182 research outputs found
Dead zone in the polar-cap accelerator of pulsars
We study plasma flows above pulsar polar caps using time-dependent
simulations of plasma particles in the self-consistent electric field. The flow
behavior is controlled by the dimensionless parameter alpha=(j/c rho_GJ) where
j is the electric current density and rho_GJ is the Goldreich-Julian charge
density. The region of the polar cap where 0<alpha<1 is a "dead zone" --- in
this zone particle acceleration is inefficient and pair creation is not
expected even for young, rapidly rotating pulsars. Pulsars with polar caps near
the rotation axis are predicted to have a hollow-cone structure of radio
emission, as the dead zone occupies the central part of the polar cap. Our
results apply to charge-separated flows of electrons (j0). In
the latter case, we consider the possibility of a mixed flow consisting of
different ion species, and observe the development of two-stream instability.
The dead zone at the polar cap is essential for the development of an outer gap
near the null surface rho_GJ=0.Comment: 10 pages, 11 figures, accepted to Ap
Dynamical chaos in the problem of magnetic jet collimation
We investigate dynamics of a jet collimated by magneto-torsional
oscillations. The problem is reduced to an ordinary differential equation
containing a singularity and depending on a parameter. We find a parameter
range for which this system has stable periodic solutions and study
bifurcations of these solutions. We use Poincar\'e sections to demonstrate
existence of domains of regular and chaotic motions. We investigate transition
from periodic to chaotic solutions through a sequence of period doublings.Comment: 11 pages, 29 figures, 1 table, MNRAS (published online
Modeling the Processes of Managing the Advertising Budget ofan Electronic Trading Platform
The paper presents the possibilities of using system-dynamic modeling to manage the advertising budget ofan electronic trading platform. A model has been developed and its mathematical description is given. Computer experiments have been carried out corresponding to various configurations of the advertising budget (distribution between different target groups) with the calculation of such efficiency indicators as profitability, total costs due to irrational advertising costs, and the cost ofattracting one client. © 2022 American Institute of Physics Inc.. All rights reserved.Simos T.E.Simos T.E.Simos T.E.Simos T.E.Tsitouras C
The origin of the diffuse non-thermal X-ray and radio emission in the Ophiuchus cluster of galaxies
We present high resolution 240 and 607 MHz GMRT radio observations,
complemented with 74 MHz archival VLA radio observations of the Ophiuchus
cluster of galaxies, whose radio mini-halo has been recently detected at 1400
MHz. We also present archival Chandra and XMM-Newton data of the Ophiuchus
cluster. Our observations do not show significant radio emission from the
mini-halo, hence we present upper limits to the integrated, diffuse non-thermal
radio emission of the core of the Ophiuchus cluster. The XMM-Newton
observations can be well explained by a two-temperature thermal model with
temperatures of ~=1.8 keV and ~=9.0 keV, respectively, which confirms previous
results that suggest that the innermost central region of the Ophiuchus cluster
is a cooling core. We also used the XMM-Newton data to set up an upper limit to
the (non-thermal) X-ray emission from the cluster.
