July 25, 2016

Hitomi reveals its legacy

The SXS spectrometer has had time to peer into the Perseus cluster before Hitomi’s demise.

Hitomi, formerly known as Astro-H, is a JAXA mission aimed at the X-ray study of hot plasma and other violent phenomena in the Universe. Launched on 17 February 2016, the satellite has unfortunately stopped working on 26 March following an anomaly, before being declared lost by JAXA.

Nevertheless, the Hitomi teams had time to commission part of its payload, including the SXS spectrometer which is a precursor of the Xifu spectrometer of ESA’s ATHENA mission, a very ambitious instrument developed under CNES responsibility.

SXS is composed of a mirror focusing X-rays on an X microcalorimeter matrix cooled to 50 mK.

Artist's view of Hitomi Credits: JAXA

The first observation of this instrument was dedicated to the Perseus cluster, where mission scientists expect to observe a wide variety of physical phenomena (hot plasma, turbulence, influence of the supermassive black hole at the centre of the cluster, etc.). Observations of the Perseus cluster using the SXS have been published on 7 July 2016 in the journal Nature.

Hitomi Persee.png

Full array spectrum of the core of the Perseus cluster obtained by the Hitomi observatory. The redshift of the Perseus cluster is z=0.01756. The inset above 7.5 keV has a logarithmic scale, which allows the weaker lines to be better seen.
© Collaboration Hitomi, Nature.

The instrument performance have kept all their promises: thanks to the collected data, all X-ray spectral emission lines from the cluster have been detailed for the first time, especially the iron fluorescence lines at different ionization degrees. These measurements give access to the state of the intracluster gas, which proves to be quieter and less turbulent than expected.


Simulated spectrum of the Athena Xifu instrument based on Hitomi observations and Athena’s expected performances. The observed region is a part (30 arcsec × 30 arcsec) of the area studied by Hitomi (3 arcmin × 3 arcmin), for a 100,000-second exposure time (230,000 seconds for Hitomi). On a region 36 times smaller and an exposure time twice as small, the energy flux collected by Athena is nearly 100 times greater in the same energy domain (5.5 - 8.5 keV). Moreover, Xifu is exploring a much larger domain close to the low energies, revealing an abundance of emission lines, physical-chemical signatures of hot gas present in the heart of the Perseus cluster.
© Barret et al.

These measurements will also be very useful for cosmology: counts of Universe clusters provide a cosmological probe which contributes among others to understanding the mechanisms of the accelerating Universe, provided their mass can be estimated; the temperature of the hot gas in the cluster is directly linked to the amount of dark matter in it.

Publication references

The Hitomi collaboration, The Quiescent Intracluster Medium in the Core of the Perseus Cluster, Nature, 535, 117-148 (2016)


  • Scientific contact: Philippe Laurent (philippe.laurent at cea.fr)
  • Astronomy and Astrophysics Programme Manager: Olivier La Marle (olivier.lamarle at cnes.fr).