95 research outputs found
Determination of pKa values for the mitomycin C redox couple by titration, pH rate profiles, and nernst-Clark Fits. studies of methanol elimination, carbocation formation, and carbocation/quinone method equilibrium
A mechanistic study of 2- vinylbenzimidazole formation from 2-(2'-haloethyl) benzimidazoles. Synthesis of highly electron rich vinylic compounds by general base and specific acid - general base
A comparison of the cytotoxic and physical properties of aziridinyl quinone derivatives based on pyrrolo[1,2-a] benzimidazole and pyrrolo[1,2-a]indole ring systems
Diffuse Galactic Soft Gamma-Ray Emission
The Galactic diffuse soft gamma-ray (30-800 keV) emission has been measured
from the Galactic Center by the HIREGS balloon-borne germanium spectrometer to
determine the spectral characteristics and origin of the emission. The
resulting Galactic diffuse continuum is found to agree well with a single
power-law (plus positronium) over the entire energy range, consistent with RXTE
and COMPTEL/CGRO observations at lower and higher energies, respectively. We
find no evidence of spectral steepening below 200 keV, as has been reported in
previous observations. The spatial distribution along the Galactic ridge is
found to be nearly flat, with upper limits set on the longitudinal gradient,
and with no evidence of an edge in the observed region. The soft gamma-ray
diffuse spectrum is well modeled by inverse Compton scattering of interstellar
radiation off of cosmic-ray electrons, minimizing the need to invoke
inefficient nonthermal bremsstrahlung emission. The resulting power requirement
is well within that provided by Galactic supernovae. We speculate that the
measured spectrum provides the first direct constraints on the cosmic-ray
electron spectrum below 300 MeV.Comment: 26 pages, 7 figure, submitted to Ap
OSSE Observations of the Soft Gamma Ray Continuum from the Galactic Plane at Longitude 95 Degrees
We present the results of OSSE observations of the soft gamma ray continuum
emission from the Galactic plane at longitude 95 degrees. Emission is detected
between 50 and 600 keV where the spectrum is fit well by a power law with
photon index -2.6+-0.3 and flux (4.0+-0.5) 10^{-2} photons/s/cm^2/rad/MeV at
100 keV. This spectral shape in this range is similar to that found for the
continuum emission from the inner Galaxy but the amplitude is lower by a factor
of four. This emission is either due to unresolved and previously unknown point
sources or it is of diffuse origin, or a combination of the two. Simultaneous
observations with OSSE and smaller field of view instruments operating in the
soft gamma ray energy band, such as XTE or SAX, would help resolve this issue.
If it is primarily diffuse emission due to nonthermal electron bremsstrahlung,
as is the >1 MeV Galactic ridge continuum, then the power in low energy cosmic
ray electrons exceeds that of the nuclear component of the cosmic rays by an
order of magnitude. This would have profound implications for the origin of
cosmic rays and the energetics of the interstellar medium. Alternatively, if
the emission is diffuse and thermal, then there must be a component of the
interstellar medium at temperatures near 10^9 K.Comment: 11 pages, Latex, requires AASTEX macros and psfig.tex, 2 postscript
figures, Accepted for publication in the Astrophysical Journal Letter
Gamma-Ray Spectral States of Galactic Black Hole Candidates
OSSE has observed seven transient black hole candidates: GRO J0422+32,
GX339-4, GRS 1716-249, GRS 1009-45, 4U 1543-47, GRO J1655-40, and GRS 1915+105.
