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X-ray astronomy
The new X-ray universe
Phil Charles and Andy Fabian review the changes brought to X-ray astronomy
by the two orbiting observatories, XMM-Newton and Chandra.
e describe here the changes
to X-ray astronomy that are
being brought about by the two new
orbiting X-ray observatories, NASA’s
Chandra and ESA’s XMM-Newton.
Between them they bring a dramatic
gain in sensitivity, spatial resolution
(now comparable to ground-based
optical and infrared telescopes) and
spectral resolution. The superb
Chandra mirrors rapidly resolved the
diffuse X-ray background and have
produced stunning detailed images
of a wide variety of cosmic objects.
Large gains have been made by
both missions in spectroscopic
performance, with individual X-ray
emission lines now detectable in hot
stellar coronae, supernova
remnants, X-ray binaries, active
galactic nuclei and galaxy clusters.
W
X
-ray astronomy took a major step in
the 1970s with the first scanning satellites, such as UHURU and Ariel V, but
it required the imaging capability (using grazing incidence mirrors) of the Einstein Observatory (launched in 1978) in order to reach the
sensitivity level necessary to study significant
samples of normal stars and galaxies. This
enabled the Einstein observatory to be used to
tackle one of the longest standing problems in
X-ray astronomy, namely the nature of the diffuse hard X-ray background, giving the first
direct indication that, at least at low energies,
it might be due to large numbers of very faint,
distant sources. X-ray astronomy is now well
established as a tool for tracing the hot universe, in the aftermath of supernova explosions, binary star systems involving white
dwarfs, neutron stars and black holes, active
galaxies and quasars, and hot intergalactic gas.
The missions that followed Einstein in the
next two decades (EXOSAT, GINGA, ROSAT,
ASCA and BeppoSAX) barely improved on
spatial resolution, which remained substantially worse (factors of 5–50) than most other
December 2001 Vol 42
1: This NASA X-ray image of the central arcminute of M31 demonstrates Chandra’s superb imaging
qualities. The two sources at the centre (blown up in the inset) are barely an arcsecond apart. The red
contours from HST show the location of the supermassive black hole. The X-ray flux is weak because
accretion onto the galaxy nucleus is very slow. The blue source is much cooler than the others, and is
probably a white dwarf accreting so fast from its companion that there is continuous nuclear burning on
the surface of the star (a “supersoft source”). (X-ray: NASA/SAO/CXC/M Garcia et al.; Optical: NASA/GSFC/T
Brown et al.)
wavelengths, particularly from the ground.
Several of the missions opened up the field of
X-ray imaging spectroscopy and all of them
enabled significant scientific advances to be
made. The area of high spectral resolution did
not, however, improve.
XMM-Newton and Chandra have changed
all this, and in highly complementary ways.
Chandra provides the sub-arcsecond resolution
equivalent to the spatial resolution of groundbased optical and infrared telescopes, but at
X-ray wavelengths (figure 1). Its mirrors are
the finest yet produced in X-ray astronomy,
having a surface figure of ~0.5 arcsec. Such
high resolution enables it to overcome the
problems of source confusion that limited earlier missions in, for example, studying individual sources in other galaxies.
XMM-Newton has a large collecting area
from its multiply nested mirrors which makes
it particularly sensitive, enabling the detection
of weaker and variable sources. Grating spectrometers have been developed for both observatories that improve dramatically on the technology used in the 1970s and 80s. Chandra
uses deployable transmission gratings that can
be introduced into the field of view (figure 2),
whereas XMM-Newton has permanently
mounted grazing-incidence reflection gratings
that intercept approximately half of the light
6.11
X-ray astronomy
ISIM
optical
bench
solar array
aspect
camera
sun shield
LETG
2: A schematic of the light path through
Chandra. X-rays are absorbed at normal
incidence; X-ray telescopes depend on
glancing incidence, and so consist of
nested barrel-shaped mirrors (NASA).
