259 research outputs found
A model-independent analysis of the variability of GRS 1915+105
We analyzed 163 observations of the microquasar GRS 1915+105 made with the
Rossi X-ray Timing Explorer (RXTE) in the period 1996-1997. For each
observation, we produced light curves and color-color diagrams. We classified
the observations in 12 separate classes, based on their count rate and color
characteristics. From the analysis of these classes, we reduced the variability
of the source to transitions between three basic states: a hard state
corresponding to the non-observability of the innermost parts of the accretion
disk, and two softer states with a fully observable disk. These two soft states
represent different temperatures of the accretion disk, related to different
local values of the accretion rate. The transitions between these states can be
extremely fast. The source moves between these three states following certain
patterns and avoiding others, giving rise to a relatively large but limited
number of variability classes. These results are the first step towards a
linking of the properties of this exceptional source with standard black-hole
systems and with accretion disk models.Comment: Accepted for publication in Astronomy & Astrophysics, 2000 January
6t
Type I X-ray bursts, burst oscillations and kHz quasi-periodic oscillations in the neutron star system IGR J17191-2821
We present a detailed study of the X-ray energy and power spectral properties
of the neutron star transient IGR J17191-2821. We discovered four instances of
pairs of simultaneous kilohertz quasi-periodic oscillations (kHz QPOs). The
frequency difference between these kHz QPOs is between 315 Hz and 362 Hz. We
also report on the detection of five thermonuclear type-I X-ray bursts and the
discovery of burst oscillations at ~294 Hz during three of them. Finally, we
report on a faint and short outburst precursor, which occurred about two months
before the main outburst. Our results on the broadband spectral and variability
properties allow us to firmly establish the atoll source nature of IGR
J17191-2821.Comment: 9 pages, 7 figures - accepted for publication in MNRA
Thermostability in endoglucanases is fold-specific
<p>Abstract</p> <p>Background</p> <p>Endoglucanases are usually considered to be synergistically involved in the initial stages of cellulose breakdown-an essential step in the bioprocessing of lignocellulosic plant materials into bioethanol. Despite their economic importance, we currently lack a basic understanding of how some endoglucanases can sustain their ability to function at elevated temperatures required for bioprocessing, while others cannot. In this study, we present a detailed comparative analysis of both thermophilic and mesophilic endoglucanases in order to gain insights into origins of thermostability. We analyzed the sequences and structures for sets of endoglucanase proteins drawn from the Carbohydrate-Active enZymes (CAZy) database.</p> <p>Results</p> <p>Our results demonstrate that thermophilic endoglucanases and their mesophilic counterparts differ significantly in their amino acid compositions. Strikingly, these compositional differences are specific to protein folds and enzyme families, and lead to differences in intramolecular interactions in a fold-dependent fashion.</p> <p>Conclusions</p> <p>Here, we provide fold-specific guidelines to control thermostability in endoglucanases that will aid in making production of biofuels from plant biomass more efficient.</p
Conquering the Solar System with CubeSat Technology â First Results of CubeSat Hardware Beyond Low Earth Orbit
This paper sets out to show the in-flight results of The Netherlands-China Low-Frequency Explorer (NCLE) â one of the first times CubeSat hardware has left low Earth Orbit.
The Netherlands-China Low-Frequency Explorer (NCLE), is a low-frequency payload which is part of the Chinese Changâe 4 mission. The NCLE instrument consists of three 5-meter long monopole antennas mounted on the Queqiao satellite and will be measuring in the 80 kHz - 80 MHz radio frequency range. The instrument is designed to address a multitude of high-profile science cases, but predominantly NCLE will open up the low-frequency regime for radio astronomy and will prepare for the ground-breaking observations of the 21-cm line emission from the Dark Ages and the Cosmic Dawn, considered to be the holy grail of cosmology.
The design of the instrument began in May 2016, with a launch scheduled May 2018. This left only 2 years to develop, build and test the instrument. Given the short development time the design is based on COTS and space qualified components as much as possible, and a design and model philosophy common to nano-satellites was adopted. Even so, special care had to be taken as one of the main challenges of this mission is EMC. This is an area which is only marginally considered during a typical CubeSat project and required a different approach.
Following the delivery in March 2018, less than 2 years after the project started, the instruments was successful launched in the 21st of May 2018 and saw its first return of telemetry January 2019. In this paper, the design of the instrument will be covered, as well as the first in flight results which were obtained. These results indicate NCLE is performing admirably after having spent over a year in interplanetary space.