Two gamma-ray spectral states are evident and, based on a limited number of
contemporaneous X-ray and gamma-ray observations, these states appear to be
correlated with X-ray states. The former three objects show hard spectra below
100 keV (photon number indices Gamma < 2) that are exponentially cut off with
folding energy ~100 keV, a spectral form that is consistent with thermal
Comptonization. This "breaking gamma-ray state" is the high-energy extension of
the X-ray low, hard state. In this state, the majority of the luminosity is
above the X-ray band, carried by photons of energy ~100 keV. The latter four
objects exhibit a "power-law gamma-ray state" with a relatively soft spectral
index (Gamma ~ 2.5-3) and no evidence for a spectral break. For GRO J1655-40,
the lower limit on the break energy is 690 keV. GRS 1716-249 exhibits both
spectral states, with the power-law state having significantly lower gamma-ray
luminosity. The power-law gamma-ray state is associated with the presence of a
strong ultrasoft X-ray excess (kT ~ 1 keV), the signature of the X-ray high,
soft (or perhaps very high) state. The physical process responsible for the
unbroken power law is not well understood, although the spectra are consistent
with bulk-motion Comptonization in the convergent accretion flow.Comment: 27 pages, 3 figures, uses aaspp.sty and psfig.st
The Case for a Low Extragalactic Gamma-ray Background
Measurements of the diffuse extragalactic gamma-ray background (EGRB) are
complicated by a strong Galactic foreground. Estimates of the EGRB flux and
spectrum, obtained by modeling the Galactic emission, have produced a variety
of (sometimes conflicting) results. The latest analysis of the EGRET data found
an isotropic flux I_x=1.45+-0.05 above 100 MeV, in units of 10^-5 s^-1 cm^-2
sr^-1. We analyze the EGRET data in search for robust constraints on the EGRB
flux, finding the gamma-ray sky strongly dominated by Galactic foreground even
at high latitudes, with no conclusive evidence for an additional isotropic
component. The gamma-ray intensity measured towards the Galactic poles is
similar to or lower than previous estimates of I_x. The high latitude profile
of the gamma-ray data is disk-like for 40<|b[deg]|<70, and even steeper for
|b|>70; overall it exhibits strong Galactic features and is well fit by a
simple Galactic model. Based on the |b|>40 data we find that I_x<0.5 at a 99%
confidence level, with evidence for a much lower flux. We show that
correlations with Galactic tracers, previously used to identify the Galactic
foreground and estimate I_x, are not satisfactory; the results depend on the
tracers used and on the part of the sky examined, because the Galactic emission
is not linear in the Galactic tracers and exhibits spectral variations across
the sky. The low EGRB flux favored by our analysis places stringent limits on
extragalactic scenarios involving gamma-ray emission, such as radiation from
blazars, intergalactic shocks and production of ultra-high energy cosmic rays
and neutrinos. We suggest methods by which future gamma-ray missions such as
GLAST and AGILE could indirectly identify the EGRB.Comment: Accepted for publication in JCAP. Increased sizes of polar regions
examined, and added discussion of spectral data. Results unchange
Dark Matter Candidates: A Ten-Point Test
An extraordinarily rich zoo of non-baryonic Dark Matter candidates has been
proposed over the last three decades. Here we present a 10-point test that a
new particle has to pass, in order to be considered a viable DM candidate: I.)
Does it match the appropriate relic density? II.) Is it {\it cold}? III.) Is it
neutral? IV.) Is it consistent with BBN? V.) Does it leave stellar evolution
unchanged? VI.) Is it compatible with constraints on self-interactions? VII.)
Is it consistent with {\it direct} DM searches? VIII.) Is it compatible with
gamma-ray constraints? IX.) Is it compatible with other astrophysical bounds?
X.) Can it be probed experimentally?Comment: 29 pages, 12 figure
Influence of porosity on lipid preservation into the wall of archaeological pottery
International audiencePorosity of archaeological pottery is a key parameter used to assess its ability to trap lipids during the use of the pot and to preserve them over time. Mercury intrusion porosimetry and gas chromatography were used to study the distribution of porosity and the preservation of lipids in different chrono-cultural contexts. The data obtained show that the porosity pattern, related to the raw materials and the savoir-faire of the potters, influences the amount of lipids accumulated in the pottery. A significant overall porosity together with a high level of small pores is generally favourable for the preservation of lipids, but variations related to the environmental context are observed
Ceramic Technology of the Asurini do Xingu, Brazil: An Ethnoarchaeological Study of Artifact Variability
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