Observations explore the nature of transitional millisecond pulsar PSR J1023+0038

28 Sep 2024, 12:50 by Tomasz Nowakowski

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Average spectrum of J1023 normalized to the emission of the continuum. The emission lines studied in this work are highlighted. Credit: Messa et al., 2024.

Using the Gran Telescopio Canarias (GTC), astronomers from Italy and Spain have carried out high-temporal-resolution optical spectroscopic observations of a transitional millisecond pulsar designated PSR J1023+0038. Results of the observational campaign, published September 19 on the pre-print server arXiv, yield essential information regarding the nature of this pulsar.

Pulsars are highly magnetized, rotating neutron stars emitting a beam of electromagnetic radiation. The most rapidly rotating pulsars, with rotation periods below 30 milliseconds, are known as millisecond pulsars (MSPs). Astronomers assume that they are formed in binary systems when the initially more massive component turns into a neutron star that is then spun up due to accretion of matter from the secondary star.

The so-called transitional millisecond pulsars (tMSPs) exhibit transitions back and forth between a rotation-powered radio pulsar state ('pulsar state') and a state characterized by X-ray pulsations and accretion disk features in the optical spectra ('disk state'). They are generally rare, as so far, only three tMSPs have been detected.

One of them is PSR J1023+0038 (or J1023 for short), first identified two decades ago. The pulsar has a spin period of 1.69 milliseconds, while its orbital period is approximately 4.75 hours. The companion star in the system is a late-type star of spectral type G5.

Previous observations of J1023 have found that it switches between an X-ray state and a radio pulsar phase powered by rotation. Now, a team of astronomers led by Marco Messa of the University of Milan in Italy, has employed GTC's Optical System for Imaging and low-Intermediate-Resolution Integrated Spectroscopy (OSIRIS) instrument to take a closer look at the nature of this pulsar and its behavior.

The observations found that J1023 shows, like other tMSPs, flux variability on short timescales (tens of seconds) in all bands. Moreover, the study found evidence for a significant variability in the emission line properties (equivalent width and full width half maximum) over a timescale of minutes. This finding marks the first time when variability in the spectral line properties of a tMSP has been observed over such short timescales.

According to the study, the episodes of variability observed in the continuum, equivalent width and full width half maximum, seem to be erratic and not correlated with each other. This makes the origin of such episodes unclear.

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

In concluding remarks, the authors of the paper add that multi-wavelength simultaneous observations should be conducted in order to assess a possible correlation between the variability in the emission line properties and the mode-switching phenomenon in J1023.

More information: M. M. Messa et al, High-temporal-resolution optical spectroscopic observations of the transitional millisecond pulsar PSR J1023+0038, arXiv (2024). DOI: 10.48550/arxiv.2409.12893

Journal information: arXiv