HRMA
ACIS/HRC
HETG
electrical
boxes
thermal shroud
3: The centre of the starburst galaxy M82, in X-rays. The bright source near the centre of the image, about 600 light years from the dynamical centre (small green +)
of the galaxy, increased dramatically in intensity over three months (compare left and right panels) after which it decreased – a pattern of variability characteristic
of a black hole. (NASA/SAO/CXC.)
oxygen VII
triplet
oxygen VIII
Lyα
He-like
H-like
neon IX neon X
triplet
Lyα
He-like
H-like
magnesium XI
triplet
direct
image,
all
energies
He-like
4: Spectral resolution illustrated by this supernova remnant E0102-723.
5: Chandra’s view of the Centaurus
galaxy cluster, revealing hitherto-unseen
details of this cooling-flow cluster,
especially in the central region.
6.12
from two of its three mirrors (hence producing
high-quality X-ray spectra as part of all observations). These instruments have brought
major improvements in spectral resolution but
with a staggering gain in throughput, which
means they can be used to study a much wider
range of objects than was possible with, for
example, the Einstein grating spectrometer.
This enables a much clearer picture of the temperature, structure and behaviour of sources,
whether galaxy clusters, active galactic nuclei
or binary stars. Both instruments – described in
more detail in the boxes on pages 6.14 and
6.16 – have a high throughput, providing a
much wider range of accessible science in the
available observing time than hitherto possible.
The spatial resolution of Chandra is especially useful in crowded source regions, such as the
huge numbers of individual sources that are
being resolved in M31, the nucleus of which has
finally been resolved in X-rays. It is now possible to monitor variability in these faint sources,
giving yet more information about the processes responsible for their emission. For example,
figure 3 shows a source identified in M82
December 2001 Vol 42
X-ray astronomy
Starburst galaxies
Extragalactic astronomy has benefited from
the sensitivity and the volume of observations
now possible. Comparisons of the number of
X-ray sources in galaxies shows a fourfold
variation from one to the next, implying different evolutionary histories. Several starburst
galaxies – galaxies undergoing extensive star
formation – have been found to contain ultraluminous X-ray sources, probably massive
black-hole systems. The first Chandra observations of NGC253 suggest that the observations
come from just a fraction of the mass in the
starburst winds, the densest parts where it
meets the galaxy’s interstellar medium. In
future, spectroscopic observations should
resolve enough of the starburst winds in order
to constrain the gas masses involved.
Galaxy clusters
The high spectral resolution and sensitivity
offered by XMM-Newton has broadened our
knowledge of clusters of galaxies, especially
those associated with cooling flows. The RGS
spectrometers have provided the first high
spectral resolution spectra of the central
regions (figure 6 shows data for Ser 159-03, as
an example). The spectra show the presence of
gas at about one third of the cluster virial temDecember 2001 Vol 42
6a: The radial distribution
of Fe, T and density in Ser
159-03. Key items are: the
temperature drop in the
centre (ascribed to cooling
gas), the temperature drop
in the outer parts, and the
clear abundance gradient.
b: RGS spectrum of this
cluster, for which the
following applies: RGS data
show O VIII, Ne X and Fe
XXIV/XXIII lines. Fe
XVII/XVIII lines are weak or
absent. Typically there is
five times less cool gas
compared to the isobaric
model.
(a) 3.0
2.5
T (keV)
2.0
1.5
Fe abundance
1.0
0.4
0.2
nH (m– 3)
0
104
1000
100
0.1
10
1
radius (arcmin)
(b) 0.06
counts/s/Å
0.04
0.02
0
(observed – model)
/model
which turned out to be highly variable (indicating that it was a single object, not a close group
of sources) and at the known distance of M82
this implied that it was “ultraluminous”
(Lx >1039 erg s–1; the source in M82 varies up
1041 erg s–1). While the existence of such objects
was hinted at by earlier missions, a substantial
number have now been found in Chandra and
XMM observations of nearby galaxies. If scaled
simply relative to accreting X-ray binaries in
our own galaxy, then this suggests that they
may be accreting black holes with a compact
object mass of around 100 Suns. If confirmed
this has important consequences for the late
stages in the evolution of the supermassive stars
that are needed to have formed these objects.
