Scientists present the first-ever detection of pulsations at optical and ultraviolet wavelengths from a millisecond pulsar in an X-ray binary system during an accretion phase. ICE's Francesco Coti Zelati and Diego F. Torres have participated in the discovery, led by researchers from the Italian National Institute of Astrophysics and based on observations made with the Galileo National Telescope in La Palma and with the Hubble Space Telescope.
The system is called SAX J1808.4-3658. It is formed by a neutron star (a rapidly gyrating, dense object) and a small star. The neutron star rotates very rapidly, causing the emission to appear pulsating, like the light of a lighthouse. In fact, the neutron star rotates faster than most pulsars.
The pulsar is in a binary system, that is, it orbits alongside another star from which it regularly removes matter. Moreover, it is an unstable celestial object, since it alternates phases of "quiescence" with periods of "activity" every 3 or 4 years. The most recent explosion, the ninth since its discovery in 1996, was recorded between August and September 2019. ICE researchers assert that, at the time of the observations at optical and ultraviolet wavelengths during this last explosion, the pulsar was surrounded by an accretion disc, displayed pulsations in the X-rays and had a high brightness, suggesting that mass accretion onto the neutron star was ongoing.
To date, about twenty systems similar to SAX J1808.4-3658 are known. Until this observation, no pulses in the UV band had been observed from pulsars in binary systems. As per the optical band, the pulses had only been seen in 5 isolated pulsars and in a single binary system.
The discovery has been published in the journal Nature Astronomy under the title “Optical and ultraviolet pulsed emission from an accreting millisecond pulsar”, and tests the theoretical models that describe the behavior of pulsars in binary systems. According to Coti Zelati and Torres, current accretion models fail to account for the luminosity of both the optical and ultraviolet pulsations that they detected, which are instead more likely driven by processes taking place in the magnetosphere of the neutron star or just outside of it.
In this context, this discovery demonstrates that acceleration of charged particles up to extremely high speeds can take place in the magnetosphere of a neutron star even when the latter is engulfed with accreting matter. Therefore, the results of the study shed new light on the properties of the magnetosphere and its interaction with accreting matter and, more in general, on the physics of millisecond pulsars in binary systems.
This study provides a novel approach to investigate accreting neutron stars in binary systems: it opens up a new perspective in searches for fast pulsations at optical and ultraviolet wavelengths from many other weakly-magnetic, accreting neutron stars in binary systems from which pulsations have never been detected at other wavelengths, despite very extensive studies. In fact, thanks to the very large photon rates and the possibility to exploit the throughput of large optical telescopes, it will be possible to attain a much higher sensitivity at optical and UV wavelengths than in the X-ray band. In this sense, neutron stars accreting at very high rates are especially important, since the detection of pulsations from them and the precise determination of their orbit would permit to increase drastically the sensitivity of searches for gravitational waves, which are expected from these systems. This would turn these neutron stars into unrivalled laboratories to study the physics of matter at supra-nuclear density and in the presence of ultra-strong magnetic fields.
The detection of optical pulsations was achieved in observations with the Silicon Fast Astronomical Photometer (SiFAP2) mounted at the Galileo National Telescope (TNG) at the Roque de los Muchachos Observatory on the island of La Palma (Canary Islands). This detection was possible thanks to the unique capabilities of this instrument, which is able to tag the time of arrival of individual photons at optical wavelengths with an accuracy of a few microseconds up to count rates as large as a few million counts every second.
