680 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
Dynamo driven accretion discs and dwarf nova eruptions
We explore the consequences of a magnetic dynamo origin for the viscosity in
accretion discs, for the structure and evolution of discs in dwarf nova
systems. We propose that the rapid cooling that sets in at the end of a dwarf
nova eruption acts to inhibit the Balbus-Hawley instability, and thereby to
quench dynamo action and so reduce disc viscosity. We demonstrate that a
modified disc instability model can reproduce the basic properties of dwarf
nova eruptions, as well as some properties of quiescent discs. We also discuss
some observational consequences of our model.Comment: uu-encoded gz-compressed Postscript file, 18 pages including 6
figures. ApJ in pres
Kepler Observations of V447 Lyr: An Eclipsing U Gem Cataclysmic Variable
We present the results of an analysis of data covering 1.5 years of the dwarf
nova V447 Lyr. We detect eclipses of the accretion disk by the mass donating
secondary star every 3.74 hrs which is the binary orbital period. V447 Lyr is
therefore the first dwarf nova in the Kepler field to show eclipses. We also
detect five long outbursts and six short outbursts showing V447 Lyr is a U Gem
type dwarf nova. We show that the orbital phase of the mid-eclipse occurs
earlier during outbursts compared to quiescence and that the width of the
eclipse is greater during outburst. This suggests that the bright spot is more
prominent during quiescence and that the disk is larger during outburst than
quiescence. This is consistent with an expansion of the outer disk radius due
to the presence of high viscosity material associated with the outburst,
followed by a contraction in quiescence due to the accretion of low angular
momentum material. We note that the long outbursts appear to be triggered by a
short outburst, which is also observed in the super-outbursts of SU UMa dwarf
novae as observed using Kepler.Comment: Accepted by MNRA
V344 Lyrae: A Touchstone SU UMa Cataclysmic Variable in the Kepler Field
We report on the analysis of the Kepler short-cadence (SC) light curve of
V344 Lyr obtained during 2009 June 20 through 2010 Mar 19 (Q2--Q4). The system
is an SU UMa star showing dwarf nova outbursts and superoutbursts, and promises
to be a touchstone for CV studies for the foreseeable future. The system
displays both positive and negative superhumps with periods of 2.20 and
2.06-hr, respectively, and we identify an orbital period of 2.11-hr. The
positive superhumps have a maximum amplitude of ~0.25-mag, the negative
superhumps a maximum amplitude of ~0.8 mag, and the orbital period at
quiescence has an amplitude of ~0.025 mag. The quality of the Kepler data is
such that we can test vigorously the models for accretion disk dynamics that
have been emerging in the past several years. The SC data for V344 Lyr are
consistent with the model that two physical sources yield positive superhumps:
early in the superoutburst, the superhump signal is generated by viscous
dissipation within the periodically flexing disk, but late in the
superoutburst, the signal is generated as the accretion stream bright spot
sweeps around the rim of the non-axisymmetric disk. The disk superhumps are
roughly anti-phased with the stream/late superhumps. The V344 Lyr data also
reveal negative superhumps arising from accretion onto a tilted disk precessing
in the retrograde direction, and suggest that negative superhumps may appear
during the decline of DN outbursts. The period of negative superhumps has a
positive dP/dt in between outbursts.Comment: ApJ, In Press (20 pages, 27 figures) A version with full-resolution
figures is available at http://www.astro.fit.edu/wood/WoodV344.pd
The Kepler light curves of V1504 Cygni and V344 Lyrae: A study of the Outburst Properties
We examine the Kepler light curves of V1504 Cyg and V344 Lyr, encompassing
~736 d at 1 min cadence. During this span each system exhibited ~64-65
outbursts, including six superoutbursts. We find that, in both systems, the
normal outbursts between two superoutbursts increase in duration over time by a
factor ~1.2-1.9, and then reset to a small value after the following
superoutburst. In both systems the trend of quiescent intervals between normal
outbursts is to increase to a local maximum about half way through the
supercycle - the interval from one superoutburst to the next - and then to
decrease back to a small value by the time of the next superoutburst. This is
inconsistent with Osaki's thermal-tidal model, which predicts a monotonic
increase in the quiescent intervals between normal outbursts during a
supercycle. Also, most of the normal outbursts have an asymmetric,
