On the origin and nature of double-double radio galaxies

TL;DR

This paper proposes a model explaining the origin and characteristics of double-double radio galaxies (DDRGs) by linking black hole spin dynamics, accretion processes, and jet morphology, revealing correlations between jet activity cycles and black hole spin states.

Contribution

It introduces a novel explanation for DDRG features based on black hole spin transitions and continuous accretion, connecting jet quiescence and reactivation to spin-up times and accretion rates.

Findings

Correlation between jet quiescence and reactivation timescales.

FRII morphology results from zero black hole spin transition.

Longer quiescent times associate with longer inner jet durations.

Abstract

Double-double radio galaxies (DDRGs) display inner and outer jets or lobes thought to result from intermittent accretion. Due to randomly triggered accretion events, the lifetime of the retriggered jet is not expected to have any connection to the time of quiescence between jets, yet we show that a correlation between the two quantities may exist, which we interpret as resulting from continued accretion through the quiescent jet phase. Despite continuous accretion, a jet is absent because its presence depends on a non-zero value of black hole spin, but accretion transitions the system from counter-rotation to corotation, and therefore through zero black hole spin where a jet cannot form. The time of jet quiescence depends on how long it takes to spin the black hole up again in corotation, which is longer for lower accretion rates. Once the black hole spin is large enough for a renewed jet, this inner jet will last longer the lower the accretion rate is. Hence, in a continuous accretion scenario, longer quiescent times tend to associate to longer inner jet times. In addition, DDRG jets are of FRII morphology which we show to result from the absence of a tilt in the accretion disk in the transition through zero black hole spin, ensuring the absence of an FRI jet in a way that connects with our understanding of X-shaped radio galaxies. Both correlated timescales as well as sameness in jet morphology offers evidence in favor of our picture.