2025ApJ...983..116Y

Among several dozen known repeating fast radio bursts, those precisely localized offer the best opportunities to explore their multiwavelength counterparts, which are key to uncovering their origins. Here we report our X-ray, ultraviolet (UV), and optical observations with the Swift satellite of the repeating FRB 20190520B, in coordination with simultaneous radio observations with the Five-hundred-meter Aperture Spherical radio Telescope (FAST). Our aim was to detect potentially associated multiwavelength bursts and identify any persistent multiwavelength counterparts to the associated persistent radio source (PRS). While a total of 10 radio bursts were detected by FAST during the Swift observations, we detected no X-ray, UV, or optical bursts in accompany with the radio bursts. We obtained the energy upper limits (3σ) on any multwavelength bursts as follows: 5.03 × 10⁴⁷ erg in the hard X-ray band (15-150 keV), 7.98 × 10⁴⁵ erg in the soft X-ray band (0.3-10 keV), and 4.51 × 10⁴⁴ erg in the U band (3465 Å), respectively. The energy ratio between soft X-ray (0.3-10 keV) and radio emission of the bursts is constrained as <6 × 10⁷, and the ratio between optical (U band) and radio as <1.19 × 10⁶. We detect no multiwavelength counterpart to the PRS. The 3σ luminosity upper limits are 1.04 × 10⁴⁷ (15-150 keV), 8.81 × 10⁴² (0.3-10 keV), 9.26 × 10⁴² (UVW1), and 2.54 × 10⁴² erg s^–1 (U), respectively. We show that the PRS is much more radio-loud than representative pulsar wind nebulae, supernova remnants, extended jets of Galactic X-ray binaries, and ultraluminous X-ray sources, suggestive of boosted radio emission of the PRS.