Ehud Behar

About 25,000 light-years from Earth, in the constellation Sagittarius, a dense veil wraps around the neutron star GX 13+1. It scatters X-ray photons the way fog scatters streetlight, and for a long time it hid the source's true power from astronomers.

When a massive star runs out of fuel, it collapses and explodes, scattering the material from its interior across space. The remnant of such an explosion, Cassiopeia A, has long been known to astronomers for its lopsided appearance: bright clumps of material are scattered unevenly.

A flashlight shines through fog, and its light changes color slightly. From that change, you can tell what the fog is made of. That's how astronomers study cold gas around X-ray sources: iron atoms in the gas absorb and re-emit X-rays at a very specific energy, like a fingerprint. But there's a catch: if you strip a few electrons from the atom, the line shifts.

In December 2024, quasar PG 1211+143 found itself in the crosshairs of two X-ray observatories at once: the spectrometers on XRISM and XMM-Newton observed it simultaneously. A quasar is the extremely bright core of a distant galaxy, where a supermassive black hole is accreting surrounding gas.

Every galaxy floats in an invisible cloud of gas, like fruit in syrup. This gas is the leftover building material from which stars were assembled, and it is constantly replenished by fresh supplies from intergalactic space. Seeing it directly is nearly impossible: it is too tenuous and too cold.

The Perseus galaxy cluster is a giant metropolis of thousands of star islands, bathed in an invisible ocean of scorching gas. Like urban smog, this gas preserves traces of all the chemical factories that have operated in galaxies for billions of years: supernova explosions that scatter freshly forged atoms across the surroundings.

Active galactic nuclei (AGN) are the universe’s powerhouses: their X-ray coronae blaze millions of times hotter than stellar atmospheres. But what strikes the spark? A ten-day X-ray campaign by the XRISM and XMM-Newton satellites, aimed at NGC 3783, provided the answer: it’s actual thunderstorms.

For years, silence reigned in the heart of galaxy 1ES 1927+654: the accreting gas merely whispered in X-rays, leaking slow winds. But in 2022–2025, the picture changed dramatically. Spectra taken by XMM-Newton and Swift lit up with emission lines — a narrow peak at 0.56 keV, likely from OVII oxygen, a broader feature near 1 keV, and a powerful iron band in the 6–7 keV region.