Two sides of the same spin: Tracking optical and radio pulsations of SAX J1808.4-3658 from outburst to quiescence
Year: 2026
Type: article
Abstract: High-time resolution optical and UV observations have recently revealed unexpectedly bright coherent pulsations from accreting millisecond X-ray pulsars. Their luminosity exceeds that expected from thermal emission from neutron-star hotspots or X-ray reflection by the accretion disk. Characterizing optical pulsations over a wide range of accretion rates through coordinated multiwavelength observations is expected to shed light on their origin. We present a timing study of the accreting millisecond pulsar SAX J1808.4-3658 during its 2022 outburst and quiescent state, using high-time-resolution optical observations with TNG/SiFAP2, simultaneous NICER X-ray data, optical monitoring from LCO, and MeerKAT radio observations in quiescence. During the outburst, we detect coherent optical pulsations, with pulsed luminosities of sim1.3 , . 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 appear to show opposite trends with X-ray luminosity, suggesting different responses of the two pulsed components to changes in the accretion rate. We also measure X-ray-to-optical delays of 0.88 ± 0.04, and 0.51 ± 0.03, on two different outburst observations, indicating that the relative timing between the two signals is not constant. Together with the spectral energy distribution, these results support different emission mechanisms for the pulsed radiation in the two bands, with nonthermal emission from relativistic particles producing optical pulsations and channeled accretion onto the neutron-star surface powering X-ray pulsations. During quiescence, we detect no coherent optical pulsations, obtaining a 95% confidence upper limit of L^ 30 -1 ms ms V, UL _ pulsed <2.2 , 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 10^ 29 -1 significantly improving previous constraints, but not excluding the presence of an active rotation-powered pulsar whose radio emission may be absorbed by intra-binary material.
Source: Astronomy and Astrophysics
Authors: C. Ballocco, A. Miraval Zanon, A. Papitto, M. Burgay, M. C. Baglio, C. Malacaria, F. Ambrosino, A. Possenti, G. Illiano, E. D. Barr, R. P. Breton, L. Burderi, S. Campana, C. J. Clark, F. Coti Zelati, D. de Martino, T. Di Salvo, M. Kramer, R. La Placa, F. Lewis, A. Marino, D. M. Russell, A. Sanna, L. Stella, B. Stappers, J. D. Turner, M. Cecconi, A. Ghedina
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $1,865
Awards: G4368239921, G6852145469, G7145178070
OpenAlex ID: W7221825732