However, there are alternative interpretations
where the high luminosity is a result of “beaming” in our direction which makes the objects
look intrinsically brighter than they really are.
In another case, the active RS CVn II Peg was
seen to flare during observation with Chandra’s high-energy transmission grating, revealing the dramatic change in temperature structure of the plasma as it moves from quiescent
to flaring conditions, a process that has links
with the much smaller-scale flares observed
from the Sun. Spectra also showed a significant
Fe deficiency.
We describe below some of the highlights of
observing with XMM-Newton and Chandra,
in fields as varied as galaxy clusters, quasars,
X-ray binaries and the Galactic Centre.
10
15
20
25
30
35
10
15
20
25
wavelength (Å)
30
35
1
0
–1
perature there, but register the absence or only
a low level of emission from Fe XVII and
Fe XVIII ions expected if gas continues to cool
below 1 keV or so. There is much less cool gas
than would be expected for most simple isobaric cooling-flow models. No unique explanation exists for this characteristic.
Chandra observations of nearby cooling-flow
clusters, the Perseus cluster, Abell 1795, and
the Centaurus cluster (figure 5), show considerable detail of the structure in the central
cooler region. Holes in the Perseus cluster
X-ray emission are formed by radio lobes, but
expanding only slowly since no shocks are evident. Two outer holes correspond to low-
frequency radio spurs, and the hot X-ray gas
shows a spiral structure that may be due to the
angular momentum of infalling material. For
A1795, a linear X-ray structure correlates with
a filament in Hα emission, and is cooler than
the surrounding cluster and may have been
formed by cooling gas. Maps of temperature,
absorption and abundance in the Centaurus
cluster indicate decreasing temperature and
decreasing absorption towards the centre of
the cluster following a peak at about 20 kpc.
Measuring mass and dark matter
Chandra’s spatial resolution makes it a robust
tool for measuring mass. So far, observations
6.13
X-ray astronomy
Chandra
have confirmed that hot gas exists within
groups of galaxies that have low X-ray luminosity, and in some with very low velocity dispersion. These groups are in the majority; if
otherwise invisible hot gas is a general feature
of them, then this may account for the bulk of
the baryonic mass at this scale.
Chandra’s resolution makes it a robust tool
for mass determination in galaxy clusters. In
Abell 2390, Abell 1835 and RXJ1347.5-1145,
all X-ray luminous, relatively relaxed clusters
of galaxies, the mass profiles determined from
the Chandra data are in good agreement with
the predictions from numerical simulations.
The best-fit X-ray mass models matched independent results from gravitational lensing studies and, where available, optical measurements
of the galaxy velocity dispersions in the clusters (figure 8). Using the Chandra measurements, the X-ray gas mass fraction as a function of radius in the clusters was determined
and, assuming that massive clusters give a fair
sample of the properties of the universe as a
whole, the results suggest the relation
Ωmatter = 0.33 – 0.40 h50–0.5. Observations have
also confirmed that hot gas exists within
groups of galaxies that have low X-ray luminosity, and in some with very low velocity dispersion. These groups are in the majority; if
6.14
7: Final polishing and checking of one of Chandra’s superb mirrors, which allow the spectacular results.
other, ACIS-S, is a spectroscopic array with
six chips, used in the main with the grating.
There is also a High Resolution Camera
(HRC; PI Steve Murray, SAO) comprising
two microchannel plate detectors. HRC-I
uses a 90 mm2 detector optimized for imaging, with a 30 arcmin field of view and
~0.5 arcsec spatial resolution. HRC-S has a
20 × 300 mm rectangular detector optimized
for use with the Low Energy Transmission
Grating experiment (PI Bert Brinkman,
Utrecht). The High Energy Transmission
Grating is used with ACIS-S (PI Claud
Canizares MIT).
8: A comparison of the projected total mass
determined from the Chandra X-ray data for Abell
2390 (68% confidence limits shown in purple) with
the strong lensing result of Pierre et al. (1996, red
circle) and the weak lensing results of Squires et al.
(1996, orange). The effective velocity dispersion
associated with the best-fit X-ray mass model is in
good agreement with the optically determined value.
