Helical magnetic fields in the M87 jet at arc-second scales

Abstract: We investigate the magnetic field configuration of the M87 jet at arc-second scales by using archival polarimetric VLA data at 8, 15, 22 and 43 GHz. By stacking images over three years in order to enhance the sensitivity, we reveal, for the first time, systematic transverse gradients of the Faraday rotation measure in several knots along the jet. Combining this result with polarization properties and the dynamics of the jet, we suggest the magnetic structure in several knots at kiloparsec scales consists of a systematically wrapped, tightly wound helical configuration. Our analysis brings us a new paradigm where the M87 jet is a fundamentally current carrying system produced in the vicinity of the supermassive black hole, transferring a huge amount of the electromagnetic energy over the host galaxy scale.