8,589 research outputs found
Imaging and burst location with the EXIST high-energy telescope
The primary instrument of the proposed EXIST mission is a coded mask high
energy telescope (the HET), that must have a wide field of view and extremely
good sensitivity. It will be crucial to minimize systematic errors so that even
for very long total integration times the imaging performance is close to the
statistical photon limit. There is also a requirement to be able to reconstruct
images on-board in near real time in order to detect and localize gamma-ray
bursts. This must be done while the spacecraft is scanning the sky. The
scanning provides all-sky coverage and is key to reducing systematic errors.
The on-board computational problem is made even more challenging for EXIST by
the very large number of detector pixels. Numerous alternative designs for the
HET have been evaluated. The baseline concept adopted depends on a unique coded
mask with two spatial scales. Monte Carlo simulations and analytic analysis
techniques have been used to demonstrate the capabilities of the design and of
the proposed two-step burst localization procedure
Very High Resolution Solar X-ray Imaging Using Diffractive Optics
This paper describes the development of X-ray diffractive optics for imaging
solar flares with better than 0.1 arcsec angular resolution. X-ray images with
this resolution of the \geq10 MK plasma in solar active regions and solar
flares would allow the cross-sectional area of magnetic loops to be resolved
and the coronal flare energy release region itself to be probed. The objective
of this work is to obtain X-ray images in the iron-line complex at 6.7 keV
observed during solar flares with an angular resolution as fine as 0.1 arcsec -
over an order of magnitude finer than is now possible. This line emission is
from highly ionized iron atoms, primarily Fe xxv, in the hottest flare plasma
at temperatures in excess of \approx10 MK. It provides information on the flare
morphology, the iron abundance, and the distribution of the hot plasma.
Studying how this plasma is heated to such high temperatures in such short
times during solar flares is of critical importance in understanding these
powerful transient events, one of the major objectives of solar physics. We
describe the design, fabrication, and testing of phase zone plate X-ray lenses
with focal lengths of \approx100 m at these energies that would be capable of
achieving these objectives. We show how such lenses could be included on a
two-spacecraft formation-flying mission with the lenses on the spacecraft
closest to the Sun and an X-ray imaging array on the second spacecraft in the
focal plane \approx100 m away. High resolution X-ray images could be obtained
when the two spacecraft are aligned with the region of interest on the Sun.
Requirements and constraints for the control of the two spacecraft are
discussed together with the overall feasibility of such a formation-flying
mission
Io: IUE observations of its atmosphere and the plasma torus
Two of the main components of the atmosphere of Io, neutral oxygen and sulfur, were detected with the IUE. Four observations yield brightnesses that are similar, regardless of whether the upstream or the downstream sides of the torus plasma flow around Io is observed. A simple model requires the emissions to be produced by the interaction of O and S columns in the exospheric range with 2 eV electrons. Cooling of the 5 eV torus electrons is required prior to their interaction with the atmosphere of Io. Inconsistencies in the characteristics of the spectra that cannot be accounted for in this model require further analysis with improved atomic data. The Io plasma torus was monitored with the IUE. The long-term stability of the warm torus is established. The observed brightnesses were analyzed using a model of the torus, and variations of less than 30 percent in the composition are observed, the quantitative results being model dependent
A new bursting X-ray transient: SAX J1750.8-2900
We have analysed in detail the discovery measurements of the X-ray burster
SAX J1750.8-2900 by the Wide Field Cameras on board BeppoSAX in spring 1997, at
a position ~1.2 degrees off the Galactic Centre. The source was in outburst on
March 13th when the first observation started and showed X-ray emission for ~ 2
weeks. A total of 9 bursts were detected, with peak intensities varying from ~
0.4 to 1.0 Crab in the 2-10 keV range. Most bursts showed a fast rise time (~
1s), an exponential decay profile with e-folding time of ~ 5s, spectral
