European astronomers have made X-ray and optical observations of a transient millisecond pulsar known as PSR J1023+0038. The results of the observation campaign, published on November 23 on arXiv.org, give important information on the origins of the pulsations of this source.
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 called millisecond pulsars (MSP). Astronomers speculate that they form in binary systems when the initially most massive component transforms into a neutron star which is then rotated due to accretion of material from the secondary star.
Some millisecond pulsars switch between low-mass X-ray binary (LMXB) states and millisecond radio pulsar (RMSP) states. These sources have been dubbed millisecond transient pulsars (tMSPs). Observations show that tMSPs are generally fast-rotating, weakly magnetized neutron stars that oscillate between the two states within days. Such behavior can be fueled by different physical mechanisms.
Located some 4,000 light-years from Earth, PSR J1023+0038 is just one of three tMSPs detected so far. It has a rotation period of about 1.688 milliseconds and its orbital period is estimated at 0.198 days. PSR J1023+0038 is the first MSP ever observed as an optical pulsar.
PSR J1023+0038 exhibits optical and X-ray pulsations that were first detected simultaneously in the high X-ray modes, but disappeared as the source transited into the low modes, suggesting a mechanism of common issue. To date, this pulsar remains the only pulsed emission tMSP detectable in the optical band.
In order to better understand the emission mechanism behind the optical and X-ray pulses of PSR J1023+0038, a team of astronomers led by Giulia Illiano from the Rome Astronomical Observatory in Italy, decided to study the phase shift between pulses in the optical and X-ray bands, in hopes of better understanding the physical mechanisms behind them.
“We performed a detailed temporal analysis of simultaneous or near-simultaneous observations in the X-ray band, acquired with the XMM-Newton and NICER satellites, and in the optical band, with the fast SiFAP2 photometers (mounted at the Telescopio Nazionale de 3 .6m Galileo) and Aqueye+ (mounted on the 1.8m Copernicus telescope),” the researchers wrote in the paper.
The study found that the optical pulses from the PSR J1023+0038 have total effective pulse amplitudes of approximately 0.1-0.8%, while the total effective pulse amplitudes of X-rays are between 4 .3 and 10.8%. It turned out that the optical pulses delay the X-ray pulses by about 150 microseconds.
The results show that the phase shift between optical and X-ray pulses is within a limited range (0 to 250 microseconds) on time scales of about five years. This discovery suggests that the two pulsations come from the same region and that their emission mechanisms are linked.
Astronomers speculate that the shock-driven pulsar mini-nebula scenario is the most plausible hypothesis that could explain the origin of the optical and X-ray pulses from PSR J1023+0038. This scenario suggests that the pulses are generated by synchrotron radiation emitted by a shock formed where the wind from the striped pulsar meets the accretion disk, about 100 km from the pulsar.
G. Illiano et al, Investigation of the origin of the optical and X-ray pulsations of the transient millisecond pulsar PSR J1023+0038, arXiv (2022). DOI: 10.48550/arxiv.2211.12975
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