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)

Contopoulos et al. 2009

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 jet due to systematically changing line-of-sight B:

BLOS B away from observer

(RM < 0)

LOS B towards observer

(RM > 0)

•

Jet axis

CCW

(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

Force on electrons

If field emerges in inner disc and closes in outer disc, gives “nested helix” structure

B fields RM gradients

(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??

Integration path passes through both regions of helical field

Direction of FR gradient depends on which region dominates FR integral

FR gradient for inner/ outer helical fields are always CW/CCW on sky

Reichstein & Gabuzda 2010

CW

(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):

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%

(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:

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!

CCW

CCW

CCW

Bonafede et al. 2010

Eilek & Owen 2002

Croke et al. 2010

When rotation is reversed, direction of “seed” poloidal B field is too — azimuthal B field remains unchanged

Mahmud 2010

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 Mahmud 2010