759 research outputs found
Searching for nova shells around cataclysmic variables
We present the results of a search for nova shells around 101 cataclysmic
variables (CVs), using Halpha images taken with the 4.2-m William Herschel
Telescope (WHT) and the 2.5-m Isaac Newton Telescope Photometric Halpha Survey
of the Northern Galactic Plane (IPHAS). Both telescopes are located on La
Palma. We concentrated our WHT search on nova-like variables, whilst our IPHAS
search covered all CVs in the IPHAS footprint. We found one shell out of the 24
nova-like variables we examined. The newly discovered shell is around V1315 Aql
and has a radius of approx.2.5 arcmin, indicative of a nova eruption
approximately 120 years ago. This result is consistent with the idea that the
high mass-transfer rate exhibited by nova-like variables is due to enhanced
irradiation of the secondary by the hot white dwarf following a recent nova
eruption. The implications of our observations for the lifetime of the
nova-like variable phase are discussed. We also examined 4 asynchronous polars,
but found no new shells around any of them, so we are unable to confirm that a
recent nova eruption is the cause of the asynchronicity in the white dwarf
spin. We find tentative evidence of a faint shell around the dwarf nova V1363
Cyg. In addition, we find evidence for a light echo around the nova V2275 Cyg,
which erupted in 2001, indicative of an earlier nova eruption approx.300 years
ago, making V2275 Cyg a possible recurrent nova.Comment: 14 pages, 50 figures, 3 Table
Assisted stellar suicide: the wind-driven evolution of the recurrent nova T Pyxidis
We show that the extremely high luminosity of the short-period recurrent nova
T Pyx in quiescence can be understood if this system is a wind-driven supersoft
x-ray source (SSS). In this scenario, a strong, radiation-induced wind is
excited from the secondary star and accelerates the binary evolution. The
accretion rate is therefore much higher than in an ordinary cataclysmic binary
at the same orbital period, as is the luminosity of the white dwarf primary. In
the steady state, the enhanced luminosity is just sufficient to maintain the
wind from the secondary. The accretion rate and luminosity predicted by the
wind-driven model for T Pyx are in good agreement with the observational
evidence. X-ray observations with Chandra or XMM may be able to confirm T Pyx's
status as a SSS.
T Pyx's lifetime in the wind-driven state is on the order of a million years.
Its ultimate fate is not certain, but the system may very well end up
destroying itself, either via the complete evaporation of the secondary star,
or in a Type Ia supernova if the white dwarf reaches the Chandrasekhar limit.
Thus either the primary, the secondary, or both may currently be committing
assisted stellar suicide.Comment: accepted for publication in A&A Letters; 5 pages; no figure
The reverberation signatures of rotating disc winds in active galactic nuclei
The broad emission lines (BELs) in active galactic nuclei (AGN) respond to
ionizing continuum variations. The time and velocity dependence of their
response depends on the structure of the broad-line region: its geometry,
kinematics and ionization state. Here, we predict the reverberation signatures
of BELs formed in rotating accretion disc winds. We use a Monte Carlo radiative
transfer and ionization code to predict velocity-delay maps for representative
high- (C) and low-ionization (H) emission lines in both high- and
moderate-luminosity AGN. Self-shielding, multiple scattering and the ionization
structure of the outflows are all self-consistently taken into account, while
small-scale structure in the outflow is modelled in the micro-clumping
approximation. Our main findings are: (1) The velocity-delay maps of
smooth/micro-clumped outflows often contain significant negative responses.
(2)~The reverberation signatures of disc wind models tend to be rotation
dominated and can even resemble the classic "red-leads-blue" inflow signature.
(3) Traditional "blue-leads-red" outflow signatures can usually only be
observed in the long-delay limit. (4) Our models predict lag-luminosity
relationships similar to those inferred from observations, but systematically
underpredict the observed centroid delays. (5) The ratio between "virial
product" and black hole mass predicted by our models depends on viewing angle.
Our results imply that considerable care needs to be taken in interpreting data
obtained by observational reverberation mapping campaigns. In particular, basic
signatures such as "red-leads-blue", "blue-leads-red" and "blue and red vary
jointly" are not always reliable indicators of inflow, outflow or rotation.
This may help to explain the perplexing diversity of such signatures seen in
observational campaigns to date.Comment: 15 pages, 17 figures, 2 tables. Accepted by MNRAS 20/7/201
On the Origin of the Absorption Features in SS433
We present high-resolution optical spectroscopy of the X-ray binary system
SS433, obtained over a wide range of orbital phases. The spectra display
numerous weak absorption features, and include the clearest example seen to
date of those features, resembling a mid-A type supergiant spectrum, that have
previously been associated with the mass donor star. However, the new data
preclude the hypothesis that these features originate solely within the
photosphere of the putative mass donor, indicating that there may be more than
one region within the system producing an A supergiant-like spectrum, probably
an accretion disc wind. Indeed, whilst we cannot confirm the possibility that
the companion star is visible at certain phase combinations, it is possible
that all supergiant-like features observed thus far are produced solely in a
wind. We conclude that great care must be taken when interpreting the behaviour
of these weak features.Comment: Accepted for publication in MNRAS, 8 pages, 6 figure
Erupting Cataclysmic Variable Stars in the Nearest Globular Cluster, NGC 6397: Intermediate Polars?
NGC 6397 is the closest globular cluster, and hence the ideal place to search
for faint stellar populations such as cataclysmic variables (CVs). HST and
Chandra observers have identified nine certain and likely CVs in this nearby
cluster, including several magnetic CV candidates. We have combined our recent
UV imagery with archival HST images of NGC 6397 to search for new CV candidates
and especially to look for dwarf nova-like eruptive events. We find remarkable
and somewhat unexpected dwarf nova-like eruptions of the two well-known
cataclysmic systems CV2 and CV3. These two objects have been claimed to be {\it
magnetic} CVs, as indicated by their helium emission-line spectra. Magnetic
fields in CVs are usually expected to prevent the disk instability that leads
to dwarf nova eruptions. In fact, most field magnetic CVs are observed to not
undergo eruptions. Our observations of the dwarf nova eruptions of CV2 and CV3
can be reconciled with these objects' HeII emission lines if both objects are
infrequently-erupting intermediate polars, similar to EX Hya. If this is the
case for most globular cluster CVs then we can reconcile the many X-ray and UV
bright CV candidates seen by Chandra and HST with the very small numbers of
erupting dwarf novae observed in cluster cores.Comment: 12 pages, 3 figures. Accepted for publication in The Astronomical
Journal. Two additional authors adde
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