Mysteriously in-sync pulsar challenges existing theories
Researchers from the Institute of Space Studies of Catalonia (IEEC) at the Institute of Space Sciences (ICE, CSIC) have contributed to a research study that, for the first time, detected synchronised pulses of optical and X-ray radiation from a mysterious pulsar. The observations indicate that a new physical mechanism might be needed to explain the behaviour of fast-spinning sources like this one, known as transitional millisecond pulsars.
The discovery was made as part of a two-day observation campaign spearheaded in 2017 by ESA’s XMM-Newton X-ray observatory and other telescopes. The group of IEEC researchers at ICE, CSIC — Francesco Coti Zelati, Nanda Rea, Santiago Serrano, and Diego Torres — have taken part in both the X-ray and optical observations, using, among others, the Montsec Observatory (Observatori Astronòmic del Montsec, OAdM) managed by IEEC. The combination of various space- and ground-based facilities [1] allowed the international team of astronomers to measure with very high temporal resolution the two types of radiation coming from the ultrafast rotating pulsar.

The pulsar [2] analysed in this study, known as PSR J1023+0038, spins around its axis within a few thousandths of a second. Such pulsars are classed as millisecond pulsars, some of which are also sucking in matter from a companion star – as is the case of this pulsar.
Earlier studies had shown that this pulsar belongs to the rare category of so-called ‘transitional millisecond pulsars’ that periodically switch between two different modes of emissions – in X-rays and radio waves.
According to the leading model explaining this behaviour, the accretion of matter from the companion star gives rise to the pulsed X-ray emissions, while the radio signal is thought to result from the rotation of the pulsar’s magnetic field. 
Further observations of PSR J1023+0038, however, revealed that an entirely different explanation might be needed to understand this class of sources.
“PSR J1023+0038 is the very first milisecond pulsar discovered with pulsations also in the optical band,” said Alessandro Papitto from INAF in Rome, Italy, lead author of the new study. 
The latest data show that the optical pulses in PSR J1023+0038 appear and disappear at exactly the same time as the X-ray ones. 
Conventional models could not explain the synchronised pulses so the team had to identify a new scenario that could explain the data. IEEC researcher Diego Torres was part of the group that put forward this new model to explain the detection, while also highlighting the existence of a small lag between the two emissions, yet to be further confirmed observationally.

“Until now, we thought that the pulsed X-ray emissions originated in a different process than the optical radiation. We also expected these processes to take place one after the other, but this is not the case for PSR J1023+0038. The synchronised pulses are an indication that they have the same origin,” says Diego Torres.

The new model states that the pulsar might be emitting a strong electromagnetic wind, which then interacts with the accretion disc around the system. As the pulsar wind hits matter in the accretion disc, it creates a massive shock, which accelerates electrons in the wind to extremely high speeds. The electrons then interact with the wind’s magnetic field, producing powerful beams of synchrotron radiation that can be observed in the optical and X-ray bands at the same time. All of this would happen at a very close distance from the pulsar, giving rise to the concept of mini pulsar wind nebula.

“The transitional pulsar J1023 + 0038 is one of the most interesting sources we know. Its multi-frequency variability is incredibly rich, and allows us to study the relationship between the magnetic field and matter in extreme conditions," concludes Torres as he is looking forward to further observations with future technologies.

For more information on this study, please visit ESA’s press release and/or access the scientific paper “Pulsating in unison at optical and X-ray energies: Simultaneous high-time resolution observations of the transitional millisecond pulsar PSR J1023+0038” by A. Papitto, F. Ambrosino, L. Stella, D. F. Torres, F. Coti Zelati et al.; published in The Astrophysical Journal.

[1] The study combines X-ray observations from ESA’s XMM-Newton and NASA’s NuSTAR and NICER X-ray observatories with ultraviolet observations from NASA’s Swift, optical observations from INAF’s optical Galileo National Telescope (TNG), equipped with the fast photometer SiFAP, and the Nordic Optical Telescope (NOT), both located at Roque de los Muchachos Observatory, in La Palma, Canary Islands, Spain, as well as from the Telescopi Joan Oró (TJO), located at the Montsec Observatory in the Catalan pre-Pyrenees, Spain, and infrared observations from the Gran Telescopio Canarias (GTC), also on the island of La Palma.
[2] Pulsars are highly magnetised, fast spinning neutron stars – the relics of massive stars. They are very dense objects, comprising up to two times the mass of the Sun within a radius of only ten km.


Title: Transitional millisecond pulsar PSR J1023+0038
Caption: This illustration shows the pulsar PSR J1023+0038 (depicted in white, on the right), which is classed as a millisecond pulsar because of its fast rotation, spinning around its axis within a few thousandths of a second. The pulsar is also part of a binary stellar system, sucking in matter from its companion star (depicted in red, on the left) via an accretion disc (also depicted in red). The illustration is not to scale; in reality, the neutron star is much smaller than the companion star.
Credit: ESA


- XMM-Newton


IEEC Communication Office
Barcelona, Spain

Rosa Rodríguez Gasén

Lead Scientist at ICE
Barcelona, Spain

Diego Torres
Institute of Space Science
Attached Documents
Generalitat de CatalunyaUniversitat de BarcelonaUniversitat Autònoma de BarcelonaUniversitat Politècnica de CatalunyaConsejo Superior de Investigaciones CientíficasCentres de Recerca de Catalunya