728 research outputs found
Recommended from our members
EAC Guidelines for the use of Geophysics in Archaeology: Questions to Ask and Points to Consider.
These guidelines provide an overview of the issues to
be considered when undertaking or commissioning
geophysical survey in archaeology. As every project diff ers
in its requirements (e.g. from fi nding sites to creating
detailed maps of individual structures) and variations in
geological and environmental conditions lead to diff erent
geophysical responses, there is no single ābestā survey
technique or methodology. Th is guide, in its European
approach, highlights the various questions to be asked
before a survey is undertaken. It does not provide recipebook
advice on how to do a geophysical survey or a tick
list of which technique is suitable under what conditions.
Experienced archaeological geophysicists should be
consulted to address the questions that are being posed.
Using geophysical techniques and methods inappropriately
will lead to disappointment and may, ultimately, result
in archaeologists not using them at all. āIf all you have is
a hammer (or magnetometer), driving a screw becomes
impossibleā.
Especially in the American literature the term āremote
sensingā is oft en used to describe geophysical as well as
air and space based exploration of underground features
(e.g. Wiseman and El-Baz 2007). By contrast, and in line
with European traditions, a clear distinction is made
here between ground-based geophysical techniques and
remote sensing techniques. Th is is based on the imaging
principles underlying the respective technologies. Ground
based systems usually collect one spatially registered data
sample from each sensor location (e.g. a single reading
for each magnetometer, or a single trace from each GPR
antenna). Remote sensing techniques, by contrast, collect
spatially resolved data from a whole area of investigation
from each sensor location, using either the systemās optical
aperture (e.g. photography) or a scanning device (e.g. laser
sampling).
These guidelines are based on the experience of the authors
in archaeological geophysics and infl uenced by various
published sources
Contemporaneous VLBA 5 GHz Observations of Large Area Telescope Detected Blazars
The radio properties of blazars detected by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope have been observed contemporaneously by the Very Long Baseline Array (VLBA). In total, 232 sources were observed with the VLBA. Ninety sources that were previously observed as part of the VLBA Imaging and Polarimetry Survey (VIPS) have been included in the sample, as well as 142 sources not found in VIPS. This very large, 5 GHz flux-limited sample of active galactic nuclei (AGNs) provides insights into the mechanism that produces strong Ī³-ray emission. In particular, we see that Ī³-ray emission is related to strong, uniform magnetic fields in the cores of the host AGN. Included in this sample are non-blazar AGNs such as 3C84, M82, and NGC 6251. For the blazars, the total VLBA radio flux density at 5 GHz correlates strongly with Ī³-ray flux. The LAT BL Lac objects tend to be similar to the non-LAT BL Lac objects, but the LAT flat-spectrum radio quasars (FSRQs) are significantly different from the non-LAT FSRQs. Strong core polarization is significantly more common among the LAT sources, and core fractional polarization appears to increase during LAT detection
Characteristics of Gamma-Ray Loud Blazars in the VLBA Imaging and Polarimetry Survey
The radio properties of blazars detected by the Large Area Telescope (LAT) on
board the Fermi Gamma-ray Space Telescope have been observed as part of the
VLBA Imaging and Polarimetry Survey (VIPS). This large, flux-limited sample of
active galactic nuclei (AGN) provides insights into the mechanism that produces
strong gamma-ray emission. At lower flux levels, radio flux density does not
directly correlate with gamma-ray flux. We find that the LAT-detected BL Lacs
tend to be similar to the non-LAT BL Lacs, but that the LAT-detected FSRQs are
often significantly different from the non-LAT FSRQs. The differences between
the gamma-ray loud and quiet FSRQs can be explained by Doppler boosting; these
objects appear to require larger Doppler factors than those of the BL Lacs. It
is possible that the gamma-ray loud FSRQs are fundamentally different from the
gamma-ray quiet FSRQs. Strong polarization at the base of the jet appears to be
a signature for gamma-ray loud AGN.Comment: 32 pages, 9 figures, accepted by Ap
Chemically Stable High Resolution Surface Patterning by Thiolated DNA for Self Assembly of Nanocircuits on a Gold Nano-Dot Surface
The Distance to Nova V959 Mon from VLA Imaging
Determining reliable distances to classical novae is a challenging but
crucial step in deriving their ejected masses and explosion energetics. Here we
combine radio expansion measurements from the Karl G. Jansky Very Large Array
with velocities derived from optical spectra to estimate an expansion parallax
for nova V959 Mon, the first nova discovered through its gamma-ray emission. We
spatially resolve the nova at frequencies of 4.5-36.5 GHz in nine different
imaging epochs. The first five epochs cover the expansion of the ejecta from
2012 October to 2013 January, while the final four epochs span 2014 February to
2014 May. These observations correspond to days 126 through 199 and days 615
through 703 after the first detection of the nova. The images clearly show a
non-spherical ejecta geometry. Utilizing ejecta velocities derived from 3D
modelling of optical spectroscopy, the radio expansion implies a distance
between 0.9 +/- 0.2 and 2.2 +/- 0.4 kpc, with a most probable distance of 1.4
+/- 0.4 kpc. This distance implies a gamma-ray luminosity much less than the
prototype gamma-ray-detected nova, V407 Cyg, possibly due to the lack of a red
giant companion in the V959 Mon system. V959 Mon also has a much lower
gamma-ray luminosity than other classical novae detected in gamma-rays to date,
indicating a range of at least a factor of 10 in the gamma-ray luminosities for
these explosions.Comment: 11 pages, 8 figures, 3 tables, submitted to ApJ 2015-01-21, under
revie
The Peculiar Multi-Wavelength Evolution Of V1535 Sco
We present multi-wavelength observations of the unusual nova V1535 Sco
throughout its outburst in 2015. Early radio observations were consistent with
synchrotron emission, and early X-ray observations revealed the presence of
high-energy (>1 keV) photons. These indicated that strong shocks were present
during the first ~2 weeks of the nova's evolution. The radio spectral energy
distribution was consistent with thermal emission from week 2 to week 6.
Starting in week 7, the radio emission again showed evidence of synchrotron
emission and there was an increase in X-ray emission, indicating a second shock
event. The optical spectra show evidence for at least two separate outflows,
with the faster outflow possibly having a bipolar morphology. The optical and
near infrared light curves and the X-ray measurements of the hydrogen column
density indicated that the companion star is likely a K giant.Comment: 20 pages, 13 figures, under review at ApJ, updated to match the most
recent version submitted to the refere
- ā¦