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Circumstantial evidence suggests that magnetism and enhanced X-ray emissionare likely correlated in early B-type stars: similar fractions of them ($\sim$10 %) are strong and hard X-ray sources and possess strong magnetic fields. Itis also known that some B-type stars have spots on their surface. Yet up to nowno X-ray activity associated with spots on early-type stars was detected. Inthis Letter we report the detection of a magnetic field on the B2V star $\rho$Oph A. Previously, we assessed that the X-ray activity of this star isassociated with a surface spot, herewith we establish its magnetic origin. Weanalyzed FORS2 ESO VLT spectra of $\rho$ Oph A taken at two epochs and detecteda longitudinal component of the magnetic field of order of $\sim500$ G in oneof the datasets. The detection of the magnetic field only at one epoch can beexplained by stellar rotation which is also invoked to explain observedperiodic X-ray activity. From archival HARPS ESO VLT high resolution spectra wederived the fundamental stellar parameters of $\rho$ Oph A and furtherconstrained its age. We conclude that $\rho$ Oph A provides strong evidence forthe presence of active X-ray emitting regions on young magnetized early typestars.

Detection of magnetic field in the B2 star $\rho$ Oph A with ESO FORS2