462 research outputs found
CTMC calculations of electron capture and ionization in collisions of multiply charged ions with elliptical Rydberg atoms
We have performed classical trajectory Monte Carlo (CTMC) studies of electron
capture and ionization in multiply charged (Q=8) ion-Rydberg atom collisions at
intermediate impact velocities. Impact parallel to the minor and to the major
axis, respectively, of the initial Kepler electron ellipse has been
investigated. The important role of the initial electron momentum distribution
found for singly charged ion impact is strongly disminished for higher
projectile charge, while the initial spatial distribution remains important for
all values of Q studied.Comment: 3 pages, 5 figure
Continued Neutron Star Crust Cooling of the 11 Hz X-Ray Pulsar in Terzan 5: A Challenge to Heating and Cooling Models?
The transient neutron star low-mass X-ray binary and 11 Hz X-ray pulsar IGR
J17480-2446 in the globular cluster Terzan 5 exhibited an 11-week accretion
outburst in 2010. Chandra observations performed within five months after the
end of the outburst revealed evidence that the crust of the neutron star became
substantially heated during the accretion episode and was subsequently cooling
in quiescence. This provides the rare opportunity to probe the structure and
composition of the crust. Here, we report on new Chandra observations of Terzan
5 that extend the monitoring to ~2.2 yr into quiescence. We find that the
thermal flux and neutron star temperature have continued to decrease, but
remain significantly above the values that were measured before the 2010
accretion phase. This suggests that the crust has not thermally relaxed yet,
and may continue to cool. Such behavior is difficult to explain within our
current understanding of heating and cooling of transiently accreting neutron
stars. Alternatively, the quiescent emission may have settled at a higher
observed equilibrium level (for the same interior temperature), in which case
the neutron star crust may have fully cooled.Comment: Accepted to ApJ without revision. Updated references and fixed few
typos to match published version. 7 pages, 3 figures, 3 table
Extreme Quiescent Variability of the Transient Neutron Star Low-mass X-ray Binary EXO 1745-248 in Terzan 5
EXO 1745-248 is a transient neutron-star low-mass X-ray binary that resides
in the globular cluster Terzan 5. We studied the transient during its quiescent
state using 18 Chandra observations of the cluster acquired between 2003 and
2016. We found an extremely variable source, with a luminosity variation in the
0.5-10 keV energy range of orders of magnitude (between
erg s and erg s) on timescales
from years down to only a few days. Using an absorbed power-law model to fit
its quiescent spectra, we obtained a typical photon index of ,
indicating that the source is even harder than during outburst and much harder
than typical quiescent neutron stars if their quiescent X-ray spectra are also
described by a single power-law model. This indicates that EXO 1745-248 is very
hard throughout the entire observed X-ray luminosity range. At the highest
luminosity, the spectrum fits better when an additional (soft) component is
added to the model. All these quiescent properties are likely related to strong
variability in the low-level accretion rate in the system. However, its extreme
variable behavior is strikingly different from the one observed for other
neutron star transients that are thought to still accrete in quiescence. We
compare our results to these systems. We also discuss similarities and
differences between our target and the transitional millisecond pulsar IGR
J18245-2452, which also has hard spectra and strong variability during
quiescence.Comment: Accepted for publication in MNRA
Determining the effects of clumping and porosity on the chemistry in a non-uniform AGB outflow
(abridged) In the inner regions of AGB outflows, several molecules have been
detected with abundances much higher than those predicted from thermodynamic
equilibrium (TE) chemical models. The presence of the majority of these species
can be explained by shock-induced non-TE chemical models, where shocks caused
by the pulsating star take the chemistry out of TE in the inner region.
