02 March 2022Powerful warm winds seen blowing from a neutron star

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Using the most powerful telescopes on Earth and in space, a team of astronomers has found for the first-time blasts of hot, warm and cold winds from a neutron star whilst it consumes matter from a nearby star. The discovery provides new insight into the behaviors of some of the most extreme objects in the universe.

Low-mass X-ray binaries (LMXBs) are systems containing a neutron star or black hole. They are fuelled by material ripped from a companion star, a process known as accretion. Most accretion occurs during violent eruptions where the systems brighten dramatically. At the same time, some of the material that spirals in is propelled back into space in the form of disc winds and jets.

Artist impression of the LMXB Swift J1858. Image credit: Gabriel Pérez (IAC).

The most common signatures of outflowing material from astronomical objects are associated with “warm” gas. Despite this, only winds of “hot” or “cold” gas have been observed in transient X-ray binaries, until now. In this new study, a team of researchers from eleven countries, led by the University of Southampton, studied the recent eruption of the X-ray binary known as Swift J1858. They used a combination of telescopes, including NASA’s Hubble Space Telescope (HST), the European Space Agency’s XMM-Newton satellite, the European Southern Observatory Organisation’s Very Large Telescope (VLT) and the Spanish Gran Telescopio Canarias (GTC).

The results, published in the journal Nature, showed persistent signatures of a warm wind at ultraviolet wavelengths occurring at the same time as signatures of a cold wind at optical wavelengths. This is the first time that winds from such a system have been seen across different bands of the electromagnetic spectrum.

Lead author Dr Noel Castro Segura, of the University of Southampton said: “Eruptions like this are rare, and each of them is unique. Normally they are heavily obscured by interstellar dust, which makes observing them really difficult. Swift J1858 was special, because even though it is located on the other side of our galaxy, the obscuration was small enough to allow for a full multiwavelength study.”

Co-author Georgios Vasilopoulos, from the Observatory of Strasbourg, added “Study of transient events has been quite challenging, but it has really progressed in this multi-wavelength era. For J1858 it has been a long journey since its first discovery with some of the largest ground-based telescopes and space observatories, with every step leading to gaining unique insights about the nature of the system and the discovery of properties seen for the first time in accreting X-ray binaries. Our curious human nature could not have asked for a more intriguing system.” 

Dr Castro Segura continues “All the astronomers in the field were incredibly excited, to the point that we combined our efforts to cover the full electromagnetic spectrum.”. This was the first time that this kind of experiment succeeded as pointed out by  co-Author Dr Hernández Santisteban from University of St Andrews “Only one other system — the black hole X-ray binary, V404 Cyg — has shown similar properties. However, our attempt to perform the same experiment on that system was unsuccessful, because the eruption ended before we could get the ground-based and space-based telescopes to observe it simultaneously.”. 

Co-author Nathalie Degenaar, from the University of Amsterdam added, “Neutron stars have an immensely strong gravitational pull that allows them to gobble up gas from other stars. The stellar cannibals are, however, messy eaters and much of the gas that neutron stars pull towards them is not consumed, but flung into space at high speed. This behaviour has a large impact both on the neutron star itself, and on its immediate surroundings. In this paper we report on a new discovery that provides key information about the messy eating patterns of these cosmic cookie monsters.”

As well as discovering the different types of winds, the team were able to study the temporal evolution of the outflowing gas. They found that the warm wind was not affected by the strong variations in the brightness of the system. The absence of such a response had previously been an unconfirmed theoretical prediction based on sophisticated simulations.

“In this research we combined the unique capabilities of the HST with the best ground-based telescopes, such as the VLT and GTC, to obtain a complete picture of the dynamics of the gas in the system, from the near-infrared to ultraviolet wavelengths. This allowed us to unveil for the first time the true nature of these powerful outflows,” Dr Castro Segura said.

“Our understanding of what causes these winds, and how fundamental they are to how these systems evolve over time, is crude at best,” said co-author Dr Knox S. Long, emeritus astronomer at the Space Telescope Science Institute. “I am excited because our discoveries give us a new window into these phenomena and might ultimately help us to build a more concrete understanding of what physical conditions are required to power winds in a wider range of astrophysical objects” he continued.

“The new insights provided by our results are key to understanding how these accreting objects interact with their environment. This is important, because by shedding energy and matter into the galaxy, they contribute to the formation of new generations of stars, and to the evolution of the galaxy itself,” Dr Castro Segura concluded.

The study was funded by grants from agencies including the Science and Technology Facilities Council (STFC) and NASA among others.

Science contact
Georgios Vassilopoulos – georgios.vasilopoulos@astro.unistra.fr

Media contact
Sébastien Derriere – sebastien.derriere@astro.unistra.fr

“A persistent ultraviolet outflow from an accreting neutron star transient” has been published in Nature : https://www.nature.com/articles/s41586-021-04324-2. Free access article https://rdcu.be/cH5AH

Other press releases on this result :
University of Southampton (english)
IAC (spanish)