357,942 research outputs found
Contemporaneous VLBA 5 GHz Observations of Large Area Telescope Detected Blazars
The radio properties of blazars detected by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope have been observed contemporaneously by the Very Long Baseline Array (VLBA). In total, 232 sources were observed with the VLBA. Ninety sources that were previously observed as part of the VLBA Imaging and Polarimetry Survey (VIPS) have been included in the sample, as well as 142 sources not found in VIPS. This very large, 5 GHz flux-limited sample of active galactic nuclei (AGNs) provides insights into the mechanism that produces strong γ-ray emission. In particular, we see that γ-ray emission is related to strong, uniform magnetic fields in the cores of the host AGN. Included in this sample are non-blazar AGNs such as 3C84, M82, and NGC 6251. For the blazars, the total VLBA radio flux density at 5 GHz correlates strongly with γ-ray flux. The LAT BL Lac objects tend to be similar to the non-LAT BL Lac objects, but the LAT flat-spectrum radio quasars (FSRQs) are significantly different from the non-LAT FSRQs. Strong core polarization is significantly more common among the LAT sources, and core fractional polarization appears to increase during LAT detection
Tame stacks and log flat torsors
We compare tame actions in the category of schemes with torsors in the
category of log schemes endowed with the log flat topology. We prove that
actions underlying log flat torsors are tame. Conversely, starting from a tame
cover of a regular scheme that is a fppf torsor on the complement of a divisor
with normal crossings, it is possible to build a log flat torsor that dominates
this cover. In brief, the theory of log flat torsors gives a canonical approach
to the problem of extending torsors into tame covers.Comment: 17 pages, LaTe
Monodromy group for a strongly semistable principal bundle over a curve, II
Let be a geometrically irreducible smooth projective curve defined over a
field . Assume that has a -rational point; fix a -rational point
. From these data we construct an affine group scheme
defined over the field as well as a principal -bundle
over the curve . The group scheme is
given by a --graded neutral Tannakian category built out of all
strongly semistable vector bundles over . The principal bundle is tautological. Let be a linear algebraic group, defined over ,
that does not admit any nontrivial character which is trivial on the connected
component, containing the identity element, of the reduced center of . Let
be a strongly semistable principal -bundle over . We associate to
a group scheme defined over , which we call the monodromy group
scheme of , and a principal -bundle over , which we call the
monodromy bundle of . The group scheme is canonically a quotient of
, and is the extension of structure group of
. The group scheme is also canonically embedded in the
fiber over of the adjoint bundle.Comment: This final version includes strengthening of the result by referee's
comments. K-Theory (to appear
Optical and Near-Infrared Imaging of the IRAS 1-Jy Sample of Ultraluminous Infrared Galaxies: II. The Analysis
The present paper discusses the results from an analysis of the images
presented in Paper I (astro-ph/0207373) supplemented with new spectroscopic
data obtained at Keck. All but one object in the 1-Jy sample show signs of a
strong tidal interaction/merger. Multiple mergers involving more than two
galaxies are seen in no more than 5 of the 118 (< 5%) systems. None of the 1-Jy
sources is in the first-approach stage of the interaction, and most (56%) of
them harbor a single disturbed nucleus and are therefore in the later stages of
a merger. Seyfert galaxies (especially those of type 1), warm ULIGs
(f_{25}/f_{60} > 0.2) and the more luminous systems (> 10^{12.5} L_sun) all
show a strong tendency to be advanced mergers with a single nucleus. An
analysis of the surface brightness profiles of the host galaxies in
single-nucleus sources reveals that about 73% of the R and K' surface
brightness profiles are fit adequately by an elliptical-like R^{1/4}-law. These
elliptical-like 1-Jy systems have luminosities, half-light radii, and R-band
axial ratio distribution that are similar to those of normal (inactive)
intermediate-luminosity ellipticals and follow with some scatter the same mu_e
- r_e relation. These elliptical-like hosts are most common among merger
remnants with Seyfert 1 nuclei (83%), Seyfert 2 optical characteristics (69%)
or mid-infrared (ISO) AGN signatures (80%). In general, the results from the
present study are consistent with the merger-driven evolutionary sequence
``cool ULIGs --> warm ULIGs --> quasars,'' although there are many exceptions.
(abridged)Comment: Correction to D.-C. Kim's affiliations. 42 pages + 3 tables + 3
multi-page jpeg figures; see
http://www.astro.umd.edu/~veilleux/pubs/paper2.tar.gz for original figure
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