4,740 research outputs found
Oral History Interview: Mr. and Mrs. W. C. Cotton
This interview is one of a series conducted concerning rural life in West Virginia and surrounding areas. At the time of the interview, Mr. and Mrs. Cotton were residing in Gallipolis, Ohio. Topics discussed in the interview include food preservation, religion, trading, entertainment, law, farming, and doctors.https://mds.marshall.edu/oral_history/1167/thumbnail.jp
A Radio Transient 0.1 pc from Sagittarius A*
We report the discovery of a transient radio source 2.7 arcsec (0.1 pc
projected distance) South of the Galactic Center massive black hole,
Sagittarius A*. The source flared with a peak of at least 80 mJy in March 2004.
The source was resolved by the Very Large Array into two components with a
separation of ~0.7 arcsec and characteristic sizes of ~0.2 arcsec. The two
components of the source faded with a power-law index of 1.1 +/- 0.1. We detect
an upper limit to the proper motion of the Eastern component of ~3 x 10^3 km
s^-1 relative to Sgr A*. We detect a proper motion of ~10^4 km s^-1 for the
Western component relative to Sgr A*. The transient was also detected at X-ray
wavelengths with the Chandra X-ray Observatory and the XMM-Newton telescope and
given the designation CXOGC J174540.0-290031. The X-ray source falls in between
the two radio components. The maximum luminosity of the X-ray source is ~10^36
erg s^-1, significantly sub-Eddington. The radio jet flux density predicted by
the X-ray/radio correlation for X-ray binaries is orders of magnitude less than
the measured flux density. We conclude that the radio transient is the result
of a bipolar jet originating in a single impulsive event from the X-ray source
and interacting with the dense interstellar medium of the Galactic Center.Comment: accepted in ApJ; 24 pages; 8 figure
Flaring Activity of Sgr A*: Expanding Hot Blobs
Sgr A* is considered to be a massive black hole at the Galactic center and is
known to be variable in radio, millimeter, near-IR and X-rays. Recent
multi-wavelength observing campaigns show a simultaneous X-ray and near-IR
flare, as well as sub-millimeter and near-IR flares from Sgr A*. The flare
activity is thought to be arising from the innermost region of Sgr A*. We have
recently argued that the duration of flares in near-IR and submillimeter
wavelengths implies that the burst of emission expands and cools on a dynamical
time scale before the flares leave Sgr A*. The detection of radio flares with a
time delay in the range of 20 and 40 minutes between 7 and 12mm peak emission
implies adiabatic expansion of a uniform, spherical hot blob due to flare
activity. We suspect that this simple outflow picture shows some of the
characteristics that are known to take place in microquasars, thus we may learn
much from comparative study of Sgr A* and its environment vs. microquasars.Comment: 10 pages, 5 figures, to be published in IV Microquasar Workshop:
Microquasars and Beyond, September 18-22 2006, Como, Ital
Radio Continuum Emission from the Magnetar SGR J1745-2900: Interaction with Gas Orbiting Sgr A*
We present radio continuum light curves of the magnetar SGR J17452900 and
Sgr A* obtained with multi-frequency, multi-epoch Very Large Array observations
between 2012 and 2014. During this period, a powerful X-ray outburst from SGR
J17452900 occurred on 2013-04-24. Enhanced radio emission is delayed with
respect to the X-ray peak by about seven months. In addition, the flux density
of the emission from the magnetar fluctuates by a factor of 2 to 4 at
frequencies between 21 and 41 GHz and its spectral index varies erratically.
Here we argue that the excess fluctuating emission from the magnetar arises
from the interaction of a shock generated from the X-ray outburst with the
orbiting ionized gas at the Galactic center. In this picture, variable
synchrotron emission is produced by ram pressure variations due to
inhomogeneities in the dense ionized medium of the Sgr A West bar. The pulsar
with its high transverse velocity is moving through a highly blue-shifted
ionized medium. This implies that the magnetar is at a projected distance of
pc from Sgr A* and that the orbiting ionized gas is partially or
largely responsible for a large rotation measure detected toward the magnetar.
Despite the variability of Sgr A* expected to be induced by the passage of the
G2 cloud, monitoring data shows a constant flux density and spectral index
during this periodComment: 12 pages, 3 figures, ApJL (in press
The galactic magnetic field in the quasar 3C216
Multifrequency polarimetric observations made with the Very Long Baseline
Array of the quasar 3C216 reveal the presence of Faraday rotation measures
(RMs) in excess of 2000 rad/m**2 in the source rest frame, in the arc of
emission located at ~ 140 mas from the core. Rotation measures in the range
-300 - +300 rad/m**2 are detected in the inner 5 mas (~30 parsecs). while the
rotation measures near the core can be explained as due to a magnetic field in
the narrow line region, we favor the interpretation for the high RM in the arc
as due to a ``local'' Faraday screen, produced in a shock where the jet is
deflected by the interstellar medium of the host galaxy. Our results indicate
that a galacit magnetic field of the order of 50 microGauss on a scale greater
than 100 pc must be present in the galactic medium.Comment: 23 pages, 3 tables, 11 figures. To appear on The Astronomical
Journal, November 1999 Issu
First Keck Nulling Observations of a Young Stellar Object: Probing the Circumstellar Environment of the Herbig Ae star MWC 325
We present the first N-band nulling plus K- and L-band V2 observations of a
young stellar object, MWC325, taken with the 85 m baseline Keck Interferometer.
