Current status of the x-ray mirror for the XL-Calibur experiment

XL-Calibur is a balloon-borne hard X-ray polarimetry mission, the first flight of which is currently foreseen for 2021. XL-Calibur carries an X-ray telescope consists of consists of 213 Wolter I grazing-incidence mirrors which are nested in a coaxial and cofocal configuration. The optics design is nearly identical to the Hard X-ray Telescope (HXT) on board the ASTRO-H satellite. The telescope was originally fabricated for the Formation Flying Astronomical Survey Telescope (FFAST) project. However, the telescope can be used for XL-Calibur, since the FFAST project was terminated before completion. The mirror surfaces are coated with Pt/C depth-graded multilayers to reflect hard X-rays above 10 keV by Bragg reflection. The effective area of the telescope is larger than 300 cm^2 at 30 keV. The mirrors are supported by alignment bars in the housing, and each of the bars has a series of 213 grooves to hold the mirrors. To obtain the best focus of the optics, the positions of the mirrors have to be adjusted by tuning the positions of the alignment bars. The tuning of the mirror positions is conducted using the X-ray beam at the synchrotron facility SPring-8 BL20B2, and this process is called optical tuning. First the positions of the second reflectors are tuned, and then those of the first reflectors are tuned. We did the first optical tuning in Jan 2020. The second tuning will be planned between April to July, 2020. This paper reports the current status of the hard X-ray telescope for XL-Calibur.