Observations explore nature of millisecond transitional pulsar PSR J1023+0038

Using the Gran Telescopio Canarias (GTC), Italian and Spanish astronomers made high temporal resolution optical spectroscopic observations of a millisecond transition pulsar designated PSR J1023+0038. Results of the observation campaign, published on September 19 on the pre-print server arXivprovide essential information on the nature of this pulsar.

Pulsars are highly magnetized rotating neutron stars that emit a beam of electromagnetic radiation. The fastest rotating pulsars, with rotation periods less than 30 milliseconds, are called millisecond pulsars (MSP). Astronomers assume that they form in binary systems when the initially more massive component transforms into a neutron star which is then rotated due to accretion of material from the secondary star.

So-called millisecond transition pulsars (tMSPs) exhibit transitions between a spin-powered radio pulsar state (“pulsar state”) and a state characterized by X-ray pulsations and accretion disk features in the spectrum optical (“disk state”). ‘). They are generally rare, since so far only three tMSPs have been detected.

One of them is PSR J1023+0038 (or J1023 for short), first identified twenty years ago. The pulsar has a rotation period of 1.69 milliseconds, while its orbital period is approximately 4.75 hours. The system’s companion star is a late-type star of spectral type G5.

Previous observations of J1023 showed that it switches between an X-ray state and a spin-powered radio pulsar phase. Now, a team of astronomers led by Marco Messa of the University of Milan in Italy used GTC’s optical imaging system and OSIRIS (Optical System for Imaging) instrument to take a closer look at the nature of this pulsar and its characteristics. behavior.

Observations revealed that J1023 shows, like other tMSPs, flux variability on short time scales (tens of seconds) in all bands. In addition, the study highlighted significant variability in the properties of the emission lines (equivalent width and half maximum width) over a period of a few minutes. This discovery marks the first time that variability in the spectral line properties of a tMSP has been observed on such short time scales.

According to the study, the episodes of variability observed in the continuum, equivalent width and half-full width, appear erratic and uncorrelated with each other. The origin of these episodes therefore remains unclear.

The observations also revealed that the Balmer and helium series emission lines in most cases exhibit a double-horned emission profile. This indicates the presence of an accretion disk, and J1023 was therefore most likely in its disk state at the time of the observation campaign.

In their concluding remarks, the authors of the article add that simultaneous observations at several wavelengths should be carried out in order to evaluate a possible correlation between the variability of the properties of the emission line and the mode switching phenomenon. in J1023.

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