softening during decay, and a spectrum which is consistent with few keV
blackbody radiation. These features identify them as type-I X-ray bursts of
thermonuclear origin. The presence of type-I bursts and the source position
close to the Galactic Centre favours the classification of this object as a
neutron star low mass X-ray binary. X-ray emission from SAX J1750.8-2900 was
not detected in the previous and subsequent Galactic bulge monitoring, and the
source was never seen bursting again.Comment: 13 pages, 3 Postscript figures, aaspp4 styl
Low-Frequency Radio Transients in the Galactic Center
We report the detection of a new radio transient source, GCRT J1746-2757,
located only 1.1 degrees north of the Galactic center. Consistent with other
radio transients toward the Galactic center, this source brightened and faded
on a time scale of a few months. No X-ray counterpart was detected. We also
report new 0.33 GHz measurements of the radio counterpart to the X-ray
transient source, XTE J1748-288, previously detected and monitored at higher
radio frequencies. We show that the spectrum of XTE J1748-288 steepened
considerably during a period of a few months after its peak. We also discuss
the need for a more efficient means of finding additional radio transients
SPI observations of positron annihilation radiation from the 4th galactic quadrant: sky distribution
During its first year in orbit the INTEGRAL observatory performed deep
exposures of the Galactic Center region and scanning observations of the
Galactic plane. We report on the status of our analysis of the positron
annihilation radiation from the 4th Galactic quadrant with the spectrometer
SPI, focusing on the sky distribution of the 511 keV line emission. The
analysis methods are described; current constraints and limits on the Galactic
bulge emission and the bulge-to-disk ratio are presented.Comment: 4 pages, 2 figures, accepted for publication in the proceedings of
the 5th INTEGRAL worksho
Evolution from protoplanetary to debris discs: The transition disc around HD 166191
HD 166191 has been identified by several studies as hosting a rare and
extremely bright warm debris disc with an additional outer cool disc component.
However, an alternative interpretation is that the star hosts a disc that is
currently in transition between a full gas disc and a largely gas-free debris
disc. With the help of new optical to mid-IR spectra and Herschel imaging, we
argue that the latter interpretation is supported in several ways: i) we show
that HD 166191 is co-moving with the ~4 Myr-old Herbig Ae star HD 163296,
suggesting that the two have the same age, ii) the disc spectrum of HD 166191
is well matched by a standard radiative transfer model of a gaseous
protoplanetary disc with an inner hole, and iii) the HD 166191 mid-IR silicate
feature is more consistent with similarly primordial objects. We note some
potential issues with the debris disc interpretation that should be considered
for such extreme objects, whose lifetime at the current brightness is mush
shorter than the stellar age, or in the case of the outer component requires a
mass comparable to the solid component of the Solar nebula. These aspects
individually and collectively argue that HD 166191 is a 4-5 Myr old star that
hosts a gaseous transition disc. Though it does not argue in favour of either
scenario, we find strong evidence for 3-5 um disc variability. We place HD
166191 in context with discs at different evolutionary stages, showing that it
is a potentially important object for understanding the protoplanetary to
debris disc transition.Comment: accepted to MNRAS, fixed typos in abstract and axis labe
Precision measurement of the n=2 triplet P J=1-to-J=0 fine structure of atomic helium using frequency-offset separated oscillatory fields
Increasing accuracy of the theory and experiment of the P fine
structure of helium has allowed for increasingly-precise tests of quantum
electrodynamics (QED), determinations of the fine-structure constant ,
and limitations on possible beyond-the-Standard-Model physics. Here we present
a 2~part-per-billion (ppb) measurement of the -to- interval.
The measurement is performed using frequency-offset
separated-oscillatory-fields. Our result of ~Hz
represents a landmark for helium fine-structure measurements, and, for the
first time, will allow for a 1-ppb determination of the fine-structure constant
when QED theory for the interval is improved.Comment: 6 pages, 6 figures. arXiv admin note: substantial text overlap with
arXiv:1807.0792
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