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.
Related
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BibTeX
@misc{ballocco2026sides,
title = {{Two sides of the same spin: tracking optical and radio pulsations of SAX J1808.4-3658 from outburst to quiescence}},
author = {C. Ballocco and A. Miraval Zanon and A. Papitto and M. Burgay and M. C. Baglio and C. Malacaria and F. Ambrosino and A. Possenti and G. Illiano and E. D. Barr and R. P. Breton and L. Burderi and S. Campana and C. J. Clark and F. Coti Zelati and D. de Martino and T. Di Salvo and M. Kramer and R. La Placa and F. Lewis and A. Marino and D. M. Russell and A. Sanna and L. Stella and B. Stappers and J. D. Turner and M. Cecconi and A. Ghedina},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.18322},
eprint = {2610.18322},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.18322},
}
RIS
TY - UNPB
TI - Two sides of the same spin: tracking optical and radio pulsations of SAX J1808.4-3658 from outburst to quiescence
AU - C. Ballocco
AU - A. Miraval Zanon
AU - A. Papitto
AU - M. Burgay
AU - M. C. Baglio
AU - C. Malacaria
AU - F. Ambrosino
AU - A. Possenti
AU - G. Illiano
AU - E. D. Barr
AU - R. P. Breton
AU - L. Burderi
AU - S. Campana
AU - C. J. Clark
AU - F. Coti Zelati
AU - D. de Martino
AU - T. Di Salvo
AU - M. Kramer
AU - R. La Placa
AU - F. Lewis
AU - A. Marino
AU - D. M. Russell
AU - A. Sanna
AU - L. Stella
AU - B. Stappers
AU - J. D. Turner
AU - M. Cecconi
AU - A. Ghedina
PY - 2026
DA - 2026/10/13
PB - arXiv
DO - 10.48550/arxiv.2610.18322
UR - https://arxiv.org/abs/2610.18322
AB - 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.
ER -