The combination of available radio and X-ray data has strong implications for
the currently proposed models of the spectral energy distribution (SED) from
the Ophiuchus cluster. In particular, a synchrotron+IC model is in agreement
with the currently available data, if the average magnetic field is in the
range (0.02-0.3) microG. A pure WIMP annihilation scenario can in principle
reproduce both radio and X-ray emission, but at the expense of postulating very
large boost factors from dark matter substructures, jointly with extremely low
values of the average magnetic field. Finally, a scenario where synchrotron and
inverse Compton emission arise from PeV electron-positron pairs (via
interactions with the CMB), can be ruled out, as it predicts a non-thermal soft
X-ray emission that largely exceeds the thermal Bremsstrahlung measured by
INTEGRAL.Comment: Accepted for publication in MNRAS; 13 pages, 8 figures. Includes
minor changes. Abridged abstrac
C, О, S, and Sr Isotope Geochemistry and Chemostratigraphy of Ordovician Sediments in the Moyero River Section, Northern Siberian Platform
© 2018, Pleiades Publishing, Inc. The 87Sr/86Sr ratio in gypsum and limestones of the Ordovician section of the Moyero River decreases from the bottom upward from 0.7091‒0.7095 in the Irbukli Formation (Nyaian Regional Stage, ~Lower Ordovician Tremadocian Stage) to 0.7080 in the upper part of the Dzherom Formation (Dolborian Regional Stage, ~Upper Ordovician Katian Stage), which is well consistent with biostratigraphic subdivision of the section and existing concept concerning the strontium isotope evolution of the World Ocean. The most characteristic feature of the carbon isotope curve is decrease of δ13С values in carbonates from weakly positive values (0.5…1.1‰) in the Irbukli Formation (Nyaian Regional Stage) to sharply negative values (–5.4..–5.8‰) in the middle part of the Kochakan Formation (top of the Kimaian Regional Stage, ~end of the Dapingian–base of the Darriwilian Stage). Increase of δ18О from 20‒22‰ to 26‒28‰, the negative correlation of δ13С and δ18О, and decrease of δ34S in gypsum from 30‒32‰ to 22‒24‰ in this interval indicate that the 13С depletion of carbonates was not related to the sulfate reduction and oxidation of organic matter during diagenesis and that the negative δ13С excursion was of primary nature. The presence of negative δ13С anomalies at this stratigraphic level in Ordovician sections of the South and North America (Buggish et al., 2003; Edwards and Saltzman, 2014; McLaughlin et al., 2016) indicates the global or subglobal distribution of this event, which was possibly related to the emergence of the oldest ground vegetation. Against the general decrease of δ13С, the lower part of the section reveals three low-amplitude (1‒2‰) positive excursions, the position of which in general confirms the existing correlation scheme of the Moyero River section with the international scale. The upper part of the section is characterized by the alternation of low-δ13С intervals (from–2 to–3‰) and brief positive excursions with amplitude of 0.5‒1.3‰. The positive δ13С excursion terminating the Ordovician section of the Moyero River correlates with the δ13С excursion in the middle Katian Stage, while the δ13С excursion in the lower part of the Baksian Regional Stage correlates with the excursion marking the Katian–Sandbian boundary
RRATs: New Discoveries, Timing Solutions & Musings
We describe observations of Rotating RAdio Transients (RRATs) that were
discovered in a re-analysis of the Parkes Multi-beam Pulsar Survey (PMPS). The
sources have now been monitored for sufficiently long to obtain seven new
coherent timing solutions, to make a total of 14 now known. Furthermore we
announce the discovery of 7 new transient sources, one of which may be
extragalactic in origin (with ) and would then be a second example of
the so-called `Lorimer burst'. The timing solutions allow us to infer neutron
star characteristics such as energy-loss rate, magnetic field strength and
evolutionary timescales, as well as facilitating multi-wavelength followup by
providing accurate astrometry. All of this enables us to consider the question
of whether or not RRATs are in any way special, i.e. a distinct and separate
population of neutron stars, as has been previously suggested. We see no reason
to consider 'RRAT' as anything other than a detection label, the subject of a
selection effect in the parameter space searched. However, single-pulse
searches can be utilised to great effect to identify pulsars difficult, or
impossible, to find by other means, in particular those with long-periods (half
of the PMPS RRATs have periods greater than 4 seconds), high-magnetic field
strengths ( G) and pulsars approaching the 'death valley'.
The detailed nulling properties of such pulsars are unknown but the mounting
evidence suggests a broad range of behaviour in the pulsar population. The
group of RRATs fit in to the picture where pulsar magnetospheres switch between
stable configurations.Comment: accepted for publication in MNRAS, 17 pages, 7 figures, 3 table
On the Pulse Intensity Modulation of PSR B0823+26
We investigate the radio emission behaviour of PSR B0823+26, a pulsar which
is known to undergo pulse nulling, using an 153-d intensive sequence of
observations. The pulsar is found to exhibit both short (~min) and unusually
long-term (~hours or more) nulls, which not only suggest that the source
possesses a distribution of nulling timescales, but that it may also provide a
link between conventional nulling pulsars and longer-term intermittent pulsars.
