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

Using the Gran Telescopio Canarias (GTC), astronomers from Italy and Spain have performed high temporal resolution optical spectroscopic observations of a millisecond transition pulsar named PSR J1023+0038. Results of the observation campaign, published on September 19 on the pre-print server arXivprovide essential information about 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 of less than 30 milliseconds, are known as millisecond pulsars (MSPs). Astronomers assume that they form in binary systems when the initially more massive component turns into a neutron star that is then spun due to the accretion of matter from the secondary star.

The so-called transitional millisecond pulsars (tMSPs) exhibit transitions back and forth between a rotationally driven 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 only three tMSPs have been detected so far.

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

Previous observations of J1023 have shown that it switches between an X-ray state and a radio pulsar phase, driven by rotation. Now, a team of astronomers led by Marco Messa from the University of Milan in Italy has used GTC’s Optical System for Imaging and OSIRIS (Optical System for Imaging) instrument to determine the nature of this pulsar and its behavior.

The observations revealed that J1023, like other tMSPs, exhibits flux variability on short timescales (tens of seconds) in all bands. Furthermore, the study found evidence for significant variability in the properties of the emission lines (equivalent width and half-width of full width) over a time scale of minutes. This finding 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 full-width half-width, appear to be erratic and not correlated with each other. This makes the origin of such episodes unclear.

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

In their closing remarks, the paper’s authors add that simultaneous observations over multiple wavelengths should be performed to assess a possible correlation between the variability in the properties of the emission lines and the mode-switching phenomenon in J1023.

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