An article published in “The Astrophysical Journal” describes a study of a binary system consisting of a millisecond pulsar known as PSR J1723-2837 and a small common star. Astrophysicist John Antoniadis of the University of Toronto and the amateur astronomer André van Staden discovered for the first time starspots and a powerful magnetic field in a millisecond pulsar’s companion. It’s a research that can help better understand this type of pulsar’s behavior.
In such a binary system, the pulsar’s gravity deforms its companion star, which takes on a teardrop shape. This companion’s brightness increases and decreases cyclically because of the presence of the pulsar and is in step with its orbital period. This doesn’t happen in the system of the pulsar PSR J1723-2837, which rotates 540 times per second, and its companion, distant about 2,500 light years from Earth.
The discovery came examining images taken by André van Staden during 15 months. He’s an an amateur astronomer who makes his observations in his home’s courtyard in South Africa using a 30 cm telescope with a CCD camera. The examin was carried out in collaboration with astrophysicist John Antoniadis with the detection of this binary system peculiarities.
The common star’s brightness peaks are not synchronized with the orbital period of 14.8 hours but are increasingly later relative to its orbital position. According to the two researchers, this anomaly is caused by starspots, the equivalent of sunspots on the Sun, on the companion star, that lower its brightness. These starspots have a size relative to the star greater than that of sunspots relative to the Sun.
The two researchers also realized that the companion star is isn’t synchronous rotation with the pulsar so it doesn’t always show it the same face as the Moon does to the Earth. The star has a rotation period of a bit shorter than its orbital period and the result is the unexpected light curve. The presence of a powerful magnetic field is a prerequisite for the existence of starspots so it can be deduced that this star has one as well.
This type of studies based on observations made over a long period can help to understand the evolution of this type of binary systems. They’re called “black widow” when the companion star has a mass that is a few hundredths of that of the Sun or “redback” when the companion star has a mass which is a few tenths of that of the Sun. The name inspired by spiders is due to the fact that pulsars devour their companions slowly stealing their gas.
According to current models, when a pulsar forms it starts emitting a strong wind that heats the face of its partner that is struck by it. In the observations made by André van Staden that doesn’s appear and the star’s magnetic field could be responsible for that but it may have effects on the pulsar too influencing its shutdown because it prevents it from stealing gas from the star.
Cases such as PSR J1723-2837 and its companion are quite rare, although in recent years astronomers have been finding more and more of them. Advances in research have been made often thanks to space telescopes such as Fermi but this time help arrived from an amateur astronomer who used his own instruments and passion.
