5,499 research outputs found
The pre-penumbral magnetic canopy in the solar atmosphere
Penumbrae are the manifestation of magnetoconvection in highly inclined (to the vertical direction)
magnetic field. The penumbra of a sunspot tends to form, initially, along the arc of the umbra
antipodal to the main region of flux emergence. The question of how highly inclined magnetic field
can concentrate along the antipodal curves of umbrae, at least initially, remains to be answered.
Previous observational studies have suggested the existence of some form of overlying magnetic canopy
which acts as the progenitor for penumbrae. We propose that such overlying magnetic canopies are
a consequence of how the magnetic field emerges into the atmosphere and are, therefore, part of the
emerging region. We show, through simulations of twisted flux tube emergence, that canopies of
highly inclined magnetic field form preferentially at the required locations above the photosphere
IRIS observations of magnetic interactions in the solar atmosphere between pre-existing and emerging magnetic fields. II. UV emission properties
Multi-wavelength ultraviolet (UV) observations by the IRIS satellite in
active region NOAA 12529 have recently pointed out the presence of long-lasting
brightenings, akin to UV bursts, and simultaneous plasma ejections occurring in
the upper chromosphere and transition region during secondary flux emergence.
These signatures have been interpreted as evidence of small-scale, recurrent
magnetic reconnection episodes between the emerging flux region (EFR) and the
pre-existing plage field. Here, we characterize the UV emission of these
strong, intermittent brightenings and we study the surge activity above the
chromospheric arch filament system (AFS) overlying the EFR. We analyze the
surges and the cospatial brightenings observed at different wavelengths. We
find an asymmetry in the emission between the blue and red wings of the Si IV
1402 \AA{} and Mg II k 2796.3 \AA{} lines, which clearly outlines the dynamics
of the structures above the AFS that form during the small-scale eruptive
phenomena. We also detect a correlation between the Doppler velocity and
skewness of the Si IV 1394 \AA{} and 1402 \AA{} line profiles in the UV burst
pixels. Finally, we show that genuine emission in the Fe XII 1349.4 \AA{} line
is cospatial to the Si IV brightenings. This definitely reveals a pure coronal
counterpart to the reconnection event.Comment: 19 pages, 8 figures + 3 figures in the Appendix; accepted in Ap
The Broad Band Spectrum of MXB 1728-34 Observed by BeppoSAX
We report on the results of a broad band (0.1-100 keV) spectral analysis of
the bursting atoll source MXB 1728-34 observed by the BeppoSAX satellite. Three
bursts were present during this observation. The spectrum during the bursts can
be fitted by a blackbody with a temperature of 2 keV. From the bursts we also
estimate a distance to the source of 5.1 kpc. MXB 1728-34 was in a rather soft
state during the BeppoSAX observation. The persistent spectrum is well fitted
by a continuum consisting of a soft blackbody emission and a comptonized
spectrum. We interpreted the soft component as the emission from the accretion
disk. Taking into account a spectral hardening factor of 1.7, we estimated that
the inner disk radius is km, where i is the
inclination angle. The comptonized component could originate in a spherical
corona, with temperature of 10 keV and optical depth of 5, surrounding the
neutron star. A broad gaussian emission line at 6.7 keV is observed in the
spectrum, probably emitted in the ionized corona or in the inner part of the
disk. Another emission line is present at 1.66 keV.Comment: 12 pages, accepted by Ap
Computer simulation of the microstructure and rheology of semi-solid alloys under shear
The rheological behavior of metallic alloys containing both solid and liquid
phases is investigated in the low solid fraction range (<50%). This behavior
depends on both the solid fraction and the shear rate. The concept of Effective
Volume Fraction (EVF) is used to decorrelate the influence of these two
parameters. At high shear rate the slurry behaves like a suspension of hard
spheres, whereas at lower shear rate, particles tend to aggregate in clusters,
entrapping liquid and thus, increasing the EVF and the viscosity. A lattice
model is introduced to simulate the aggregation / break-up processes within a
slurry under shear. When the steady state is reached, the entrapped liquid
fraction is calculated, leading to a viscosity estimation. Simulation results
for the viscosity and 3D cluster structure are in good agreement with
experimental results.Comment: 30 pages, 17 figures, to be published in Acta Mate
Measuring the spin up of the Accreting Millisecond Pulsar XTE J1751-305
We perform a timing analysis on RXTE data of the accreting millisecond pulsar
XTE J1751-305 observed during the April 2002 outburst. After having corrected
for Doppler effects on the pulse phases due to the orbital motion of the
source, we performed a timing analysis on the phase delays, which gives, for
the first time for this source, an estimate of the average spin frequency
derivative = (3.7 +/- 1.0)E-13 Hz/s. We discuss the torque resulting
from the spin-up of the neutron star deriving a dynamical estimate of the mass
accretion rate and comparing it with the one obtained from X-ray flux.
