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OCR for page 297
X-Ray Emission from Active Galactic Nuclei
RICHARD MUSHOTZKY
NASA/Goddard Space Flight Center
INTRODUCTION
There have recently been a large number of good reviews in this subject
(Mushotzky 1987; Elvis and Lawrence 1984; Urry 1988; Pounds and Turner
1989; McHardy 1989) which have extensively covered the X-ray spectral
and temporal properties of Seyfert galaxies and quasars In the 0.1-20 kev
band. Rather than reviewing this material again I would like to concentrate
on several observation issues which have been raised recently and some
theoretical consequences. I shall also stress what the next year will bring
from Ginga, Rosat and BBXRT observations and re-analysis of Einstein
data.
THE FORM OF THE CONTINUUM
E < 10 key
Recently Elvis and co-workers (Elvis e! al. 1986; Valves and Wins
19~) have stressed that the form of the continuum in the 0.14 kev band as
determined from Einstein imaging proportional counter (IPC) observations
of quasars, differs markedly from that seen at higher energies in Seyfert
~ galaxies. Men a powerlaw model is fit to the IPC data they find a
correlation between X-ray spectral index and radio properties, with radio
loud objects having an X-ray energy index ~ ~ 0.5 and radio quiet objects
have cat ~ 1.0. Their data also show a broad distribution in X-ray specUal
indices. This is in contrast with the narrow distn~ution of spectral indices,
strongly peaked around ~ ~ 0.7, seen in Me 2-20 kev data (Musho~zky
297
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AMERICAN AND SOVIET PERSPECTIVES
1982, 1984; Halpern 1982; Turner and Pounds 1989), independently of
radio "loudness." WiLkes and Elvis also note that they find that their
fits indicate a systematically low column density to many of their quasars
which they feel Is an indication of an additional extremely soft component
which is only visible at very low energies, E < 0.25 key. However, much
of the comparison between the 0.1~ kev and 2-20 kev data Is between
samples rather than objects. The objects in the Wilkes and Elvis IPC
sample have considerably lower fluxes than the Seyfert galaxies observed
by the Exosat and HEAD-1 proportional counters in the 2-20 kev band and
thus comparison on an object by object basis (or even individual objects
observed at different times) has been diffictlL (but see below).
Recent analysis of the Einstein IPC Seyfert galaxy sample and the
ability of the Japanese large area proportional counter experiment on the
Ginga satellite to go to lower fluxes then previous experiments has allowed
direct comparison of 2-20 kev and 0.1-4 kev data for the same objects.
Kruper et al. (1989) have determined the 0.14 kev spectral properties of
a large sample of Seyfert galaxies. The mean 0.2-4 kev spectral index of
a well determined sub-sample is = 0.81+/~.03 compared to < cat
> = 0.71+/-0.05 in the 2-10 kev band (Ibrner and Pounds 1989~. This
small difference is caused, Unmanly, (Figure 1) by the few objects (in the
overlapping sample) with very steep spectra. As shown in Figure 1 many of
these objects have a soft excess over a simple power law fit ~ the Exosat
data. It thus seems that a simple power law model is not an adequate fit
to the spectrum of Seyfert galaxies in the 0.2-4.0 kev band and that both
Exosat and the Einstein IPC frequently "agree" as to which objects are
steep at low energies. At present the overlap between the published Exosat
and Ginga quasar samples with the published IPC data are too small to have
any definitive conclusions since there are only 4 objects in common (Exosat
observations of IIIZW2 and 3C273, Ginga data on Mk205, 3C273,3C279~i
However, it is clear (Making et al. 1988) that Ginga has sufficient
sensitivity to determine the spectral indices of enough quasars and Seyfert
galaxies to confirm or deny the IPC/Exosat result for Seyfert I's.
