The Importance of High Spectral Resolution in Studying the
X-Ray Diffuse Background
K. A. Barger
Department of Physics/Astronomy, Western Washington University
Department of Astronomy, University of Wisconsin-Madison
In present day cosmology, there are still many unanswered questions regarding
the matter in the universe. The abundances of different types of matter in the universe are
currently unknown. Observations so far have not been able to answer all of these
questions and leave much to be understood. The amount of baryonic matter that has been
detected is not equal to the amount of matter required to create the gravitational effects
that are observed. The observations that have been taken so far show that there is not
enough luminous matter in the universe to explain these gravitational. There are a couple
theories why this could be. One is that our understanding of gravity could be wrong or
incomplete, another is that that there is the necessary amount of matter required for the
gravitational interactions but most of the matter is non-baryonic and does interact
gravitationally but extremely weakly with other conventional matter (Liddle, A., 2003),
and another is that maybe not all of the baryons have been detected yet. One way to
search for the missing baryons is by studying the diffuse X-ray background with high
spectral resolution detectors.
The galactic interstellar medium (ISM) contains a substantial amount of hot X-ray
emitting gas. Chandra, the X-ray observatory, has found that this gas is not uniformed,
and that it is instead clumpy with some patches that could be remnants of supernovas
(Tess, J. et al. 2005), or of quasars. The hot clumps of gas that are found in intergalactic
space could contain a large portion of baryons (McCammon, D., Almy, R., Apodaca, E.,
et al. 2002). Most of the luminous mass in clusters of galaxies is hot gas. This gas is
warmed by the surrounding galaxies and has temperatures ranging from 106-to-7K. This
energetic gas emits synchrotron and thermal radiation in the form of X-rays. Synchrotron
radiation is caused created when a relativistic charged particle is accelerated in a
magnetic field. These X-rays make up the diffusive X-ray background.
The soft X-rays in the diffuse X-ray background is dominated by partially ionized
metals. This plasma should have enough energy to escape the gravitational pull of
galaxies, but it is confined by some force. One possibility is that the ions could be
confined by magnetic fields which would justify the synchrotron radiation. It is estimated
that ~30% of the baryons in clusters of galaxies are contained in the Galactic ISM (Fang,
T., et al, 2005). There is an extensive distribution of this plasma in the halo of the Milky
Way and in surrounding galaxies. Studying these X-ray spectral lines provides a
description of the composition of material that is found in the Galactic ISM as well as
information about the physical state of the emitted material. The history of the gas can be
studied by comparing the electron temperature to the ionization state (McCammon, D.,
Almy, R., Apodaca, E., et al. 2002).
X-rays have small wavelengths that are on the order of angstroms. This makes
studying them from Earth’s surface impossible because the Earth’s atmosphere. The
particles in Earth’s atmosphere absorb photons because the photons have short
wavelengths when compared to the size of the particle. It is essential that X-ray
astronomy is done at least 90% above of Earth’s atmosphere. For this reason, satellites
and rockets are used to study this region of the spectra. The development of the
equipment used to study X-rays is not easy and is very expensive. Both satellites and
rockets commonly used to study X-rays. Even though a rocket’s flight time is far less
then that of a satellite, rockets still have a major advantage over the satellites. They are
able to make observations at low altitudes at specific locations that can be chosen such
that the Earth’s magnetic field can act as a shield and protect the equipment from
energetic charged particles (P. A. Charles, and F. D. Seward, 1995).
The first deep all-sky survey of the soft X-ray sky was done by the satellite
Roenttgen-Satellit (ROSAT) which was launched in 1990. This telescope had a 2° field
of view and a 1 arcminute spatial resolution. This satellite used a low-internalbackground gas-filled position-sensitive proportional counter detector, making it suitable
for studying the background radiation. The satellite also was equipped with a high
resolution imager that could be used to aid in identifying X-ray sources.
Once ROSAT completed an all-sky survey, scientist where able to pinpoint
specific area of interest to study (Charles, P. A., and Seward, F. D. 1995). Information
acquired by ROSAT was able to find that active galaxy nucleus (AGN) produce a large
amount of the observed X-rays. These AGNs can produce a very substantial amount of
the observed X-ray background XRB over a broad range of energies. The data that has
from ROSAT has been used to determine that at least 10% of the background has to come
from galaxies with some enhanced star formation processes (Calzetti, D., Livio, M.,
Madau P., 1995). The information gathered ROSAT can provide significant information
that could lead to a better understanding of the evolution of the clusters of galaxies and
the galactic ISM. The data taken from this satellite has been used to put constraints on the
upper limit of cosmological constraints. One study used a power law of the soft XRB
spectrum to calculate cosmological constraints (Diego, J. M., Sliwa, W., Silk, J., et al.
