The Japanese Hitomi mission (formerly called ASTRO-H) was a high-energy astrophysics space observatory, developed by the Japan Aerospace Exploration Agency (JAXA) in collaboration with institutions in Japan, the US, Canada, and Europe. After a perfect launch on 17 February 2016 and a few weeks of excellent operations, Hitomi was unfortunately considered to be lost by the end April 2016.
Hitomi was equipped with a Soft X-ray Spectrometer (SXS). The SXS was built by JAXA/ISAS in collaboration with NASA/GSFC, with contributions from Europe.
SRON provided the filter wheel for SXS, as well as the calibration source (MXS, together with the University of Geneva). In addition, SRON contributed to the system design, calibrations, and astronomical data analysis.
The SXS spectrometer was the first instrument to simultaneously provide maps and accurate spectra of clusters of galaxies and the remnants of supernovae.
In June 2017 ESA decided to contribute to the Japanese/American satellite XARM (X-ray Astronomy Recovery Mission). This mission - scheduled for launch in 2021 - will compensate the loss of Hitomi. Just like Hitomi, XARM will make X-ray observations of unprecedented accuracy. The ESA contribution to XARM also means a green light for a Dutch/Swiss contribution. This will once again consist of a filter wheel with X-ray sources.
Using observations of cosmic X-rays Hitomi was designed to study collapsing material in the vicinity of black holes, turbulences in clusters of galaxies, the shockwaves caused by supernova explosions and large-scale structures in the universe. Dark matter and the acceleration of cosmic particles to high energies shall also be investigated during the mission.
The SXS spectrometer, which consisted of an array of X-ray calorimeters that allow for spatially resolved spectroscopy with unprecedented spectral resolution (about 5 eV over the SXS energy band), was developed specifically to study the dynamics (velocities, turbulences) in extended objects such as clusters of galaxies and supernova remnants.
Hitomi spectrum Perseus cluster
Hitomi's first and only scientific observation consisted of a close X-ray look at the Perseus cluster, a cluster formed by thousands of galaxies. From previous observations made in the X-ray band, it was known that the hot gas in the cluster is carved with loops, bubbles and ripples. At the center of the cluster sits an active galaxy hosting a supermassive black hole. By virtue of its relativistic jets ejection, the black hole has shaped the surrounding medium forming giant cavities (several times larger than the central galaxy itself) within the hot gas.
Using SXS data SRON researchers did not find a turbulent gas, which was a surprise. The data showed beyond any doubt that a quiet gas had spread over a wide region, extending between 100 and 200 thousand light years. This suggested that large scale turbulence is not that easy to produce nor very easy to dissipate. This in turn means that clusters of galaxies, that can be detected up to large distances, can be used as standard candles for cosmological models. These models depend on the estimated mass of the cluster, which in turn is accurately measured only if the gas is not particularly turbulent.
The Hitomi spectrum of the Perseus cluster is revolutionary in many other respects. It gives a precious insight into the gas dynamics, but also shows emission lines from rare elements, important to study the production of elements in the universe. On top of that it shows a richness of emission lines that will be used by atomic physicist to better understand the gas conditions in extreme-temperature environments.
SRON developed a ‘filter wheel’ for ASTRO-H's Soft X-ray Spectrometer (SXS) in collaboration with the University of Geneva. The spectra of the X-ray sources that ASTRO-H planned to observe vary considerably in intensity. Strong X-ray sources can result in an overload of the spectrometer. The special filters in the wheel were going to prevent this.The filter wheel therefore enhanced the instrument’s scientific performance.
SRON also developed the onboard calibration source for the space instrument. The unique energy-separating capacity of the spectrometer required a continuous correction for small fluctuations in the instrument’s energy scale. This can only be done effectively if there is a new type of X-ray source on board - the local calibration source with precisely known spectral lines - that can be switched on and off.