The importance of these "soft" X-ray spectral components for models
of the origin of the X-ray continuum, the connection between the X-ray
and UV spectral bands and the ionization of the broad line clouds have
been discussed in the above review papers. Depending on the exact nature
and luminosity of the "soft" components, there is a large number of likelier
physical sites for its production viz: 1) an extension to higher energy of
1 Recent analysis of both Ginga (M. Ibrner 1989) and Exosat results (Mushotzly 1990) indicate
that the 2-15 kev spectral indices of quasars are very similar to those of Spyfert I galaxies and do
not show any trends with redshift. The overlap betwen the IPC and Exosat and Ginga samples is
~ 12 objects and thus these results are similar to those for the IPC Seyfert sample.
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HIGH-ENERGY ASTROPHYSICS
299
. 6
1 .4 - equal seal indices /
/ AKN 120
'3 1 2 ~A/
tat A NGC7469
O 0.8 - ~' ~
X 0.6 - ·/ .. MKN1040
0.4
0 .2 -
o
. . . . . .
0 0.2 0.4 0.6 0.8
IPC a
1 1.2 1.4 1.6 1.8 2
FIGURE 1 Comparison of the X-ray spectral indices of the overlapping sample of Spyfert
I galaxies between the Einstein IPC and Exosat detectors The objects indicated by a
shaded box have soft components detected by Exosat CIDn~er 1988~.
the radiation of an accretion disk (Czerny and Elvis 1987~; 2) an extension
of a "synchrotron" power law visible in the IR band; 3) a thermal photo-
ionized plasma associated with the gas confining the broad line clouds;
4) an x-ray jet, associated with the radio jet frequently seen in so-called
radio quiet objects (Haniff et al. 1989); 5) a very large number of x-ray
binaries and SNR associated with a starbursts in these objects; 6) a "partial
covering" of the envision line region by an absorbing cloud; 7) a galactic
wind associated with either a starburst or nuclear phenomena; or 8) the
signature of reprocessing of the nuclear radiation by a thermal accretion
disk or "cold" accreting matter. It is, in fact, anticipated that all or most
of these components have already been detected. However, it requires
detailed spectroscopy, imaging and timing anab,rsis to determine which or
how many of these components are prominent in which objects. Many of
these putative components can only be detected in the soft X-ray band. It is
thus important to know the nature of the spectra of these soft components
in demil. If possibility #1 is true in a fair number of objects it implies that
a very significant fraction of the total energy in these objects is radiated in
an accretion disk component (Sun and MaLkan 19893.
It is possible to fit more complex models man single power laws to the
combined simultaneous Einstein solid state spectrometer (SSS) and monitor
proportional counter (MPC) data, or to the combined IPC and MPC data.
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AMERICAN AND SOVIET PERSPECTIVES
This extends the bandwidth for fitting the spectra to ~ 0.1-15 kev for the
IPC+MPC combination (with E/^E ~ 1 for the IPC and K/A E ~ 3 for the
MPC) or -to 0.6-15 kev for the SSS+MPC data (with however E/^E ~ 10
for the SSS). These projects are underway at present. Preliminary results
from the IPC+MPC fitting for 26 objects indicate that, for those objects
for which the IPC data have been fit by single "steep" power laws that
complex models are preferred systematically over single power laws. The
indications are that an additional soft component is required for ~ 2/3 of
the observations and this component is very soft for only ~ 1/3 of the these
observations (e.g., ~ 10% of the total sample). Most of the observations
can be modeled by a hat power law of energy index ~ 0.7 and a soft
component which can be represented by a black body of kT ~ 0.25 kev
or thermal bremsstrahlung of kT < 1/2 key. However, it seems likely that
the observed distribution of kTs is biased by the detailed form of the IPC
transmission function and its poor energy resolution. For a few objects,
these results are confirmed by a similar analysis of the Einstein SSS and
MPC data and a re-analysis of the combined low energy (LE) and medium
energy (ME) Exosat data. Use of these two component models eliminates
the artifact of systematically low galactic column densities noted by Spokes
and Elvis and results in a good fit to the data. However neither the IPC
nor SSS data can determine the form of the soft excess, in particular it is
not clear if it a "pure" continuum or if spectral line radiation is important
ate< lkev.
