4,723 research outputs found
Properties of the redback millisecond pulsar binary 3FGL J0212.1+5320
Linares et al. (2016) obtained quasi-simultaneous g', r' and i-band light
curves and an absorption line radial velocity curve of the secondary star in
the redback system 3FGL J0212.1+5320. The light curves showed two maxima and
minima primarily due to the secondary star's ellipsoidal modulation, but with
unequal maxima and minima. We fit these light curves and radial velocities with
our X-ray binary model including either a dark solar-type star spot or a hot
spot due to off-centre heating from an intrabinary shock, to account for the
unequal maxima. Both models give a radial velocity semi-amplitude and
rotational broadening that agree with the observations. The observed secondary
star's effective temperature is best matched with the value obtained using the
hot spot model, which gives a neutron star and secondary star mass of =1.85 and =0.50
, respectively.Comment: 10 pages, 8 figues, accepted by MNRA
Low-frequency QPO from the 11 Hz accreting pulsar in Terzan 5: not frame dragging
We report on 6 RXTE observations taken during the 2010 outburst of the 11 Hz
accreting pulsar IGR J17480-2446 located in the globular cluster Terzan 5.
During these observations we find power spectra which resemble those seen in
Z-type high-luminosity neutron star low-mass X-ray binaries, with a
quasi-periodic oscillation (QPO) in the 35-50 Hz range simultaneous with a kHz
QPO and broad band noise. Using well known frequency-frequency correlations, we
identify the 35-50 Hz QPOs as the horizontal branch oscillations (HBO), which
were previously suggested to be due to Lense-Thirring precession. As IGR
J17480-2446 spins more than an order of magnitude more slowly than any of the
other neutron stars where these QPOs were found, this QPO can not be explained
by frame dragging. By extension, this casts doubt on the Lense-Thirring
precession model for other low-frequency QPOs in neutron-star and perhaps even
black-hole systems.Comment: 6 pages, 5 figures, Accepted for publication in ApJ
La sedimentación deltaica actual en el embalse de Cellers (Río Noguera Pallaresa, provincia de Lérida)
[Resumen] Condicionados por diversos factores, dependientes principalmente de las características morfodinámicas de la zona y del régimen de explotación, la sedimentación costera muestra un considerable desarrollo en el embalse de Cellers (río Noguera Pallaresa, provincia de Lérida). A partir de datos geomorfológicos y sedimentológicos obtenidos en superficie, en este trabajo se aborda el estudio de los cuerpos sedimentarios originados. Se describen sus características y el mecanismo de sedimentación. Por último, se incide en la previsible evolución de estos depósitos deltaicos ante hipotéticas variaciones del nivel de base.[Abstract] The Cellers reservoir at the Noguera Pallaresa river (Lérida) shows a elldeveloped sedimentacion along its shores, due to the regional dominant geomorphic processes and the exploitation regime. Using geomorphological and sedimentological data, we describe the carachteristics of the sedimentary bodies and their depositional mechanisms. We also discuss
possible evolutionary trends of these deltaic deposits as a response to base level changes
Constraining the properties of neutron star crusts with the transient low-mass X-ray binary Aql X-1
Aql X-1 is a prolific transient neutron star low-mass X-ray binary that
exhibits an accretion outburst approximately once every year. Whether the
thermal X-rays detected in intervening quiescent episodes are the result of
cooling of the neutron star or due to continued low-level accretion remains
unclear. In this work we use Swift data obtained after the long and bright 2011
and 2013 outbursts, as well as the short and faint 2015 outburst, to
investigate the hypothesis that cooling of the accretion-heated neutron star
crust dominates the quiescent thermal emission in Aql X-1. We demonstrate that
the X-ray light curves and measured neutron star surface temperatures are
consistent with the expectations of the crust cooling paradigm. By using a
thermal evolution code, we find that ~1.2-3.2 MeV/nucleon of shallow heat
release describes the observational data well, depending on the assumed
mass-accretion rate and temperature of the stellar core. We find no evidence
for varying strengths of this shallow heating after different outbursts, but
this could be due to limitations of the data. We argue that monitoring Aql X-1
for up to ~1 year after future outbursts can be a powerful tool to break model
degeneracies and solve open questions about the magnitude, depth and origin of
shallow heating in neutron star crusts.Comment: 14 pages, 5 figures, 3 tables, accepted to MNRA
Low-level accretion in neutron-star X-ray binaries
We search the literature for reports on the spectral properties of
neutron-star low-mass X-ray binaries when they have accretion luminosities
between 1E34 and 1E36 ergs/s. We found that in this luminosity range the photon
index (obtained from fitting a simple absorbed power-law in the 0.5-10 keV