2003). However, the data taken from ROSAT left many questions unanswered.
Some of the early diffuse X-ray background (XRB) observations used gas
scintillation counters and solid-state detectors. Dispersive instruments, such as the Bragg
crystals, offer high resolution but result is a large loss of signal flux. The highest
resolution observation made with a Bragg crystal Diffuse X-ray Spectrometer resulted in
a crowed spectra that made the peaks in the spectra difficult to distinguish. Nondispersive detectors, such as solid-state diodes and proportional counters are also
inadequate in resolving the spectra. A new kind of detector is currently implemented to
solve many of the problems faced with other detectors. This new detector is
microcalorimeter detectors and is currently being implemented in rockets and is now
being proposed to be in larger scale missions such as in satellites (McCammon, D., Almy,
R., et al. 2002).
A group headed by Dr. McCammon at the University of Wisconsin Madison
(UW-Madison) is using microcalorimeter detectors a new alternate approach to enhance
X-ray spectroscopy. ROSAT has left many unresolved questions and this team of
scientist is trying to answer some of them. This team of scientist studies the diffusive
XRB with rockets and has been doing so for many years. The last rocket flown by this
group of scientist consists of 36 of these microcalorimeter detectors to increase the
resolution of the spectrum. These detectors are designed specifically to detect small
changes in temperatures coinciding with small changes in energy. These detectors have
the advantage being highly sensitive to small fluctuations in the desired energy range
(McCammon, D., Almy, R., et al. 2002).
The high resolution instrumentation that was used to construct this rocket was not
is not easy to design. There were many sensitive specifications that had to be fulfilled in
order for the rocket to function properly. The microcalorimeter thermal detectors were
composed of superconductors that operate at temperatures around 0.06K. The highly
sensitive detectors required the use of infrared (IR) filters. Unfortunately, these filters
also block out some of the desired spectra (McCammon, D., Almy, R., et al. 2002). This
and the background radiation made the spectra difficult to resolve. The rocket’s total
flight time was less then 15min. In this short amount of time, the rocket was able to
detect O VII, O VIII, C VI ion spectral lines as well as some silicon ions in the Galactic
ISM.
Knowing the types of baryons that are found in the Galactic ISM is an important
clue to knowing the baryon density parameter Ωb. With accurate information about the
baryons found in the Galactic ISM, theoretical models can be constructed to aid in
determining where that mass has originated from and the amount of ions that should be
detected in different regions. This gives the scientists crucial clues on the best ways to
detect them and the type of equipment that is best suited for the task.
Another rocket is currently being developed UW-Madison by the same scientists
and is scheduled to launch during the fall of 2005. The scientists hope to resolve the
problems that were faced by the previous rockets. The high spectral resolution results
from the work done by these scientists using the microcalorimeter detectors has resulted
in promising proposals of other missions that incorporated these detectors.
A new X-ray mission has been proposed to NASA to aid in the search for
baryons. This mission consists of a small explorer satellite called the Missing Baryon
Explorer (MBE). The MBE main objective will be to study soft X-ray emissions from the
missing baryonic matter in the local universe which is presumed to be in the warm-hot
intergalactic medium (WHIM). This mission is being planed by the UW-Madison Space
Science and Engineering Center, with collaboration with partnering teams at the NASA
Goddard Space Flight Center, Spectrum Astro, and Lockheed Martin/Advanced
Technology Laboratory. The satellite will be equipped with an imaging X-ray
spectrometer that consists of an array of microcalorimeters detectors which will provide
high spectral resolution. A conical-foil optic with a 1.4-m focal length will provide a
large collecting area and 5-arcminute image quality which will match the spatial
resolution of the detectors (Sanders, W. T., et al 2003). NASA has currently chose to not
fund this mission, but even with this set back the team of scientists plans to continue their
work and resubmit their proposal to NASA at a future date.
The results from past and current X-ray missions have been able to confirm the
presents of baryons in the Galactic ISM. With the results that have been collected from
these detectors used and the results from future detectors, scientists will be able to
determine the types of elements and ions that are in the Galactic ISM. This information
will aid in models to determine the abundances of the observed elements and what
elements that they should expect to observe as well as where they originated from. By
knowing abundance of baryons in the Galactic ISM, cosmologist can use this information
to create a better model of the universe and better understand its history and how it works
today. The abundance information is also an important parameter in knowing the events
of the early universe. This information is crucial to the Hot Big Bang theory. The
information can also be used to better understand the shape of the present and past
universe.