The Ginga and Exosat timing data indicate that there are several
physical origins of the soft component. Sometimes the hard (E > 2 kev3
flux varies faster than the "soft" (as in ~335, Turner and Pounds 1988),
while sometimes the soft component varies faster than the hard (NGC 4051
Kunieda et al. 1989) and sometimes the soft flux does not vary at all while
the hard flux changes (NGC4151 Pounds et al. 1986 or NGC 7469, Barr
1986~. However these data (mostly Exosat results) have suffered badly
from a lack of spectral resolution at E < 2 kev and lack of sensitivity and
bandwidth at E > 10 key.
It is clear that what is needed to resolve this issue is a broad band
telescope and sensitive spectrometer in the 0.2-12 kev band such as BBXRT
(the Goddard Space Flight Center Broad Band X-ray Telescope Spectrom-
eter) or SODART on Spectrum-X We anticipate that the BBXRT spectra
of ~ 30 AGN will resolve the issue of the form of the continuum for this
selected sample but will not be extensive enough to attack the issues of the
dependence of the X-ray continuum form on optical and UV luminosity,
redshift, IR or radio properties.2 Concentrating on the low~nergy data
2 Results presented at the Bologna 1989 conference on active galaxies and the X-ray background
indicate that there may be another continuum component which is becoming important at E >
OCR for page 301
HIGH-ENERGY ASTROPHYSICS
301
alone, the spectral resolution (K/A E ~ Z5 at 1 key) and sensitivity of the
Rosat PSPC will be able to test whether complex spectral models in the
0.1-2 kev band are necessary for a large sample of objects and will allow
direct comparison with the (non-sunultaneous) Ginga results. However the
lack of simultaneous E > 2 kev data will make the interpretation model
dependent. I thus anticipate that ~ ~ 2 years from now that the form of
the 0.1-10 kev continuum will be well determined for a reasonable sample
of QSOs and Seyfert I galaxies.
With these data we will be able to start to understand the nature of
spectral evolution and the statistical connections between the form of the
X-ray continuum and the other properties of these objects (Mushotzly and
Wandel 1989~. The BBXRT data should be of high enough quality that the
spectral nature of the soft component is clear and thus its connection (if
any) to the UV bump and an "extension" of the IR "power law" or any
of the other suggested models can be tested. BBXRrs great sensitivity
to spectral lines will test whether any of the optically thin models for the
formation of the soft component are "correct".
The frequent presence of "soft excess" in the spectra of active galaxies
implies that if they are a major contn~utor to the cosmic X-ray background
that the spectral form of the background should change at E < 2 kev
(Boldt 1987~. Thus comparison of the contn~ution of classes of object
to the background must include an additional spectral uncertainty. If the
temperature of this component is related to its luminosity or to cosmological
epoch one might expect a strong change in the form of the continuum with
redshift which has not been seen so far (Canizares and White 1989~.
The Continuum from 1~100 key
By analogy with recent results on galactic black hole candidates perhaps
the best clues to understanding the nature of the physical mechanisms
responsible for the high-energy emission from AGN lies in the modeling
of the E > 10 kev continuum. In particular the case for the effects of e+
-e~ pairs and reprocessing due to cold material cannot be strongly made
until high quaky spectral and temporal data are available in this energy
range.
At these energies the data from HEAD-1 are still the best available
for a "large" data set. While they are not of very high quality, the E
> 20 kev data indicated that the form of the continuum was consistent
with a single powerlaw being a good description of the continuum from
2-100 kev (Rothschild et al. 1983~. In one object, NGC4151 (Batty et al.
15 key. This component was hypothesized to be due to the reflection of the power law continuum
off an accretion disk see next section.
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302
AMERICAN AND SOVIET PERSPECTIVES
1984) there was evidence for a change (a steepening) in the slope of the
high-energy continuum. Recent theoretical models (Lightman and White
1988; Ferland and Rees 1988; Guilbert and Rees 1988) indicate that if
an accretion disk or other source of "cold material" exists close to the
central engine, as is required in almost any accretion model of AGN,
that significant reprocessing can take place which should distort He 10-200
kev spectrum in a particular fashion depending on whether reflectance or
transmission dominates. There are indications from Toga data (Matsuoka
et al. 1989) that such an effect has been detected in > 3 objects.
