1,215 research outputs found
The Chandra Dust Scattering Halo of Galactic Center transient Swift J174540.7-290015
We report the detection of a dust scattering halo around a recently
discovered X-ray transient, Swift J174540.7-290015, which in early February of
2016 underwent one of the brightest outbursts (F_X ~ 5e-10 erg/cm^2/s) observed
from a compact object in the Galactic Center field. We analyze four Chandra
images that were taken as follow-up observations to Swift discoveries of new
Galactic Center transients. After adjusting our spectral extraction for the
effects of detector pileup, we construct a point spread function for each
observation and compare it to the GC field before the outburst. We find
residual surface brightness around Swift J174540.7-290015, which has a shape
and temporal evolution consistent with the behavior expected from X-rays
scattered by foreground dust. We examine the spectral properties of the source,
which shows evidence that the object transitioned from a soft to hard spectral
state as it faded below L_X ~ 1e36 erg/s. This behavior is consistent with the
hypothesis that the object is a low mass X-ray binary in the Galactic Center.Comment: Accepted for publication in Ap
Development of a regional GPS-based ionospheric TEC model for South Africa
Advances in South African space physics research and related disciplines require better spatial and time resolution ionospheric information than was previously possible with the existing ionosonde network. A GPS-based, variable degree adjusted spherical harmonic (ASHA) model was developed for near real-time regional ionospheric total electron content (TEC) mapping over South Africa. Slant TEC values along oblique GPS signal paths are quantified from a network of GPS receivers and converted to vertical TEC by means of the single layer mapping function. The ASHA model coefficients and GPS differential biases are estimated from vertical TEC at the ionospheric pierce points and used to interpolate TEC at any location within the region of interest. Diurnal TEC variations with one minute time resolution and time-varying 2D regional TEC maps are constructed. In order to validate the ASHA method, simulations with an IRI ionosphere were performed, while the ASHA results from actual data were compared with two independent GPS-based methodologies and measured ionosonde data
Pinkeye of cattle
1 online resource (PDF, 4 pages)This archival publication may not reflect current scientific knowledge or recommendations. Current information available from the University of Minnesota Extension: https://www.extension.umn.edu
Footrot of cattle
1 online resource (PDF, 2 pages)This archival publication may not reflect current scientific knowledge or recommendations. Current information available from the University of Minnesota Extension: https://www.extension.umn.edu
Simultaneous Multi-band Radio & X-ray Observations of the Galactic Center Magnetar SGR 17452900
We report on multi-frequency, wideband radio observations of the Galactic
Center magnetar (SGR 17452900) with the Green Bank Telescope for 100
days immediately following its initial X-ray outburst in April 2013. We made
multiple simultaneous observations at 1.5, 2.0, and 8.9 GHz, allowing us to
examine the magnetar's flux evolution, radio spectrum, and interstellar medium
parameters (such as the dispersion measure (DM), the scattering timescale and
its index). During two epochs, we have simultaneous observations from the
Chandra X-ray Observatory, which permitted the absolute alignment of the radio
and X-ray profiles. As with the two other radio magnetars with published
alignments, the radio profile lies within the broad peak of the X-ray profile,
preceding the X-ray profile maximum by 0.2 rotations. We also find that
the radio spectral index is significantly negative between 2 and
9 GHz; during the final 30 days of our observations ,
which is typical of canonical pulsars. The radio flux has not decreased during
this outburst, whereas the long-term trends in the other radio magnetars show
concomitant fading of the radio and X-ray fluxes. Finally, our wideband
measurements of the DMs taken in adjacent frequency bands in tandem are
stochastically inconsistent with one another. Based on recent theoretical
predictions, we consider the possibility that the dispersion measure is
frequency-dependent. Despite having several properties in common with the other
radio magnetars, such as , an
increase in the radio flux during the X-ray flux decay has not been observed
thus far in other systems.Comment: 15 pages, 9 figures, 3 tables; accepted to Ap
A tree-decomposed transfer matrix for computing exact Potts model partition functions for arbitrary graphs, with applications to planar graph colourings
Combining tree decomposition and transfer matrix techniques provides a very
general algorithm for computing exact partition functions of statistical models
defined on arbitrary graphs. The algorithm is particularly efficient in the
case of planar graphs. We illustrate it by computing the Potts model partition
functions and chromatic polynomials (the number of proper vertex colourings
using Q colours) for large samples of random planar graphs with up to N=100
vertices. In the latter case, our algorithm yields a sub-exponential average
running time of ~ exp(1.516 sqrt(N)), a substantial improvement over the
exponential running time ~ exp(0.245 N) provided by the hitherto best known
algorithm. We study the statistics of chromatic roots of random planar graphs
in some detail, comparing the findings with results for finite pieces of a
regular lattice.Comment: 5 pages, 3 figures. Version 2 has been substantially expanded.
