The value of the EM radiation we receive from the Universe can be realized by considering the following: Temperatures in the Universe today range from 1010 Kelvin to 2.7 Kelvin (in the cores of stars going supernova and in intergalactic space, respectively). Densities range over 45 orders of magnitude between the centers of neutron stars to the virtual emptiness of intergalactic space. Magnetic field strengths can range from the 1013 Gauss fields around neutron stars to the 1 Gauss fields of planets such as Earth to the 10-7 Gauss fields of intergalactic space. It is not possible to reproduce these enormous ranges in a laboratory on Earth and study the results of controlled experiments; we must use the Universe as our laboratory in order to see how matter and energy behave in these extreme conditions. Understanding this behavior is central to our ability to take our current models and extrapolate them successfully into "what will become of our Universe?"
As we develop better observing technologies and techniques, we can ask and answer fundamental questions, such as:
What happens to pressure, to temperature, and to the states of matter in the intense gravity near a black hole? Black holes give us access to conditions that exist no where else in the Universe and future X-ray missions will be able to probe very close to the event horizon of a black hole. An emission line in the gravitational field of a black hole has a characteristic, identifiable shape; it is redshifted (gravitational redshift) with a peculiar double peak. Current instruments do not have enough collecting area to gather enough information on the short timescales required by the changing emission from a supermassive black hole. On timescales of an hour, you need to get enough photons for a good observation with enough resolution to be able to observe the characteristic shape of the line in order to be an effective probe.
How does matter flow in an accretion disk around a black hole? Less powerful, but closer, active galaxies will allow us to probe the outer portions of the accretion disks. Recent Very Long Baseline Interferometry (VLBI) observations have enabled us to measure the mass of the central black hole with unprecedented accuracy. Extending this technique to more distant and smaller galaxies, requiring a full-scale space-based VLBI capability, is the next step to fully understand how gas in the outer part of the disk is fed inward to the black hole.
What causes jets? Numerous observations show that cosmic jets are a frequent natural consequence of accretion disks. These collimated beams shoot out perpendicular to the accretion disk around the compact object. First discovered by radio astronomers, they are now regularly seen at optical, X-ray, and gamma-ray wavelengths as well. Understanding how these jets are formed and what role they play in the accretion process is a major unsolved question. In particular, we need to determine if they are launched and collimated by magnetic stresses or if the pressure of the intense radiation fields (or some other phenomenon) is responsible. The jets accelerate electrons up to nearly the speed of light, producing gamma-rays which can be used as a probe of the jet environment. In addition, from high resolution X-ray spectra, we can estimate the velocity distribution of high energy electron populations and deduce magnetic field strengths.
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