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. Some of that gas does not fall into the hole but is ejected outward as an ultrafast outflow — at speeds up to 40% of the speed of light.

In X-ray light, this outflow leaves its mark: iron and oxygen atoms absorb light at specific wavelengths, and these "dips" in the spectrum reveal the gas's velocity and density. It turns out the outflow is not uniform. At least three zones are discernible, moving at different speeds—7.4%, 12%, and 33% of the speed of light. The slower zones are denser and cooler, while the faster ones are more tenuous and hotter, like layers of different temperatures in a steam plume above a pot.

This "clumpiness" explains how the black hole accelerates the wind. Simple radiation pressure on free electrons could not impart such speeds to the gas. But if the flow contains dense clumps rich in absorption lines, light pushes on them much more strongly, just as a gust of wind shoves a sail with patches harder than it does smooth fabric. Such clumps are likely what carry matter away from the black hole.

Quasar PG 1211+143 is already the second example where the XRISM X-ray telescope has resolved the clumpy structure of an accretion wind. Previously, the same was seen in quasar PDS 456. It seems that ultrafast outflows from black holes are not smooth rivers but rather billowing jets of steam escaping from a boiling cauldron.