540 research outputs found
The Impact of Accretion Disk Winds on the Optical Spectra of Cataclysmic Variables
Many high-state non-magnetic cataclysmic variables (CVs) exhibit blue-shifted
absorption or P-Cygni profiles associated with ultraviolet (UV) resonance
lines. These features imply the existence of powerful accretion disk winds in
CVs. Here, we use our Monte Carlo ionization and radiative transfer code to
investigate whether disk wind models that produce realistic UV line profiles
are also likely to generate observationally significant recombination line and
continuum emission in the optical waveband. We also test whether outflows may
be responsible for the single-peaked emission line profiles often seen in
high-state CVs and for the weakness of the Balmer absorption edge (relative to
simple models of optically thick accretion disks). We find that a standard disk
wind model that is successful in reproducing the UV spectra of CVs also leaves
a noticeable imprint on the optical spectrum, particularly for systems viewed
at high inclination. The strongest optical wind-formed recombination lines are
H and He II . We demonstrate that a higher-density outflow
model produces all the expected H and He lines and produces a recombination
continuum that can fill in the Balmer jump at high inclinations. This model
displays reasonable verisimilitude with the optical spectrum of RW Trianguli.
No single-peaked emission is seen, although we observe a narrowing of the
double-peaked emission lines from the base of the wind. Finally, we show that
even denser models can produce a single-peaked H line. On the basis of
our results, we suggest that winds can modify, and perhaps even dominate, the
line and continuum emission from CVs.Comment: 15 pages, 13 figures. Accepted to MNRA
Calcium-Rich Gap Transients: Tidal Detonations of White Dwarfs?
We hypothesize that at least some of the recently discovered class of
calcium-rich gap transients are tidal detonation events of white dwarfs (WDs)
by black holes (BHs) or possibly neutron stars. We show that the properties of
the calcium-rich gap transients agree well with the predictions of the tidal
detonation model. Under the predictions of this model, we use a follow-up X-ray
observation of one of these transients, SN 2012hn, to place weak upper limits
on the detonator mass of this system that include all intermediate-mass BHs
(IMBHs). As these transients are preferentially in the stellar haloes of
galaxies, we discuss the possibility that these transients are tidal
detonations of WDs caused by random flyby encounters with IMBHs in dwarf
galaxies or globular clusters. This possibility has been already suggested in
the literature but without connection to the calcium-rich gap transients. In
order for the random flyby cross-section to be high enough, these events would
have to be occurring inside these dense stellar associations. However, there is
a lack of evidence for IMBHs in these systems, and recent observations have
ruled out all but the very faintest dwarf galaxies and globular clusters for a
few of these transients. Another possibility is that these are tidal
detonations caused by three-body interactions, where a WD is perturbed toward
the detonator in isolated multiple star systems. We highlight a number of ways
this could occur, even in lower-mass systems with stellar-mass BHs or neutron
stars. Finally, we outline several new observational tests of this scenario,
which are feasible with current instrumentation.Comment: 10 pages, 1 figure, accepted for publication in MNRA
Line-driven Disk Winds in Active Galactic Nuclei: The Critical Importance of Ionization and Radiative Transfer
Accretion disk winds are thought to produce many of the characteristic
features seen in the spectra of active galactic nuclei (AGN) and quasi-stellar
objects (QSOs). These outflows also represent a natural form of feedback
between the central supermassive black hole and its host galaxy. The mechanism
for driving this mass loss remains unknown, although radiation pressure
mediated by spectral lines is a leading candidate. Here, we calculate the
ionization state of, and emergent spectra for, the hydrodynamic simulation of a
line-driven disk wind previously presented by Proga & Kallman (2004). To
achieve this, we carry out a comprehensive Monte Carlo simulation of the
radiative transfer through, and energy exchange within, the predicted outflow.
