629 research outputs found
Constrains on parameters of magnetic field decay for accreting isolated neutron stars
The influence of exponential magnetic field decay (MFD) on the spin evolution
of isolated neutron stars is studied. The ROSAT observations of several X-ray
sources, which can be accreting old isolated neutron stars, are used to
constrain the exponential and power-law decay parameters. We show that for the
exponential decay the ranges of minimum value of magnetic moment, , and
the characteristic decay time, , , are excluded assuming the
standard initial magnetic moment, . For these
parameters, neutron stars would never reach the stage of accretion from the
interstellar medium even for a low space velocity of the stars and a high
density of the ambient plasma. The range of excluded parameters increases for
lower values of .
We also show, that, contrary to exponential MFD, no significant restrictions
can be made for the parameters of power-law decay from the statistics of
isolated neutron star candidates in ROSAT observations.
Isolated neutron stars with constant magnetic fields and initial values of
them less than never come to the stage
of accretion.
We briefly discuss the fate of old magnetars with and without MFD, and
describe parameters of old accreting magnetars.Comment: 18 pages, 6 PostScript figures, to be published in the Proceedings of
the XXVIII ITEP Winter Schoo
Graphene Transport at High Carrier Densities using a Polymer Electrolyte Gate
We report the study of graphene devices in Hall-bar geometry, gated with a
polymer electrolyte. High densities of 6 are
consistently reached, significantly higher than with conventional back-gating.
The mobility follows an inverse dependence on density, which can be correlated
to a dominant scattering from weak scatterers. Furthermore, our measurements
show a Bloch-Gr\"uneisen regime until 100 K (at 6.2 ),
consistent with an increase of the density. Ubiquitous in our experiments is a
small upturn in resistivity around 3 , whose origin is
discussed. We identify two potential causes for the upturn: the renormalization
of Fermi velocity and an electrochemically-enhanced scattering rate.Comment: 13 pages, 4 figures, Published Versio
Plasma biomarkers of brain injury in COVID-19 patients with neurological symptoms
Objective: Neurological symptoms (NS) were often reported in COVID-19 infection. We examined the plasma levels of glial fibrillary acidic protein (GFAP) and S100B together, as brain injury biomarkers, in relation to persistent NS in a cohort of patients with COVID-19 during the acute phase of the disease.Methods: A total of 20 healthy controls and 58 patients with confirmed COVID-19 were enrolled in this prospective study. Serum GFAP and S100B levels were measured by using enzymle linked immunoassay method from blood samples.Results: Serum GFAP levels were found to be significantly higher in the severe group than in the controls (p = 0.007). However, serum S100B levels were similar between control and disease groups (p > 0.05). No significant results for GFAP and S100B were obtained between the disease groups depending on whether the sampling time was below or above 5 days (p > 0.05). We did not find a correlation between serum GFAP and S100B levels and the presence of NS (p > 0.05). However, serum S100B levels were slightly higher in patients with multiple NS than in those with a single symptom (p = 0.044).Conclusions: Elevated GFAP was associated with disease severity but not with NS in COVID-19 patients. Whereas, high serum S100B was associated with the multipl NS in these patients. Our data suggest that GFAP and S100B may be of limited value currently in order to represent the neuronal damage, though serving a basis for the future work
Revisiting Combinatorial Ambiguities at Hadron Colliders with MT2
We present a method to resolve combinatorial issues in multi-particle final
states at hadron colliders. The use of kinematic variables such as MT2 and
invariant mass significantly reduces combinatorial ambiguities in the signal,
but at a cost of losing statistics. We illustrate this idea with gluino pair
production leading to 4 jets +\met in the final state as well as
production in the dilepton channel. Compared to results in recent studies, our
method provides greater efficiency with similar purityComment: 20 pages, 7 figures, 10 table
The Evolution of Relativistic Binary Progenitor Systems
Relativistic binary pulsars, such as B1534+12 and B1913+16 are characterized
by having close orbits with a binary separation of ~ 3 R_\sun. The progenitor
of such a system is a neutron star, helium star binary. The helium star, with a
strong stellar wind, is able to spin up its compact companion via accretion.