1015
mass (M)
The Chandra X-ray Observatory started life
as NASA’s Advanced X-ray Astrophysics
Facility (AXAF). It was renamed in honour
of Subrahmanyan Chandrasekhar. It was
launched and deployed by the Space Shuttle
Columbia on 23 July 1999, into an elliptical
high-Earth orbit that allows long-duration
uninterrupted exposures of celestial objects.
The combination of high resolution, large
collecting area and sensitivity to higher
energy X-rays make it possible for Chandra
to study extremely faint sources, sometimes
strongly absorbed, in crowded fields.
The observatory is expected to have a
ten-year mission.
Chandra has a spatial resolution of
< 1 arcsec over an energy range from 0.1 to
10 keV. It follows a 64-hour, highly eccentric
Earth orbit. The observatory carries a
Wolter Type 1 grazing incidence iridiumcoated imaging telescope with a field of
view ~30 arcmin across and an effective
area of 800 and 400 cm2 at 0.25 and 5 keV
respectively. The detectors are as follows.
The AXAF Charge Coupled Imaging
Spectrometer (ACIS; PI Gordon Garmire,
Penn State University) has two CCD arrays
for a total of 10 chips. One array (ACIS-I) is
primarily for imaging and uses four chips; the
1014
100
200
500
1000
radius (kpc)
2000
otherwise invisible hot gas is a general feature
among them, then this may account for the
bulk of the baryonic mass at this scale.
Quasars
High-resolution spectroscopy has changed
researchers’ perceptions of quasars, providing
some localization of the sources of particular
lines. This is another field in which combining
observations from both Chandra and XMMNewton brings benefits in terms of details and
in the number of objects examined. Combining
data from 10 AGN shows a correlation
between the ionization state of the disk and the
AGN luminosity. Largely neutral iron gives a
strong line at 6.4 keV; ionized iron produces
lines at 6.7 keV and 6.97 keV. Iron appears
largely as a neutral line in many Seyfert 1 type
galactic nuclei; it becomes more ionized as the
luminosity of the central source increases. The
high sensitivity and spectral resolution is
enabling weak narrow line components to be
readily found: there is a narrow, neutral iron
line of similar width and relative strength in all
but the most luminous sources, implying some
common origin in a molecular torus or possibly the optical broad-line-region (BLR).
The EPIC consortium has examined several
December 2001 Vol 42
X-ray astronomy
December 2001 Vol 42
Markarian 766
counts s–1 Å–1
0.08
0.06
C VI
Lyα
0.04
0.02
0 VIII
Lyα
N VII
Lyα
residuals
0
0.02
0
–0.02
5
10
15
20
25
wavelength (Å)
35
30
MCG – 6 – 30 –15
counts s–1 Å–1
0.08
0.06
C VI
Lyα
0.04
0.02
0 VIII
Lyα
N VII
Lyα
0
residuals
AGNs including the luminous quasar PKS
0558-504, which shows a featureless X-ray
continuum in which a large, soft X-ray excess
dominates the high-energy features. The data
are consistent with a multitemperature accretion-disk corona. Variations in the X-ray continuum, such as an increase of 35% in less than
an hour, suggest that there is a high accretion
rate. In this source there is no sign yet of a neutral iron line at 6.4 keV, although there is a
highly ionized iron line at 6.97 keV which may
be variable. Mrk 205, an intermediate-luminosity QSO, has a two-component iron line, which
can be fitted by a narrow 6.4 keV (neutral)
component, possibly arising from the molecular
torus and/or BLR, and a broad 6.7 keV (Helike) component, possibly arising from a partially ionized accretion disk. A similar two-component profile is seen in the lower luminosity
Narrow-Line Seyfert 1 galaxy, Mrk 359,
although here the narrow line appears stronger
and the broad component is more neutral.
