Observation of an ultraviolet and optical signal that challenges pulsar models
The study is led by researchers from the Italian National Institute of Astrophysics (INAF) and based on observations made with the Galileo National Telescope in La Palma and with the Hubble Space Telescope.
Researchers from the Institute for Space Studies of Catalonia (IEEC) at the Institute of Space Sciences (ICE, CSIC) have participated in the discovery.
An study led by the Italian National Institute of Astrophysics (INAF), and in which the researchers from the Institute of Space Studies of Catalonia (IEEC — Institut d’Estudis Espacials de Catalunya) at the Institute of Space Sciences (ICE, CSIC) Francesco Coti Zelati and Diego F. Torres participated, has presented the first-ever detection of pulsations at optical and ultraviolet (UV) wavelengths from a millisecond pulsar in an X-ray binary system during an accretion phase.
The system — called SAX J1808.4-3658 — is formed by a neutron star and a small star. The neutron star, a dense object, 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 object, since it alternates phases of quiescence with periods of activity every three or four years. The most recent explosion, the ninth since its discovery in 1996, was recorded between August and September 2019. The researchers Coti Zelati and Torres assert that, at the time of the observations at optical and UV 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 five isolated pulsars and in a single binary system.
The discovery tests the theoretical models that describe the behavior of pulsars in binary systems: according to the IEEC-ICE researchers who have participated in the finding, current accretion models fail to account for the luminosity of both the optical and UV 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 UV 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 drastically increase 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 La Palma island (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.
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More information
This research is presented in a paper entitled “Optical and ultraviolet pulsed emission from an accreting millisecond pulsar”, by F. Ambrosino, A. Miraval Zanon, A. Papitto, F. Coti Zelati, S. Campana, P. D'Avanzo, L. Stella, T. Di Salvo, L. Burderi, P. Casella, A. Sanna, D. de Martino, M. Cadelano, A. Ghedina, F. Leone, F. Meddi, P. Cretaro, MC Baglio, E. Poretti, RP Mignani, DF Torres, et al. to appear in the journal Nature Astronomy on 22 February 2021.
The Institute of Space Studies of Catalonia (IEEC — Institut d’Estudis Espacials de Catalunya) promotes and coordinates space research and technology development in Catalonia for the benefit of society. IEEC fosters collaborations both locally and worldwide and is an efficient agent of knowledge, innovation and technology transfer. As a result of over 20 years of high-quality research, done in collaboration with major international organisations, IEEC ranks among the best international research centers, focusing on areas such as: astrophysics, cosmology, planetary science, and Earth Observation. IEEC’s engineering division develops instrumentation for ground- and space-based projects, and has extensive experience in working with private or public organisations from the aerospace and other innovation sectors.
IEEC is a private non-profit foundation, governed by a Board of Trustees composed of Generalitat de Catalunya and four other institutions that each have a research unit, which together constitute the core of IEEC R&D activity: the University of Barcelona (UB) with the research unit ICCUB — Institute of Cosmos Sciences; the Autonomous University of Barcelona (UAB) with the research unit CERES — Center of Space Studies and Research; the Polytechnic University of Catalonia (UPC) with the research unit CTE — Research Group in Space Sciences and Technologies; the Spanish Research Council (CSIC) with the research unit ICE — Institute of Space Sciences. IEEC is a CERCA (Centres de Recerca de Catalunya) center.
Image
Caption: An x-ray bright pulsar accreting matter from a companion star (archival artistic image)
Credits: ESA
Contacts
IEEC Communication Office
Barcelona, Spain
Ana Montaner and Rosa Rodríguez
E-mail: comunicacio@ieec.cat
Lead Researchers at the Institute of Space Sciences (ICE, CSIC)
Barcelona, Spain
Francesco Coti Zelati
E-mail: cotizelati@ice.csic.es
Diego F. Torres
E-mail:dtorres@ice.csic.es