Description
SAX J1808.4-3658 is the prototype of accreting millisecond X-ray pulsars and the first of this class
to show optical and UV pulsations during an accretion outburst. These pulsations challenge standard
rotation- or accretion-powered scenarios and provide a unique window into the emission
mechanisms of pulsating neutron stars. I will present a multi-band timing study based on XMMNewton,
HST/STIS, and TNG/SiFAP2 observations obtained during the 2022 outburst. Optical and
UV pulsations observed in 2022 are compared with those first detected in 2019, allowing us to
study their evolution across different outbursts. HST observations confirmed the presence of significant
UV pulsations at an X-ray luminosity approximately a factor of two lower than during
the 2019 outburst, extending the range of mass accretion rates at which UV pulsations have been
detected. Analyzing optical observations performed with the fast optical photometer SiFAP2, we
find an anti-correlation between the X-ray flux and the amplitude of the optical pulsations, with
the latter increasing as the X-ray flux decreases. I will also discuss optical observations obtained
during the quiescent state of the source, probing the pulse emission mechanism over a large range
of mass accretion rates.