The X-ray astronomy satellite Astro-H was successfully launched on 17 February 2016 at 17H 45 (UT 8H45) from the JAXA Tanegashima Space Center in Japan. The satellite was put into orbit 14 min later and the first tests started. ASTRO-H is a JAXA / NASA mission under the Japanese leadership and with an ESA participation. This new observatory aims to observe and study the hot and energetic Universe. Astro-H embeds several instruments that when combined cover the spectral band 0.3-600 keV. Among the instruments on board, the spectrometer SXS is equipped with a state-of-the-art X-ray calorimeter. This detector, cooled to a temperature close to absolute zero, will achieve unmatched performances in terms of spectral resolution. CEA is involved in the mission, technically and scientifically, through the ESA contribution and support from the French space agency CNES.
After this sucessful launch, JAXA decided to rename Astro-H "Hitomi".
The Astro-H satellite was successfully launched by a H-IIA rocket from the Tanegashima Space Center at 8h45 Universal Time (17h45 civil time in Japan, 9h45 in France). 14 minutes after liftoff, the separation with the launch vehicle occurred and the satellite was then injected into a circular orbit at an altitude of 575 km altitude and with an inclination of 31 degrees. With this orbit the period of revolution of the satellite is 96 minutes. Immediately, the first operations of the different systems began. They will continue for 2 weeks and will then be followed by a calibration phase. Once these steps validated, the scientific observations will begin. The lifetime of the mission is 3 years minimum. Astro-H is the sixth Japanese satellite dedicated to X-ray Astronomy, in the tradition of high-energy observatories of this country.
Astro-H is a space mission dedicated to the study of the hot and energetic universe. Astro-H will address the question of the large structures of the universe in particular the interaction between the hot gas in galaxies clusters and dark matter. The mission will investigate the formation and evolution of supermassive black holes in galaxies and their impact in the structure of the universe. Astro-H will study the chemical evolution of galaxies at high redshift and its unique performance in spectroscopy will allow examining in detail the properties of the iron lines in active galactic nuclei. Astro-H will also study the laws of physic in extreme conditions at play in compact objects such neutron stars and black holes, and their environment. The particle acceleration processes and shocks in supernova remnants or the origin of cosmic rays will also be among its scientific objectives.
The payload of the Astro-H observatory has six telescopes, four based on focalisation of X-rays and two Compton telescopes.
The principle of grazing incidence used to focus the X-ray requires appropriate focal lengths depending on the energy of the incident photons. It is not today possible to focus with a single device and with a good efficiency both the low and high-energy photons. To cover the spectral band concerned, 0.3-100 keV, Astro-H team has split this energy field by using two types of mirrors.
The two soft X-ray telescopes have a focal length of 6m and are sensible in the 0.3-12 keV band. Each telescope is equipped with a special detection plane. The first is a start-of-the-art spectrometer (SXS) consisting of X-rays microcalorimeters, devices that measure a tiny temperature increase when a photon interacts with the detector. This principle, used successfully in the field of far infrared (like for the PACS instrument on board the Herschel satellite) is here applied for the first time in X-ray Astronomy. The spectral resolution achieved with SXS (a few eV) is sensibly improved compared to previous mission currently in operation as Chandra, XMM-Newton or NuSTAR. This performance nevertheless has a cost: the detectors must be cooled to a temperature of 50 milli-Kelvin above absolute zero and maintained in this condition for at least the 3 years minimum duration of the mission. The second low-energy X-rays telescope is an imager with four CCD X-rays cameras. This equipment is called SXI for Soft X-ray Imager and offers a moderate spatial resolution of the order of one arcminute.
