187 research outputs found
Digitial Readout for Microwave Kinetic Inductance Detectors and Applications in High Time Resolution Astronomy
This dissertation spans two topics relating to optical tonear-infrared astronomical cameras built around Microwave KineticInductance Detectors (MKIDs). The first topic is the development of adigital readout system for 10- to 30-kilopixel arrays of MKIDs. MKIDs aresuperconducting detectors that can detect individual photons with a widerange of wavelengths with high time resolution (\SI{2}{\micro s}) and low energyresolution. The advantage of MKIDs over other low temperature detectors with similar capabilities is that it is relatively straightforward to multiplex MKIDs into largearrays. All the complexity of readout is in room temperature electronics.This work discusses the implementation and programming of theseelectronics.The second part of this work demonstrates the capabilities of the prototypeoptical and near-infrared MKID instrument with observations ofpulsars. Detecting optical pulsations in these objects require high timeresolution and low noise. The discovery of a correlation between thebrightness of optical pulses from the Crab pulsar and the time of arrivalof coincident giant radio pulses is presented. The search for opticalpulses from a millisecond pulsar J0337+1715 is discussed along with a newupper limit on the brightness of its optical pulses
Aggressive Aerial Grasping using a Soft Drone with Onboard Perception
Contrary to the stunning feats observed in birds of prey, aerial manipulation
and grasping with flying robots still lack versatility and agility.
Conventional approaches using rigid manipulators require precise positioning
and are subject to large reaction forces at grasp, which limit performance at
high speeds. The few reported examples of aggressive aerial grasping rely on
motion capture systems, or fail to generalize across environments and grasp
targets. We describe the first example of a soft aerial manipulator equipped
with a fully onboard perception pipeline, capable of robustly localizing and
grasping visually and morphologically varied objects. The proposed system
features a novel passively closing tendon-actuated soft gripper that enables
fast closure at grasp, while compensating for position errors, complying to the
target-object morphology, and dampening reaction forces. The system includes an
onboard perception pipeline that combines a neural-network-based semantic
keypoint detector with a state-of-the-art robust 3D object pose estimator,
whose estimate is further refined using a fixed-lag smoother. The resulting
pose estimate is passed to a minimum-snap trajectory planner, tracked by an
adaptive controller that fully compensates for the added mass of the grasped
object. Finally, a finite-element-based controller determines optimal gripper
configurations for grasping. Rigorous experiments confirm that our approach
enables dynamic, aggressive, and versatile grasping. We demonstrate fully
onboard vision-based grasps of a variety of objects, in both indoor and outdoor
environments, and up to speeds of 2.0 m/s -- the fastest vision-based grasp
reported in the literature. Finally, we take a major step in expanding the
utility of our platform beyond stationary targets, by demonstrating
motion-capture-based grasps of targets moving up to 0.3 m/s, with relative
speeds up to 1.5 m/s
The Flare-dominated Accretion Mode of a Radio-bright Candidate Transitional Millisecond Pulsar
© 2020. The American Astronomical Society. All rights reserved.. We report new simultaneous X-ray and radio continuum observations of 3FGL J0427.9-6704, a candidate member of the enigmatic class of transitional millisecond pulsars. These XMM-Newton and Australia Telescope Compact Array observations of this nearly edge-on, eclipsing low-mass X-ray binary were taken in the sub-luminous disk state at an X-ray luminosity of erg s-1. Unlike the few well-studied transitional millisecond pulsars, which spend most of their disk state in a characteristic high or low accretion mode with occasional flares, 3FGL J0427.9-6704 stayed in the flare mode for the entire X-ray observation of ∼20 hr, with the brightest flares reaching ∼2 × 1034 erg s-1. The source continuously exhibited flaring activity on timescales of ∼10-100 s in both the X-ray and optical/ultraviolet (UV). No measurable time delay between the X-ray and optical/UV flares is observed, but the optical/UV flares last longer, and the relative amplitudes of the X-ray and optical/UV flares show a large scatter. The X-ray spectrum can be well-fit with a partially absorbed power law (Γ ∼ 1.4-1.5), perhaps due to the edge-on viewing angle. Modestly variable radio continuum emission is present at all epochs, and is not eclipsed by the secondary, consistent with the presence of a steady radio outflow or jet. The simultaneous radio/X-ray luminosity ratio of 3FGL J0427.9-6704 is higher than any known transitional millisecond pulsars and comparable to that of stellar-mass black holes of the same X-ray luminosity, providing additional evidence that some neutron stars can be as radio-loud as black holes