We find that the wind is much more ionized than originally estimated. This is
in part because it is much more difficult to shield any wind regions
effectively when the outflow itself is allowed to reprocess and redirect
ionizing photons. As a result, the calculated spectrum that would be observed
from this particular outflow solution would not contain the ultraviolet
spectral lines that are observed in many AGN/QSOs. Furthermore, the wind is so
highly ionized that line-driving would not actually be efficient. This does not
necessarily mean that line-driven winds are not viable. However, our work does
illustrate that in order to arrive at a self-consistent model of line-driven
disk winds in AGN/QSO, it will be critical to include a more detailed treatment
of radiative transfer and ionization in the next generation of hydrodynamic
simulations.Comment: 13 pages, 10 figures - Accepted for publication in Ap
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
Deep near-IR observations of the Globular Cluster M4: Hunting for Brown Dwarfs
We present an analysis of deep HST/WFC3 near-IR (NIR) imaging data of the
globular cluster M4. The best-photometry NIR colour-magnitude diagram (CMD)
clearly shows the main sequence extending towards the expected end of the
Hydrogen-burning limit and going beyond this point towards fainter sources. The
white dwarf sequence can be identified. As such, this is the deepest NIR CMD of
a globular cluster to date. Archival HST optical data were used for
proper-motion cleaning of the CMD and for distinguishing the white dwarfs (WDs)
from brown dwarf (BD) candidates. Detection limits in the NIR are around F110W
approx 26.5 mag and F160W approx27 mag, and in the optical around F775W approx
28 mag. Comparing our observed CMDs with theoretical models, we conclude that
we have reached beyond the H-burning limit in our NIR CMD and are probably just
above or around this limit in our optical-NIR CMDs. Thus, any faint NIR sources
that have no optical counterpart are potential BD candidates, since the optical
data are not deep enough to detect them. We visually inspected the positions of
NIR sources which are fainter than the H-burning limit in F110W and for which
the optical photometry did not return a counterpart. We found in total five
sources for which we did not get an optical measurement. For four of these five
sources, a faint optical counterpart could be visually identified, and an upper
optical magnitude was estimated. Based on these upper optical magnitude limits,
we conclude that one source is likely a WD, one source could either be a WD or
BD candidate, and the remaining two sources agree with being BD candidates. For
only one source no optical counterpart could be detected, which makes this
source a good BD candidate. We conclude that we found in total four good BD
candidates.Comment: ApJ accepted, 28 pages including 16 figure
Testing Hydrodynamic Models of LMC X-4 with UV and X-ray Spectra
We compare the predictions of hydrodynamic models of the LMC X-4 X-ray binary
system with observations of UV P Cygni lines with the GHRS and STIS
spectrographs on the Hubble Space Telescope. The hydrodynamic model determines
density and velocity fields of the stellar wind, wind-compressed disk,
accretion stream, Keplerian accretion disk, and accretion disk wind. We use a
Monte Carlo code to determine the UV P Cygni line profiles by simulating the
radiative transfer of UV photons that originate on the star and are scattered
in the wind. The qualitative orbital variation predicted is similar to that
observed, although the model fails to reproduce the strong orbital asymmetry
(the observed absorption is strongest for phi>0.5). The model predicts a
mid-eclipse X-ray spectrum, due almost entirely to Compton scattering, with a
factor 4 less flux than observed with ASCA. We discuss how the model may need
to be altered to explain the spectral variability of the system.Comment: 11 figures, accepted by Ap
The origin of optical emission lines in the soft state of X-ray binary outbursts: the case of MAXI J1820+070
The optical emission line spectra of X-ray binaries (XRBs) are thought to be
produced in an irradiated atmosphere, possibly the base of a wind, located
above the outer accretion disc. However, the physical nature of - and physical
conditions in - the line-forming region remain poorly understood. Here, we test
the idea that the optical spectrum is formed in the transition region between
the cool, geometrically thin part of the disc near the mid-plane and a hot,
vertically extended atmosphere or outflow produced by X-ray irradiation. We
first present a VLT X-Shooter spectrum of XRB MAXI J1820+070 in the soft state
associated with its 2018 outburst, which displays a rich set of double-peaked
hydrogen and helium recombination lines. Aided by ancillary X-ray spectra and
reddening estimates, we then model this spectrum with the Monte Carlo radiative
transfer code Python, using a simple biconical disc wind model inspired by
radiation-hydrodynamic simulations of irradiation-driven outflows from XRB
discs. Such a model can qualitatively reproduce the observed features; nearly
all of the optical emission arising from the transonic 'transition region' near
the base of the wind. In this region, characteristic electron densities are on
the order of 10 cm, in line with the observed flat Balmer
decrement (H/H). We conclude that strong irradiation
can naturally give rise to both the optical line-forming layer in XRB discs and
an overlying outflow/atmosphere that produces X-ray absorption lines.Comment: 17 pages, 11 figures. This article has been accepted for publication
in MNRAS published by Oxford University Press on behalf of the Royal
Astronomical Societ
Autonomous Unmanned Ground Vehicles for Urban Logistics: Optimization of Last Mile Delivery Operations
In an era dominated by ongoing urbanization and rising e-commerce, the efficient delivery of goods within cities becomes a major challenge. As a new element of urban logistics, we discuss the potential of autonomous unmanned ground vehicles (AUGV) regarding the last mile delivery of shipments to customers. We propose an optimization model to minimize the delivery costs of urban shipments using AUGV. Simultaneously, best locations from a set of existing stations are selected for AUGV positioning and optimal route determination. With our developed Location Routing Problem, we provide decision support for parcel service providers, city authorities, and other relevant decision makers. Regarding the Green Information Systems domain, we tackle the lack of solution-oriented research addressing a more sustainable and locally emission free supply of goods within urban areas
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