The high~ualibr Ginga data that is available should serve to better de-
fine the continuum in the 10-37 kev band. However, the Ginga background
in this energy band is relatively large (and variable) compared to the signal
from most active galaxies and thus care must to taken in the analysis and
interpretation of these data.
Progress in this field has been slow and it appears that the next
improvements for weak sources may be a while in coming. It is quite
possible that the mask technique used on Granat may make a major
advance in this area. If this proves not to be true for unforeseen reasons,
we must wait until the ATE and SAX missions which will provide chopping
detectors of sufficient sensitimbr to obtain samples of > 20 objects out to
E > 50 key.
As in the lower energy bands, the form of the AGN spectra appears
to deviate from that of the X-ray background ~B) radiation. In both
the E 2 kev domain the spectra of sources is
steeper than the background (at E sot~rce5 > 1.0 (Maccacaro
et al. 1988; Canizares and White 1989), while backg~oun`, 0.8 (Fabian et
al. 1983~; at E>2 key, ,ource`, ~ 0.7 while background ~ 0.4 (Boldt
1987~.
As opposed to the 0.5-3 Rev band where both luminosity and spec-
tral evolution are needed for AGN to produce the background flux and
spectrum in the E > 10 kev band only spectral evolution is needed. As
many authors have pointed Out (e.g., Rothschild et al. 1983), the flat AGN
spectrum would predict too much sky flux at E > 50 kev if there is strong
luminosity or density evolution at these energies. There is thus a strong re-
quirement to measure the high-energy continuum of those objects thought
to be responsible for the 0.5-3 kev background radiation The spectral and
density evolution of these objects must be very well tuned (Mor~sawa and
Takahara 1989) in order to reproduce the high~ualin,r XRB spectral data.
X-RAY SPECTRAL FEATURES
As stressed by Elvis and Lawrence (1985) the energy resolution of most
present day X-ray counters is sufficient to reveal only the very strongest
OCR for page 303
HIGH-ENER~Y ASTROPHYSICS
303
features. The only significant X-ray spectral feature seen in more than one
object has been the "6.4 key" line due to iron. Before the launch of G~ga
this line had only been determined with > 3~ significance in ~ 6 objects.
However it was only for the two brightest AGN, NGC4151 and Cen-A,
that the error bars were sufficiently small that detailed information could
be gleaned.
As shown in detail by Maxima 1986, the iron line emission from
Cen-A was consistent with fluorescence from a spherically uniform cold
absorber with "solar" iron abundance which was also responsible for the
low energy photoelectric absorption seen in this source. However, in the
case of NGC 4151 the iron line was a factor of 3-6 times too strong for
this to be the case. Part of this enhancement was most likely due to a
greater than solar iron abundance (as is common in the central regions of
spiral galaxies) but this could not reproduce the whole effect. The other
explanations relied on either geometry or time variability. The geometrical
explanations required that either the reprocessor or the radiation field be
non-spherical The time variability explanation relied on the fact that the
region responsible for the iron line was most likely several light days from
the nucleus and thus the observer, who sees the average over the whole
iron emission line region, was seeing enhanced iron line radiation due to
a brightening in the continuum that occurred prior to the observations lit
up the Fe line producing region and had decayed away. However this
explanation could not explain the systematically high iron line flux seen in
this object (Warwick et al. 1989).
More recent observations (summanzed by Warwick et al. 1989) show
a very complex pattern with none of the expected behavior between source
intensity, photoelectric absorption and iron line intensity. It Is clear that
the origin of the Pe line Is not well understood, even for this the best
observed AGN.
Pounds et al. (1989) have used Ginga data to study iron lines in three
objects. In one of them, NGC5548 there is a weak ~ 120 ev equivalent width
line (similar to Cen-A), but no evidence for absorption by cold material
either at iron or at low energies (NGC5548, in fact, has a very strong,
very soft "excess"~. This is the second example of a fluorescent iron line
without low energy absorption, Mk509 (Monni et al. 1987), being the other.
