Striking New Evidence for the Action of a Cosmic Battery in AGN

1. Force on electrons Striking New Evidence for the Action of a Cosmic Battery in AGN Denise Gabuzda (University College Cork) Dimitris Christodoulou (University of Massachusetts Lowell) Ioannis Contopoulos (Academy of Athens) Demosthenes Kazanas (NASA/Goddard) (3) Parsec scales Transverse FR gradients are observed across a number of AGN jets on parsec (VLBI) scales. Two examples (black star is centre of activity, arrow shows direction of gradient): Meier, Koide & Uchida 2001 (1) Faraday rotation (FR) and helical/toroidal fields Rotation of central supermassive black hole and accretion disc + jet outflow gives rise to helical jet B field. Yields FR gradient across the jetdue to systematically changing line-of-sight B: CCW CW  Reichstein & Gabuzda 2010 LOS B away from observer (RM < 0) B Contopoulos et al. (2009)found an excess of CW gradients. For FR gradients detected relatively close to VLBI core: 21 CW, 7 CCW Prob of 21 or more being CW by chance: ~ 0.6% Jet axis LOS B towards observer (RM > 0) (4) Kiloparsec scales Examination of 85 published FR maps by Christodoulou et al. (2011) yielded firm transverse RM gradients in 7 sources, tentative gradients in 4 more. 9 of these 11 have CCW gradients! Two examples: • • (2) The “Poynting—Robertson Battery” (Contopoulos et al. 2009 & refs therein) •  Radiation absorbed by charges in rotating accretion disc is re-radiated isotropically in rest frame, “beamed” in direction of motion (rotation) in observer’s frame • · Charges thus feel a reaction force: • · Deceleration of e– >> deceleration of p due to dependence on mass (Thomson cross section ~ m-2) • Electric current in direction of rotation • Couples direction of rotation and poloidal B field wound up by differential rotation • Azimuthal and poloidal B fields ae coupled Eilek & Owen 2002 CCW CCW Bonafede et al. 2010 • With 4 firm decaparsec-scale transverse FR gradients (right), have 13/15 CCW: • Prob of 13 or more of 15 being CCW by chance: ~ 0.4% • Predominance of CW/CCW gradients on pc/kpc scales! Croke et al. 2010 Integration path passes through both regions of helical field Direction of FR gradient depends on which region dominates FR integral If field emerges in inner disc and closes in outer disc, gives “nested helix” structure CCW • (5) Conclusion: • Excess of CW/CCW transverse FR gradients across AGN jets on smaller/larger scales, requires coupling between poloidal and azimuthal jet B fields; mechanism must also explain reversal from CW to CCW. • The Poynting—Robertson battery can provide this! • Currents must be involved — any other ideas?? FR gradient for inner/ outer helical fields are always CW/CCW on sky B fields RM gradients When rotation is reversed, direction of “seed” poloidal B field is too — azimuthal B field remains unchanged Asada et al. 2002, PASJ, 54, L39; Bonafede et al. 2010, A&A, 513, 30; Christodoulou et al. 2011, ApJ, submitted; Contopoulos et al. 2009, ApJ, 702, L148; Croke et al. 2010, MNRAS, 402, 259; Eilek & Owen 2002, ApJ, 567, 202; Mahmud 2009, PhD Thesis, Physics, UCC; Meier et al. 2001, Science, 291, 84; Reichstein & Gabuzda 2011, in prep Contopoulos et al. 2009 Mahmud 2010 Mahmud 2010