4.1. Past and present X-ray instruments
With the advent of the Chandra and XMM-Newton observatories,
high resolution X-ray spectroscopy of a wide variety of cosmic sources
became feasible for the first time.
Among the possible results most eagerly speculated upon was the detection
of intergalactic absorption lines from highly ionised metals in the
continuum spectra of bright extragalactic sources. After all, one of
the most striking results from the Einstein observatory had been
the detection of a very significant broad absorption feature at ~ 600
eV in the spectrum of PKS 2155-304 with the Objective Grating Spectrometer
(Canizares
& Kruper 1984).
Ironically, if interpreted as intergalactic
H-like O Ly
absorption,
redshifted and broadened by the expansion of the Universe, the strength
of the feature implied the presence of a highly ionised IGM of
near-critical density, a possibility that has of course definitively
been discounted since then.
The High Energy Transmission Grating Spectrometer (HETGS;
Canizares et
al. 2005)
and the Low Energy Transmission Grating Spectrometer (LETGS;
Brinkman et
al. 2000)
on Chandra, and the Reflection Grating Spectrometer (RGS) on
XMM-Newton
(den Herder et
al. 2001)
were the first instruments to provide
sensitivity to weak interstellar and intergalactic X-ray absorption lines.
The 'traditional' ionisation detectors (proportional counters, CCD's) do
not have sufficient energy resolution for this application.
But the angular resolution provided by an X-ray telescope can be used to
produce a high resolution spectrum, by the use of diffracting
elements. Laboratory X-ray spectroscopy is typically performed with
crystal diffraction spectrometers, and the use of crystal spectrometers
for general use in astrophysics was pioneered
on the Einstein observatory (e.g.
Canizares et
al. 1979).
The Focal Plane Crystal Spectrometer indeed detected the first ever
narrow X-ray absorption line in a
cosmic source, the 1s - 2p absorption by neutral oxygen in
the interstellar medium towards the Crab
(Canizares
& Kruper 1984).
Previous grating spectrometers (the Objective Grating Spectrometer on
Einstein and the two Transmission Grating Spectrometers on
EXOSAT) had only limited resolution and sensitivity. But there is
no fundamental limit to the resolution of a diffraction grating
spectrometer, and the high angular resolution of the Chandra
telescope has allowed for high resolution spectroscopy using
transmission gratings. The focusing optics on XMM-Newton have
more modest angular resolution, but they are used with a fixed array of
grazing incidence reflection gratings, which produce
very large dispersion angles and thus high spectral resolution.
The HETGS provides a spectral resolution of
= 0.0125 Å over the
1.5-15 Å band
with the high line density grating, and
= 0.025 Å over the
2-20 Å band with the medium line density grating. The LETGS has
= 0.05 Å over the
2-170 Å band,
while the RGS has = 0.06
Å over the 5-38 Å band. These numbers
translate to resolving powers of R = 400-1500 in the O K band,
and with a sufficiently bright continuum source, one should be able to
detect equivalent widths of order 0.1-0.5 eV (5-20 mÅ), or below in
spectra with very high signal-to-noise. The predictions for H- and
He-like O resonance absorption line strengths are generally smaller than
these thresholds, but not grossly so, and so a search for intergalactic
O was initiated early on. Given that the current spectrometers are not
expected to resolve the absorption lines, the only freedom we have to
increase the sensitivity of the search is to increase the
signal-to-noise ratio in the continuum, and it becomes crucial to find
suitable, very bright sources, at redshifts that are large enough that
there is a reasonable a priori probability of finding a filament with
detectable line absorption.
As we will see when we discuss the results of the observational searches
for X-ray absorption lines, the problem is made considerably more
difficult by the very sparseness of the expected absorption
signature. Frequently, when absorption features of marginal statistical
significance are detected in astrophysical data, plausibility is greatly
enhanced by simple, unique spectroscopic arguments. For instance, for
all plausible parameter configurations, an absorbing gas cloud detected
in N V should also produce detectable C IV absorption; or, both members
of a doublet should appear in the correct strength ratio if
unsaturated. In the early stages of
Ly forest astrophysics,
it was arguments of this type, rather than the crossing of formal
statistical detection thresholds alone, that guided the field (e.g.,
Lynds 1971).
But for the 'X-ray Forest' absorption, we expect a very
different situation. The detailed simulations confirm what simple
analytical arguments had suggested: in most cases, Intergalactic
absorption systems that are in principle detectable with current or
planned X-ray instrumentation will show just a single absorption line,
usually the O VII n = 1-2 resonance line, at an unknown redshift.
