Figure 2
Accretion in Manko-Novikov spacetimes with
and
,
,
, and
. The solid black line at small radii is the partial horizon
, the region where closed timelike curves exist (i.e. the region where
) is shown in green/light gray, the red dashed line is the boundary of the ergoregion (i.e.
on that line), while the red dot on the equatorial plane marks the position of the ISCO, i.e. the inner edge of the thin accretion disk. The thick disk, if it forms, sheds from the ISCO and is denoted by concentric contours, whose label is an upper limit to the
of the gas temperature in
(see text for details). In blue/dark grey is the region accessible to the gas shedding from the inner edges of the thick disk [i.e. the region where
,
, and
being the energy and angular momentum of the gas at the inner edges of the thick disk]. If no thick disk is present, in blue/dark grey is the region accessible to the gas shedding from the inner edge of the thin disk [i.e. the region where
]: if this plunge region does not reach the ISCO, we also show (with solid black lines around the ISCO) the contours
and
, which represent the region where the gas reaching the ISCO can shed if subject to a small perturbation. For
the ISCO is radially unstable, and the gas plunges directly into the compact object remaining roughly on the equatorial plane, as in the Kerr case [scenario (1a)]. For
the ISCO is radially unstable and the gas plunges, but does not reach the compact object; instead, it gets trapped between the object and the ISCO and forms a thick disk [scenario (1b)]. For
the ISCO is vertically unstable, and the gas plunges directly into the compact object outside the equatorial plane [scenario (2a)]. For
the ISCO is vertically unstable, and the gas does not plunge directly but remains trapped in the vicinities of the ISCO [because
near the ISCO]. However, the potential barrier is not tall enough to allow a thick disk to form, and a small perturbation is enough to cause the gas to fall into the central object [scenario (2b)].
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