Free Astronomy Magazine November-December 2023

38 NOVEMBER-DECEMBER 2023 ASTRO PUBLISHING stems from an intricate interplay between the pulsar wind, a flow of high-energy particles blowing away from the pulsar, and matter flow- ing towards the pul- sar,” says Coti Zelati, who is also affiliated with INAF. In the low mode, mat- ter flowing towards the pulsar is expelled in a narrow jet perpendicu- lar to the disc. Gradually, this matter accumulates closer and closer to the pulsar and, as this happens, it is hit by the wind blowing from the pulsating star, causing the matter to heat up. The system is now in a high mode, glowing brightly in the X-ray, ultraviolet and visible light. Eventually, blobs of this hot matter are removed by the pulsar via the jet. With less hot matter in the disc, the system glows less brightly, switching back into the low mode. While this discovery has unlocked the mystery of J1023’s strange be- haviour, astronomers still have much to learn from studying this unique system and ESO’s telescopes will continue to help astronomers ob- serve this peculiar pulsar. In particu- lar, ESO’s Extremely Large Telescope (ELT), currently under construction in Chile, will offer an unprece- dented view of J1023’s switching mechanisms. “The ELT will allow us to gain key in- sights into how the abundance, dis- tribution, dynamics, and energetics of the inflowing matter around the pulsar are affected by the mode switching behavior,” concludes Ser- gio Campana, Research Director at the INAF Brera Observatory and coauthor of the study. ! W ith a remarkable observational campaign that involved 12 telescopes both on the ground and in space, including three European Southern Observatory (ESO) facilities, astronomers have uncovered the strange behaviour of a pulsar, a super-fast-spinning dead star. This video summarises the discovery. [ESO] tromagnetic radiation into space. As it rotates, this beam sweeps across the cosmos — much like a light- house beam scanning its surround- ings — and is detected by as- tronomers as it intersects the line of sight to Earth. This makes the star appear to pulse in brightness as seen from our planet. PSR J1023+0038, or J1023 for short, is a special type of pulsar with a bizarre behaviour. Located about 4500 light-years away in the Sextans constellation, it closely orbits an- other star. Over the past decade, the pulsar has been actively pulling mat- ter off this companion, which accu- mulates in a disc around the pulsar and slowly falls towards it. Since this process of accumulating matter began, the sweeping beam virtually vanished and the pulsar started incessantly switching be- tween two modes. In the ‘high’ mode, the pulsar gives off bright X- rays, ultraviolet and visible light, while in the ‘low’ mode it’s dimmer at these frequencies and emits more radio waves. The pulsar can stay in each mode for several seconds or minutes, and then switch to the other mode in just a few seconds. This switching has thus far puzzled astronomers. “Our unprecedented observing cam- paign to understand this pulsar’s be- haviour involved a dozen cutting- edge ground-based and space- borne telescopes,” says Francesco Coti Zelati, a researcher at the Insti- tute of Space Sciences, Barcelona, Spain, and co-lead author of the paper. The campaign included ESO’s Very Large Telescope (VLT) and ESO’s New Technology Telescope (NTT), which detected visible and near-in- frared light, as well as the Atacama Large Millimeter/submillimeter Array (ALMA), in which ESO is a partner. Over two nights in June 2021, they observed the system make over 280 switches between its high and low modes. “We have dis- covered that the mode switching