Revisiting hypervelocity stars after Gaia DR2
Hypervelocity stars are intriguing rare objects traveling at speeds large enough to be unbound from the Milky Way. Several mechanisms have been proposed for producing them, including the interaction of the Galaxy’s super-massive black hole (SMBH) with a binary; rapid mass-loss from a companion to a star in a short-period binary; the tidal disruption of an infalling galaxy and finally ejection from the Large Magellanic Cloud. While previously discovered high-velocity early-type stars are thought to be the result of an interaction with the SMBH, the origin of high-velocity late type stars is ambiguous. The second data release of Gaia (DR2) enables a unique opportunity to resolve this ambiguity and determine whether any late-type candidates are truly unbound from the Milky Way. In this paper, we utilize the new proper motion and velocity information available from DR2 to re-evaluate a collection of historical data compiled on the newly-created Open Fast Stars Catalog. We find that almost all previously-known high-velocity late-type stars are most likely bound to the Milky Way. Only one late-type object (LAMOST J115209.12+120258.0) is unbound from the Galaxy. Performing integrations of orbital histories, we find that this object cannot have been ejected from the Galactic centre and thus may be either debris from the disruption of a satellite galaxy or a disc runaway.D. Boubert thanks the UK Science and Technology Facilities Council for supporting his PhD. K. Hawkins thanks the Simons Society of Fellows and the Flatiron Institute Center for Computational Astrophysics. I. Ginsburg was supported in part by Harvard University funds and the Institute for Theory and Computation. J.S. acknowledges support from the Packard Foundation. This work has made use of data from the European Space Agency (ESA) mission Gaia(https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. Based on observations obtained at the Southern Astrophysical Research (SOAR) telescope, which is a joint project of the Ministério da Ciência, Tecnologia, Inovações e Comunicações (MCTIC) do Brasil, the U.S. National Optical Astronomy Observatory (NOAO), the University of North Carolina at Chapel Hill (UNC), and Michigan State University (MSU)
Colour-colour diagrams and extragalactic globular cluster ages. Systematic uncertainties using the (V-K)-(V-I) diagram
(abridged) We investigate biases in cluster ages and [Fe/H] estimated from
the (V-K)-(V-I) diagram, arising from inconsistent Horizontal Branch
morphology, metal mixture, treatment of core convection between observed
clusters and the theoretical colour grid employed for age and metallicity
determinations. We also study the role played by statistical fluctuations of
the observed colours, caused by the low total mass of typical globulars.
Horizontal Branch morphology is potentially the largest source of uncertainty.
A single-age system harbouring a large fraction of clusters with an HB
morphology systematically bluer than the one accounted for in the theoretical
colour grid, can simulate a bimodal population with an age difference as large
as 8 Gyr. When only the redder clusters are considered, this uncertainty is
almost negligible, unless there is an extreme mass loss along the Red Giant
Branch phase. The metal mixture affects mainly the redder clusters; the effect
of colour fluctuations becomes negligible for the redder clusters, or when the
integrated Mv is brighter than -8.5 mag. The treatment of core convection is
relevant for ages below ~4 Gyr. The retrieved [Fe/H] distributions are overall
only mildly affected. Colour fluctuations and convective core extension have
the largest effect. When 1sigma photometric errors reach 0.10 mag, all biases
found in our analysis are erased, and bimodal age populations with age
differences of up to ~8 Gyr go undetected. The use of both (U-I)-(V-K) and
(V-I)-(V-K) diagrams may help disclosing the presence of blue HB stars
unaccounted for in the theoretical colour calibration.Comment: 20 pages, including 26 figures. A&A in pres
An Observationally-Derived Kick Distribution for Neutron Stars in Binary Systems
Understanding the natal kicks received by neutron stars (NSs) during
formation is a critical component of modelling the evolution of massive
binaries. Natal kicks are an integral input parameter for population synthesis
codes, and have implications for the formation of double NS systems and their
subsequent merger rates. However, many of the standard observational kick