This strongly suggests that geometrical effects can dominate the observed
Fe line emission. Pounds et al. speculate that the iron line ~ NGC5548 is
due to reprocessing in a "face-on" disk and that such re-processing is also
associated with the presence of soft components.
As the observational situation has improved so the theoretical expla-
nations for the origin of this line have increased in complexity. At present
we have > 6 strong candidates for the physical origin of the iron line in
AGN (some of which are directly connected to models of the low energy
OCR for page 304
304
AMERICAN AND SOVIET PERSPECTIVES
continuum) viz: 1) fluorescence from the same clouds that produce the
broad optical and I]V lines; 2) fluorescence from the innermost regions
of an accretion disk (Fabian e' al. 1989~; 3) recombination radiation from
a cloud of "cold" electrons that are responsible for the "redacted" broad
optical lines seen in Seyfert I} galaxies (Knoll and Begelman 1988~; 4)
fluorescence from "cold" material being accreted by the central object
(Ferland and Rees 1988~; 5) recombination in a hot photo-ionized inter-
cloud medium or ~ a galactic wind; or 6) in a jet (ala SS433~. The situation
is similar to the confusion that reigned ten Years ado over the origin of the
iron line in X-ray binaries.
High spectral resolution (AK/E ~ 30) and signal to noise data are
required to discriminate amongst these models. Fluorescence lines origi-
nating in an accretion disk should be broad and show velocity shifts which
are easily resolved with detectors with 200 ev resolution. The energies
and ratios of recombination lines originating in a photo-ionized plasma
(Krolik and Begelman 1988) have characteristic energies and line ratios of
a photo-ionized gas.
TIME VAR[ABILI1Y
McHardy (1989) has recent reviewed the Exosat data on time Yari-
ability in both the 2-10 kev and E < 1 kev bands. He concludes that while
there may exist a variety of power density spectra (PDS) power law slopes,
all the Exosat data are consistent with no cutoff in the high frequency
spectrum and most sources exhibit power law slopes ~ 1 over a broad
frequency range. He also concludes that the time series are stationary, e.g.
that the form of the PDS is the same no matter what stretch of data is
being analyzed.
A perhaps contrary observation is that of Urry et al. (1988) from
analysis of the Einstein IPC data. For a large number of AGN observations
of average length of a few thousand seconds with good signal to noise,
only a very small number showed significant variability. We (Weaver e!
al. 1989) have recently analyzed a large number (A 2003 of Einstein Solid
State Spectrometer observations of AGN and have obtained similar results.
Taken at face value this might indicate that, like low-mass X-ray binaries,
sometimes AGN are in an active state and sometimes they are quiescent.
Higher signal to noise and longer observations are necessary to test this
hypothesis.
We (Awaki et al. 19893 have obtained long Ginga observations of
two Seyfert I galaxies (NGC4593 and NGC68143 which were known to be
variable from Exosat, HEAD-1 and Einstein data. Preliminary analysis of
the NGC4593 data shows that during one of the two Ginga observations,
on tune scales less than 1000 seconds, me source showed no evidence for
OCR for page 305
HIGH-ENERGY ASTROPHYSICS
305
variability at > 20% level. In strong contrast, the Toga observation of
NGC6814 showed extremely large amplitude variations (~I/I > 5) on time
scales < 500 seconds. It thus seems that there exists a wide variety of
time variability behavior ~ AGN which is only beginning to be explored.
There are indications from Ginga and Exosat data that some sources (such
as ~335; Turner and Pounds 1988 and NGC4051; Lawrence e' al. 1987)
show different time variability behaviors at low and high energies thus
giving vital clues as to the origin of the continuum which may be connected
to their spectral differences.
Ginga observations of Seyfert galaxies can explore the domain of
fast (At < 1~ second) mnability of AGN to emmine me characteristic
time scales expect with the innermost regions of an accretion disk around
massive blackholes, T ~ SOM6 sec. However the absence of low-energy X-
ray detectors on Ginga (and XrE) will not allow follow-up of the exciting
Exosat results. It will await the large collecting area and broad bandwidth
of SODART and XbiM to attack this area of research.
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Representative terms from entire chapter:
accretion disk
