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Optical and ultraviolet pulsed emission from an accreting millisecond pulsar

Another important result of SiFAP2 that defies the known physical models of accreting pulsars has been published in Nature Astronomy.

SiFAP2, the Silicon Fast Astronomical Photometer and Polarimeter mounted at the TNG, has been used with STIS on the HST to detect the first-ever coherent pulsations at optical and UV wavelengths, respectively. The target was SAX J1808.4-3658, a millisecond pulsar in a X-ray binary system monitored during an accretion phase.

In 1996, SAX J1808.4-3658 has been the first X-ray binary system discovered with an accreting millisecond pulsar, that is a neutron star spinning hundreds of times per second. Every 3-4 years it experiences outburst phases (9 since its discovery) during which its luminosity increases of several orders of magnitude and the system enters an accreting phase. Thanks to the mass and angular momentum transferred from the accretion disc, the pulsar is accelerated and rotates with a frequency of ~401 Hz (2.5ms period) generating pulsations in the X band. We now know 20 other binary systems like SAX J1808.

A team composed of researchers from Italian and international Institutes, including our TNG staff observed SAX J1808 from TNG. The target was in outburst and visible only few hours, being at a declination -37deg it is always very low on the horizon. Using SiFAP2 astronomers were able to detect the pulsations of the ms-pulsar in the visible band. Towards the end of the burst they also detected the pulsations at UV wavelengths using the STIS/MAMA photometer mounted on the Hubble Space Telescope (Figure 1).

Current accretion models fail to account for the luminosity in the visible and ultraviolet pulsed emissions, which are supposed to be driven from processes in the magnetosphere of the neutron star. It was widely believed that the charge density of accreting matter would shut off the acceleration of particles from the magnetosphere. "Our study shows that the interaction of accreting matter with the magnetosphere allows for X-ray, visible and UV pulsed emissions and that the rotation-powered and accretion-powered mechanisms are not mutually exclusive" says Arianna Miraval Zanon, PhD student at the University of Insubria (Como, Italy) and INAF associate.

"What is also worth noticing is that the visible pulsations are nearly half period off-phase with the X pulsations, as if they were produced in opposite poles of the neutron star. Our discovery provides a novel observational window to study accreting neutron star in X-ray binaries and a powerful tool to probe the physics of millisecond pulsars in close binary systems." says Dr. Filippo Ambrosino, INAF researcher at Roma Observatory.

The TNG is at the forefront of time domain astronomy thanks to the unique capabilities of SiFAP2, a silicon based photometer and polarimeter able to time tag single photons with a time resolution of 8 ns and having an absolute accuracy better than 60 µs . This instrument has been developed by the University of La Sapienza, Rome and improved by a team lead by the FGG staff (including Univ.of Catania and Observatory of Roma) in order to be mounted at the TNG with increased versatility and polarimetric capabilities (Figure 2). With SiFAP2 it will be possible to study the most rapid and violents events of the Universe, from FRB to GRB and hopefully also the EM counterparts of GW events.

SiFAP2 has been offered for the next semester as a PI instrument and it is the instrument which has received the highest number of proposals at the TNG (excluding the granted time observations).

This study has been published on Nature Astronomy with the title of Optical and ultraviolet pulsed emission from an accreting millisecond pulsar 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, M. C. Baglio, E. Poretti, R. P. Mignani, D. F. Torres, G. L. Israel, M. Cecconi, D. M. Russell, M. D. Gonzalez Gomez, A. L. Riverol Rodriguez, H. Perez Ventura, M. Hernandez Diaz, J. J. San Juan, D. M. Bramich, F. Lewis.

Figure 1. Detection and shape of coherent optical and ultraviolet signals from SAX J1808.4-3658. Upper panel: Fourier power density spectrum of the optical (320-900 nm) light curve from the 3.3 ks observation carried out with the SiFAP2 photometer mounted at the TNG, starting on August 7, 2019 at 22:31 (Coordinated Universal Time, UTC). Lower panel: Fourier power density spectrum over the same frequency range from the ultraviolet (165-310 nm) light curve collected with STIS on board HST during a 2.2 ks observation starting on August 28, 2019 at 21:47 UTC. The dotted red vertical line marks the spin frequency of SAX J1808.4-3658 from the X-ray ephemeris, whose uncertainty on the spin frequency is small enough that only a single trial frequency has to be examined. The insets show the background-subtracted, normalised pulse profiles obtained by folding the optical and UV light curves at the X-ray period, two cycles are plotted for clarity.

Figure 2. The 2 co-first-authors of the paper during their observations with SiFAP2 at the TNG during summer 2019. Left to Right Massimo Cecconi (TNG staff), Arianna Miraval-Zanon (PhD Student), Filippo Ambrosino (INAF-Roma) and Adriano Ghedina (TNG staff). Backwards is the telescope operator Gianni Mainella.

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DOI number: 10.1038/s41550-021-01308-0