Multi-band analysis of the pulsed profiles of the accreting millisecond pulsar SAX J1808.4-3658

With spin periods shorter than 10 ms, millisecond pulsars are the fastest-spinning pulsars known. Often found in binary systems, they are ideal probes for investigating the complex interplay between mass accretion and magnetic field rotation. Millisecond pulsars achieve their very high spin frequencies through a Gyr-long evolutionary phase involving the accretion of matter and angular momentum from a low-mass companion star, a process known as ‘recycling’. This phase ends when mass transfer declines, transforming the pulsar into a radio/gamma-ray millisecond pulsar powered by the rotation of its magnetic field. The discovery of the accreting millisecond pulsar SAX J1808.4-3658 confirmed the recycling scenario, showing that mass accretion in a low-mass X-ray binary can spin up a neutron star to millisecond periods.

SAX J1808.4-3658 was also the first of this class to show optical/UV pulsations during a bright accretion event. Challenging conventional rotation or accretion-powered scenarios, these pulsations offer key insights into the different emission mechanisms of pulsating neutron stars. Before returning to quiescence at the end of its accretion outbursts, this source shows several reflares of particular interest to give insights into the physics of the accretion disk at low accretion rates. I will present the first simultaneous X-ray/UV timing and spectral study of XMM-Newton and Hubble Space Telescope observations of the source during the final stage of its 2022 outburst, focusing on the flaring phase. I will also discuss optical observations performed with the fast optical photometer SiFAP2, mounted at the Telescopio Nazionale Galileo, aimed at investigating the nature of the pulsations at these energies.

Caterina Ballocco is a Ph.D candidate at Sapienza Università di Roma in collaboration with Osservatorio Astronomico di Roma (INAF)