Self-intersection of the Fallback Stream in Tidal Disruption Events. (arXiv:1904.12018v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Lu_W/0/1/0/all/0/1">Wenbin Lu</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bonnerot_C/0/1/0/all/0/1">Clément Bonnerot</a>
We propose a semi-analytical model for the self-intersection of the fallback
stream in tidal disruption events (TDEs). When the blackhole mass exceeds a
critical value M_cr, a large fraction of the shocked gas becomes unbound, in
the form of a collision-induced outflow (CIO). This is because the large
apsidal precession causes the stream to self-intersect near the local escape
speed at radius much below the apocenter. The rest of the fallback gas is left
in more tightly bound orbits and quickly joins the accretion flow. We propose
that the CIO is responsible for reprocessing the hard emission from the
accretion flow into the optical band. This picture naturally explains the large
photospheric radius (or low blackbody temperature) and typical widths of the H
and/or He emission lines seen in optically selected TDEs. We predict the
CIO-reprocessed spectrum in the infrared to be L_nu ~ nu^{~0.5}, shallower
than a blackbody. The partial sky coverage of the CIO also provides a
unification of the diverse X-ray behaviors of optically bright TDEs. According
to this picture, optical surveys filter out those TDEs with blackhole mass less
than M_cr due to lack of a reprocessing layer. This filtering causes the
optical TDE rate to be lower than the total rate by a factor of ~10 or more. We
also predict that the volumetric rate of optically selected TDEs is nearly flat
with respect to the blackhole mass for M < 10^7 solar masses. When the CIO is
decelerated by the ambient medium, radio emission at the level of that in
ASASSN-14li may be produced, but the timescales and peak luminosities can be
highly diverse. Finally, our method paves the way for global simulations of the
disk formation process by injecting gas at the intersection point according to
the prescribed velocity and density profiles.
We propose a semi-analytical model for the self-intersection of the fallback
stream in tidal disruption events (TDEs). When the blackhole mass exceeds a
critical value M_cr, a large fraction of the shocked gas becomes unbound, in
the form of a collision-induced outflow (CIO). This is because the large
apsidal precession causes the stream to self-intersect near the local escape
speed at radius much below the apocenter. The rest of the fallback gas is left
in more tightly bound orbits and quickly joins the accretion flow. We propose
that the CIO is responsible for reprocessing the hard emission from the
accretion flow into the optical band. This picture naturally explains the large
photospheric radius (or low blackbody temperature) and typical widths of the H
and/or He emission lines seen in optically selected TDEs. We predict the
CIO-reprocessed spectrum in the infrared to be L_nu ~ nu^{~0.5}, shallower
than a blackbody. The partial sky coverage of the CIO also provides a
unification of the diverse X-ray behaviors of optically bright TDEs. According
to this picture, optical surveys filter out those TDEs with blackhole mass less
than M_cr due to lack of a reprocessing layer. This filtering causes the
optical TDE rate to be lower than the total rate by a factor of ~10 or more. We
also predict that the volumetric rate of optically selected TDEs is nearly flat
with respect to the blackhole mass for M < 10^7 solar masses. When the CIO is
decelerated by the ambient medium, radio emission at the level of that in
ASASSN-14li may be produced, but the timescales and peak luminosities can be
highly diverse. Finally, our method paves the way for global simulations of the
disk formation process by injecting gas at the intersection point according to
the prescribed velocity and density profiles.
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