Both agencies which were involved in the project of Hitomi (Astro-H) satellite decided to create replacement for one of the most advanced X-ray astronomy satellite in recent years. To remind, mission of the Hitomi finished with disaster of the satellite few weeks after launch.
Problems with Astro-H (Hitomi) satellite started few weeks after launch from Tanegashima Space Center, Japan at 17 February 2016 on the top of the H-IIA rocket. On March 27, 2016, satellite with one of the most advanced instruments for observing space in X ray, falled apart to few pieces after exceeding spinning speed. Disaster was beginning of the investigation. It showed that falling apart was caused by Attitude Control System and it was possible to be avoided in the phase of design of the Hitomi by JAXA. In spite of the fact, that investigation pointed, that disaster of the Astro-H was fault of JAXA, NASA still was interested in creating next X ray telescope and sending it together with JAXA to space.
According to JAXA the X-ray Astronomy Recovery Mission (XARM) will be smaller comparing to Hitomi and equipped with less advanced instruments. Still most crucial the Soft X-ray Imager and the Soft X-ray Spectrometer will be made using parts provided by NASA. It will be also able to observe space through less wide angle of view. This means that profile of the mission will not be identical comparing to Hitomi's mission plan, still participation of the NASA will not be changed - most important instruments will be provided thanks to the NASA.
One of the most important difference between XARM and Hitomi is lack of Hard X Ray telescope and extending boom in the front of the satellite. This will make construction of the satellite simpler and less expensive comparing to Hitomi, which cost exceeded $273 million.
JAXA claims, that it is possible to launch XARM in 2021. NASA agreed to provide X ray telescope and spectrometer for XARM and still keep JAXA as supervisor of the project. Project already started in April, in Japanese 2017 fiscal year.