fast-rise/slower-decline shape, consistent with outbursts triggered at large
radii. The exponential rate of decay of the plateau phase of the superoutbursts
is 8 d/mag for V1504 Cyg and 12 d/mag for V344 Lyr. This time scale gives a
direct measure of the viscous time scale in the outer accretion disk given the
expectation that the entire disk is in the hot, viscous state during
superoutburst. The resulting constraint on the Shakura-Sunyaev parameter,
alpha_{hot} ~ 0.1, is consistent with the value inferred from the fast dwarf
nova decays. By looking at the slow decay rate for superoutbursts, which occur
in systems below the period gap, in combination with the slow decay rate in one
long outburst above the period gap (in U Gem), we infer a steep dependence of
the decay rate on orbital period for long outbursts. This implies a steep
dependence of alpha_{cold} on orbital period, consistent with tidal torquing as
being the dominant angular momentum transport mechanism in quiescent disks in
interacting binary systems.Comment: 11 pages, 11 figures; accepted by the Astrophysical Journa
Short gamma-ray bursts within 200 Mpc
We present a systematic search for short-duration gamma-ray bursts (GRBs) in the local Universe based on 14 yr of observations with the Neil Gehrels Swift Observatory. We cross-correlate the GRB positions with the GLADE catalogue of nearby galaxies, and find no event at a distance ≲100 Mpc and four plausible candidates in the range 100 Mpc ≲ D ≲ 200 Mpc. Although affected by low statistics, this number is higher than the one expected for chance alignments to random galaxies, and possibly suggests a physical association between these bursts and nearby galaxies. By assuming a local origin, we use these events to constrain the range of properties for X-ray counterparts of neutron star mergers. Optical upper limits place tight constraints on the onset of a blue kilonova, and imply either low masses (≲10−3M⊙) of lanthanide-poor ejecta or unfavorable orientations (θ_(obs) ≳ 30 deg). Finally, we derive that the all-sky rate of detectable short GRBs within 200 Mpc is 1.3^(+1.7)_(−0.8) yr⁻¹ (68 per cent confidence interval), and discuss the implications for the GRB outflow structure. If these candidates are instead of cosmological origin, we set a upper limit of ≲2.0 yr⁻¹ (90 per cent confidence interval) to the rate of nearby events detectable with operating gamma-ray observatories, such as Swift and Fermi
Search for Gravitational Waves from Intermediate Mass Binary Black Holes
We present the results of a weakly modeled burst search for gravitational waves from mergers of non-spinning intermediate mass black holes (IMBH) in the total mass range 100-450 solar Mass and with the component mass ratios between 1:1 and 4:1. The search was conducted on data collected by the LIGO and Virgo detectors between November of 2005 and October of 2007. No plausible signals were observed by the search which constrains the astrophysical rates of the IMBH mergers as a function of the component masses. In the most efficiently detected bin centered on 88 + 88 solar Mass , for non-spinning sources, the rate density upper limit is 0.13 per Mpc(exp 3) per Myr at the 90% confidence level
The outburst duration and duty-cycle of GRS 1915+105
The extraordinarily long outburst of GRS 1915+105 makes it one of the most
remarkable low-mass X-ray binaries (LMXBs). It has been in a state of constant
outburst since its discovery in 1992, an eruption which has persisted ~100
times longer than those of more typical LXMBs. The long orbital period of GRS
1915+105 implies that it contains large and massive accretion disc which is
able to fuel its extreme outburst. In this paper, we address the longevity of
the outburst and quiescence phases of GRS 1915+105 using Smooth Particle
Hydrodynamics (SPH) simulations of its accretion disc through many outburst
cycles. Our model is set in the two-alpha framework and includes the effects of
the thermo-viscous instability, tidal torques, irradiation by central X-rays
and wind mass loss. We explore the model parameter space and the examine the
impact of the various ingredients. We predict that the outburst of GRS 1915+105
should last a minimum of 20 years and possibly up to ~100 years if X-ray
irradiation is very significant. The predicted recurrence times are of the
order of 10^4 years, making the X-ray duty cycle a few 0.1%. Such a low duty
cycle may mean that GRS 1915+105 is not an anomaly among the more standard
LMXBs and that many similar, but quiescent, systems could be present in the
Galaxy.Comment: 10 pages, 9 figures, accepted for publication by MNRA
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