Constraints on the distance to the source are discussed, leading to a lower
limit of \sim 6.7 kpc.Comment: 7 pages, 3 figures, Accepted for publication by MNRA
Chandra Observation of the Persistent Emission from the Dipping Source XB 1916-053
We present the results of a 50 ks long Chandra observation of the dipping
source XB 1916-053. During the observation two X-ray bursts occurred and the
dips were not present at each orbital period. From the zero-order image we
estimate the precise X-ray coordinates of the source with a 90% uncertainty of
0.6''. In this work we focus on the spectral study of discrete absorption
features, during the persistent emission, using the High Energy Transmission
Grating Spectrometer on board the Chandra satellite. We detect, for the first
time in the 1st-order spectra of XB 1916-053, absorption lines associated to Ne
X, Mg XII, Si XIV, and S XVI, and confirm the presence of the Fe XXV and Fe
XXVI absorption lines with a larger accuracy with respect to the previous XMM
EPIC pn observation. Assuming that the line widths are due to a bulk motion or
a turbulence associated to the coronal activity, we estimate that the lines are
produced in a photoionized absorber distant from the neutron star 4 x 10^{10}
cm, near the disk edge.Comment: 20 pages, 10 figures, submitted to ApJ on 2005-09-22, accepted by ApJ
on 2006-05-0
The Status of a PA Endangered Bird- the Upland Sandpiper
The upland sandpiper (Bartramia Longuardia) has experienced a steep population decline in the northeastern U.S. since the mid-20th Century. In Pennsylvania it was found in less than 0.5% of atlas blocks during the Second Atlas of Breeding Birds in Pennsylvania project (2nd PBBA; 2004-09) and breeding was confirmed at only two locations. Due to continued declines and a small population size, the upland sandpiper was listed as PA endangered in 2012. During May 2012 the areas around 15 2nd PBBA upland sandpiper sightings were resurveyed by Gettysburg College students and volunteer birdwatchers. The aim was to establish whether the atlas records related to persisting populations. We used five-minute audio playback at up to 10 locations within 4km of the atlas sightings. A maximum of 19 pairs/calling male upland sandpipers were found across the state in 2012, most of them on or close to reclaimed surface mines. However, locating such a scarce species can be problematic, and it is still not known to what extent the species is under-reported. To help direct future surveys we analyzed data from the 2nd PBBA and the 2012 survey to produce a habitat suitability model for the upland sandpiper in Pennsylvania. We used a GIS framework to determine areas of suitable habitat and then stratified these by proximity to recent (2004-2012) upland sandpiper sightings. We recommend that our suitability model be used to establish a sampling protocol for more thorough statewide upland sandpiper survey every five years, in order that the speciesâ precarious status can be closely monitored
Discovery of periodic dips in the light curve of GX 13+1: the X-ray orbital ephemeris of the source
The bright low-mass X-ray binary (LMXB) GX 13+1 is one of the most peculiar
Galactic binary systems. A periodicity of 24.27 d with a formal statistical
error of 0.03 d was observed in its power spectrum density obtained with RXTE
All Sky Monitor (ASM) data spanning 14 years. Starting from a recent study,
indicating GX 13+1 as a possible dipping source candidate, we systematically
searched for periodic dips in the X-ray light curves of GX 13+1 from 1996 up to
2013 using RXTE/ASM, and MAXI data to determine for the first time the X-ray
orbital ephemeris of GX 13+1. We searched for a periodic signal in the ASM and
MAXI light curves, finding a common periodicity of 24.53 d. We folded the 1.3-5
keV and 5-12.1 keV ASM light curves and the 2-4 and 4-10 keV MAXI light curves