On left, payload description of Astro-H. The mirrors are mounted on a platform at one end of the satellite. The first focal plane, located 6 meters away, is the home of the low energy instruments (SXI and SXS) and of the 2 detectors SGD (Soft Gamma-ray Detector). The two high-energy instruments (HXI) are in turn located on a platform at the end of an extensible optical bench (EOB), allowing the hard X-rays to be focussed since the instruments HXI require a focal length of 12 meters. The extensible mast (EOB) is deployed in space and the fine control of the mast length is provided by a laser system. Right, the satellite before the vibration tests. Once set in space, the satellite will measure 14 meters long for a total weight of 2700 kg. (Credit: JAXA)
The two high-energy telescopes (HXT) have, in turn, different mirror designs. With a focal length of 12 m, they are each equipped with a hybrid detection plane made of silicon /cadmium-telluride (CdTe) and sensitive in the 5-80 keV band. The spatial resolution of the two imagers HXI is of the order of one arcminute.
Two identical soft gamma-ray imagers (SGD or Soft Gamma-ray Detector) complete the instruments on board and cover the energy range 60-600 keV. These instruments do not use focusing telescopes, impossible to realize today at these energies, but are Compton mini-telescopes based on silicon and CdTe detectors.
CEA participates in Astro-H trough the ESA contribution, both for the technical aspect of the mission and its scientific follow-up. Two teams from CEA-IRFU (the Astrophysics Division - SAp and the Electronics, Detectors and Computing Division - Sedi) were involved in the HXI (Hard X-ray Imager) and SGD (Soft-Gamma- ray Detector) instruments. With the support of ESA, they conducted studies of radiation effects on its most critical components, i.e. their front-end electronic circuits (ASICs) and their detectors based on CdTe crystals. The CEA also provided BGO crystals for the anticoincidence systems of HXI and SGD. To study the BGO and CdTe behaviours, the teams conducted proton radiation tests.
Through its participation to the mission, the Astrophysics Division is in charge of studying the long-term behaviour, over one year, of the HXI CdTe detectors. The aim is to check in the stability of the components, in conditions as close as they will appear in space. For this, the laboratory has developed a test bench that includes a vacuum chamber cooled to -15 degrees Celsius in which are installed the detector and its associated electronics (left). The tested detector is a fraction (1 / 16th) of the total surface of HXI detection plane and comprises 32x32 strips of CdTe. The detector is visible on the right image in the form of a square 10x10 mm (bottom of the picture and golden colour) and connectors linking the frontal electronic circuits (ASICs) of IDeF-X type. Each day, the high voltage supply is switched-off for 20 minutes, then the performance of the detector evaluated. This cycle, similar to what will be done in space, is a necessary step to allow this kind of detector to recover its optimal operation. (Photo credit: CEA-SAp / Daniel Maier)
Thanks to his expertise, CEA has designed and built a flight calibration source to control during the observations the gain of the HXI detectors. The very low radioactive activity of the source (required not to pollute beyond reasonable the weak signal from celestial objects) and the implementation of a system which meets the space environment constraints required special developments.
Researchers from the Astrophysics Division are involved with the support of CNES in two key areas of research: the study of the polarization of light in black hole candidates and pulsars and the central regions of the Galaxy. These two scientific topics will be investigated in detail with the instruments HXI and SGD in complement to previous studies conducted with the Integral and XMM-Newton observatories. As co-investigators (Co-I) of the project, scientists from the Astrophysics Division will have in agreement with their Japanese colleagues a special access to the data during the first calibration phase of the mission.
The Astro-H observatory is open to the scientific community through call for competitive proposals. However, the phases of the mission meet a schedule defined by the consortium and follow a number of rules (observed sources, data access - restricted at the beginning of the mission to the Astro-H consortium members, etc).
The timetable is spread over four phases described below (function, percentage, duration of the phase):
- Phase 0, satellite testing, instrument calibration, 100%, 3 months
- Phase 1 Performance Verification (PV), 100%, 6 months
- Phase 2 PV (25%), Guest Observation (GO) (75%) 12 months
- Phase 3, PV (10%), GO (90%), rest of the mission
The first call for proposals, open to the entire scientific community, is expected in mid-2016 for an observation period starting early 2017.
The data will be made public on a dedicated database archive after a one-year period.
Rédaction : C. Gouiffès