Quasi-periodic oscillations of GHz-band polarization in a black hole

Relativistic jets from accreting black holes radiate non-thermal emission which is highly variable in different time scales. Previous studies on black holes of different mass scales, including supermassive and stellar-mass black holes, only report flux quasi-periodic oscillations in radio, optical, X-ray and gamma-ray bands. No quasi-periodic variations in polarization have yet been detected in any black hole systems. Here, we report the first detection of GHz-band radio polarization oscillations in GRS 1915+105 which harbors a fast-spinning stellar-mass black hole with a relativistic jet. Our observations show that during the increasing phase of radio emission, linear polarization and flux exhibit similar oscillation periods of $17$ and $33$ seconds, and their variation patterns anti-correlate with each other. These rare, short-period oscillations in both polarization and flux would be important to understand instabilities and special dynamics in magnetized relativistic jets.