In late 2018, the Neil Gehrels Swift observatory (Swift), discovered a new bright source lighting up the X-ray sky. It was called Swift J1858.6-0814, or shortly Swift J1858, and soon realized to be an X-ray binary: a system of two stars orbiting around each other where one of the two is a black hole or a neutron star and the other a regular star. These objects shine bright in X-rays (and at other wavelengths) when the black hole or neutron star is able to pull gas from its companion towards itself. Often this happens only sporadically during episodes that we call outbursts.

About two hundred X-ray binaries are currently known in our Galaxy and many of these have been extensively studied since the dawn of X-ray astronomy in the late 1960s. Swift J1858 immediately stood out, however, by displaying extreme behavior in which the X-ray emission changed by orders of magnitude on short (hours) time scales. Only a handful of other X-ray binaries had ever been observed to display similarly volatile behavior as Swift J1858. Perhaps the most prominent one of those is the infamous black hole V404 Cygni. Based on this analogy, Swift J1858 was therefore expected to habor a black hole too.

The extreme behavior of Swift J1858 drew a lot of attention in the X-ray binary community and motivated a massive multi-wavelength campaign involving many ground-based and space-based observatories. The fleet of facilities pointing to Swift J1858 involved, for instance, ESA’s XMM-Newton satellite (X-rays), NASA’s NICER mission located on the International Space Station (X-rays), the Hubble Space Telescope (UV), the Very Large Telescope in Chile (UV/optical/infrared), the 10-m Grantecan telescope on La Palma (optical/infrared), the Very Large Array in New Mexico USA (radio) and the Atacama Telescope Compact Array in Australia (radio). All these efforts allowed for an unprecedented characterization of the binary and its extreme variability.

X-ray studies suggested that Swift J1858 was very rapidly swallowing gas from its companion, but our radio studies showed that it was also blasting a bright collimated jet into space. Moreover, our X-ray and optical studies showed that it was also blowing material into space via a disk wind. One of the most surprising discoveries was that Swift J1858 turned out to harbor a neutron star rather than a black hole. This was established by the detection of a thermonuclear explosion from the source, a so-called type-I X-ray burst, which cannot be produced by a black hole because they lack a surface. Neutron stars might be tiny, but they can truly be as violent as black holes!

Swift J1858 is now dormant, but our ambitious multi-wavelength campaign has delivered an incredibly rich data set for us to analyze and interpret. A first series of papers reporting on the findings at different wavelengths has already been published, but the analysis is ongoing. In particular, correlating all the data sets obtained at different wavelengths is expected to result in new discoveries that will help us understand how accretion and associated outflows work, and why Swift J1858 showed such extreme behavior. So there is more to come!

Paper links (ADS):

• detection of a cold optical disk wind
• observations of an extremely varying radio jet
• possible detection of a hot X-ray disk wind
• revealing the binary parameters