The Keck nuller was designed for the study of faint dust signatures associated
with debris disks, but it also has a unique capability for studying the
temperature and density distribution of denser disks found around young stellar
objects. Interferometric observations of MWC 325 at K, L and N encompass a
factor of five in spectral range and thus, especially when spectrally dispersed
within each band, enable characterization of the structure of the inner disk
regions where planets form. Fitting our observations with geometric models such
as a uniform disk or a Gaussian disk show that the apparent size increases
monotonically with wavelength in the 2-12 um wavelength region, confirming the
widely held assumption based on radiative transfer models, now with spatially
resolved measurements over broad wavelength range, that disks are extended with
a temperature gradient. The effective size is a factor of about 1.3 and 2
larger in the L-band and N-band, respectively, compared to that in the K-band.
The existing interferometric measurements and the spectral energy distribution
can be reproduced by a flat disk or a weakly-shadowed nearly flat-disk model,
with only slight flaring in the outer regions of the disk, consisting of
representative "sub-micron" (0.1 um) and "micron" (2 um) grains of a 50:50
ratio of silicate and graphite. This is marked contrast with the disks
previously found in other Herbig Ae/Be stars suggesting a wide variety in the
disk properties among Herbig Ae/Be stars.Comment: Accepted for publication in the Ap
The Variability of Polarized Radiation from Sgr A*
Sgr A* is variable at radio and submillimeter wavelengths on hourly time
scales showing time delays between the peaks of flare emission as well as
linearly polarized emission at millimeter and sub-mm wavelengths. To determine
the polarization characteristics of this variable source at radio frequencies,
we present VLA observations of Sgr A* and report the detection of polarized
emission at a level of 0.77\pm0.01% and 0.2\pm0.01% at 43 and 22 GHz,
respectively. The change in the time averaged polarization angle between 22 and
43 GHz corresponds to a RM of -2.5\pm0.6 x10^3 rad m{-2} with no phase wrapping
(or \sim 5x10^4 rad m^2 with 2\pi phase wrap). We also note a rise and fall
time scale of 1.5 -- 2 hours in the total polarized intensity. The light curves
of the degree of linearly polarized emission suggests a a correlation with the
variability of the total intensity at 43 GHz. The available polarization data
at radio and sub-mm wavelengths suggest that the rotation measure decreases
with decreasing frequency. This frequency dependence, and observed changes in
polarization angle during flare events, may be caused by the reduction in
rotation measure associated with the expansion of synchrotron-emitting blobs.Comment: 11 pages, 3 figures, ApJL (in press
Spatial and Temporal Variations in Small-Scale Galactic HI Structure Toward 3C~138
We present three epochs of VLBA observations of Galactic HI absorption toward
the quasar 3C~138 with resolutions of 20 mas (~ 10 AU). This analysis includes
VLBA data from observations in 1999 and 2002 along with a reexamination of 1995
VLBA data. Improved data reduction and imaging techniques have led to an order
of magnitude improvement in sensitivity compared to previous work. With these
new data we confirm the previously detected milliarcsecond scale spatial
variations in the HI opacity at the level of Delta(tau_{max}) =0.50 \pm 0.05.
The typical size scale of the optical depth variations is ~ 50 mas or 25 AU. In
addition, for the first time we see clear evidence for temporal variations in
the HI opacity over the seven year time span of our three epochs of data. We
also attempted to detect the magnetic field strength in the HI gas using the
Zeeman effect. From this analysis we have been able to place a 3 sigma upper
limit on the magnetic field strength per pixel of ~45 muG. We have also been
able to calculate for the first time the plane of sky covering fraction of the
small scale HI gas of ~10%. We also find that the line widths of the
milliarcsecond sizescale HI features are comparable to those determined from
previous single dish measurements toward 3C~138, suggesting that the opacity
variations cannot be due to changes in the HI spin temperature. From these
results we favor a density enhancement interpretation for the small scale HI
structures, although these enhancements appear to be of short duration and are
unlikely to be in equilibrium.Comment: 34 pages, 8 figures. Figures 3 & 4 are in color. Accepted to A
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