614 research outputs found
The Kepler Light Curve of V344 Lyrae: Constraining the Thermal-Viscous Limit Cycle Instability
We present time dependent modeling based on the accretion disk limit cycle
model for a 270 d light curve of the short period SU UMa-type dwarf nova V344
Lyr taken by Kepler. The unprecedented precision and cadence (1 minute) far
surpass that generally available for long term light curves. The data encompass
two superoutbursts and 17 normal (i.e., short) outbursts. The main decay of the
superoutbursts is nearly perfectly exponential, decaying at a rate ~12 d/mag,
while the much more rapid decays of the normal outbursts exhibit a
faster-than-exponential shape. Our modeling using the basic accretion disk
limit cycle can produce the main features of the V344 Lyr light curve,
including the peak outburst brightness. Nevertheless there are obvious
deficiencies in our model light curves: (1) The rise times we calculate, both
for the normal and superoutbursts, are too fast. (2) The superoutbursts are too
short. (3) The shoulders on the rise to superoutburst have more structure than
the shoulder in the observed superoutburst and are too slow, comprising about a
third to half of the total viscous plateau, rather than the ~10% observed.
However, one of the alpha_{cold} -> alpha_{hot} interpolation schemes we
investigate (one that is physically motivated) does yield longer superoutbursts
with suitably short, less structured shoulders.Comment: 39 pages, 9 figures, accepted in the Astrophysical Journa
Outburst Properties of V1504 Cyg and V344 Lyr
I begin by reviewing dwarf novae and the disk instability theory, and then present an overview of three ideas for producing superoutbursts in the SU UMa stars − the thermal tidal instability, irradiation-induced secondary mass overflow, and the plain vanilla disk limit cycle instability. I discuss the properties of the outbursts in two SU UMa systems observed by Kepler in the context of the three theories. I conclude with a look beyond the SU UMa systems
Gamma Ray Bursts - Observations
We are in an exciting period of discovery for gamma-ray bursts. The Swift observatory is detecting 100 bursts per year, providing arcsecond localizations and sensitive observations of the prompt and afterglow emission. The Fermi observatory is observing 250 bursts per year with its medium-energy GRB instrument and about 10 bursts per year with its high-energy LAT instrument. In addition, rapid-response telescopes on the ground are providing new capabilities to study optical emission during the prompt phase and spectral signatures of the host galaxies. The combined data set is enabling great advances in our understanding of GRBs including afterglow physics, short burst origin, and high energy emission
Fall-Back Disks in Long and Short GRBs
We present numerical time-dependent calculations for fall-back disks relevant
to GRBs in which the disk of material surrounding the black hole (BH) powering
the GRB jet modulates the mass flow, and hence the strength of the jet. Given
the initial existence of a small mass <10^{-4} msun near the progenitor with a
circularization radius ~10^{10}-10^{11} cm, an unavoidable consequence will be
the formation of an "external disk" whose outer edge continually moves to
larger radii due to angular momentum transport and lack of a confining torque.
For long GRBs, if the mass distribution in the initial fall-back disk traces
the progenitor envelope, then a radius ~10^{11} cm gives a time scale ~10^4 s
for the X-ray plateau. For late times t>10^7 s a steepening due to a cooling
front in the disk may have observational support in GRB 060729. For short GRBs,
one expects most of the mass initially to lie at small radii <10^8 cm; however
the presence of even a trace amount ~10^{-9} msun of high angular momentum
material can give a brief plateau in the light curve. By studying the plateaus
in the X-ray decay of GRBs, which can last up to ~10^4 s after the prompt
emission, Dainotti et al. find an apparent inverse relation between the X-ray
luminosity at the end of the plateau and the duration of the plateau. We show
that this relation may simply represent the fact that one is biased against
detecting faint plateaus, and therefore preferentially sampling the more
energetic GRBs. If, however, there were a standard reservoir in fall-back mass,
our model can reproduce the inverse X-ray luminosity-duration relation. We
emphasize that we do not address the very steep, initial decays immediately
following the prompt emission, which have been modeled by Lindner et al. as
fall-back of the progenitor core, and may entail the accretion of > 1 msun.Comment: 8 pages, 6 figures, to appear in the Astrophysical Journal, May 10,
2011, v. 73
A New Paradigm for Gamma Ray Bursts: Long Term Accretion Rate Modulation by an External Accretion Disk
We present a new way of looking at the very long term evolution of GRBs in
which the disk of material surrounding the putative black hole powering the GRB
jet modulates the mass flow, and hence the efficacy of the process that
extracts rotational energy from the black hole and inner accretion disk. The
pre-Swift paradigm of achromatic, shallow-to-steep "breaks" in the long term
GRB light curves has not been borne out by detailed Swift data amassed in the
past several years. We argue that, given the initial existence of a fall-back
disk near the progenitor, an unavoidable consequence will be the formation of
an "external disk" whose outer edge continually moves to larger radii due to
angular momentum transport and lack of a confining torque. The mass reservoir
at large radii moves outward with time and gives a natural power law decay to
the GRB light curves. In this model, the different canonical power law decay
segments in the GRB identified by Zhang et al. and Nousek et al. represent
different physical states of the accretion disk. We identify a physical disk
state with each power law segment.Comment: 12 pages, 1 figure, accepted into the Astrophysical Journal: 23 May
200
The global structure of thin, stratified "alpha"-discs and the reliability of the one layer approximation
We report the results of a systematic comparison between the vertically
averaged model and the vertically explicit model of steady state, Keplerian,
optically thick "alpha"-discs. The simulations have concerned discs currently
found in three different systems: dwarf novae, young stellar objects and active
galactic nuclei. In each case, we have explored four decades of accretion rates
and almost the whole disc area
(except the narrow region where the vertically averaged model has degenerate
solutions). We find that the one layer approach gives a remarkably good
estimate of the main physical quantities in the disc, and specially the
temperature at the equatorial plane which is accurate to within 30% for cases
considered. The major deviations (by a factor < 4) are observed on the disc
half-thickness. The sensitivity of the results to the "alpha"-parameter value
has been tested for 0.001 < alpha < 0.1 and appears to be weak. This study
suggests that the ``precision'' of the vertically averaged model which is easy
to implement should be sufficient in practice for many astrophysical
applications.Comment: 4 pages, PostScript. Accepted in Astronomy & Astrophysic
The thermal-viscous disk instability model in the AGN context
Accretion disks in AGN should be subject to the same type of instability as
in cataclysmic variables (CVs) or in low-mass X-ray binaries (LMXBs), which
leads to dwarf nova and soft X-ray transient outbursts. It has been suggested
that this thermal/viscous instability can account for the long term variability
of AGNs. We test this assertion by presenting a systematic study of the
application of the disk instability model (DIM) to AGNs. We are using the
adaptative grid numerical code we have developed in the context of CVs,
enabling us to fully resolve the radial structure of the disk. We show that,
because in AGN disks the Mach numbers are very large, the heating and cooling
fronts are so narrow that they cannot be resolved by the numerical codes that
have been used until now. In addition, these fronts propagate on time scales
much shorter than the viscous time. As a result, a sequence of heating and
cooling fronts propagate back and forth in the disk, leading only to small
variations of the accretion rate onto the black hole, with short quiescent
states occurring for very low mass transfer rates only. Truncation of the inner
part of the disk by e.g. an ADAF does not alter this result, but enables longer
quiescent states. Finally we discuss the effects of irradiation by the central
X-ray source, and show that, even for extremely high irradiation efficiencies,
outbursts are not a natural outcome of the model.Comment: Astronomy & Astrophysics - in pres
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