The NCLE instrument represents one of the first times the CubeSat methodology and hardware left Low Earth Orbit. This, together with the strict EMC requirements have resulted in CubeSat hardware which can be used in future interplanetary missions. The promising results give strong confidence in the technology and enables new mission opportunities which could not be served by CubeSats in the past. This will fuel the next phase of the CubeSat revolution where they will venture out into interplanetary space in support of bigger missions
Exploring Accretion and Disk-Jet Connections in the LLAGN M81*
We report on a year-long effort to monitor the central supermassive black
hole in M81 in the X-ray and radio bands. Using Chandra and the VLA, we
obtained quasi-simultaneous observations of M81* on seven occasions during
2006. The X-ray and radio luminosity of M81* are not strongly correlated on the
approximately 20-day sampling timescale of our observations, which is
commensurate with viscous timescales in the inner flow and orbital timecales in
a radially-truncated disk. This suggests that short-term variations in black
hole activity may not be rigidly governed by the "fundamental plane", but
rather adhere to the plane in a time-averaged sense. Fits to the X-ray spectra
of M81* with bremsstrahlung models give temperatures that are inconsistent with
the outer regions of very simple advection-dominated inflows. However, our
results are consistent with the X-ray emission originating in a transition
region where a truncated disk and advective flow may overlap. We discuss our
results in the context of models for black holes accreting at small fractions
of their Eddington limit, and the fundamental plane of black hole accretion.Comment: Accepted for publication in Ap
X-ray time variability across the atoll source states of 4U 1636--53
We have studied the rapid X-ray time variability in 149 pointed observations
with the \textit{Rossi X-ray Timing Explorer} (RXTE)'s Proportional Counter
Array of the atoll source 4U~1636--53 in the banana state and, for the first
time with RXTE, in the island state. We compare the frequencies of the
variability components of 4U~1636--53 with those in other atoll and Z-sources
and find that 4U~1636--53 follows the universal scheme of correlations
previously found for other atoll sources at (sometimes much) lower
luminosities. Our results on the hectohertz QPO suggest that the mechanism that
sets its frequency differs from that for the other components, while the
amplitude setting mechanism is common. A previously proposed interpretation of
the narrow low-frequency QPO frequencies in different sources in terms of
harmonic mode switching is not supported by our data, nor by some previous data
on other sources and the frequency range that this QPO covers is found not to
be related to spin, angular momentum or luminosity.Comment: 16 pages, 13 figures, accepted for publication in Ap
Correlated X-ray Spectral and Timing Behavior of the Black Hole Candidate XTE J1550-564: A New Interpretation of Black Hole States
We present an analysis of RXTE data of the X-ray transient XTE J1550-564. The
source went through several states, which were divided into spectrally soft and
hard states. These states showed up as distinct branches in the color-color
diagram, forming a structure with a comb-like topology; the soft state branch
forming the spine and the hard state branches forming the teeth. Variability
was strongly correlated with the position on the branches. The broad band noise
became stronger, and changed from power law like to band limited, as the
spectrum became harder. Three types of QPOs were found: 1-18 Hz and 102-284 Hz
QPOs on the hard branches, and 16-18 Hz QPOs on and near the soft branch. The
frequencies of the high and low frequency QPOs on the hard branches were
correlated with each other, and anti-correlated with spectral hardness. The
changes in QPO frequency suggest that the inner disc radius only increases by a
factor of 3-4 as the source changes from a soft to a hard state. Our results on
XTE J1550-564 strongly favor a 2-dimensional description of black hole
behavior, where the regions near the spine of the comb in the color-color
diagram can be identified with the high state, and the teeth with transitions
from the high state, via the intermediate state (which includes the very high
state) to the low state, and back. The two physical parameters underlying this
behavior vary to a large extent independently and could for example be the mass
accretion rate through the disk and the size of a Comptonizing region.Comment: 49 pages (inlcuding 26 figures and 4 tables), accepted for
publication in ApJ Supplement
High frequency quasi-periodic oscillations in the black hole X-ray transient XTE J1650-500
We report the detection of high frequency variability in the black hole X-ray
transient XTE J1650-500. A quasi-periodic oscillation (QPO) was found at 250 Hz
during a transition from the hard to the soft state. We also detected less
coherent variability around 50 Hz, that disappeared when the 250 Hz QPO showed
up. There are indications that when the energy spectrum hardened the QPO
frequency increased from ~110 Hz to ~270 Hz, although the observed frequencies
are also consistent with being 1:2:3 harmonics of each other. Interpreting the
250 Hz as the orbital frequency at the innermost stable orbit around a
Schwarzschild black hole leads to a mass estimate of 8.2 Msun. The spectral
results by Miller et al.(2002, ApJ, 570, L69), which suggest considerable black
hole spin, would imply a higher mass.Comment: Submitted to ApJ, 12 pages including 2 figure
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