The Athena X-ray observatory is the second large mission (L2) of ESA's Cosmic Vision-programme. Athena is scheduled to launch in the early 2030s. The space telescope will answer two big scientific questions: How does ordinary matter glue together to form large-scale structures such as the Milky Way and clusters of galaxies? And how do black holes grow and influence their surroundings?
Athena will observe the hot universe for at least four years. The telescope will carry two instruments: the camera annex spectrometer X-ray Integral Field Unit (X-IFU) and the Wide Field Imager (WFI). SRON is co-Principal Investigator for X-IFU. Our hardware contribution to X-IFU is the focal plane assembly including its cold electronics. SRON also carries out the parallel development of back-up ultra-sensitive detector arrays for X-IFU.
To answer the two big scientific questions mentioned above, Athena will map and study large-scale gas structures in the Universe and survey supermassive black holes and explore high-energy astrophysical events such as supernova explosions and energetic stellar flares.
Athena will map the distribution and content of hot gas clouds within galaxies and galaxy clusters at various distances, so at different points in cosmic history. Astronomers can use those data to understand how these large-scale structures formed and evolved. The bulk of ordinary matter in the Universe comprises hot gas which can only be observed by space-based facilities operating in the X-ray band.
One of the bigger mysteries in astronomy is how supermassive black holes are created. One possibility is that the first generation of stars in the Universe turned into stellar black holes at the end of their life, and eventually grew into the supermassive versions that we see today. Athena will search for these events—supernovae shining bright in X-rays—at distances where the Universe appears to be less than 1 billion years old.
Athena will also study fully grown supermassive black holes as their cosmic feedback is linked with galaxy formation and evolution. Luckily for Athena, their accretion produces X-rays. Athena will look for them at distances where the Universe appears to be between 1.5 and 6 billion years old, to cover the era where star formation and accretion processes were at a peak.
SRON is one of the leading institutes in the development of the X-ray Integral Field Unit (X-IFU) for Athena. X-IFU will be an imaging camera annex spectrograph with unprecedented energy resolution. The camera is cooled to near absolute zero Kelvin. The spectrograph will be able to produce a spectrum for each pixel of the captured image and derive the characteristics of gas as hot as 10 million degrees Celsius.
Crucial to the instrument is the superconducting Transition Edge Sensor (TES) based detectors array (developed at NASA Goddard Space Flight Center) and its read-out (developed at SRON and VTT in Finland). Apart from the readout, SRON is responsible for the development of the back-up TES detectors array.
XIFU will host an array of ~3000 pixels read-out by 100 Superconducting QUantum Interference Devices (SQUID), which are the core of the amplification chain in the Frequency Division Multiplexing (FDM) read-out.
A TES detector is cooled to a base temperature of 50mK, well below its critical temperature, and operated at its superconducting transition by a stable MHz bias voltage. When one photon from space is absorbed by the detector, the TES temperature rises and with that also its resistance. The electrical current that was previously able to run with very low dissipation, all of a sudden experiences resistance and decreases in value. This is read out and registered as an incoming photon by the SQUID amplifier. The higher the induced current drop, the more energy the photon turns out to have had.
TES based detectors can achieve a spectral resolution of a few electronVolts at X-ray energies from 200 eV up to 10 keV. They will provide astronomers with images and spectra containing valuable information about the abundance of elements and the temperature and density of gas in many sources ranging from supernovae remnants, the gas near supermassive black holes and the gas within galaxies and galaxy clusters.