8 research outputs found
Stellar disruption by a supermassive black hole: is the light curve really proportional to ?
In this paper we revisit the arguments for the basis of the time evolution of
the flares expected to arise when a star is disrupted by a supermassive black
hole. We present a simple analytic model relating the lightcurve to the
internal density structure of the star. We thus show that the standard
lightcurve proportional to only holds at late times. Close to the
peak luminosity the lightcurve is shallower, deviating more strongly from
for more centrally concentrated (e.g. solar--type) stars. We test
our model numerically by simulating the tidal disruption of several stellar
models, described by simple polytropic spheres with index . The
simulations agree with the analytical model given two considerations. First,
the stars are somewhat inflated on reaching pericentre because of the effective
reduction of gravity in the tidal field of the black hole. This is well
described by a homologous expansion by a factor which becomes smaller as the
polytropic index becomes larger. Second, for large polytropic indices wings
appear in the tails of the energy distribution, indicating that some material
is pushed further away from parabolic orbits by shocks in the tidal tails. In
all our simulations, the lightcurve is achieved only at late stages.
In particular we predict that for solar type stars, this happens only after the
luminosity has dropped by at least two magnitudes from the peak. We discuss our
results in the light of recent observations of flares in otherwise quiescent
galaxies and note the dependence of these results on further parameters, such
as the star/hole mass ratio and the stellar orbit.Comment: 10 pages, 10 figures, MNRAS accepte
Black hole mergers: the first light
The coalescence of supermassive black hole binaries occurs via the emission
of gravitational waves, that can impart a substantial recoil to the merged
black hole. We consider the energy dissipation, that results if the recoiling
black hole is surrounded by a thin circumbinary disc. Our results differ
significantly from those of previous investigations. We show analytically that
the dominant source of energy is often potential energy, released as gas in the
outer disc attempts to circularize at smaller radii. Thus, dimensional
estimates, that include only the kinetic energy gained by the disc gas,
underestimate the real energy loss. This underestimate can exceed an order of
magnitude, if the recoil is directed close to the disc plane. We use three
dimensional Smooth Particle Hydrodynamics (SPH) simulations and two dimensional
finite difference simulations to verify our analytic estimates. We also compute
the bolometric light curve, which is found to vary strongly depending upon the
kick angle. A prompt emission signature due to this mechanism may be observable
for low mass (10^6 Solar mass) black holes whose recoil velocities exceed about
1000 km/s. Emission at earlier times can mainly result from the response of the
disc to the loss of mass, as the black holes merge. We derive analytically the
condition for this to happen.Comment: 16 pages, accepted by MNRAS. Animations of the simulations are
available at http://jilawww.colorado.edu/~pja/recoil.htm
Disc instability in RS Ophiuchi: a path to Type Ia supernovae?
We study the stability of disc accretion in the recurrent nova RS Ophiuchi.
We construct a one-dimensional time-dependent model of the binary-disc system,
which includes viscous heating and radiative cooling and a self-consistent
treatment of the binary potential. We find that the extended accretion disc in
this system is always unstable to the thermal-viscous instability, and
undergoes repeated disc outbursts on ~10-20yr time-scales. This is similar to
the recurrence time-scale of observed outbursts in the RS Oph system, but we
show that the disc's accretion luminosity during outburst is insufficient to
explain the observed outbursts. We explore a range of models, and find that in
most cases the accretion rate during outbursts reaches or exceeds the critical
accretion rate for stable nuclear burning on the white dwarf surface.
Consequently we suggest that a surface nuclear burning triggered by disc
instability may be responsible for the observed outbursts. This allows the
white dwarf mass to grow over time, and we suggest that disc instability in RS
Oph and similar systems may represent a path to Type Ia supernovae.Comment: 8 pages, 5 figures. Accepted for publication in MNRA
Tidally-induced thermonuclear Supernovae
We discuss the results of 3D simulations of tidal disruptions of white dwarfs
by moderate-mass black holes as they may exist in the cores of globular
clusters or dwarf galaxies. Our simulations follow self-consistently the
hydrodynamic and nuclear evolution from the initial parabolic orbit over the
disruption to the build-up of an accretion disk around the black hole. For
strong enough encounters (pericentre distances smaller than about 1/3 of the
tidal radius) the tidal compression is reversed by a shock and finally results
in a thermonuclear explosion. These explosions are not restricted to progenitor
masses close to the Chandrasekhar limit, we find exploding examples throughout
the whole white dwarf mass range. There is, however, a restriction on the
masses of the involved black holes: black holes more massive than M swallow a typical 0.6 M dwarf before their tidal forces
can overwhelm the star's self-gravity. Therefore, this mechanism is
characteristic for black holes of moderate masses. The material that remains
bound to the black hole settles into an accretion disk and produces an X-ray
flare close to the Eddington limit of _\odot$), typically lasting for a few months. The combination
of a peculiar thermonuclear supernova together with an X-ray flare thus
whistle-blows the existence of such moderate-mass black holes. The next
generation of wide field space-based instruments should be able to detect such
events.Comment: 8 pages, 2 figures, EuroWD0
Black hole mergers: Can gas discs solve the 'final parsec' problem?
We compute the effect of an orbiting gas disc in promoting the coalescence of a central supermassive black hole binary. Unlike earlier studies, we consider a finite mass of gas with explicit time dependence: we do not assume that the gas necessarily adopts a steady state or a spatially constant accretion rate, i.e. that the merging black hole was somehow inserted into a pre-existing accretion disc. We consider the tidal torque of the binary on the disc, and the binary's gravitational radiation. We study the effects of star formation in the gas disc in a simple energy feedback framework.
The disc spectrum differs in detail from that found before. In particular, tidal torques from the secondary black hole heat the edges of the gap, creating bright rims around the secondary. These rims do not in practice have uniform brightness either in azimuth or time, but can on average account for as much as 50 per cent of the integrated light from the disc. This may lead to detectable high-photon-energy variability on the relatively long orbital time-scale of the secondary black hole, and thus offer a prospective signature of a coalescing black hole binary.
We also find that the disc can drive the binary to merger on a reasonable time-scale only if its mass is at least comparable with that of the secondary black hole, and if the initial binary separation is relatively small, i.e. a0≲ 0.05 pc. Star formation complicates the merger further by removing mass from the disc. In the feedback model we consider, this sets an effective limit to the disc mass. As a result, binary merging is unlikely unless the black hole mass ratio is ≲0.001. Gas discs thus appear not to be an effective solution to the ‘last parsec’ problem for a significant class of mergers