The winds of a neutron star devouring its companion

 Using the most powerful telescopes on the ground and in space, a team of astronomers have discovered for the first time gusts of hot, warm and cold winds blown by a neutron star as it devours material from a nearby star. This discovery provides insight into the behavior of some of the most extreme objects in the universe.

Low-mass X-ray binaries (LMXB) are systems containing a compact object: a neutron star or a black hole. They are fed by material stripped from a companion star, a process known as accretion. Most of the accretion takes place during violent eruptions, during which the luminosity of the systems increases dramatically. At the same time, some of the material that spirals around the compact object is propelled into space in the form of winds and jets.

The most common signatures of matter escaping from astronomical objects are associated with "warm" gas. Despite this, only "hot" or "cold" gas winds have been observed in variable X binaries, so far. In this new study, a team of researchers from eleven countries investigated the recent flare 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's Very Large Telescope (VLT), and the Gran Telescopio Canarias (GTC) Spanish.

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

Lead author Dr Noel Castro Segura, from the University of Southampton, says: "Eruptions like this are rare, and each one is unique. Normally, they are heavily obscured by interstellar dust, which makes observing them very difficult. Swift J1858 was special because even though it is located on the far side of our Galaxy, the obscuration was low enough to allow full study at multiple wavelengths."

Co-author Georgios Vasilopoulos, from the Strasbourg Observatory, adds: "The study of transient events was quite difficult, but it is really progressing today in the era of multi-wavelength studies. In the case of J1858, it has been a long journey since its first discovery with some of the largest ground-based telescopes and space observatories, and each step has provided unique insights into the nature of the system and uncovered observed properties for the first time in accreting X-ray binaries. Our curiosity could not have dreamed of 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 entire electromagnetic spectrum". It was the first time this type of experiment had succeeded, as co-author Dr Hernández Santisteban of the University of St Andrews points out: "Only one other system – the binary X-ray black hole, V404 Cyg – showed similar properties. However, our attempt to perform the same experiment on this system failed, as the eruption ended before we could observe it simultaneously with ground and space telescopes".

Co-author Nathalie Degenaar, from the University of Amsterdam, adds: "Neutron stars have considerable gravitational pull which allows them to gobble up gas from other stars. These stellar cannibals eat dirty, however, and much of the gas that neutron stars attract to them is not consumed, but shot out into space at high speed. This behavior has a significant impact on both the neutron star itself and its immediate surroundings. In this article, we report a new discovery that provides critical insight into the disordered eating habits of these cosmic gluttons."

In addition to discovering the different types of winds, the team was able to study the time evolution of the escaping gas. They found that the lukewarm wind was unaffected by large variations in system brightness. The lack of such a response was previously an unconfirmed theoretical prediction based on sophisticated simulations.

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

"Our understanding of what causes these winds, and their fundamental role in how these systems evolve over time, is sketchy at best," said co-author Dr. Knox S. Long, astronomer emeritus at the Space Telescope Science Institute. "I am very excited because our findings open a new window for us into these phenomena and could ultimately help us better understand the physical conditions required to power the winds in more astrophysical objects," he said. he continued.

"The new insights offered by our results are essential for understanding how these accreting objects interact with their environment. This is important, because by throwing energy and matter back into the Galaxy, they contribute to the formation of new generations of stars and to the evolution of the Galaxy itself", concludes Dr Castro Segura.

https://astro.unistra.fr/fr/2022/03/02/les-vents-dune-etoile-a-neutrons-devorant-son-compagnon/