Moreover, a non-uniform density structure has been detected in several AGB
outflows. A detailed parameter study on the quantitative effects of a
non-homogeneous outflow has so far not been performed. We implement a porosity
formalism for treating the increased leakage of light associated with radiation
transport through a clumpy, porous medium. The effects from the altered UV
radiation field penetration on the chemistry, accounting also for the increased
reaction rates of two-body processes in the overdense clumps, are examined. We
present a parameter study of the effect of clumping and porosity on the
chemistry throughout the outflow. Both the higher density within the clumps and
the increased UV radiation field penetration have an important impact on the
chemistry, as they both alter the chemical pathways. The increased amount of UV
radiation in the inner region leads to photodissociation of parent species,
releasing the otherwise deficient elements. We find an increased abundance in
the inner region of all species not expected to be present assuming TE
chemistry, such as HCN in O-rich outflows, HO in C-rich outflows, and
NH in both. Outflows whose clumps have a large overdensity and that are
very porous to the interstellar UV radiation field yield abundances comparable
to those observed in O- and C-rich outflows for most of the unexpected species
investigated. The inner wind abundances of HO in C-rich outflows and of
NH in O- and C-rich outflows are however underpredicted.Comment: 33 pages, 20 figures, 15 tables, accepted for publication in
Astronomy & Astrophysic
The 2005 outburst of GRO J1655-40: spectral evolution of the rise, as observed by Swift
We present Swift observations of the black hole X-ray transient, GRO
J1655-40, during the recent outburst. With its multiwavelength capabilities and
flexible scheduling, Swift is extremely well-suited for monitoring the spectral
evolution of such an event. GRO J1655-40 was observed on 20 occasions and data
were obtained by all instruments for the majority of epochs. X-ray spectroscopy
revealed spectral shapes consistent with the ``canonical'' low/hard, high/soft
and very high states at various epochs. The soft X-ray source (0.3-10 keV) rose
from quiescence and entered the low/hard state, when an iron emission line was
detected. The soft X-ray source then softened and decayed, before beginning a
slow rebrightening and then spending weeks in the very high state. The
hard X-rays (14-150 keV) behaved similarly but their peaks preceded those of
the soft X-rays by up to a few days; in addition, the average hard X-ray flux
remained approximately constant during the slow soft X-ray rebrightening,
increasing suddenly as the source entered the very high state. These
observations indicate (and confirm previous suggestions) that the low/hard
state is key to improving our understanding of the outburst trigger and
mechanism. The optical/ultraviolet lightcurve behaved very differently from
that of the X-rays; this might suggest that the soft X-ray lightcurve is
actually a composite of the two known spectral components, one gradually
increasing with the optical/ultraviolet emission (accretion disc) and the other
following the behaviour of the hard X-rays (jet and/or corona).Comment: Accepted for publication in MNRA
Recommended from our members
Spring 1956
This year the newly organized Turf Management Club has undertaken the publication of a booklet featuring various aspects of turf work. Through this we are attempting to present material that will be of interest to those who are familiar with the Massachusetts Turf Schools. At the same time we hope that it may have some educational value by presenting the view of both of those of us on the job and others engaged in research and promotion and selling.
The publication will include information about the current years winter school and turf conference, articles about some of the professors here at the University who are responsible for the course work, reports about activities and honors earned by the Stockbridge Turf Majors while on campus; and reports on research in fine turf conducted here at the University of Massachusetts. There will also be articles written by men connected with turf work such as yourselves and articles written by staff members at the University.
