Neutron Star Wind Challenges Existing Space Physics Models

The X-Ray Imaging and Spectroscopy Mission, or XRISM, has revealed a significant contrast in the winds emanating from the neutron star GX13+1, compared to those found near supermassive black holes. This discovery challenges existing models of how these cosmic winds form and interact with their environments.

On February 25, 2024, XRISM utilized its Resolve instrument to observe GX13+1, a neutron star surrounded by an accretion disk of superheated material. This disk produces X-rays as material spirals inward and collides with the star's surface, generating powerful outflows that can reshape surrounding space.

The team targeted GX13+1 to gain insights into these processes, expecting to capture unprecedented details due to Resolve's ability to measure individual X-ray photon energies. Matteo Guainazzi, ESA XRISM project scientist, expressed the excitement of witnessing what he describes as a game-changing result after years of pursuit.

The significance of these winds extends beyond curiosity; they play a crucial role in cosmic evolution. Similar winds from supermassive black holes can compress molecular clouds, initiating star formation, or disperse those clouds, halting star formation altogether.

This feedback mechanism can even regulate the growth of entire galaxies. Just before the observations began, GX13+1 unexpectedly brightened, reaching or surpassing the Eddington limit, a critical threshold where radiation pressure can expel most infalling material back into space.

Chris Done, lead researcher from Durham University, noted that this unexpected outburst generated a thicker-than-previously observed wind, despite its speed remaining relatively low at around one million kilometers per hour.

This is notably slower compared to the faster winds observed near supermassive black holes, which can approach 20 to 30 percent of light speed. The team's observations highlighted a stark difference, as previous studies of supermassive black holes at the Eddington limit revealed ultra-fast, clumpy winds, while the outflow from GX13+1 was characterized as slow and smooth.

The researchers propose that these differences may be attributed to the temperature of the accretion disks surrounding the objects. Counterintuitively, supermassive black holes have cooler disks compared to neutron stars, despite being more luminous due to their larger sizes.

The emitted radiation from these disks peaks in ultraviolet light, which interacts more effectively with matter than the X-rays emitted by neutron star systems. If validated, this hypothesis could refine our understanding of energy and matter exchange in extreme environments, impacting theories on galaxy growth and cosmic evolution.

The XRISM mission, launched on September 7, 2023, is a collaboration between JAXA, NASA, and ESA, aiming to explore celestial phenomena with unprecedented detail using its two instruments, Resolve and Xtend.