One group of theorists that are the data that has been collected from several X-ray
observations is being used to create simulations of the soft XRB. Specifically, this study
is main mission is to search for the presents of baryons that are missing from the current
observations. This study was done for the soft X-ray emissions that come from the
WHIM in a hydrodynamic simulation of a Cold Dark Matter universe. In particular, this
group selected three representative regions of sky for their simulation. These regions
included a galaxy group, a filament region, and a void-like under luminous region. From
this study they were able to determine where the X-rays could have originated from. This
information gives scientists clues about the evolutionary and thermal processes that are
taking place. They concluded that the galaxy group was dominated by the hot intragroup
medium, the void-like area was completely dominated by AGNs and the Galactic
foreground, and that the majority of the filament region comes from the AGNs plus the
Galactic foreground. They also concluded that it was possible to detect the filament
region from emission lines in the WHIM (Fang, T., et al. 2005).
The thermal parameters that were used in their model were taken from the data
collected and published in 2002 by the rocket developed by the McCammon group at
UW-Madison. This particular rocket did use the microcalorimeters to detect the diffuse
X-rays. They also used the spectrum constructed by McCammon to reflect all the data on
the resolved AGN contribution of the soft X-ray background that was taken from the
ROSAT satellite the Chandra X-ray Observatory. Their model found that the XRB from
the WHIM should contain Ne IX, Fe XVII, O VII, O VII, N VII, and C VI ions. They
also analyze three types of X-ray emissions and the locations in which they are typically
found (Fang, T., et at 2005). This information gives scientist an idea of the ideal locations
that they should make their observations. With accurate models, the scientists are also
able to determine what the types of equipment will be most effective to collect the
desired data. Once the scientist have made sufficient observations, they are able to verify
or discount the theoretical models which leads to a better understanding of the processes
that are taking place.
These theorists have deduced that the current X-ray telescopes that are in use,
such as Chandra the X-ray Observatory and the X-ray Multi-Mirror Mission (XMMNewton) are inadequate for achieving high resolution spectroscopy of the extended
structures of the diffuse X-ray emission from WHIM. These telescopes do offer high
spectral resolution; however, they are ineffective in studying this particular region of the
sky. The reasons for their ineffectiveness vary from the effective area of the detector
being too small or that the detector has a large effective area but has insufficient
resolving power.
These theorists also analyzed future X-ray missions and proposed mission and
compared their effectiveness at studying this region of the X-rays. One of the satellites
that analyzed was the MBE. Their results showed that the MBE able to meet with the
required specifications to detect the missing baryons. The MBE is planed have the
required high resolution spectroscopy needed by using the X-ray Calorimeter Telescope
(XCT). This observatory also consists of a moderate resolution imager to help identify
objects. Their study showed that the MBE would be able to detect X-ray emission from
filaments without ambiguity (Fang, T., et al. 2005). The filaments and the hot clumps
may be the repository for a majority of the present-day baryons (McCammon, D., et al.
2002). They also showed that the MBE showed the most promise when compared to the
other proposed missions.
The baryons that have been detected in the Galactic ISM could account for some
of the missing mass in the universe, but here is still much more mass that is missing. This
is why it is important to continue the search for the missing baryons in the universe. The
high resolution X-ray detectors have demonstrated the ability to resolve ion spectral lines
in the X-ray diffuse background which can be used to determine the abundances of
baryons in this region of space. This would give scientist important clues on the types of
baryons that are in this region of space and the surrounding region including the
surrounding clusters of galaxies. This information can then used to get a more accurate
estimate of the matter density parameters of the universe. Current estimates of the
universe predict that the universe has a matter density parameter Ωm of ~0.35 and of that
only ~0.01 to 0.04 has been found to be from baryons and ~0.30 is hypostasized to be
dark matter. This model of the matter in universe agrees with the current observed
distribution of matter found so far (Liddle, A., 2003). This large percentage of dark
matter means that most of the matter in the universe is not fully understood. Projects such
as the ones at UW-Madison have shown great promise in detecting the baryons in the Xray diffuse background. This information can be used to develop better models of the
matter distribution in the universe giving scientist important clues about the evolution of
the universe and its shape.
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