The International Collaborative Research Group* consists of Dr. MIHARA Tatehiro (Senior researcher at the High Energy Astrophysics Laboratory, Pioneering Research Division, RIKEN), Dr. IWAKIRI Wataru (Assistant Professor at the Faculty of Science and Engineering, Chuo University), Dr. NEGORO Hitoshi (Professor at the Faculty of Science and Engineering, Nihon University), Dr. SERINO Motoko (Assistant Professor at College of Science and Engineering, Aoyama Gakuin University), and Dr. NAKAHIRA Satoshi (Chief Scientist at the Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (JAXA)), started OHMAN (on-orbit hookup of MAXI and NICER, also known as Orbiting High-energy Monitor Alert Network), a prompt observation project for X-ray emission sources on the International Space Station (ISS), on August 10, 2022, and successfully performed a coordinated observation on September 13.

 The OHMAN project links instruments on the International Space Station (ISS) to provide real-time observations. The project links MAXI^[1], an all-sky X-ray monitor, and NICER^[2] of the U.S. National Aeronautics and Space Administration (NASA), which specializes precision observations with a narrow field-of-view. This project was sponsored by the Japan-US Open Platform Partnership Program (JP-US OP3), which is a framework for Japan and the US to cooperate in maximizing ISS utilization and is based on an April 2021 agreement.

 Previously, MAXI data had been analyzed on Earth, then information about sudden X-ray phenomena was emailed to other satellites for follow-up observation. This took more than three hours from the discovery until the follow-up. With OHMAN, information on a new X-ray star emergence is processed by MAXI on a laptop PC aboard the ISS then forwarded to NICER. This enables on the ISS follow-up observations without supports of a ground base. This creates an international observatory capable of wide-area discoveries and detailed follow-up observations on the ISS, allowing discoveries to be followed up within ten minutes.
 Future OHMAN observations are expected to identify MAXI-unidentified soft X-ray transients (MUSSTs), which had been detected by MAXI but were not followed up.

* International Collaborative Research Group

• MIHARA Tatehiro, Senior researcher
• TAMAGAWA Toru, Principal Investigator
• Tamagawa High Energy Astrophysics Laboratory, Pioneering Research Division, RIKEN


• IWAKIRI Wataru, Assistant Professor
• Department of Physics, Faculty of Science and Engineering, Chuo University
• (Visiting Researcher at the Tamagawa High Energy Astrophysics Laboratory, Pioneering Research Division, RIKEN)


• NEGORO Hitoshi, Professor
• Department of Physics, College of Science and Technology, Nihon University
• SERINO Motoko, Assistant Professor
• Department of Physical Sciences , College of Science and Engineering, Aoyama Gakuin University


• NAKAHIRA Satoshi, Chief Scientist
• Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (JAXA)


• Exposed Facility Experiment Team, JEM Utilization Center, Japan Aerospace Exploration Agency (JAXA)


• MAXI Team in Japan
•  RIKEN, JAXA/ISAS, Tokyo Institute of Technology, Nihon University, Kyoto University, Aoyama Gakuin University, University of Miyazaki, Chuo University, Ehime University


• U.S. National Aeronautics and Space Administration (NASA)
•  Goddard space flight center (GSFC)
•   NICER Team Representative, Keith Gendreau
•  ISS team, Johnson Space Center (JSC)

Research Support

 The following research was supported by the FY 2017 JAXA/ISAS Small-scale project OHMAN-JP (On-orbit Hookup of MAXI and NICER – Japan), the Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for Scientific Research (B): “Revealing MAXI unidentified short transient by prompt follow-up” (Principal Investigator: MIHARA Tatehiro) , “Advanced study on short X-ray transients with MAXI alerts” (Principal Investigator: MIHARA Tatehiro), JSPS Grant-in-Aid for Scientific Research on Innovative Areas (Research area proposal type)“Genesis of Gravitational waves physics and astronomy” (Collaborative Investigator: MIHARA Tatehiro); JSPS Grant-in-Aid for Young Scientists (B) “Observational study of nucleosynthesis in X-ray superburst using the MAXI-NICER cooperation” (Principal Investigator: IWAKIRI Wataru), the JSPS Grant-in-Aid for Scientific Research (C) “On-orbit Hookup of MAXI And NICER for exploring and localizing X-ray counterparts of gravitational wave sources” (Principal Investigator: SERINO Motoko), JSPS New Academic Area Research (Research Area Proposal Type) “Construction of assistant observation systems for continuous gravitational wave candidates using microsats and MAXI-NICER collaboration” (Principal Investigator: IWAKIRI Wataru), and RIKEN's Program for Special Postdoctoral Researchers (SPDR) (FY 2016 – 2017: IWAKIRI Wataru).

