XRISM sees surprisingly slow and dense wind from neutron star

Last year, XRISM used its Resolve instrument to look at neutron star GX13+1, the burnt-out core of a once larger star. A disc of hot matter around GX13+1, known as an accretion disc, is gradually spiralling down to strike the neutron star’s surface, thereby emitting bright X-rays. Such inflows also power outflows that influence and transform the cosmic environment. Yet the details of how these outflows are produced remain a matter of ongoing research. Which is why XRISM pointed its focus towards GX13+1.

Outflows appear also from supermassive black holes in the centres of galaxies, and can cause stars to form by triggering the collapse of giant molecular clouds, or they can stop star formation by heating and blowing those clouds apart. Astronomers call this ‘feedback’, and it can be so powerful that the winds from a supermassive black hole can control the growth of its entire host galaxy.

Since the mechanisms generating the winds from supermassive black holes may be fundamentally the same as those at work around neutron star GX13+1, the XRISM team chose to look at GX13+1 as it is closer and therefore appears brighter than the supermassive black hole varieties, meaning that it can be studied in more detail.

There was a surprise. A few days before their observations were due to take place, GX13+1 unexpectedly brightened – reaching or even exceeding a theoretical ceiling known as the Eddington limit. The principle behind this limit is that as more matter falls onto a compact object such as a black hole or a neutron star, more energy is released. The faster the energy is released, the greater the pressure it exerts on other infalling material, pushing more of it back into space. At the Eddington limit, the amount of high-energy light being produced is essentially enough to transform almost all of the infalling matter into a cosmic wind.

Resolve happened to be watching GX13+1 as this staggering event took place. ‘We could not have scheduled this if we had tried,’ said Chris Done, Durham University, UK, the lead researcher on the study. ‘The system went from about half its maximum radiation output to something much more intense, creating a wind that was thicker than we’d ever seen before.’

But mysteriously, the wind was not travelling at the speed that the XRISM scientists were expecting. It remained around 1 million km/h. This is slow compared to the cosmic winds produced near the Eddington limit around a supermassive black hole. In that situation, the winds can reach 20 to 30 percent the speed of light, more than 200 million km/h.

‘The winds were utterly different but they’re from systems which are about the same in terms of the Eddington limit. So if these winds really are just powered by radiation pressure, why are they different?’ asks Done.

The team has proposed that it comes down to the temperature of the accretion disc that forms around the central object. Counterintuitively, supermassive black holes tend to have accretion discs that are lower in temperature than those around stellar mass binary systems with stellar black holes or neutron stars.

This is because the accretion discs around supermassive black holes are larger. They are also more luminous, but their power is spread across a larger area. Therefore the typical kind of radiation released by a supermassive black hole accretion disc is ultraviolet, which carries less energy than X-rays.

Since ultraviolet light interacts with matter much more readily than X-rays do, the XRISM research team speculates that this may push the matter more efficiently, creating the faster winds observed in black hole systems. If so, the discovery promises to reshape our understanding of how energy and matter interact in some of the most extreme environments in the Universe.

‘Stratified wind from a super-Eddington X-ray binary is slower than expected’, Nature