The neutron star's magnetic field is then lowered to observed values of about
10^{10} Gauss. As the pulsar lifetime is inversely proportional to its magnetic
field, the possibility of observing such a system is, thus, enhanced by this
type of evolution. We will show that a nascent (Crab-like) pulsar in such a
system can, through accretion-braking torques (i.e. the "propeller effect") and
wind-induced spin-up rates, reach equilibrium periods that are close to
observed values. Such processes occur within the relatively short helium star
lifetimes. Additionally, we find that the final outcome of such evolutionary
scenarios depends strongly on initial parameters, particularly the initial
binary separation and helium star mass. It is, indeed, determined that the
majority of such systems end up in the pulsar "graveyard", and only a small
fraction are strongly recycled. This fact might help to reconcile theoretically
expected birth rates with limited observations of relativistic binary pulsars.Comment: 24 pages, 10 Postscript figures, Submitted to The Astrophysical
Journa
Neurological symptoms and neuronal damage markers in acute COVID-19: Is there a correlation? A pilot study
A wide spectrum of neurological symptoms (NS) has been described in patients with COVID-19. We examined the plasma levels of neuron-specific enolase (NSE) and neurofilament light chain (NFL) together, as neuronal damage markers, and their relationships with clinical severity in patients with NS at acute COVID-19. A total of 20 healthy controls and 59 patients with confirmed COVID-19 were enrolled in this pilot prospective study. Serum NSE and NFL levels were measured by using the enzyme-linked immunoassay method from serum samples. Serum NSE levels were found to be significantly higher in the severe group than in the nonsevere group (p = 0.034). However, serum NFL levels were similar between the control and disease groups (p > 0.05). For the mild group, serum NFL levels were significantly higher in patients with the sampling time >= 5 days than in those with the sampling time 0.05). Increased serum NSE levels were associated with disease severity regardless of accompanied NS in patients with acute COVID-19 infection. However, serum NFL levels may have a role at the subacute phase of COVID-19
A multifrequency study of giant radio sources-II. Spectral ageing analysis of the lobes of selected sources
Multifrequency observations with the GMRT and the VLA are used to determine
the spectral breaks in consecutive strips along the lobes of a sample of
selected giant radio sources (GRSs) in order to estimate their spectral ages.
The maximum spectral ages estimated for the detected radio emission in the
lobes of our sources range from 6 to 36 Myr with a median value of
20 Myr using the classical equipartition fields. Using the magnetic field
estimates from the Beck & Krause formalism the spectral ages range from 5
to 38 Myr with a median value of 22 Myr. These ages are significantly
older than smaller sources. In all but one source (J1313+6937) the spectral age
gradually increases with distance from the hotspot regions, confirming that
acceleration of the particles mainly occurs in the hotspots. Most of the GRSs
do not exhibit zero spectral ages in the hotspots, as is the case in earlier
studies of smaller sources. This is likely to be largely due to contamination
by more extended emission due to relatively modest resolutions. The injection
spectral indices range from 0.55 to 0.88 with a median value of
0.6. We discuss these values in the light of theoretical expectations,
and show that the injection spectral index appears to be correlated with
luminosity and/or redshift as well as with linear size.Comment: 12 Pages, 13 Figures, 9 Tables, Accepted for publication in MNRA
Restrictions on parameters of power-law magnetic field decay for accreting isolated neutron stars
In this short note we discuss the influence of power-law magnetic field decay
on the evolution of old accreting isolated neutron stars. We show, that,
contrary to exponential field decay (Popov & Prokhorov 2000), no additional
restrictions can be made for the parameters of power-law decay from the
statistics of isolated neutron star candidates in ROSAT observations.
We also briefly discuss the fate of old magnetars with and without field
decay, and describe parameters of old accreting magnetars.Comment: 8 pages including 3 PostScript figure
On the Origin of X-ray Emission From Millisecond Pulsars in 47 Tuc
The observed spectra and X-ray luminosities of millisecond pulsars in 47 Tuc
can be interpreted in the context of theoretical models based on strong, small
scale multipole fields on the neutron star surface. For multipole fields that
are relatively strong as compared to the large scale dipole field, the emitted
X-rays are thermal and likely result from polar cap heating associated with the
return current from the polar gap. On the other hand, for weak multipole
fields, the emission is nonthermal and results from synchrotron radiation of
pairs created by curvature radiation. The X-ray luminosity, , is
related to the spin down power, , expressed in the form with and for strong and weak
multipole fields respectively. If the polar cap size is of the order of the
length scale of the multipole field, and , the polar cap
temperature is .
A comparison of the X-ray properties of millisecond pulsars in globular
clusters and in the Galactic field suggests that the emergence of relatively
strong small scale multipole fields from the neutron star interior may be
correlated with the age and evolutionary history of the underlying neutron
star.Comment: 25 pages, 2 figures, accepted for publication in Ap
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