When assessing the possibility that a given apparent absorption feature
is 'real', one has to allow for the number of independent trial
redshifts (very roughly given by the width of the wavelength band
surveyed, divided by the nominal spectral resolution of the
spectrometer), and with a wide band and a high spectral resolution, this
tends to dramatically reduce the significance of even fairly impressive
apparent absorption dips. For instance, an apparent absorption line
detected at a formal '3
significance' (or p = 0.0015 a priori probability for a negative
deviation this large to occur due to statistical fluctuation) with
Chandra LETGS in the 21.6-23.0 Å band pales to '1.7
' if we assume it is the
O VII resonance line in the redshift range z = 0 - 0.065; with
N 30
independent trials, the chances of not
seeing a 3 excursion are
(1 - p)N = 0.956, or: one will see such a feature
one in twenty times if one tries this experiment (we are assuming a
Gaussian distribution of fluctuations here). If we allow for confusion
with O VIII Ly at higher
redshift, or even other transitions, the significance is even further
reduced. And the larger the number of sources surveyed, the larger the
probability of false alarm. Clearly, more
reliable statistics on intervening X-ray absorbers and detections at higher
significance are desired, but the required high-quality data will not be
available until the next-generation X-ray facilities such as XEUS
and Constellation X are installed (see
Paerels et
al. 2008
- Chapter 19, this volume).
Nevertheless, even with these odds, the above discussed high-ion measurements are important observations to do with the currently available instruments Chandra and XMM-Newton. Given the predicted strengths of the absorption lines (e.g., Chen et al.2003; see also Sect. 5.1), attention has naturally focused on a handful of very bright BL Lac- and similar sources. Below, we discuss the results of the searches. Note that the subject has recently also been reviewed by Bregman (2007).
4.2. Intervening WHIM absorbers at low redshift
The first attempt at detecting redshifted X-ray O absorption lines was performed by Mathur et al. (2003) with a dedicated deep observation (470 ks) with the Chandra LETGS of the quasar H 1821+643, which has several confirmed intervening O VI absorbers. No significant X-ray absorption lines were found at the redshifts of the O VI systems, but this was not really surprising in view of the modest signal to noise ratio in the X-ray continuum. Since it requires very bright continua to detect the weak absorption, it is also not surprising that the number of suitable extragalactic sources is severely limited. Early observations of a sample of these (e.g. S5 0836+710, PKS 2149-306; Fang et al. 2001; PKS 2155-304, Fang et al. 2002) produced no convincing detections. Nicastro and his colleagues then embarked on a campaign to observe Mrk 421 during its periodic X-ray outbursts, when its X-ray flux rises by an order of magnitude (e.g. Nicastro 2005). The net result of this has been the accumulation of a very deep spectrum with the Chandra LETGS, with a total of more than 7 million continuum counts, in about 1000 resolution elements. Nicastro et al. (2005) have claimed evidence for the detection of two intervening absorption systems in these data, at z = 0.011 and z = 0.027. But the spectrum of the same source observed with the XMM-Newton RGS does not show these absorption lines (Rasmussen et al. 2007), despite higher signal-to-noise and comparable spectral resolution (Mrk 421 is observed by XMM-Newton for calibration purposes, and by late 2006, more than 1 Ms exposure had been accumulated). Kaastra et al. (2006) have reanalysed the Chandra LETGS data, and find no significant absorption. Other sources, less bright but with larger redshifts, have been observed (see for instance Steenbrugge et al. (2006) for observations of 1ES 1028+511 at z = 0.361), but to date no convincing evidence for intervening absorption has materialised.
Observations have been conducted to try and detect the absorption
by intergalactic gas presumably associated with known locations of
cosmic overdensity, centred on massive clusters.
Fujimoto et
al. (2004)
attempted to detect absorption in the quasar LBQS 1228+1116,
located behind the Virgo cluster. An XMM-Newton RGS spectrum
revealed a marginal feature at the (Virgo) redshifted position of
O VIII Ly,
but only at the ~ 95% confidence level. Likewise,
Takei et
al. (2007)
took advantage of the location of X Comae behind the Coma cluster to try and detect absorption from Coma
or its surroundings,
but no convincing, strong absorption lines were detected in a deep
observation with XMM-Newton RGS. The parallel CCD imaging data
obtained with EPIC show weak evidence for Ne IX n = 1-2 line
emission at the redshift of Coma, which, if real, would most likely be
associated with WHIM gas around the cluster, seen in projection (the
cluster virial temperature
is too high for Ne IX). In practice, the absence of very bright point
sources behind clusters, which makes absorption studies difficult, and the
bright foregrounds in emission, will probably make this approach to
detecting and characterizing the WHIM not much easier than the random
line-of-sight searches.
The conclusion from the search for intergalactic X-ray absorption is that there is no convincing, clear detection for intervening absorption. This is, in retrospect, not that surprising, given the sensitivity of the current X-ray spectrometers, the abundance of suitably bright and sufficiently distant continuum sources, and the predicted properties of the WHIM.
4.3. The Milky Way halo and Local Group gas
The first positive result of the analysis of bright continuum spectra was
the detection of O VII and O VIII n = 1 - 2 resonance line
absorption at redshift zero.
Nicastro et
al. (2002)
first identified the
resonance lines in the Chandra LETGS spectrum of PKS 2155-304 (O VII
n = 1 - 2, n = 1 - 3, O VIII
Ly, Ne IX n = 1 -
2).
Rasmussen et
al. (2003)
detected resonance absorption in the XMM-Newton RGS spectra of
3C 273, Mrk 421,
and PKS 2155-304. Since then, at least O VII n =
1 - 2 has been
detected in effectively all sufficiently bright continuum sources, both
with Chandra and XMM-Newton; a recent compilation appears
in
Fang et al. (2006).
