci (1997).
Ghisellini, Haardt Matt (2004) proposed a model
in which blobs are launched with a velocity
smaller than the escape velocity. Collisions
between the outcoming and returning blobs provide
the heating
7
X-ray reprocessing
X-ray illumination of cold matter produces the
so-called Compton reflection continuum plus
several fluorescent lines, by far the most
important being the Fe Ka (e.g. Matt et al. 1991,
George Fabian 1991)
(Reynolds et al. 1995)
8
Iron lines
If reprocessing occurs in the accretion disc, SR
and GR effects modify the line profile in a
characteristic and well-recognizable way
(Fabian et al. 2000)
9
Iron lines
(Matt et al. 1992)
i20 i50 i87
a0
a1
Fabian et al. (2000)
Isotropic illumination (e.g. George Fabian
1991, Matt et al. 1991, 1992)
10
Observations
ASCA (Tanaka et al. 1995)
MCG-6-30-15
agt0 !!!
BeppoSAX (Guainazzi et al. 1999)
XMM-Newton (Wilms et al. 2001)
11
How common are relativistic lines in AGN ?
XMM-Newton observations of many other bright
Seyfert galaxies, however, found only the narrow
line (which is almost ubiquitous) (e.g. Pounds
Reeves 2002, Bianchi et al. 2004)
MCG-8-11-11 (Matt et al. 2005)
The relativistic line is certainly not
ubiquitous, at least in nearby Seyfert galaxies.
12
Next questions therefore are

• How frequent are the relativistic lines in AGN?
• Is the presence of a rel. line related to other
  AGN
• properties?

Guainazzi et al. (2006) started to address these
questions by analysing all radio-quiet,
unobscured (or moderately obscured) AGN publicly
available in the XMM-Newton archive. The sample
is composed of 102 sources. Only a handful of
them, however, are bright and well-exposed enough
to permit to search for a relativistic line.
13
How common are relativistic lines in AGN ?
The fraction of sources with a relativistic line
in the subsample of sources with enough counts
(10000) is 42 13 Hopefully, before the end
of the mission, we will have enough sufficiently
exsposed sources from an unbiased sample to
significantly improve this measure. This number
is very important to address the question of if
and how the standard picture ought to be
modified.
14
In the meantime, further information can be
obtained from stacked spectra. A strong
L-dependence is found!!
L1 Lxlt1043
L2 1043 ltLxlt 5x1043
L3 5x1043 ltLxlt1.5 x1044
L4 Lxgt 1.5x1044
(Guainazzi et al. 2006)
15
A difference between type 1 and 2 objects seems
to be present. If real, is it an inclination
effect? Is the disc co-aligned with the
absorbing matter , whatever this matter is (the
torus, the Galactic disc, etc)?
(Guainazzi et al. 2006)
16
Why are rel. lines not ubiquitous?
Relatvistic lines are expected in the standard
accretion disc scenario, so the fact that
they are not always present poses a problem. A
few possible solutions (none fully
satisfactory) a) Ionized discs b) Truncated
discs c) Very broad lines (extreme Kerr)
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Ionized discs
Hydrostatic equilibrium (Nayakshin Kallman 2001)
Costant density model (Matt et al. 1996)
For a given BH mass, ? increases with the accr.
rate, hence with L. But for a given L/Ledd, ?
decreases with M, then with L!!
18
Truncated discs
Accretion discs are believed to be truncated in
the low/hard state of Galactic BH systems
(Fender et al. 2004) (but the issue is
still controversial, Miller et al. 2006) It has
sometimes been suggested that normal Seyfert
Galaxies are the analogue of hard state
GBH (NLSy1 being possibly the analogue of soft
state GBH). But this issue is also highly
controversial (e.g. Uttley McHardy
2005) Recently, we have found possible evidence
for disc truncation in a quasar, Q0056-383 (Matt
et al. 2005)
19
Q0056-383
Q0056-383 was observed twice by XMM-Newton, about
3 years apart. In the second observation, the
soft X-ray emission is fainter, the hard X-ray
emission is flatter, the iron line EW is halved.
Is the disc truncated !?!
(Matt et al. 2005)
20
Extreme Kerr lines
Let us assume that emission comes from very close
to the BH. This is possible if the primary X-ray
source is close to the event horizon (Martocchia
Matt 1996, Martocchia et al. 2002) In this
case, lines are very broad and difficult to
separate from the continuum. Their EW are also
very large, however, because of light bending and
gravitational redshift of the primary
emission (Martocchia Matt 1996, Miniutti et al.
2003)
XMM-Newton observations of MCG-6-30-15 have
confirmed the presence of the relativistic line,
and have shown that the BH is spinning (Wilms et
al. 2001, Fabian et al. 2002). First evidence
for a spinning BH in a radio-quiet AGN
(Fabian et al. 2002)
21
Let us assume a lamppost on the BH axis, as in
the aborted jet model (Ghisellini, Haardt Matt
2004)
(Martocchia Matt 1996)
(Iwasawa et al. 1996)
Upper limits on extreme Kerr lines (Bianchi et
al. 2004) generally rule out this solution, at
least for BL Seyfert 1s, even if such an extreme
line has already been observed in a deep low
state of MCG-6-30-15
22
Light bending model
Indeed, Miniutti et al. (2004) explain the
puzzling temporal behaviour of MCG-6-30-15 (the
line varies much less than the power law) in
terms of an X-ray source close to the BH rotation
axis with a variable height (the so called light
bending model).
They showed that in this model the primary
emission varies more than the reprocessed
emission
23
Polarization of reflected radiation
In this scenario, the polarization degree and
angle of the reflected radiation strongly depend
on h, and hence on time. X-ray polarization
is a powerful tools to probe strong
gravity. Sensitive enough X-ray polarimeters do
exist!!! (Costa et al. 2005)
(Dovciak, Karas Matt 2004)
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Obscuration in AGN
Malkan et al. 1998
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Obscuration and reprocessing from cold distant
matter
I will call Torus the circumnuclear, pc-scale
matter, whatever its real shape (not necessarily
doughnut-like!).
(Matt, Guainazzi Maiolino 2003)
gt1024
There is increasing evidence that the Torus is
Compton-thick, the Compton-thin absorbers
probably related to more distant matter.
1023
1022
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Is the torus C-thick?
We analysed 8 Sey1s/C-thin Sey2s observed
simultaneously by XMM-Newton and BeppoSAX
(Bianchi et al. 2004). In 7 sources there are
both a narrow (and constant) iron line and the
Compton reflection (CR) continuum
C-Thick Torus
In the last source (NGC 7213) there is no CR and
the line may be produced either in a C-thin torus
or in the BLR
In many (most?) C-thin Sey2s there is also
evidence of reflection by C-thick
matter. Evidence come either from broad
band spectroscopy or from variability
27
Example NGC 5506

