Press Release

ESO telescopes help unravel pulsar puzzle

30 August 2023

With 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 mysterious object is known to switch between two brightness modes almost constantly, something that until now has been an enigma. But astronomers have now found that sudden ejections of matter from the pulsar over very short periods are responsible for the peculiar switches.

“We have witnessed extraordinary cosmic events where enormous amounts of matter, similar to cosmic cannonballs, are launched into space within a very brief time span of tens of seconds from a small, dense celestial object rotating at incredibly high speeds,” says Maria Cristina Baglio, researcher at New York University Abu Dhabi, affiliated with the Italian National Institute for Astrophysics (INAF), and the lead author of the paper published today in Astronomy & Astrophysics

A pulsar is a fast-rotating, magnetic, dead star that emits a beam of electromagnetic radiation into space. As it rotates, this beam sweeps across the cosmos — much like a lighthouse beam scanning its surroundings — and is detected by astronomers 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 another star. Over the past decade, the pulsar has been actively pulling matter off this companion, which accumulates 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 between 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 campaign to understand this pulsar’s behaviour involved a dozen cutting-edge ground-based and space-borne telescopes," says Francesco Coti Zelati, a researcher at the Institute 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-infrared 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 discovered that the mode switching stems from an intricate interplay between the pulsar wind, a flow of high-energy particles blowing away from the pulsar, and matter flowing towards the pulsar,” says Coti Zelati, who is also affiliated with INAF.

In the low mode, matter flowing towards the pulsar is expelled in a narrow jet perpendicular 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 behaviour, astronomers still have much to learn from studying this unique system and ESO’s telescopes will continue to help astronomers observe this peculiar pulsar. In particular, ESO’s Extremely Large Telescope (ELT), currently under construction in Chile, will offer an unprecedented view of J1023’s switching mechanisms. “The ELT will allow us to gain key insights into how the abundance, distribution, dynamics, and energetics of the inflowing matter around the pulsar are affected by the mode switching behavior,” concludes Sergio Campana, Research Director at the INAF Brera Observatory and coauthor of the study.

More information

This research was presented in a paper to appear in Astronomy & Astrophysics (doi:10.1051/0004-6361/202346418)

The team is composed of M. C. Baglio (Center for Astro, Particle, and Planetary Physics, New York University Abu Dhabi, UAE [NYU Abu Dhabi]; INAF – Osservatorio Astronomico di Brera, Merate, Italy [INAF Brera]), F. Coti Zelati (Institute of Space Sciences, Campus UAB, Barcelona, Spain [ICE–CSIC]; Institut d’Estudis Espacials de Catalunya (IEEC), Barcelona, Spain [IEEC]; INAF Brera), S. Campana (INAF Brera), G. Busquet (Departament de Física Quànticai Astrofísica, Universitat de Barcelona, Spain; Institut de Ciències del Cosmos, Universitat de Barcelona, Spain; IEEC), P. D’Avanzo (INAF Brera), S. Giarratana (INAF – Istituto di Radioastronomia, Bologna, Italy [INAF Bologna]; Department of Physics and Astronomy, University of Bologna, Italy [Bologna]), M. Giroletti (INAF Bologna; Bologna), F. Ambrosino (INAF – Osservatorio Astronomico di Roma, Rome, Italy [INAF Roma]); INAF – Istituto Astrofisica Planetologia Spaziali, Rome, Italy; Sapienza Università di Roma, Rome, Italy), S.Crespi (NYU Abu Dhabi), A. Miraval Zanon (Agenzia Spaziale Italiana, Rome, Italy; INAF Roma), X. Hou (Yunnan Observatories, Chinese Academy of Sciences, Kunming, China; Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, Kunming, China), D. Li (National Astronomical Observatories, Chinese Academy of Sciences, Beijing, China; University of Chinese Academy of Sciences, Beijing, China; Research Center for Intelligent Computing Platforms, Zhejiang Laboratory, Hangzhou, China), J. Li (CAS Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei, China; School of Astronomy and Space Science, University of Science and Technology of China, Hefei, China), P. Wang (Institute for Frontiers in Astronomy and Astrophysics, Beijing Normal University, Beijing, China), D. M. Russell (NYU Abu Dhabi), D. F. Torres (INAF Brera; IEEC; Institució Catalana de Recercai Estudis Avançats, Barcelona, Spain), K. Alabarta (NYU Abu Dhabi), P. Casella (INAF Roma), S. Covino (INAF Brera), D. M. Bramich (NYU Abu Dhabi; Division of Engineering, New York University Abu Dhabi, UAE), D. de Martino (INAF − Osservatorio Astronomico di Capodimonte, Napoli, Italy), M. Méndez (Kapteyn Astronomical Institute, University of Groningen, Groningen, The Netherlands), S. E. Motta (INAF Brera), A. Papitto (INAF Roma), P. Saikia (NYU Abu Dhabi), and F. Vincentelli (Instituto de Astrofísica de Canarias, Tenerife, Spain; Departamento de Astrofísica, Universidad de La Laguna, Tenerife, Spain).