Portions of a deeper spectrum that shows the zero redshift absorption are
shown in Fig. 7.
 |
Figure 7. Chandra LETGS spectrum of
Mrk 421. Crosses are the data,
the solid line is a model. The labels identify z
|
Nicastro et
al. (2002)
initially interpreted the absorption as arising in an extended
intergalactic filament. The argument that drives this interpretation is
based on the assumption that O VI, O VII, and O VIII are all located in
a single phase of the absorbing gas. The simultaneous appearance of
finite amounts of O VI and O VIII only occurs in photoionised gas, not
in gas in collisional ionisation equilibrium, and this requires that the
gas has very low density (the photoionisation is produced by the local
X-ray background radiation field; for a measured ionisation parameter,
the known intensity of the ionising field fixes the
gas density). The measured ionisation balance then implies a length
scale on the order of l ~ 10
(Z0.1)-1 Mpc, where Z0.1
is the metallicity in units
0.1 Solar. This is a very large length, and even for 0.3
Z
metallicity, the structure still would not fit in the Local Group (and
it is unlikely to have this
high a metallicity if it were larger than the Local Group). In fact, the
absorption lines should have been marginally resolved in this case, if
the structure expands
with a fair fraction of the expansion of the Universe.
Rasmussen et
al. (2003)
constrained the properties of the absorbing gas by dropping the O VI,
and by taking into account the intensity of the
diffuse O VII and O VIII line emission as measured by the
Wisconsin/Goddard rocket-borne Quantum X-ray Calorimeter (XQC)
experiment
(McCammon et
al. 2002).
The cooling timescale of O VI-bearing gas is much smaller than that of
gas with the higher ionisation stages of O, and this justifies the
assumption that O VI is located in a different, transient phase of the
gas. With only O VII and O VIII, the medium can be denser and more
compact, and be in collisional ionisation equilibrium. Treating the
measured O VII emission line intensity as an upper limit to the emission
from a uniform medium, and constraining the ionisation balance from the
measured ratio of O VII and O VIII column densities in the lines of
sight to Mrk 421 and PKS 2155-304,
Rasmussen et al. derived an upper limit on the density of the medium of
ne
2 ×
10-4 cm-3 and a length scale l
100 kpc.
Bregman (2007) favours a different solution, with a lower electron temperature and hence a higher O VII ion fraction. If one assumes Solar abundance and sets the O VII fraction to 0.5, the characteristic density becomes ne ~ 10-3 cm-3, and the length scale (l ~ 20 kpc) suggests a hot Galactic halo, rather than a Local Group intragroup medium.
Arguments for both type of solution (a small compact halo and a more
tenuous Local Group medium) can be given. The most direct of these is a
measurement of the O VII line absorption towards the LMC by
Wang et al. (2005)
in the spectrum of the X-ray binary LMC X-3, which indicates that a
major fraction of the O VII column in that direction is in fact in front
of the LMC.
Bregman (2007)
points out that the distribution of column densities of highly ionised
O on the sky is not strongly correlated with the likely projected mass
distribution of the Local Group, and that the measured velocity centroid
of the absorption lines
appears characteristic of Milky Way gas, rather than Local Group gas. On
the other hand, a direct measurement of the Doppler broadening of the O
VII gas, from the curve of growth of the n = 1 - 2 and n =
1 - 3 absorption lines in the spectrum of Mrk 421 and PKS 2155-304
(Williams et
al. 2005,
Williams et
al. 2007),
indicates an ion temperature of Ti
106.0-6.3
K (Mrk 421) and Ti
106.2-6.4
K (PKS 2155-304), and these
values favour the low-density, Local Group solution. Regardless, the
prospect of directly observing hot gas expelled from the Galactic disk,
or measuring the virial
temperature of the Galaxy and/or the Local Group is exciting enough to
warrant further attention to redshift zero absorption and emission.
Finally, the spectrum of Mrk 421 shows the expected z = 0
innershell O VI absorption, at 22.019 Å, both with Chandra
and with XMM-Newton. There
has been some confusion regarding an apparent discrepancy between the O
VI column densities derived from the FUV and from the X-ray absorption
lines, in the sense that the X-ray column appeared to be significantly
larger than the FUV column
(Williams et
al. 2005).
Proposed physical explanations for this
effect involve a depletion of the lower level of the FUV transitions
(1s2 2s) in favour of (at least)
1s2 2p, which weakens the
1032, 1038 Å
absorption but does not affect the 1s - 2p X-ray
absorption. However, it requires very high densities to maintain a
finite excited state population, and, more directly,
the measured wavelength of the X-ray line is actually not consistent
with the wavelength calculated for 1s - 2p in excited O
VI, off by about 0.03-0.05 Å, on
the order of a full resolution element of both the Chandra LETGS
and the XMM-Newton RGS (Raassen 2007, private communication). The
conclusion is that the
discrepancy is due to an authentic statistical fluctuation in the X-ray
spectrum - or, more ironically, to the presence of a weak, slightly
redshifted O VII absorption line.