range) increases with decreasing 0.5-10 keV X-ray luminosity (i.e., the
spectrum softens). Such behaviour has been reported before for individual
sources, but here we demonstrate that very likely most (if not all)
neutron-star systems behave in a similar manner and possibly even follow a
universal relation. When comparing the neutron-star systems with black-hole
systems, it is clear that most black-hole binaries have significantly harder
spectra at luminosities of 1E34 - 1E35 erg/s. Despite a limited number of data
points, there are indications that these spectral differences also extend to
the 1E35 - 1E36 erg/s range. This observed difference between the neutron-star
binaries and black-hole ones suggests that the spectral properties (between
0.5-10 keV) at 1E34 - 1E35 erg/s can be used to tentatively determine the
nature of the accretor in unclassified X-ray binaries. We discuss our results
in the context of properties of the accretion flow at low luminosities and we
suggest that the observed spectral differences likely arise from the
neutron-star surface becoming dominantly visible in the X-ray spectra. We also
suggest that both the thermal component and the non-thermal component might be
caused by low-level accretion onto the neutron-star surface for luminosities
below a few times 1E34 erg/s.Comment: Accepted for publication in MNRA
A complex state transition from the black hole candidate Swift J1753.5-0127
We present our monitoring campaign of the outburst of the black-hole
candidate Swift J1753.5-0127, observed with the Rossi X-ray Timing Explorer and
the Swift satellites. After ~4.5 years since its discovery, the source had a
transition to the hard intermediate state. We performed spectral and timing
studies of the transition showing that, unlike the majority of the transient
black holes, the system did not go to the soft states but it returned to the
hard state after a few months. During this transition Swift J1753.5-0127
features properties which are similar to those displayed by the black hole
Cygnus X-1. We compared Swift J1753.5-0127 to one dynamically confirmed black
hole and two neutron stars showing that its power spectra are in agreement with
the binary hosting a black hole. We also suggest that the prolonged period at
low flux that followed the initial flare is reminiscent of that observed in
other X-ray binaries, as well as in cataclysmic variables.Comment: Accepted for publication in MNRA
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
A return to strong radio flaring by Circinus X-1 observed with the Karoo Array Telescope test array KAT-7
Circinus X-1 is a bright and highly variable X-ray binary which displays
strong and rapid evolution in all wavebands. Radio flaring, associated with the
production of a relativistic jet, occurs periodically on a ~17-day timescale. A
longer-term envelope modulates the peak radio fluxes in flares, ranging from
peaks in excess of a Jansky in the 1970s to an historic low of milliJanskys
during the years 1994 to 2007. Here we report first observations of this source
with the MeerKAT test array, KAT-7, part of the pathfinder development for the
African dish component of the Square Kilometre Array (SKA), demonstrating
successful scientific operation for variable and transient sources with the
test array. The KAT-7 observations at 1.9 GHz during the period 13 December
2011 to 16 January 2012 reveal in temporal detail the return to the
Jansky-level events observed in the 1970s. We compare these data to
contemporaneous single-dish measurements at 4.8 and 8.5 GHz with the HartRAO
26-m telescope and X-ray monitoring from MAXI. We discuss whether the overall
modulation and recent dramatic brightening is likely to be due to an increase
in the power of the jet due to changes in accretion rate or changing Doppler
boosting associated with a varying angle to the line of sight.Comment: 7 pages, 5 figures, accepted for publication in MNRAS 14 May 201
Definition of Low Earth Orbit slotting architectures using 2D lattice flower constellations
This work proposes the use of 2D Lattice Flower Constellations (2D-LFCs) to facilitate the design of a Low Earth Orbit (LEO) slotting system to avoid collisions between compliant satellites and to optimize the available orbital volume. Specifically, this manuscript proposes the use of concentric orbital shells of admissible “slots” with stacked intersecting orbits that preserve a minimum separation distance between satellites at all times. The problem is formulated in mathematical terms and three approaches are explored: random constellations, single 2D-LFCs, and unions of 2D-LFCs. Each approach is evaluated in terms of several metrics including capacity, Earth coverage, orbits per shell, and symmetries. Additionally, a rough estimate for the capacity of LEO is generated, subject to certain minimum separation and station-keeping assumptions, and several trade-offs are identified to guide policy-makers interested in the adoption of a LEO slotting scheme for space traffic management
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