at the period of 24.53 d finding a periodic dip. To refine the value of the
period we used the timing technique dividing the ASM light curve in eight
intervals and the MAXI light curve in two intervals, obtaining four and two dip
arrival times from the ASM and MAXI light curves, respectively. We improved the
X-ray position of GX 13+1 using a recent Chandra observation. The new X-ray
position is discrepant by \sim 7\arcsec from the previous one, while it is
compatible with the infrared and radio counterpart positions. We detected an
X-ray dip, that is totally covered by the Chandra observation, in the light
curve of GX 13+1 and showed, a-posteriori, that it is a periodic dip. We
obtained seven dip arrival times from ASM, MAXI, and Chandra light curves. We
calculated the delays of the detected dip arrival times with respect to the
expected times for a 24.52 d periodicity. Fitting the delays with a linear
function we find that the orbital period and the epoch of reference of GX 13+1
are 24.5274(2) days and 50,086.79(3) MJD, respectively.(Abridged)Comment: 12 pages, including 16 figures. Accepted for publication in A&
Timing of the Accreting Millisecond Pulsar XTE J1814-338
We present a precise timing analysis of the accreting millisecond pulsar XTE
J1814-338 during its 2003 outburst, observed by RXTE. A full orbital solution
is given for the first time; Doppler effects induced by the motion of the
source in the binary system were corrected, leading to a refined estimate of
the orbital period, P_orb=15388.7229(2)s, and of the projected semimajor axis,
a sini/c= 390.633(9) lt-ms. We could then investigate the spin behaviour of the
accreting compact object during the outburst. We report here a refined value of
the spin frequency (nu=314.35610879(1) Hz) and the first estimate of the spin
frequency derivative of this source while accreting (nu^dot=(-6.7 +/- 0.7)
10^(-14) Hz/s). This spin down behaviour arises when both the fundamental
frequency and the second harmonic are taken into consideration. We discuss this
in the context of the interaction between the disc and the quickly rotating
magnetosphere, at accretion rates sufficiently low to allow a threading of the
accretion disc in regions where the Keplerian velocity is slower than the
magnetosphere velocity. We also present indications of a jitter of the pulse
phases around the mean trend, which we argue results from movements of the
accreting hotspots in response to variations of the accretion rate.Comment: 7 pages, 4 figures, Accepted for publication by MNRA
Searching for pulsed emission from XTE J0929-314 at high radio frequencies
The aim of this work is to search for radio signals in the quiescent phase of
accreting millisecond X-ray pulsars, in this way giving an ultimate proof of
the recycling model, thereby unambiguously establishing that accreting
millisecond X-ray pulsars are the progenitors of radio millisecond pulsars.
To overcome the possible free-free absorption caused by matter surrounding
accreting millisecond X-ray pulsars in their quiescence phase, we performed the
observations at high frequencies. Making use of particularly precise orbital
and spin parameters obtained from X-ray observations, we carried out a deep
search for radio-pulsed emission from the accreting millisecond X-ray pulsar
XTE J0929-314 in three steps, correcting for the effect of the dispersion due
to the interstellar medium, eliminating the orbital motions effects, and
finally folding the time series.
No radio pulsation is present in the analyzed data down to a limit of 68
microJy at 6.4 GHz and 26 microJy at 8.5 GHz.
We discuss several mechanisms that could prevent the detection, concluding
that beaming factor and intrinsic low luminosity are the most likely
explanations.Comment: 7 pages, 4 figures. Accepted for publication in Astronomy &
Astrophysic
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