Extinction as a tool to find distances to X-ray sources

Finding distances in astronomy is notoriously difficult. With accurate optical spectroscopy, both the distance to and extinction (reddening) of normal stars can be found, but this is not possible for X-ray sources. It is possible, however, to measure the extincting column from an X-ray spectrum from photoelectric ionisation edges. It is also possible to construct the function of reddening along the line of sight using field red-clump (helium-burning giants) stars in the infrared. I will demonstrate this method to derive distances for the Anomalous X-ray Pulsars (AXPs). With these new distances, it turns out that all the AXPs have remarkably similar 2-10 keV luminosities, a clue to their internal processes. Our method succeeds for the AXPs, despite its many-step nature and poorly defined conversions. This hints that the two measurables involved - soft-X-ray extinction and infrared reddening - are far better correlated than the separate relations linking them would initially suggest. Such a correlation would be very useful, and the observational resources exist to be able to establish it.