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-internal-background 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 spectrumto 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. Non-dispersive 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 (XMM-Newton) 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 X-ray 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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