The HETG is also probing circumsource
regions in AGN. Extended emission from the
Seyfert 1.5 galaxy NGC 4151 shows evidence
for both photoionized and collisionally ionized
material in a multicomponent narrow line
region. The emission line spectrum of the
Seyfert 2 galaxy NGC 1068 looks remarkably
similar to that of NGC 4151, but with a relatively weaker continuum. The spectra of two
high-redshift quasars were searched for evidence of resonance absorption lines from an
enriched warm intergalactic medium, but no
such lines have been seen to date. This nondetection does not yet set significant constraints on cosmological models of the intergalactic medium. Results from the grating
spectrometers on board Chandra and XMMNewton vividly demonstrate the power of
high-resolution spectroscopy in the soft X-ray
band. Never before have AGN spectra been
measured in such detail: chemical composition,
temperatures, densities, ionization states and
velocities of the emitting gas can be derived
directly from the spectra, and the location of
the gas in the AGN can be pinpointed. A variety of physical scenarios have already been
found and studied quantitatively for the different AGN classes.
An exciting but still-controversial RGS result
is the identification of emission lines from a
relativistic disk around a massive rotating
black hole in the Narrow Line Seyfert 1 galaxies MCG-6-30-15 and Mrk 766 (figure 9a). A
model for these sources has been constructed
involving a single power-law absorbed by ionized gas with very strong, He-like, oxygen,
nitrogen and carbon emission lines, gravitationally redshifted and broadened by relativistic effects in the vicinity of a Kerr black hole
(fig 9b). An alternative interpretation of the
spectrum obtained with the HETG from the
0.02
0
–0.02
5
10
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20
30
25
35
wavelength (Å)
9 (a and b): XMM-Newton RGS spectra of the Narrow Line Seyfert 1 galaxies MCG-6-30-15 and
Mrk766, fitted with a model composed of a power-law, warm absorption and emission lines of H-like
oxygen, nitrogen and carbon, gravitationally redshifted and broadened by relativistic effects in the
vicinity of a Kerr black hole.
10: A simulation of an X-ray binary. (R Hynes, University of Southampton.)
6.15
X-ray astronomy
XMM-Newton
The European Space Agency’s X-ray
Multi-Mirror satellite, known as
XMM-Newton, was launched from Kourou,
French Guiana, on 10 December 1999, into
a 48-hour elliptical orbit around the Earth,
inclined at 40° with a southern apogee at
114 000 km and the perigee altitude of
7000 km. The eccentric orbit means that
very long, uninterrupted observations can
be made. It has an operational lifetime of
up to 10 years.
XMM-Newton is a three-axis stabilized
spacecraft with a pointing accuracy of one
arcsec. It carries three X-ray telescope
modules, each shaped like a barrel and
containing 58 high-precision concentric
mirrors, delicately nested to offer the largest
collecting area possible. Each mirror is made
of nickel, coated with gold, and sits just
millimetres from its neighbour. Together
they have an area greater than that of a
tennis court. The observatory covers a range
of 0.1–12 keV (0.1–10 nm spectral range),
first of these objects is that this feature can be
wholly explained by a dusty warm absorber
alone. The apparent redshifted OVII edge can
be fully explained by a combination of the
overlapping 1s2 – 1s2p (n > 5) OVII resonance
line series from a warm absorber, and the Fe L3
and L2 edges from embedded dust.
X-ray binaries
Chandra and XMM-Newton are ideally suited
to observations of X-ray binaries, particularly
able to exploit the high orbits of both spacecraft which offer long, uninterrupted viewing
times (conventional near-Earth orbit missions
can only view any given point on the sky for
typically 40 min out of every 100 min orbit, as
a result of Earth-occultation and other constraints). This is because most powerful X-ray
binaries have orbital periods of a few hours to
a day or so. The improved sensitivity has
enabled observations to be made of transient
X-ray sources in quiescence, for which spectral
and variability studies can be performed that
were not previously possible. These are providing valuable insights into the differences
between black holes and neutron stars where
the latter systems appear to be brighter than
equivalent objects thought to contain black
holes. This is explained as possible direct evidence for the existence of an event horizon
around the black hole, in that hot gas that
crosses this region is lost from view for ever,
whereas in a neutron star binary this hot
material will come to rest on the surface of the
neutron star and radiate its energy back out.
