13,737 research outputs found
The resummed thrust distribution in DIS
We present preliminary results on the resummation of leading and
next-to-leading logarithms for the thrust distribution in deep inelastic
scattering. Our predictions, expanded to O(alpha_s^2), are compared to
corresponding results from the Monte Carlo programs DISASTER++ and DISENT.Comment: 5 pages; talk presented by V. Antonelli at the UK Phenomenology
Workshop on Collider Physics, September 1999, St. John's College, Durha
Spectral libraries and their uncertainties
Libraries of stellar spectra are fundamental tools in the study of stellar
populations and in automatic determination of atmospheric parameters for large
samples of observed stars. In the context of the present volume, here I give an
overview of the current status of stellar spectral libraries from the
perspective of stellar population modeling: what we have currently available,
how good they are, and where we need further improvement
Weak solutions to problems involving inviscid fluids
We consider an abstract functional-differential equation derived from the
pressure-less Euler system with variable coefficients that includes several
systems of partial differential equations arising in the fluid mechanics. Using
the method of convex integration we show the existence of infinitely many weak
solutions for prescribed initial data and kinetic energy
The BeppoSAX HELLAS survey: on the nature of faint hard X-ray selected sources
The BeppoSAX 4.5-10 keV High Energy Large Area Survey has covered about 80
square degrees of sky down to a flux of F(5-10keV)~5E-14 cgs. Optical
spectroscopic identification of about half of the sources in the sample (62)
shows that many (~50%) are highly obscured AGN, in line with the predictions of
AGN synthesis models for the hard X-ray background (XRB, see e.g. Comastri et
al. 1995). The X-ray data, complemented by optical, near-IR and radio
follow-up, indicate that the majority of these AGN are ``intermediate''
objects, i.e. type 1.8-1.9 AGN,`red' quasars, and even a few broad line, blue
continuum quasars, obscured in X-rays by columns of the order of
logNH=22.5-23.5 cm-2, but showing a wide dispersion in optical extinction. The
optical and near-IR photometry of the obscured objects are dominated by galaxy
starlight, indicating that a sizeable fraction of the accretion power in the
Universe may actually have been missed in optical color surveys. This also
implies that multicolor photometry techniques may be efficiently used to assess
the redshift of the hard X-ray selected sources.Comment: 9 pages, Invited talk to appear in the Proceedings of the Conference
X-ray Astronomy '999: Stellar Endpoints, AGNs and the Diffuse X-ray
Background. (September 6-10 - 1999
The causal structure of spacetime is a parameterized Randers geometry
There is a by now well-established isomorphism between stationary
4-dimensional spacetimes and 3-dimensional purely spatial Randers geometries -
these Randers geometries being a particular case of the more general class of
3-dimensional Finsler geometries. We point out that in stably causal
spacetimes, by using the (time-dependent) ADM decomposition, this result can be
extended to general non-stationary spacetimes - the causal structure (conformal
structure) of the full spacetime is completely encoded in a parameterized
(time-dependent) class of Randers spaces, which can then be used to define a
Fermat principle, and also to reconstruct the null cones and causal structure.Comment: 8 page
A comparative study of the X-ray afterglow properties of optically bright and dark GRBs
We have examined the complete set of X-ray afterglow observations of dark and
optically bright GRBs performed by BeppoSAX until February 2001. X-ray
afterglows are detected in 90% of the cases. We do not find significant
differences in the X-ray spectral shape, in particular no higher X-ray
absorption in GRBs without optical transient (dark GRBs) compared to GRBs with
optical transient (OTGRBs). Rather, we find that the 1.6-10 keV flux of OTGRBs
is on average about 5 times larger than that of the dark GRBs. A K-S test shows
that this difference is significant at 99.8% probability. Under the assumption
that dark and OTGRB have similar spectra, this could suggest that the first are
uncaught in the optical band because they are just faint sources. In order to
test this hypothesis, we have determined the optical-to-X ray flux ratios of
the sample. OTGRBs show a remarkably narrow distribution of flux ratios, which
corresponds to an average optical-to-x spectral index 0.794\pm 0.054. We find
that, while 75% of dark GRBs have flux ratio upper limits still consistent with
those of OT GRBs, the remaining 25% are 4 - 10 times weaker in optical than in
X-rays. The significance of this result is equal to or higher than 2.6 sigma.
If this sub-population of dark GRBs were constituted by objects assimilable to
OTGRBs, they should have shown optical fluxes higher than upper limits actually
found. We discuss the possible causes of their behaviour, including a possible
occurrence in high density clouds or origin at very high redshift and a
connection with ancient, Population III stars.Comment: 19 pages, 5 figures. To be published in The Astrophysical Journa
The End of Amnesia: Measuring the Metallicities of Type Ia SN Progenitors with Manganese Lines in Supernova Remnants
The Mn to Cr mass ratio in supernova ejecta has recently been proposed as a
tracer of Type Ia SN progenitor metallicity. We review the advantages and
problems of this observable quantity, and discuss them in the framework of the
Tycho Supernova Remnant. The fluxes of the Mn and Cr Kalpha lines in the X-ray
spectra of Tycho observed by the Suzaku satellite suggests a progenitor of
supersolar metallicity.Comment: 4 pages, 1 figure, Proceedings of 'Probing Stellar Populations out to
the Distant Universe'. September 15-19 2008, Cefalu, Sicily, Ital
Multi-wavelength analysis of the field of the dark burst GRB 031220
We have collected and analyzed data taken in different spectral bands (from
X-ray to optical and infrared) of the field of GRB031220 and we present results
of such multiband observations. Comparison between images taken at different
epochs in the same filters did not reveal any strong variable source in the
field of this burst. X-ray analysis shows that only two of the seven Chandra
sources have a significant flux decrease and seem to be the most likely
afterglow candidates. Both sources do not show the typical values of the R-K
colour but they appear to be redder. However, only one source has an X-ray
decay index (1.3 +/- 0.1) that is typical for observed afterglows. We assume
that this source is the best afterglow candidate and we estimate a redshift of
1.90 +/- 0.30. Photometric analysis and redshift estimation for this object
suggest that this GRB can be classified as a Dark Burst and that the
obscuration is the result of dust extinction in the circum burst medium or
inside the host galaxy.Comment: 7 pages, 5 figures, accepted for publication on A&
INTEGRAL and XMM-Newton Spectral Studies of NGC 4388
We present first INTEGRAL and XMM-Newton observations of a Seyfert galaxy,
the type 2 AGN NGC 4388. Several INTEGRAL observations performed in 2003 allow
us to study the spectrum in the 20 - 300 keV range. In addition two XMM-Newton
observations give detailed insight into the 0.2 - 10 keV emission. The
measurements presented here and comparison with previous observations by
BeppoSAX, SIGMA and CGRO/OSSE show that the overall spectrum from soft X-rays
up to the gamma-rays can be described by a highly absorbed (N_H = 2.7e23
1/cm^2) and variable non-thermal component in addition to constant non-absorbed
thermal emission (T = 0.8 keV) of low abundance (7% solar), plus a constant Fe
K-alpha and K-beta line. The hard X-ray component is well described by a simple
power law with a mean photon index of 1.7. During the INTEGRAL observations the
20 - 100 keV flux increased by a factor of 1.4. The analysis of XMM-Newton data
implies that the emission below 3 keV is decoupled from the AGN and probably
due to extended emission as seen in Chandra observations. The constant iron
line emission is apparently also decoupled from the direct emission of the
central engine and likely to be generated in the obscuring material, e.g. in
the molecular torus.Comment: 11 pages, 5 figures, accepted for publication in Ap
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