In 2026, a black hole in the binary system GS 1354-64 woke up: it began pulling matter from its companion star, and a scorching disk swirled around it. The XRISM X-ray telescope watched this disk through the Resolve microcalorimeter — an instrument that catches individual photons and measures their energy with precision previously unimagined. Over the observation period, 3.5 million photons were collected, and their energies revealed a clear picture.
The spectrum showed a broad iron line. Iron in the disk emits X-rays at a strictly defined energy, but due to the disk's rapid rotation and the black hole's strong gravity, the line smears into a wide band. From the shape of this band, one can calculate how fast the black hole itself is spinning. It turned out to be spinning faster than 98% of the maximum possible rate, like a top spun almost to its limit.
The disk's inclination angle to the line of sight, derived from the reflection data, came out different in different models — meaning the method still needs refinement. But the main achievement is elsewhere: the microcalorimeter showed it can discern fine details in X-ray light that previously blurred into noise. Such instruments will pave the way for precise measurements of black holes across the sky.
as written by the authors arXiv:2603.06883
We present a spectroscopic analysis of XRISM and NuSTAR observations of the black hole X-ray binary GS~1354--64 during its 2026 outburst. A total number of 3.5 million photons are collected by the microcalorimeter Resolve on board XRISM, providing an unprecedented high-resolution view of the iron line profile. A clean broad iron line is found in the data, without significant narrow features. Modeling the broad iron line with relativistic reflection from the inner accretion disk suggests a rapidly spinning black hole (a>0.98) in the system. Measurements of the disk inclination angle from the reflection method are model-dependent. This work demonstrates the power of X-ray microcalorimeters in studying the inner accretion flow and constraining black hole parameters.
The Resolve microcalorimeter aboard XRISM is cooled to 0.05 degrees above absolute zero, colder than open space, so that thermal noise doesn't interfere with catching individual photons.
In the movie "Interstellar," the black hole Gargantua spins at nearly the maximum rate: that's why time flows so slowly on Miller's planet.