4,252 research outputs found
Stellar Wind Accretion in GX301-2: Evidence for a High-density Stream
The X-ray binary system GX301-2 consists of a neutron star in an eccentric
orbit accreting from the massive early-type star WRAY 977. It has previously
been shown that the X-ray orbital light curve is consistent with existence of a
gas stream flowing out from Wray 977 in addition to its strong stellar wind.
Here, X-ray monitoring observations by the Rossi X-ray Timing Explorer (RXTE)/
All-Sky-Monitor (ASM) and pointed observations by the RXTE/ Proportional
Counter Array (PCA) over the past decade are analyzed. We analyze both the flux
and column density dependence on orbital phase. The wind and stream dynamics
are calculated for various system inclinations, companion rotation rates and
wind velocities, as well as parametrized by the stream width and density. These
calculations are used as inputs to determine both the expected accretion
luminosity and the column density along the line-of-sight to the neutron star.
The model luminosity and column density are compared to observed flux and
column density vs. orbital phase, to constrain the properties of the stellar
wind and the gas stream. We find that the change between bright and medium
intensity levels is primarily due to decreased mass loss in the stellar wind,
but the change between medium and dim intensity levels is primarily due to
decreased stream density. The mass-loss rate in the stream exceeds that in the
stellar wind by a factor of 2.5. The quality of the model fits is significantly
better for lower inclinations, favoring a mass for WRAY 977 of 53 to 62 Msun.Comment: 19 pages, 6 figure
Multi-epoch Analysis of Pulse Shapes from the Neutron Star SAX J1808.4-3658
The pulse shapes detected during multiple outbursts of SAX J1808 are analyzed
in order to constrain the neutron star's mass and radius. We use a hot-spot
model with a small scattered-light component to jointly fit data from two
different epochs, under the restriction that the star's mass and radius and the
binary's inclination do not change from epoch to epoch. All other parameters
describing the spot location, emissivity, and relative fractions of blackbody
to Comptonized radiation are allowed to vary with time. The joint fit of data
from the 1998 "slow decay" and the 2002 "end of outburst maximum" epochs using
the constraint i<90 degrees leads to the 3 sigma confidence constraint on the
neutron star mass 0.8 M_sun < M < 1.7 M_sun and equatorial radius 5 km < R < 13
km. Inclinations as low as 41 degrees are allowed. The best-fit models with M >
1.0 M_sun from joint fits of the 1998 data with data from other epochs of the
2002 and 2005 outbursts also fall within the same 3 sigma confidence region.
This 3 sigma confidence region allows a wide variety of hadronic equations of
state, in contrast with an earlier analysis (Leahy et al 2008) of only the 1998
outburst data that only allowed for extremely small stars.Comment: 12 pages, 9 figures, accepted by ApJ. This revised version includes
an expanded Section
Transfer learning for radio galaxy classification
In the context of radio galaxy classification, most state-of-the-art neural
network algorithms have been focused on single survey data. The question of
whether these trained algorithms have cross-survey identification ability or
can be adapted to develop classification networks for future surveys is still
unclear. One possible solution to address this issue is transfer learning,
which re-uses elements of existing machine learning models for different
applications. Here we present radio galaxy classification based on a 13-layer
Deep Convolutional Neural Network (DCNN) using transfer learning methods
between different radio surveys. We find that our machine learning models
trained from a random initialization achieve accuracies comparable to those
found elsewhere in the literature. When using transfer learning methods, we
find that inheriting model weights pre-trained on FIRST images can boost model
performance when re-training on lower resolution NVSS data, but that inheriting
pre-trained model weights from NVSS and re-training on FIRST data impairs the
performance of the classifier. We consider the implication of these results in
the context of future radio surveys planned for next-generation radio
telescopes such as ASKAP, MeerKAT, and SKA1-MID
A Monte Carlo Study of the 6.4 keV Emission at the Galactic Center
Strong fluorescent Fe line emission at 6.4 keV has been observed from the Sgr
B2 giant molecular cloud located in the Galactic Center region. The large
equivalent width of this line and the lack of an apparent illuminating nearby
object indicate that a time-dependent source, currently in a low-activity
state, is causing the fluorescent emission. It has been suggested that this
illuminator is the massive black hole candidate, Sgr A*, whose X-ray luminosity
has declined by an unprecedented six orders of magnitude over the past 300
years. We here report the results of our Monte Carlo simulations for producing
this line under a variety of source configurations and characteristics. These
indicate that the source may in fact be embedded within Sgr B2, although
external sources give a slightly better fit to the data. The weakened
distinction between the internal and external illuminators is due in part to
the instrument response function, which accounts for an enhanced equivalent
width of the line by folding some of the continuum radiation in with the
intrinsic line intensity. We also point out that although the spectrum may be
largely produced by K emission in cold gas, there is some evidence in
the data to suggest the presence of warm (~10^5 K) emitting material near the
cold cloud.Comment: 11 pages, 4 figure
RACE pulls for shared control
Maintaining and supporting an aircraft fleet, in a climate of reduced manpower and financial resources, dictates effective utilization of robotics and automation technologies. To help develop a winning robotics and automation program the Air Force Logistics Command created the Robotics and Automation Center of Excellence (RACE). RACE is a command wide focal point. Race is an organic source of expertise to assist the Air Logistic Center (ALC) product directorates in improving process productivity through the judicious insertion of robotics and automation technologies. RACE is a champion for pulling emerging technologies into the aircraft logistic centers. One of those technology pulls is shared control. Small batch sizes, feature uncertainty, and varying work load conspire to make classic industrial robotic solutions impractical. One can view ALC process problems in the context of space robotics without the time delay. The ALC's will benefit greatly from the implementation of a common architecture that supports a range of control actions from fully autonomous to teleoperated. Working with national laboratories and private industry, we hope to transition shared control technology to the depot floor. This paper provides an overview of the RACE internal initiatives and customer support, with particular emphasis on production processes that will benefit from shared control technology
Apologies for Blanks or Laments for Dumbness: Tina Darragh’s <i>Opposable Dumbs</i> as Open Source and/or Open Content
This essay engages in a reading of Tina Darragh’s publication 'Opposable Dumbs' (2010). This reading is carried out in pursuit of a number of critical and theoretical questions, that include asking what sort of text this is, and how we might read it. The essay considers how Darragh’s work connects to the debate around open source and free software, and to the politics and poetics of that debate. Taking up the call for creative responses in Darragh’s anti-rights or inverted copyright statement, the writing takes a route through the text that parallels some of Darragh’s strategies as a writer. This creative reading is linked to a reading of Stephen Voyce’s essay on open source poetics (2011), with some reference to a wider discourse around FLOSS, creative commons, and copyleft strategies. This essay proposes Darragh’s work as a case study for Voyce’s proposals, and suggests that her practice may in fact go further than he proposes in moving from a position of ‘open source’ to one of ‘open content.
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