776 research outputs found
A Deep Pulse Search in Eleven Low Mass X-Ray Binaries
We present a systematic coherent X-ray pulsation search in eleven low mass
X-ray binaries (LMXBs). We select a relatively broad variety of LMXBs,
including persistent and transient sources and spanning orbital periods between
0.3 and 17 hours. We use about 3.6 Ms of data collected by the Rossi X-Ray
Timing Explorer (RXTE) and XMM-Newton and apply a semi-coherent search strategy
to look for weak and persistent pulses in a wide spin frequency range. We find
no evidence for X-ray pulsations in these systems and consequently set upper
limits on the pulsed sinusoidal semi-amplitude between 0.14% and 0.78% for ten
outbursting/persistent LMXBs and 2.9% for a quiescent system. These results
suggest that weak pulsations might not form in (most) non-pulsating LMXBs.Comment: submitted to ApJ, 8 page
A Stress/Displacement Virtual Element Method for Plane Elasticity Problems
The numerical approximation of 2D elasticity problems is considered, in the
framework of the small strain theory and in connection with the mixed
Hellinger-Reissner variational formulation. A low-order Virtual Element Method
(VEM) with a-priori symmetric stresses is proposed. Several numerical tests are
provided, along with a rigorous stability and convergence analysis
1 Hz Flaring in the Accreting Millisecond Pulsar NGC 6440 X-2: Disk Trapping and Accretion Cycles
The dynamics of the plasma in the inner regions of an accretion disk around
accreting millisecond X-ray pulsars is controlled by the magnetic field of the
neutron star. The interaction between an accretion disk and a strong magnetic
field is not well-understood, particularly at low accretion rates (the
so-called ``propeller regime'). This is due in part to the lack of clear
observational diagnostics to constrain the physics of the disk-field
interaction. Here we associate the strong ~1 Hz modulation seen in the
accreting millisecond X-ray pulsar NGC 6440 X-2 with an instability that arises
when the inner edge of the accretion disk is close to the corotation radius
(where the stellar rotation rate matches the Keplerian speed in the disk). A
similar modulation has previously been observed in another accreting
millisecond X-ray pulsar (SAX J1808.4-3658) and we suggest that the two
phenomena are related and that this may be a common phenomenon among other
magnetized systems. Detailed comparisons with theoretical models suggest that
when the instability is observed, the interaction region between the disk and
the field is very narrow -- of the order of 1 km. Modelling further suggests
that there is a transition region (~1-10 km) around the corotation radius where
the disk-field torque changes sign from spin up to spin down. This is the first
time that a direct observational constraint has been placed on the width of the
disk-magnetosphere interaction region, in the frame of the trapped-disk
instability model.Comment: Accepted by ApJ, minor revisio
The Optical Counterpart to the Accreting Millisecond X-ray Pulsar SAX J1748.9-2021 in the Globular Cluster NGC 6440
We used a combination of deep optical and Halpha images of the Galactic
globular cluster NGC 6440, acquired with the Hubble Space Telescope, to
identify the optical counterpart to the accreting millisecond X-ray pulsar SAX
J1748.9-2021during quiescence. A strong Halpha emission has been detected from
a main sequence star (hereafter COM-SAX J1748.9-2021) located at only 0.15"
from the nominal position of the X-ray source. The position of the star also
agrees with the optical counterpart found by Verbunt et al. (2000) during an
outburst. We propose this star as the most likely optical counterpart to the
binary system. By direct comparison with isochrones, we estimated that COM-SAX
J1748.9-2021 has a mass of 0.70 Msun - 0.83 Msun, a radius of 0.88 pm 0.02 Rsun
and a superficial temperature of 5250pm80 K. These parameters combined with the
orbital characteristics of the binary suggest that the system is observed at a
very low inclination angle (~8 deg -14 deg) and that the star is filling or
even overflowing its Roche Lobe. This, together with the equivalent width of
the Halpha emission (~20 Ang), suggest possible on-going mass transfer. The
possibile presence of such a on-going mass transfer during a quiescence state
also suggests that the radio pulsar is not active yet and thus this system,
despite its similarity with the class of redback millisecond pulsars, is not a
transitional millisecond pulsar.Comment: 8 pages, 6 figures. Accepted for publication in Ap
The Radiative Efficiency of a Radiatively Inefficient Accretion Flow
A recent joint XMM-Newton/Nuclear Spectroscopic Telescope Array (NuSTAR)
observation of the accreting neutron star Cen X-4 () revealed a hard power-law component
(-) with a relatively low cut-off energy (~10 keV),
suggesting bremsstrahlung emission. The physical requirements for
bremsstrahlung combined with other observed properties of Cen X-4 suggest the
emission comes from a boundary layer rather than the accretion flow. The
accretion flow itself is thus undetected (with an upper limit of ). A deep search for coherent pulsations (which
would indicate a strong magnetic field) places a 6 per cent upper limit on the
fractional amplitude of pulsations, suggesting the flow is not magnetically
regulated. Considering the expected energy balance between the accretion flow
and the boundary layer for different values of the neutron star parameters
(size, magnetic field, and spin) we use the upper limit on to
set an upper limit of for the intrinsic radiative
efficiency of the accretion flow for the most likely model of a fast-spinning,
non-magnetic neutron star. The non-detection of the accretion flow provides the
first direct evidence that this flow is indeed 'radiatively inefficient', i.e.
