Measure X-rays with cold detectors

XRISM is a Japan-led X-ray observatory developed with NASA and contributions from ESA. It studies energetic cosmic gas from Earth orbit. It launched with SLIM on H-IIA Flight 47 at 08:42:11 Japan Standard Time on September 7, 2023, equivalent to 23:42:11 UTC on September 6. The observatory separated about fourteen minutes after liftoff; the lunar lander separated later. A shared rocket did not make their science objectives or destinations the same.

The launch date illustrates why a timeline needs a time zone. September 6 in NASA’s UTC-based account and September 7 in JAXA’s Japanese account describe the same launch. Compare the timestamp before interpreting the different calendar dates as a conflict.

Resolve absorbs individual X-ray photons in a very cold detector. Each absorption creates a small temperature pulse; calibration relates the pulse to photon energy. Many events build a spectrum: how the measured signal changes with energy. Cooling the detector helps distinguish these small pulses from thermal fluctuations. It does not cool the distant source.

Think of two measuring tools: a camera that maps where light arrives and an energy meter that distinguishes similar photon energies. The tools answer different questions. A sharp spectrum can separate nearby atomic features even when the target occupies only a few spatial detector elements. More image pixels alone do not guarantee better energy measurements.

Two kinds of detail answer different questions

In an original arithmetic example, one photon carries 2 kiloelectronvolts and another carries 6 kiloelectronvolts. The second carries three times the energy, not three times the wavelength. A thousand detected photons all at 2 keV carry a total of 2,000 keV; that total is different from the energy of each photon. These invented values illustrate bookkeeping, not a measured XRISM spectrum.

Resolve is often described as a 36-pixel system. The technical guide distinguishes 35 science pixels from a separate calibration pixel used for tracking the energy scale. Calibration measurements are part of making reliable science; they should not be counted as an additional sky image. The spacecraft photograph below shows ground testing in 2022, not an image of the observatory operating in orbit.