24,843 research outputs found
Occultation of the Quiescent Emission from Sgr A* by IR Flares
We have investigated the nature of flare emission from Sgr A* during
multi-wavelength observations of this source that took place in 2004, 2005 and
2006. We present evidence for dimming of submm and radio flux during the peak
of near-IR flares. This suggests that the variability of Sgr A* across its
wavelength spectrum is phenomenologically related. The model explaining this
new behavior of flare activity could be consistent with adiabatically cooling
plasma blobs that are expanding but also partially eclipsing the background
quiescent emission from Sgr A*. When a flare is launched, the plasma blob is
most compact and is brightest in the optically thin regime whereas the emission
in radio/submm wavelengths has a higher opacity. Absorption in the observed
light curve of Sgr A* at radio/submm flux is due to the combined effects of
lower brightness temperature of plasma blobs with respect to the quiescent
brightness temperature and high opacity of plasma blobs. This implies that
plasma blobs are mainly placed in the magnetosphere of a disk-like flow or
further out in the flow. The depth of the absorption being larger in submm than
in radio wavelengths implies that the intrinsic size of the quiescent emission
increases with increasing wavelength which is consistent with previous size
measurements of Sgr A*. Lastly, we believe that occultation of the quiescent
emission of Sgr A* at radio/submm by IR flares can be used as a powerful tool
to identify flare activity at its earliest phase of its evolution.Comment: 11 pages, 5 figures, accepted by ApJ
A Fuzzy Logic Based Algorithm for Finding Astronomical Objects in Wide-Angle Frames
Accurate automatic identification of astronomical objects in an imperfect
world of non-linear wide-angle optics, imperfect optics, inaccurately pointed
telescopes, and defect-ridden cameras is not always a trivial first step. In
the past few years, this problem has been exacerbated by the rise of digital
imaging, providing vast digital streams of astronomical images and data. In the
modern age of increasing bandwidth, human identifications are many times
impracticably slow. In order to perform an automatic computer-based analysis of
astronomical frames, a quick and accurate identification of astronomical
objects is required. Such identification must follow a rigorous transformation
from topocentric celestial coordinates into image coordinates on a CCD frame.
This paper presents a fuzzy logic based algorithm that estimates needed
coordinate transformations in a practical setting. Using a training set of
reference stars, the algorithm statically builds a fuzzy logic model. At
runtime, the algorithm uses this model to associate stellar objects visible in
the frames to known-catalogued objects, and generates files that contain
photometry information of objects visible in the frame. Use of this algorithm
facilitates real-time monitoring of stars and bright transients, allowing
identifications and alerts to be issued more reliably. The algorithm is being
implemented by the Night Sky Live all-sky monitoring global network and has
shown itself significantly more reliable than the previously used non-fuzzy
logic algorithm.Comment: Accepted for publication in PAS
The Nature of Nonthermal X-ray Filaments Near the Galactic Center
Recent Chandra and XMM-{\it Newton} observations reported evidence of two
X-ray filaments G359.88-0.08 (SgrA-E) and G359.54+0.18 (the ripple filament)
near the Galactic center. The X-ray emission from these filaments has a
nonthermal spectrum and coincides with synchrotron emitting radio sources.
Here, we report the detection of a new X-ray feature coincident with a radio
filament G359.90-0.06 (SgrA-F) and show more detailed VLA, Chandra and BIMA
observations of the radio and X-ray filaments. In particular, we show that
radio emission from the nonthermal filaments G359.90-0.06 (SgrA-F) and
G359.54+0.18 (the ripple) has a steep spectrum whereas G359.88-0.08 (SgrA-E)
has a flat spectrum. The X-ray emission from both these sources could be due to
synchrotron radiation. However, given that the 20 \kms molecular cloud, with
its intense 1.2mm dust emission, lies in the vicinity of SgrA-F, it is possible
that the X-rays could be produced by inverse Compton scattering of far-infrared
photons from dust by the relativistic electrons responsible for the radio
synchrotron emission. The production of X-ray emission from ICS allows an
estimate of the magnetic field strength of ~0.08 mG within the nonthermal
filament. This should be an important parameter for any models of the Galactic
center nonthermal filaments.Comment: 14 pages, 9 figures, in Cospar 2004 session E1.4; editors: Cara
Rakowski and Shami Chatterjee; "Young Neutron Stars and Supernova Remnants",
publication: Advances in Space Research (in press
- …