6.16
11: The nested mirrors of XMM-Newton.
giving a collecting area of 4300 cm2 at
1.5 keV and 1800 cm2 at 8 keV. The resolution is 5 arcsec (full width half-maximum),
14 arcsec (half energy width) at all wavelengths. There are three main scientific
instruments on board.
The three European Photon Imaging
Cameras (EPIC) were produced by a
consortium of 10 institutes in four nations:
the UK, Italy, France and Germany. Principal Investigator is Martin Turner of the
X-ray Astronomy Group at Leicester
University. One of the cameras uses a new
type of CCD (PN) developed by the Max
An especially exciting observation of an
X-ray binary was made by Chandra which
found the first X-ray “P Cygni” profiles from
the extreme object Circinus X-1. Such profiles
indicate that there is dense, outflowing material, but to see this in such highly ionized lines as
silicon XIV means that we are watching the
interaction of matter very close to the compact
object (probably a neutron star). This is
because the line profiles (seen in X-rays for the
very first time here) require velocities of
around 2000 km s–1 in material that must be at
temperatures of typically tens of millions of
degrees (to produce the level of ionization).
This is the analogy in X-ray binaries, according
to the observers of this event, to Broad Absorption Line quasars, where similar (but much
larger scale) outflows are believed to be driven
by a powerful central source.
One example is afforded by the Chandra coverage of the recent outbursting transient XTE
J1118+480. Combined with EUVE and RXTE
data, it was possible to construct an unbroken
spectral energy distribution from extreme-UV
to hard X-rays. High spectral resolution observations have revealed richly structured spectra
containing both emission lines and absorption
edges. These contain valuable clues about
photoionized winds, disk atmospheres and
other structures within the binary; recent
XMM results on EXO 0748-676 show the
power of such observations. Even when no
lines are seen, it is of interest to rule out geometries that predict significant line emission. For
example, the lack of emission lines in LMC
Planck Institute of Extraterrestrial Physics in
Garching, Germany.
There are two Reflection Grating Spectrometers (RGS). The Principal Investigator
is Prof. Bert Brinkman of the High-Energy
Astronomy division, SRON, Utrecht,
Netherlands, with co-Investigator Steven
Kahn from Columbia University, NY, USA.
The Optical Monitor (OM), co-aligned
with the main X-ray telescope, will give the
XMM-Newton mission a multiwavelength
capacity. The Mullard Space Science Laboratory (MSSL) has supplied this 30 cm aperture Ritchey-Chretien telescope (with a
170–600 nm spectral range). OM Principal
Investigator is Keith Mason of MSSL.
In addition, XMM-Newton has a particle
detector, the EPIC Radiation Monitor System (ERMS), developed by the Centre
d’Etude Spatiale des Rayonnements (CESR)
in Toulouse, France. It will measure radiation levels in the Earth’s radiation belts and
from solar flares, radiation that can perturb
the sensitive CCD detectors of the main
science instruments.
X-3 has been used to argue that accretion in
this high-mass X-ray binary and black hole
candidate is by Roche lobe overflow rather
than by wind accretion.
The future
We can only hope here to have whetted the
appetite for the feast of new results and insight
that will follow the opening of the frontier of
high-resolution X-ray spectroscopy. Already
there are indications of exciting new results
such as X-ray spectroscopy of the twin-jet
source SS433 revealing a forest of X-ray emission lines, including components associated
with the relativistic jets. And deep X-ray imaging of globular clusters is finally helping to
resolve the variety of populations that are present. We already know of a dozen or so luminous X-ray binaries associated with globular
clusters (in itself an indication of their enhanced
formation rates in the dense stellar environments of the core), and one of these (M15) has
now been shown to contain two bright X-ray
sources, separated by only 2.7 arcsec. But it is
the study of the faint X-ray source population
that Chandra and XMM-Newton are contributing to most, as their sensitivity allows
them to reach almost the level of normal stellar
coronal activity as displayed by the Sun! ●
Prof. Phil Charles, University of Southampton;
Prof. Andy Fabian, Institute of Astronomy,
University of Cambridge. This article was based on
the RAS Discussion Meeting on Advances in X-ray
astronomy, reported in The Observatory.
December 2001 Vol 42