During the past week NuSTAR observed the "Rosetta Stone" of accreting millisecond X-ray pulsars (AMXP)—SAX J1808.4-3658. This observation was crucial in understanding the spectral timing properties during the decay phase of its 2025 outburst, one of the longest, if not the longest ever observed. SAX J1808 is also the only known AMXP to show coherent optical and X-ray pulsations and so the NuSTAR observation was coordinated with optical fast photometry using the LightSpeed instrument on the Magellan telescope. Analysis of these coordinated observations will be very important to understand the physics behind these coherent optical pulsations. NuSTAR will aid in comparing the X-ray pulse period, pulse shape, and any phase lag with the same in the optical domain. Moreover, this observation, together with a previous NuSTAR Director's Discretionary Time observation, has provided high-energy X-ray coverage to an ongoing multiwavelength campaign on SAX J1808 including lower energy X-ray observations by NASA’s Gehrels-Swift and ESA’s XMM-Newton telescopes, optical observations using the 6.5 meter Megellan telescopes at the Las Campanas Observatory in Chile, and the Giant Metrewave Radio Telescope (GMRT) near Pune, India.

Authors: Sayantan Bhattacharya (Research Associate, Tata Institute of Fundamental Research, Mumbai)