Pulsed-laser calibration of high-resolution microcalorimeters for x-ray astronomy

The resolving power of single-photon microcalorimeters for high-resolution x-ray spectroscopy has improved to the point that uncertainties in the energy calibration can limit their scientific capability. X-ray fluorescent lines have been the usual standard for calibrations, but they have intrinsic widths much broader than the current detector resolution. Since microcalorimeters respond to total energy deposited as heat, we have investigated the idea that five hundred 3 eV photons from an ultraviolet laser delivered in a pulse much shorter than the thermal integration time of the detectors should look the same as a single 1500 eV x-ray photon. We have illuminated devices that have 290 μm square gold absorbers, superconducting transition edge sensor thermometers, and about 1 eV FWHM (Full Width at Half Maximum) resolution with ∼100ns pulses from an ultraviolet laser. This produces combs of lines with 3 eV spacing and negligible intrinsic width that extends to at least 1500 eV. The accuracy of the line energies is limited only by our knowledge of the laser wavelength. Simultaneous illumination with oxygen and aluminum K fluorescent lines shows that the response to an x-ray photon is indistinguishable from the response to a burst of UV photons with the same total energy to better than 0.4 eV at 1500 eV, limited by our knowledge of the fluorescent energy distribution.