 Monitor of All-sky X-ray Image (MAXI) on the International Space Station (ISS) was proposed by RIKEN and has discovered 34 new celestial bodies through continuous observations since its launch in August 2009. Fourteen of these are black holes. However, some of the new celestial bodies remain unidentified. In particular, a group of rapidly dimming sources shine only within MAXI's 90-minute scan and disappear by the time follow-up observations can be made with the Swift satellite XRT instruments a few hours later (Figure. 1). We call these MAXI unidentified short soft X-ray transient (MUSST) ^[3]. Recently, it has been suggested that one of the MUSST objects is a supernova explosion of a type predicted by theory but never observed, in which a black hole or neutron star[4] enters the core of a star and causes a supernova^[5]. Thus, MUSST celestial bodies may be completely new astronomical phenomena. However, while MAXI has a wide field of view, its positioning accuracy is poor. More precise position information is necessary for detailed observations to study their true nature with large telescopes and satellites. Therefore, it is necessary to pinpoint the positions of MUSST objects and make follow-up observations before they disappear.

MAXI detected these example objects, shown in the upper figures. However, in the follow-up observations by the Swift satellite about half a day later, they were not detected at all, as shown in the lower figures.

 MAXI data have been analyzed at a ground station, then potentially useful information is sent by email or other means to a satellite for follow-up observations. Because of the manual processing involved, this sequence of events can cause a delay of several hours. This will remain true until MUSST detection is fully implemented. However, the ISS carries two X-ray astronomy instruments, MAXI (Japan) and NICER (U.S.). MAXI is a wide-field monitor that detects X-ray phenomena over a wide but shallow field of view. NICER is a tracking X-ray telescope with a narrow field of view and no ability to detect prominences, but it can make very detailed observations. Currently, these two instruments are controlled independently from the ground. If these instruments, with different yet complementary characteristics, could work together, an innovative international observatory would be realized on the ISS to detect sudden phenomena in the x-ray band and immediately make detailed follow-up observations. The MAXI team in Japan and the NICER team in the U.S. launched the OHMAN project to realize this goal.
 To achieve this, we installed the MAXI data nova detection software previously used on Earth on an ISS PC so that information on sudden phenomena could be transmitted directly to NICER automatically. No human intervention is required, and short-duration events will be observed in detail within ten minutes. (The final delay target is within two minutes.)
 Preparations for OHMAN by NASA/JSC (the ISS team) and JAXA (the Exposed Facility Team) began in November 2020, and OHMAN became operational on May 26, 2022. Then, after thirteen days of verification, test operations were carried out for 2 and a half months. On August 10, OHMAN started normal operation. On September 13, MAXI detected an X-ray burst from the globular cluster M 15. NICER made automatic observations after 5 minutes and 30 seconds (Figure. 2). Since the X-ray burst was short this time, the burst had gone out when the NICER started observation. However, this event realized the prompt follow-up observation and was a large advance towards future observations.

(upper) X-ray intensity with MAXI. (middle) movement of NICER from the trigger time 0 sec. (lower) X-ray count rate of NICER. NICER detected M15 after 330 sec. The X-ray burst had ended by that time. 430 counts/sec is the normal X-ray count rate from M15. It corresponds to 0.07counts/sec with MAXI.s

 The accuracy of the OHMAN operation will be improved by parameter adjustments especially for the frequent solar flares since 2022 April. Although no new MUSST objects have been detected since implementation, MUSST may include undiscovered gamma-ray bursts caused by ancient explosive events by the first stars in the universe. The project is also expected to detect X-ray bursts caused by runaway nuclear fusion reactions on the surface of neutron stars, including superbursts with unusually long durations and stellar flares that are several thousand to several hundred million times larger than solar flares. These have been difficult to observe in detail from their initiation. MAXI operation has been approved through December 2024, and future prompt, coordinated observations are expected.