The European Southern Observatory (ESO) enables scientists worldwide to discover the secrets of the Universe for the benefit of all. We design, build and operate world-class observatories on the ground — which astronomers use to tackle exciting questions and spread the fascination of astronomy — and promote international collaboration for astronomy. Established as an intergovernmental organisation in 1962, today ESO is supported by 16 Member States (Austria, Belgium, Czechia, Denmark, France, Finland, Germany, Ireland, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom), along with the host state of Chile and with Australia as a Strategic Partner. ESO’s headquarters and its visitor centre and planetarium, the ESO Supernova, are located close to Munich in Germany, while the Chilean Atacama Desert, a marvellous place with unique conditions to observe the sky, hosts our telescopes. ESO operates three observing sites: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope and its Very Large Telescope Interferometer, as well as survey telescopes such as VISTA. Also at Paranal ESO will host and operate the Cherenkov Telescope Array South, the world’s largest and most sensitive gamma-ray observatory. Together with international partners, ESO operates ALMA on Chajnantor, a facility that observes the skies in the millimetre and submillimetre range. At Cerro Armazones, near Paranal, we are building “the world’s biggest eye on the sky” — ESO’s Extremely Large Telescope. From our offices in Santiago, Chile we support our operations in the country and engage with Chilean partners and society.

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of ESO, the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the National Science and Technology Council (NSTC) in Taiwan and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI). ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.



Maria Cristina Baglio
New York University Abu Dhabi and Italian National Institute for Astrophysics (INAF)
Abu Dhabi, United Arab Emirates
Tel: +97126287089
Email: ;

Francesco Coti Zelati
Institute of Space Sciences
Barcelona, Spain
Tel: (+34) 937379788 430416

Sergio Campana
INAF Brera Observatory
Merate, Italy
Tel: +39 02 72320418

Bárbara Ferreira
ESO Media Manager
Garching bei München, Germany
Tel: +49 89 3200 6670
Cell: +49 151 241 664 00

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About the Release

Release No.:eso2315
Name:PSR J1023+0038
Type:Milky Way : Star : Evolutionary Stage : Neutron Star : Pulsar
Facility:Atacama Large Millimeter/submillimeter Array, New Technology Telescope, Very Large Telescope
Science data:2023A&A...677A..30B


The dark background of this image is broken by the fascinating artist's impression in the central region of the small pulsar and the big companion star. At the very centre is the pulsar, a small white sphere with two narrow white jets coming vertically out of it on either side — one is extending upwards to the top of the frame, the other downwards to the bottom. Grey semi-spherical blobs of hot gas are being expelled in the same directions as the jets, although the bottom blob is barely visible underneath a gas disc close to the pulsar. The disc is swirling into the pulsar and it is bright and white close to it. The disc is connected with a curved arm to the companion star, a much larger and brighter circle in the upper right of the image. From this star, the pulsar is stealing gas.
Artist’s impression of the pulsar PSR J1023+0038


ESO telescopes help solve pulsar puzzle (ESOcast 266 Light)
ESO telescopes help solve pulsar puzzle (ESOcast 266 Light)
Artist’s animation of the pulsar PSR J1023+0038
Artist’s animation of the pulsar PSR J1023+0038