Speaker
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 XMM-Newton, HST/STIS and TNG/SiFAP2 observations obtained during the 2022 outburst. Using XMM-Newton data collected during the final flaring stage of the outburst, we find pulse phase and amplitude shifts, including a phase jump of approximately 0.4 cycles, tightly correlated with the X-ray flux. We interpret such a correlation in terms of hot-spot drift on the neutron star surface, driven by an increase in the inner-disk radius when the mass accretion rate decreases.
Simultaneous HST observations confirmed the presence of significant UV pulsations, with a luminosity exceeding the predictions of standard emission models. Analyzing optical observations, 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.
Finally, I will present optical observations obtained during the quiescent state, probing the pulse emission mechanism over a large range of mass accretion rate.