The main objective of this publication is to form a bond of common interest and friendship between the alumni and other friends of our turf schools. Those of us who graduate this year are looking forward to getting the news from the university in years to come. We hope you feel the same
Mode Spectroscopy and Level Coupling in Ballistic Electron Waveguides
A tunable quantum point contact with modes occupied in both transverse
directions is studied by magnetotransport experiments. We use conductance
quantization of the one-dimensional subbands as a tool to determine the mode
spectrum. A magnetic field applied along the direction of the current flow
couples the modes. This can be described by an extension of the Darwin-Fock
model. Anticrossings are observed as a function of the magnetic field, but not
for zero field or perpendicular field directions, indicating coupling of the
subbands due to nonparabolicity in the electrical confinement.Comment: 4 pages, 3 figure
Variability of X-ray binaries from an oscillating hot corona
The spectral and timing properties of an oscillating hot thermal corona are
investigated. This oscillation is assumed to be due to a magneto-acoustic wave
propagating within the corona and triggered by an external, non specified,
excitation. A cylindrical geometry is adopted and, neglecting the rotation, the
wave equation is solved in for different boundary conditions. The resulting
X-ray luminosity, through thermal comptonization of embedded soft photons, is
then computed, first analytically, assuming linear dependence between the local
pressure disturbance and the radiative modulation. These calculations are also
compared to Monte-Carlo simulations. The main results of this study are: (1)
the corona plays the role of a low band-pass medium, its response to a white
noise excitation being a at top noise Power Spectral Density (PSD) at low
frequencies and a red noise at high frequency, (2) resonant peaks are present
in the PSD. Their powers depend on the boundary conditions chosen and, more
specifically, on the impedance adaptation with the external medium at the
corona inner boundary. (3) The flat top noise level and break as well as the
resonant peak frequencies are inversely proportional to the external radius rj.
(4) Computed rms and f-spectra exhibit an overall increase of the variability
with energy. Comparison with observed variability features, especially in the
hard intermediate states of X-ray binaries are discussed.Comment: 12 pages, 7 figures, accepted for publication in MNRA
Variability as a Predictor for the Hard-to-soft State Transition in GX 339â4
During the outbursts of black hole X-ray binaries (BHXRBs), their accretion flows transition through several states. The source luminosity rises in the hard state, dominated by nonthermal emission, before transitioning to the blackbody-dominated soft state. As the luminosity decreases, the source transitions back into the hard state and fades to quiescence. This picture does not always hold, as â40% of the outbursts never leave the hard state. Identifying the physics that govern state transitions remains one of the outstanding open questions in black hole astrophysics. In this paper we present an analysis of archival RXTE data of multiple outbursts of GX 339â4. We compare the properties of the X-ray variability and time-averaged energy spectrum and demonstrate that the variability (quantified by the power spectral hue) systematically evolves â10â40 days ahead of the canonical state transition (quantified by a change in spectral hardness); no such evolution is found in hard-state-only outbursts. This indicates that the X-ray variability can be used to predict if and when the hard-to-soft state transition will occur. Finally, we find a similar behavior in 10 outbursts of four additional BHXRBs with more sparse observational coverage. Based on these findings, we suggest that state transitions in BHXRBs might be driven by a change in the turbulence in the outer regions of the disk, leading to a dramatic change in variability. This change is only seen in the spectrum days to weeks later, as the fluctuations propagate inwards toward the corona
Continued cooling of the accretion-heated neutron star crust in the X-ray transient IGR J17480-2446 located in the globular cluster Terzan 5
We present a new Chandra observation (performed in July 2016) of the neutron
star X-ray transient IGR J17480-2446, located in the globular cluster Terzan 5.
We study the continued cooling of the neutron star crust in this system that
was heated during the 2010 outburst of the source. This new observation was
performed two years after the last observation of IGR J17480-2446, hence,
significantly extending the cooling baseline. We reanalysed all available
Chandra observations of the source (but excluding observations during which one
of the known transients in Terzan 5 was in outburst) and fitted the obtained
cooling curve with our cooling code NSCool, which allows for much improved
modelling than what was previously performed for the source. The data and our
fit models indicate that the crust was still cooling ~5.5 years after the
outburst ended. The neutron star crust has likely not reached crust-core
thermal equilibrium yet, and further cooling is predicted (which can be
confirmed with additional Chandra observations in >5 years). Intriguingly, we
find indications that the thermal conductivity might be relatively low in part
of the crust compared to what has been inferred for other crust-cooling sources
and tentatively suggest that this layer might be located around the neutron
drip. The reason for this difference is unclear, but might be related to the
fact that IGR J17480-2446 harbours a relatively slowly rotating neutron star
(with a spin of 11 Hz) that has a relatively strong inferred surface magnetic
field ( Gauss) compared to what is known or typically assumed for
other cooling sources.Comment: 17 pages, 10 figures, 4 tables, accepted for publication in MNRA
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