423 research outputs found
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
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
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
Gamma-Ray Astronomy Technology Needs
In recent decades gamma-ray observations have become a valuable tool for studying the universe. Progress made in diverse 8re1lS such as gamma-ray bursts (GRBs), nucleosynthesis, and active galactic nuclei (AGNs) has complimented and enriched our astrophysical understanding in many ways. We present an overview of current and future planned space y-ray missions and discussion technology needs for- the next generation of space gamma-ray instruments
Recent Progress on GRBs with Swift
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. In addition, rapid-response telescopes on the ground have 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 supernova physics
The Nature of the Giant Outbursts in the Bursting Pulsar GRO J 1744-28
We investigate the possible role of an accretion disk instability in
producing the giant outbursts seen in GRO J1744-28. Specifically, we study the
global, time dependent evolution of the Lightman-Eardley instability which can
develop near the inner edge of an accretion disk when the radiation pressure
becomes comparable to the gas pressure. Broadly speaking, our results are
compatible with earlier works by Taam \& Lin and by Lasota \& Pelat. The
uniqueness of GRO J1744-28 appears to be associated with the constraint that,
in order for outbursts to occur, the rate of accretion at the inner edge must
be within a narrow range just above the critical accretion rate at which
radiation pressure is beginning to become significant.Comment: 11 pages in .tex file, 4 Postscript figures, .tex file uses
aasms.sty; Ap. J. L. 1996, in pres
SWIFT Observations of a Far UV Luminosity Component in SS433
SS433 is a binary system showing relativistic Doppler shifts in its two sets of emission lines. The origin of its UV continuum is not well established. We observed SS433 to determine the emission mechanism responsible for its far UV spectrum. The source was observed at several different phases of both its 13 d orbital period and 162.5 d precession period using the UVOT and XRT detector systems on Swift. The far UV spectrum down to 1880 Angstrom lies significantly above the spectral flux distribution predicted by extrapolating the reddened blackbody continuum that fits the spectrum above 3500 Angstroms. The intensity of the far UV flux varies over a period of days and the variability is correlated with the variability of the soft X-ray flux from the source. An emission mechanism in addition to those previously detected in the optical and X-ray regions must exist in the far UV spectrum of SS433
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