Two sides of the same spin: tracking optical and radio pulsations of SAX J1808.4-3658 from outburst to quiescence

Abstract

High-time-resolution optical and UV observations have revealed bright coherent pulsations from accreting millisecond pulsars, exceeding the luminosity expected from thermal emission from NS hotspots or X-ray reflection by the accretion disk. Characterizing optical pulsations over a wide range of accretion rates through coordinated multi-wavelength observations can shed light on their origin. We present a timing study of the AMSP SAX J1808.4-3658 during its 2022 outburst and quiescence, using optical TNG/SiFAP2 observations, simultaneous NICER X-ray data, LCO optical monitoring, and MeerKAT radio observations in quiescence. During the outburst, we detect coherent optical pulsations with pulsed luminosities of 1.3x10^(30)erg/s. Combined with previous detections, our results suggest that the optical pulsed amplitude increases as the X-ray flux decreases. The X-ray and optical pulsed luminosities show opposite trends with X-ray luminosity, suggesting different responses to changes in the accretion rate. We measure X-ray-to-optical delays of 0.88+-0.04ms and 0.51+-0.03 ms in two outburst observations, indicating that the relative timing between the signals is not constant. Together with the spectral energy distribution, these results support different emission mechanisms: non-thermal emission from relativistic particles for optical pulsations and channeled accretion onto the NS surface for X-ray pulsations. During quiescence, we detect no coherent optical pulsations, obtaining a 95% confidence upper limit of <2.2x10^(29)erg/s on the optical pulsed luminosity, corresponding to an optical efficiency comparable to those of spider radio millisecond pulsars. Simultaneous MeerKAT observations reveal no radio pulsations down to ~10μJy, significantly improving previous constraints but not excluding an active rotation-powered pulsar whose radio emission may be absorbed by intra-binary material.