618 research outputs found
Discovery of X-rays from Mars with Chandra
On 4 July 2001, X-rays from Mars were detected for the first time. The
observation was performed with the ACIS-I detector onboard Chandra and yielded
data of high spatial and temporal resolution, together with spectral
information. Mars is clearly detected as an almost fully illuminated disk, with
an indication of limb brightening at the sunward side, accompanied by some
fading on the opposite side. The morphology and the X-ray luminosity of ~4 MW
are fully consistent with fluorescent scattering of solar X-rays in the upper
Mars atmosphere. The X-ray spectrum is dominated by a single narrow emission
line, which is most likely caused by O-K_alpha fluorescence. No evidence for
temporal variability is found. This is in agreement with the solar X-ray flux,
which was almost constant during the observation. In addition to the X-ray
fluorescence, there is evidence for an additional source of X-ray emission,
indicated by a faint X-ray halo which can be traced to about three Mars radii,
and by an additional component in the X-ray spectrum of Mars, which has a
similar spectral shape as the halo. Within the available limited statistics,
the spectrum of this component can be characterized by 0.2 keV thermal
bremsstrahlung emission. This is indicative of charge exchange interactions
between highly charged heavy ions in the solar wind and exospheric hydrogen and
oxygen around Mars. Although the observation was performed at the onset of a
global dust storm, no evidence for dust-related X-ray emission was found.Comment: 11 pages, 16 figure
Closed-loop input impedance of PWM buck-derived DC-DC convertersProceedings of IEEE International Symposium on Circuits and Systems - ISCAS '94
Discovery of X-rays from Venus with Chandra
On January 10 and 13, 2001, Venus was observed for the first time with an
X-ray astronomy satellite. The observation, performed with the ACIS-I and
LETG/ACIS-S instruments on Chandra, yielded data of high spatial, spectral, and
temporal resolution. Venus is clearly detected as a half-lit crescent, with
considerable brightening on the sunward limb. The morphology agrees well with
that expected from fluorescent scattering of solar X-rays in the planetary
atmosphere. The radiation is observed at discrete energies, mainly at the
O-K_alpha energy of 0.53 keV. Fluorescent radiation is also detected from
C-K_alpha at 0.28 keV and, marginally, from N-K_alpha at 0.40 keV. An
additional emission line is indicated at 0.29 keV, which might be the signature
of the C 1s --> pi* transition in CO_2 and CO. Evidence for temporal
variability of the X-ray flux was found at the 2.6 sigma level, with
fluctuations by factors of a few times indicated on time scales of minutes. All
these findings are fully consistent with fluorescent scattering of solar
X-rays. No other source of X-ray emission was detected, in particular none from
charge exchange interactions between highly charged heavy solar wind ions and
atmospheric neutrals, the dominant process for the X-ray emission of comets.
This is in agreement with the sensitivity of the observation.Comment: 12 pages, 9 figure
A first search of excited states double beta and double electron capture decays of Pd110 and Pd102
A search for double beta decays of the palladium isotopes 110Pd and 102Pd
into excited states of their daughters was performed and first half-life limits
for the 2{\nu}{\beta}{\beta} and 0{\nu}{\beta}{\beta} decays into first excited
0+ and 2+ states of 5.89e19 yr and 4.40e19 yr (95% CL) for the 110Pd decay were
obtained. The half-life limits for the corresponding double electron capture
transition of 102Pd are 7.64e18 yr and 2.68e18 yr (95% CL) respectively. These
are the first measurements for 102Pd.Comment: Updated to published version. Refined analysis and minor text
changes. Half-life limits change
Chandra's Close Encounter with the Disintegrating Comets 73P/2006 (Schwassmann--Wachmann--3) Fragment B and C/1999 S4 (LINEAR)
On May 23, 2006 we used the ACIS-S instrument on the Chandra X-ray
Observatory (CXO) to study the X-ray emission from the B fragment of comet
73P/2006 (Schwassmann-Wachmann 3) (73P/B). We obtained a total of 20 ks of CXO
observation time of Fragment B, and also investigated contemporaneous ACE and
SOHO solar wind physical data. The CXO data allow us to spatially resolve the
detailed structure of the interaction zone between the solar wind and the
fragment's coma at a resolution of ~ 1,000 km, and to observe the X-ray
emission due to multiple comet--like bodies. We detect a change in the spectral
signature with the ratio of the CV/OVII line increasing with increasing
collisional opacity as predicted by Bodewits \e (2007). The line fluxes arise
from a combination of solar wind speed, the species that populate the wind and
the gas density of the comet. We are able to understand some of the observed
X-ray morphology in terms of non-gravitational forces that act upon an actively
outgassing comet's debris field. We have used the results of the Chandra
observations on the highly fragmented 73P/B debris field to re-analyze and
interpret the mysterious emission seen from comet C/1999 S4 (LINEAR) on August
1st, 2000, after the comet had completely disrupted. We find the physical
situations to be similar in both cases, with extended X-ray emission due to
multiple, small outgassing bodies in the field of view. Nevertheless, the two
comets interacted with completely different solar winds, resulting in
distinctly different spectra.Comment: accepted by ApJ, 44 Pages, including 4 tables and 14 figure
A Single Deformed Bow Shock for Titan-Saturn System
During periods of high solar wind pressure, Saturn's bow shock is pushed inside Titan's orbit exposing the moon and its ionosphere to the solar wind. The Cassini spacecraft's T96 encounter with Titan occurred during such a period and showed evidence for shocks associated with Saturn and Titan. It also revealed the presence of two foreshocks: one prior to the closest approach (foreshock 1) and one after (foreshock 2). Using electromagnetic hybrid (kinetic ions and fluid electrons) simulations and Cassini observations, we show that the origin of foreshock 1 is tied to the formation of a single deformed bow shock for the Titan‐Saturn system. We also report the observations of a structure in foreshock 1 with properties consistent with those of spontaneous hot flow anomalies formed in the simulations and previously observed at Earth, Venus, and Mars. The results of hybrid simulations also show the generation of oblique fast magnetosonic waves upstream of the outbound Titan bow shock in agreement with the observations of large‐amplitude magnetosonic pulsations in foreshock 2. We also discuss the implications of a single deformed bow shock for new particle acceleration mechanisms and also Saturn's magnetopause and magnetosphere
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