NGC 5506 is a C-thin Sey2 (NH1022 cm-2 ) with
A narrow (slt40 eV) iron line
Chandra (Bianchi et al. 2003)
28
Example NGC 5506

NGC 5506 is a C-thin Sey2 (NH1022 cm-2 ) with
a constant Fe line (despite large continuum
variations)
(Bianchi et al. 2003)
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Example NGC 5506

NGC 5506 is a C-thin Sey2 (NH1022 cm-2 ) with
a strong Reflection Component
XMM and BeppoSAX (Matt et al. 2001)
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How many Compton-thick Sey2 are there?

If the C-thin matter is not the torus, we cannot
use the fraction of Sey2 to derive the torus
opening angle. We need to look directly at X-ray
absorption. In the local Universe, about half of
optically selected Sey2s are C-thick To go beyond
the local Universe, sensitive (imaging) hard
X-ray detectors are needed ? SIMBOL-X
(Guainazzi, Matt Perola 2005)
(Risaliti et al. 1999)
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How many Compton-thick Sey2 are there?

Elusive (i.e. without optical evidence of
nuclear activity) AGN may account for almost half
of local AGN. Most of them are C-thick
(Maiolino et al. 2003)
32
The X-ray Baldwin effect

Iwasawa Taniguchi (1993), using GINGA data,
found an anticorrelation between the EW of the
iron line and the X-ray luminosity (called X-ray
Baldwin effect). The spectral resolution of
GINGA was insufficient to separate broad and
narrow lines. Page et al. (2004) confirmed
this effect using narrow (torus?) lines from
XMM-Newton observations of both RQ and RL
objects. Jimenez-Bailon et al. (2005) analysed a
sample of PG quasars and found that the effect is
statistically significant only when RL objects
are included. Jiang et al. (2006, see also poster
070), using Chandra and XMM data, found instead a
(weaker) effect also for RQ sources
only. Bianchi et al. (2006 see also poster
002) analysed all the RQ AGN publicly available
in the XMM-Newton archive (130 objects).
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The X-ray Baldwin effect

log(EW)(1.710.01) (-0.180.01)log(Lx) Bianchi
et al. (2006 see also poster 002) Possible
explanations are a) Variability (Jiang et al.
2006). It certainly explains part of the effect,
but unlikely one spread over several decades in
L. b) Decreasing of the covering factor of the
torus with L (dust sublimation? Ionization?)
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Where is the C-thin absorber?

The C-thin absorbing matter may be the galactic
disc (Maiolino Rieke 1995), or the dust lanes
(Malkan et al. 1998, Matt 2000), or starburst
regions (Weaver 2001) However, in a few cases
(NGC 3227, Lamer et al. 2004 NGC 4388, Elvis et
al. 2004, NGC 1365, Risaliti et al. 2005, and the
classical case of NGC 4151), the C-thin absorber
varies on short time-scales, implying much
smaller distances. BLR ?
NGC 4388 (Elvis et al. 2004)
NGC 1365 (Risaliti et al. 2005)
35
Beyond the nucleus. Absorption in Compton-thin
AGN
Ueda et al. (2003) and La Franca et al. (2005)
have found that the fraction of absorbed AGN
decreases with X-ray luminosity. These results
refer mainly to C-thin AGN, as C-thick AGN are
rare in lt10 keV surveys. As said before, it is
likely that the C-thin absorption is not related
to the torus So we (Lamastra, Perola Matt
2006, see also poster 075) searched for a
solution related to the host galaxy.
La Franca et al. 2005
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Beyond the nucleus. Absorption in Compton-thin
AGN
The inner molecular galactic disc is affected by
the gravitational forces of the disc itself, of
the Black Hole, and of the Bulge (which in turn
depends on the BH mass).
The height of the disc, Z(r), then depends on
the BH mass, as well as the opening angle
(and then the obscuration)
Lamastra et al. (2006)
37
Beyond the nucleus. Absorption in Compton-thin
AGN
Assuming L0.1xLedd, a good agreement with the
data is obtained for S gt 150-200 MT/pc2,. Note
that S500 MT/pc2 in the Galaxy, and that half of
sources in the BIMA sample (Helfer et al. 2003)
have S gt 150 MT/pc2
Lamastra et al. (2006)
38
Summary

• Relativistic lines are present in some (40
  ??)
• AGN, but not in all. Still an open problem.
• The torus is likely C-thick. Possible
  dependence
• of its covering factor on X-ray luminosity
• (X-ray Baldwin effect)
• C-thin absorption likely unrelated to the
  torus,
• and possibly related to the host galaxy.