Version 3 shows that the worst-case running time is sub-exponential in the
number of vertice
The X-ray outburst of the Galactic Center magnetar over six years of Chandra observations
The magnetar SGR J1745-2900 discovered at parsecs distance from the Milky Way
central black hole, Sagittarius A*, represents the closest pulsar to a
supermassive black hole ever detected. Furthermore, its intriguing radio
emission has been used to study the environment of the black hole, as well as
to derive a precise position and proper motion for this object. The discovery
of SGR J1745-2900 has opened interesting debates about the number, age and
nature of pulsars expected in the Galactic center region. In this work, we
present extensive X-ray monitoring of the outburst of SGR J1745-2900 using the
Chandra X-ray Observatory, the only instrument with the spatial resolution to
distinguish the magnetar from the supermassive black hole (2.4" angular
distance). It was monitored from its outburst onset in April 2013 until August
2019, collecting more than fifty Chandra observations for a total of more than
2.3 Ms of data. Soon after the outburst onset, the magnetar emission settled
onto a purely thermal emission state that cooled from a temperature of about
0.9 to 0.6 keV over 6 years. The pulsar timing properties showed at least two
changes in the period derivative, increasing by a factor of about 4 during the
outburst decay. We find that the long-term properties of this outburst
challenge current models for the magnetar outbursts.Comment: 11 pages, 6 figures. Accepted by Ap
The X-ray outburst of the Galactic Centre magnetar SGR J1745-2900 during the first 1.5 year
In 2013 April a new magnetar, SGR 1745-2900, was discovered as it entered an
outburst, at only 2.4 arcsec angular distance from the supermassive black hole
at the Centre of the Milky Way, Sagittarius A*. SGR 1745-2900 has a surface
dipolar magnetic field of ~ 2x10^{14} G, and it is the neutron star closest to
a black hole ever observed. The new source was detected both in the radio and
X-ray bands, with a peak X-ray luminosity L_X ~ 5x10^{35} erg s^{-1}. Here we
report on the long-term Chandra (25 observations) and XMM-Newton (8
observations) X-ray monitoring campaign of SGR 1745-2900, from the onset of the
outburst in April 2013 until September 2014. This unprecedented dataset allows
us to refine the timing properties of the source, as well as to study the
outburst spectral evolution as a function of time and rotational phase. Our
timing analysis confirms the increase in the spin period derivative by a factor
of ~2 around June 2013, and reveals that a further increase occurred between
2013 Oct 30 and 2014 Feb 21. We find that the period derivative changed from
6.6x10^{-12} s s^{-1} to 3.3x10^{-11} s s^{-1} in 1.5 yr. On the other hand,
this magnetar shows a slow flux decay compared to other magnetars and a rather
inefficient surface cooling. In particular, starquake-induced crustal cooling
models alone have difficulty in explaining the high luminosity of the source
for the first ~200 days of its outburst, and additional heating of the star
surface from currents flowing in a twisted magnetic bundle is probably playing
an important role in the outburst evolution.Comment: 16 pages, 12 figures; accepted for publication on MNRA
A strongly magnetized pulsar within grasp of the Milky Way's supermassive black hole
The center of our Galaxy hosts a supermassive black hole, Sagittarius (Sgr)
A*. Young, massive stars within 0.5 pc of SgrA* are evidence of an episode of
intense star formation near the black hole a few Myr ago, which might have left
behind a young neutron star traveling deep into SgrA*'s gravitational
potential. On 2013 April 25, a short X-ray burst was observed from the
direction of the Galactic center. Thanks to a series of observations with the
Chandra and the Swift satellites, we pinpoint the associated magnetar at an
angular distance of 2.4+/-0.3 arcsec from SgrA*, and refine the source spin
period and its derivative (P=3.7635537(2) s and \dot{P} = 6.61(4)x10^{-12}
s/s), confirmed by quasi simultaneous radio observations performed with the
Green Bank (GBT) and Parkes antennas, which also constrain a Dispersion Measure
of DM=1750+/-50 pc cm^{-3}, the highest ever observed for a radio pulsar. We
have found that this X-ray source is a young magnetar at ~0.07-2 pc from SgrA*.
Simulations of its possible motion around SgrA* show that it is likely (~90%
probability) in a bound orbit around the black hole. The radiation front
produced by the past activity from the magnetar passing through the molecular
clouds surrounding the Galactic center region, might be responsible for a large
fraction of the light echoes observed in the Fe fluorescence features.Comment: ApJ Letters in pres
The Chandra High Resolution X-ray Spectrum of Quiescent Emission from Sgr A*
In quiescence, Sgr A* is surprisingly dim, shining 100,000 times less than
expected for its environment. This problem has motivated a host of theoretical
models to explain radiatively inefficient accretion flows (RIAFs). The Chandra
Galactic Center (GC) X-ray Visionary Program obtained approximately 3 Ms (one
month) of Chandra HETG data, offering the only opportunity to examine the
quiescent X-ray emission of Sgr A* with high resolution spectroscopy. Utilizing
custom background regions and filters for removing overlapping point sources,
this work provides the first ever look at stacked HETG spectra of Sgr A*. We
model the background datasets with a cubic spline and fit the unbinned Sgr A*
spectra with a simple parametric model of a power law plus Gaussian lines under
the effects of interstellar extinction. We detect a strong 6.7 keV iron
emission line in the HEG spectra and a 3.1 keV emission line in the MEG
spectra. In all cases, the line centroids and equivalent widths are consistent
with those measured from low-resolution CCD spectra. An examination of the
unbinned, stacked HEG+/-1 spectrum reveals fine structure in the iron line
complex. In addition to resolving the resonant and forbidden lines from He-like
iron, there are apparent emission features arising with higher statistical
significance at lower energy, potentially associated with FeXX-XXIV ions in a
~1 keV plasma arising near the Bondi radius of Sgr A*. With this work, we
release the cleaned and stacked Sgr A* and background HETG spectra to the
public as a special legacy dataset.Comment: Accepted by Ap
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