18,494 research outputs found
Pixelated Lenses and H_0 from Time-delay QSOs
Observed time delays between images of a lensed QSO lead to the determination
of the Hubble constant by Refsdal's method, provided the mass distribution in
the lensing galaxy is reasonably well known. Since the two or four QSO images
usually observed are woefully inadequate by themselves to provide a unique
reconstruction of the galaxy mass, most previous reconstructions have been
limited to simple parameterized models, which may lead to large systematic
errors in the derived H_0 by failing to consider enough possibilities for the
mass distribution of the lens. We use non-parametric modeling of galaxy lenses
to better explore physically plausible but not overly constrained galaxy mass
maps, all of which reproduce the lensing observables exactly, and derive the
corresponding distribution of H_0's. Blind tests - where one of us simulated
galaxy lenses, lensing observables, and a value for H_0, and the other applied
our modeling technique to estimate H_0 indicate that our procedure is reliable.
For four simulated lensed QSOs the distribution of inferred H_0 have an
uncertainty of \simeq 10% at 90% confidence. Application to published
observations of the two best constrained time-delay lenses, PG1115+080 and
B1608+656, yields H_0=61 +/- 11 km/s/Mpc at 68% confidence and 61 +/- 18
km/s/Mpc at 90% confidence.Comment: 27 pages, including 17 figs, LaTeX; accepted to A
Superconducting and ferromagnetic phases induced by lattice distortions in SrFe2As2
Single crystals of SrFe2As2 grown using a self-flux solution method were
characterized via x-ray, transport and magnetization studies, revealing a
superconducting phase below T_c = 21 K characterized by a full electrical
resistivity transition and partial diamagnetic screening. The reversible
destruction and reinstatement of this phase by heat treatment and mechanical
deformation studies, along with single-crystal X-ray diffraction measurements,
indicate that internal crystallographic strain originating from c-axis-oriented
planar defects plays a central role in promoting the appearance of
superconductivity under ambient pressure conditions in ~90% of as-grown
crystals. The appearance of a ferromagnetic moment with magnitude proportional
to the tunable superconducting volume fraction suggests that these phenomena
are both stabilized by lattice distortion.Comment: 4 pages, 4 figure
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