most of the gravitational potential energy lost by the flow before it hits the
star is not emitted as radiation.Comment: 15 pages, 3 figures - minor modifications to match published versio
Discovery of coherent millisecond X-ray pulsations in Aql X-1
We report the discovery of an episode of coherent millisecond X-ray pulsation
in the neutron star low-mass X-ray binary Aql X-1. The episode lasts for
slightly more than 150 seconds, during which the pulse frequency is consistent
with being constant. No X-ray burst or other evidence of thermonuclear burning
activity is seen in correspondence with the pulsation, which can thus be
identified as occurring in the persistent emission. The pulsation frequency is
550.27 Hz, very close (0.5 Hz higher) to the maximum reported frequency from
burst oscillations in this source. Hence we identify this frequency with the
neutron star spin frequency. The pulsed fraction is strongly energy dependent,
ranging from 10% (16-30 keV). We discuss possible physical
interpretations and their consequences for our understanding of the lack of
pulsation in most neutron star low-mass X-ray binaries. If interpreted as
accretion-powered pulsation, Aql X-1 might play a key role in understanding the
differences between pulsating and non-pulsating sources.Comment: 5 pages, 3 figures, accepted by ApJ Letters after minor revisions.
Slightly extended discussion. One author added. Uses emulateapj.cl
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
X-ray Observations of XSS J12270-4859 in a New Low State: A Transformation to a Disk-Free Rotation-Powered Pulsar Binary
We present XMM-Newton and Chandra observations of the low-mass X-ray binary
XSS J12270--4859, which experienced a dramatic decline in optical/X-ray
brightness at the end of 2012, indicative of the disappearance of its accretion
disk. In this new state, the system exhibits previously absent
orbital-phase-dependent, large-amplitude X-ray modulations with a decline in
flux at superior conjunction. The X-ray emission remains predominantly
non-thermal but with an order of magnitude lower mean luminosity and
significantly harder spectrum relative to the previous high flux state. This
phenomenology is identical to the behavior of the radio millisecond pulsar
binary PSR J1023+0038 in the absence of an accretion disk, where the X-ray
emission is produced in an intra-binary shock driven by the pulsar wind. This
further demonstrates that XSS J12270-4859 no longer has an accretion disk and
has transformed to a full-fledged eclipsing "redback" system that hosts an
active rotation-powered millisecond pulsar. There is no evidence for diffuse
X-ray emission associated with the binary that may arise due to outflows or a
wind nebula. An extended source situated 1.5' from XSS J12270--4859 is unlikely
to be associated, and is probably a previously uncatalogued galaxy cluster.Comment: 8 pages, 6 figures; accepted for publication in the Astrophysical
Journa
Enhancing the Sustainability of the Aviation Industry: Airlines’ Commitment to “Green” Practices
The aviation industry represents an important polluter, being responsible for increasing
environmental impacts on global scale. Aiming to approach the adoption of suitable policies
in the aviation industry towards the achievement of the national and international
sustainability goals, the present research tackles airlines’ commitment to aviation-related
environmental issues, as well as their willingness to adopt sustainable aviation fuel (i.e., bio
jet fuel) and sustainable development strategies, focusing on those companies operating
flights in the Karol Wojtyła Airport (Bari, Italy). The paper adopts the χ2 test and the logistic
regression to investigate three different hypotheses related to airlines’ headquarters, carriers’
typology (i.e., low-cost or not, flag carriers or not) and years of service. Results outline that
traditional airlines, either flag carriers or not, as well as South and North American
companies, are more likely to be aware of aviation environmental consequences, publishing
environmental reports and offering to passengers the chance to participate to climate change
reduction (e.g., through online carbon offset programs or more expensive ticket to produce
bio jet fuels). In addition, airlines transiting in Karol Wojtyła Airport show a small
willingness to share information through environmental reports and are scarcely intentioned
to make use of bio jet fuels, confirming that low-cost companies are still less attentive
towards aviation environmental issues. The present research contributes to the empirical
studies on sustainable aviation and carriers’ commitment to environmental strategies,
highlighting the need to enhance carbon offsets programs and digital technologies as the
online compensation of CO2 emissions
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