The On-orbit Hookup of MAXI and NICER (OHMAN)

As part of NICER's 1^st mission extension, the NICER team worked with the MAXI team to develop the OHMAN target-of-opportunity (TOO) enhancements for NICER and MAXI. OHMAN (formally the Orbiting High-energy Monitor Alert Network) is a software payload that connects JAXA's MAXI payload on the JEM-EF to the NICER payload at the ISS external ELC2 mount using an Express Laptop Computer for Express Rack 5.

The OHMAN flight software scans live data from MAXI before it is fed into the telemetry multiplexer (LEHX) inside the JEM. The OHMAN software finds X-ray enhancements observed by MAXI, taking advantage of the huge (~900 square degree) instantaneous MAXI field of view (FOV) to monitor many well known as well as unknown celestial sources. If an enhancement is seen, the OHMAN software checks whether NICER has an unobstructed view of its location on the sky. This visibility calculation takes into account the ISS orbit ephemeris, attitude, and the joint angles of the ISS solar and radiator panels, as well as NICER’s Earth, Sun, and Moon avoidance constraints. If the visibility is good, the OHMAN software generates an alert that is sent to NICER for prompt follow up. The alert is encoded in a Health and Status data packet sent through the Express Rack Computer to the Payload MDM on ISS. The Payload MDM then passes this along to NICER via the Unique Ancillary Data (UAD) channel on the ISS 1553 communication network. Visibility permitting, NICER can react within a few minutes and follow up on the MAXI transient, carrying out a grid search over the MAXI source localization region, if necessary, and delivering massive collecting area, throughput, time and energy resolution in the soft X-ray band. OHMAN became operational during the summer of 2022 and has reacted to several MAXI triggers including GRB221009A.