distributions that are used are obtained from samples created only from
isolated NSs. Kick distributions derived in this way overestimate the intrinsic
NS kick distribution. For NSs in binaries, we can only directly estimate the
effect of the natal kick on the binary system, instead of the natal kick
received by the NS itself. Here, for the first time, we present a binary kick
distribution for NSs with low-mass companions. We compile a catalogue of 145
NSs in low-mass binaries with the best available constraints on proper motion,
distance, and systemic radial velocity. For each binary, we use a
three-dimensional approach to estimate its binary kick. We discuss the
implications of these kicks on system formation, and provide a parametric model
for the overall binary kick distribution, for use in future theoretical
modelling work. We compare our results with other work on isolated NSs and NSs
in binaries, finding that the NS kick distributions fit using only isolated
pulsars underestimate the fraction of NSs that receive low kicks. We discuss
the implications of our results on modelling double NS systems, and provide
suggestions on how to use our results in future theoretical works.Comment: Accepted for publication in MNRAS. 28 pages, 19 figures, 8 table
A Comprehensive Archival Search for Counterparts to Ultra-Compact High Velocity Clouds: Five Local Volume Dwarf Galaxies
We report five Local Volume dwarf galaxies (two of which are presented here
for the first time) uncovered during a comprehensive archival search for
optical counterparts to ultra-compact high velocity clouds (UCHVCs). The UCHVC
population of HI clouds are thought to be candidate gas-rich, low mass halos at
the edge of the Local Group and beyond, but no comprehensive search for stellar
counterparts to these systems has been presented. Careful visual inspection of
all publicly available optical and ultraviolet imaging at the position of the
UCHVCs revealed six blue, diffuse counterparts with a morphology consistent
with a faint dwarf galaxy beyond the Local Group. Optical spectroscopy of all
six candidate dwarf counterparts show that five have an H-derived
velocity consistent with the coincident HI cloud, confirming their association,
the sixth diffuse counterpart is likely a background object. The size and
luminosity of the UCHVC dwarfs is consistent with other known Local Volume
dwarf irregular galaxies. The gas fraction () of the five
dwarfs are generally consistent with that of dwarf irregular galaxies in the
Local Volume, although ALFALFA-Dw1 (associated with ALFALFA UCHVC
HVC274.68+74.70123) has a very high 40. Despite the
heterogenous nature of our search, we demonstrate that the current dwarf
companions to UCHVCs are at the edge of detectability due to their low surface
brightness, and that deeper searches are likely to find more stellar systems.
If more sensitive searches do not reveal further stellar counterparts to
UCHVCs, then the dearth of such systems around the Local Group may be in
conflict with CDM simulations.Comment: 18 pages, 4 tables, 4 figures, ApJ Accepte
The flickering radio jet from the quiescent black hole X-ray binary A0620-00
Weakly accreting black hole X-ray binaries launch compact radio jets that
persist even in the quiescent spectral state, at X-ray luminosities <1e-5 of
the Eddington luminosity. However, radio continuum emission has been detected
from only a few of these quiescent systems, and little is known about their
radio variability. Jet variability can lead to misclassification of accreting
compact objects in quiescence, and affects the detectability of black hole
X-ray binaries in next-generation radio surveys. Here we present the results of
a radio monitoring campaign of A0620-00, one of the best-studied and
least-luminous known quiescent black hole X-ray binaries. We observed A0620-00
at 9.8 GHz using the Karl G. Jansky Very Large Array on 31 epochs from 2017 to
2020, detecting the source ~75% of the time. We see significant variability
over all timescales sampled, and the observed flux densities follow a lognormal
distribution with a mean of 12.5 uJy and standard deviation of 0.22 dex. In no
epoch was A0620-00 as bright as in 2005 (51 +/- 7 uJy), implying either that
this original detection was obtained during an unusually bright flare, or that
the system is fading in the radio over time. We present tentative evidence that
the quiescent radio emission from A0620-00 is less variable than that of V404
Cyg, the only other black hole binary with comparable data. Given that V404 Cyg
has a jet radio luminosity ~20 times higher than A0620-00, this comparison
could suggest that less luminous jets are less variable in quiescence.Comment: 9 pages, 7 figures, accepted for publication in MNRA
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