Despite seeing evidence for periodicities in the pulsar radio emission, we are
uncertain whether they are intrinsic to the source, due to the influence of
observation sampling on the periodicity analysis performed. Remarkably, we find
evidence to suggest that the pulsar may undergo pre-ignition periods of
'emission flickering', that is rapid changes between radio-on (active) and -off
(null) emission states, before transitioning to a steady radio-emitting phase.
We find no direct evidence to indicate that the object exhibits any change in
spin-down rate between its radio-on and -off emission modes. We do, however,
place an upper limit on this variation to be <= 6 % from simulations. This
indicates that emission cessation in pulsars does not necessarily lead to large
changes in spin-down rate. Moreover, we show that such changes in spin-down
rate will not be discernible in the majority of objects which exhibit
short-term (<= 1 d) emission cessation. In light of this, we predict that many
pulsars could exhibit similar magnetospheric and emission properties to PSR
B0823+26, but which have not yet been observed.Comment: 13 pages, 11 figures, accepted for publication in MNRAS; 1 reference
correcte
A Tale of Two Current Sheets
I outline a new model of particle acceleration in the current sheet
separating the closed from the open field lines in the force-free model of
pulsar magnetospheres, based on reconnection at the light cylinder and
"auroral" acceleration occurring in the return current channel that connects
the light cylinder to the neutron star surface. I discuss recent studies of
Pulsar Wind Nebulae, which find that pair outflow rates in excess of those
predicted by existing theories of pair creation occur, and use those results to
point out that dissipation of the magnetic field in a pulsar's wind upstream of
the termination shock is restored to life as a viable model for the solution of
the "" problem as a consequence of the lower wind 4-velocity implied by
the larger mass loading.Comment: 17 pages, 6 figures, Invited Review, Proceedings of the "ICREA
Workshop on The High-Energy Emission from Pulsars and their Systems", Sant
Cugat, Spain, April 12-16, 201
An annular gap acceleration model for -ray emission of pulsars
If the binding energy of the pulsar's surface is not so high (the case of a
neutron star), both the negative and positive charges will flow out freely from
the surface of the star. The annular free flow model for -ray emission
of pulsars is suggested in this paper. It is emphasized that: (1). Two kinds of
acceleration regions (annular and core) need to be taken into account. The
annular acceleration region is defined by the magnetic field lines that cross
the null charge surface within the light cylinder. (2). If the potential drop
in the annular region of a pulsar is high enough (normally the cases of young
pulsars), charges in both the annular and the core regions could be accelerated
and produce primary gamma-rays. Secondary pairs are generated in both regions
and stream outwards to power the broadband radiations. (3). The potential drop
in the annular region grows more rapidly than that in the core region. The
annular acceleration process is a key point to produce wide emission beams as
observed. (4). The advantages of both the polar cap and outer gap models are
retained in this model. The geometric properties of the -ray emission
from the annular flow is analogous to that presented in a previous work by Qiao
et al., which match the observations well. (5). Since charges with different
signs leave the pulsar through the annular and the core regions, respectively,
the current closure problem can be partially solved.Comment: 11 pages 2 figures, accepted by Chinese Journal of Astronomy and
Astrophysic
INTEGRAL Observations of SS433: Analysis of Precessional and Orbital X-ray Periodicities
Hard X-ray INT observations of SS 433 carried out during 2003-2005 years with
an analysis of precessional and orbital variability is presented. The width of
X-ray eclipse in the 25-50 keV range at the precessional phase
(accretion disk is open to observer) is higher than that in the Ginga 18.4-27.6
keV range. This fact suggests existance the presence of hot extended corona
around the supercritical accretion disk. Spectrum of hard X-rays in the range
10-200 keV does not change with the precessional phase which also suggests that
hard X-ray flux is generated in the hot extended corona around the accretion
disk. The parameters of this hot corona are: kT=23-25 keV, \tau = 1.8-2.8. Mass
ratio estimated from the analysis of the ingress part of the eclipse light
curve is in the range q=m_x/m_v=0.3-0.5.Comment: 5 pages, 10 figure
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