46 research outputs found
Optical/IR from ground
Optical/infrared (O/IR) astronomy in the 1990's is reviewed. The following subject areas are included: research environment; science opportunities; technical development of the 1980's and opportunities for the 1990's; and ground-based O/IR astronomy outside the U.S. Recommendations are presented for: (1) large scale programs (Priority 1: a coordinated program for large O/IR telescopes); (2) medium scale programs (Priority 1: a coordinated program for high angular resolution; Priority 2: a new generation of 4-m class telescopes); (3) small scale programs (Priority 1: near-IR and optical all-sky surveys; Priority 2: a National Astrometric Facility); and (4) infrastructure issues (develop, purchase, and distribute optical CCDs and infrared arrays; a program to support large optics technology; a new generation of large filled aperture telescopes; a program to archive and disseminate astronomical databases; and a program for training new instrumentalists
General Relativistic Magnetohydrodynamic Simulations of Magnetically Choked Accretion Flows around Black Holes
Black hole (BH) accretion flows and jets are qualitatively affected by the
presence of ordered magnetic fields. We study fully three-dimensional global
general relativistic magnetohydrodynamic (MHD) simulations of radially extended
and thick (height to cylindrical radius ratio of )
accretion flows around BHs with various dimensionless spins (, with BH
mass ) and with initially toroidally-dominated (-directed) and
poloidally-dominated ( directed) magnetic fields. Firstly, for toroidal
field models and BHs with high enough , coherent large-scale (i.e. ) dipolar poloidal magnetic flux patches emerge, thread the BH, and generate
transient relativistic jets. Secondly, for poloidal field models, poloidal
magnetic flux readily accretes through the disk from large radii and builds-up
to a natural saturation point near the BH. For sufficiently high or low
the polar magnetic field compresses the inflow into a geometrically
thin highly non-axisymmetric "magnetically choked accretion flow" (MCAF) within
which the standard linear magneto-rotational instability is suppressed. The
condition of a highly-magnetized state over most of the horizon is optimal for
the Blandford-Znajek mechanism that generates persistent relativistic jets with
% efficiency for . A magnetic Rayleigh-Taylor
and Kelvin-Helmholtz unstable magnetospheric interface forms between the
compressed inflow and bulging jet magnetosphere, which drives a new jet-disk
quasi-periodic oscillation (JD-QPO) mechanism. The high-frequency QPO has
spherical harmonic mode period of for
with coherence quality factors . [abridged]Comment: 32 pages + acks/appendix/references, 22 figures, 10 tables. MNRAS in
press. High-Res Version: http://www.slac.stanford.edu/~jmckinne/mcaf.pdf .
Fiducial Movie: http://youtu.be/V2WoJOkIin
HST and VLT observations of the neutron star 1E 1207.4-5209
1E 1207.4-5209, the peculiar Central Compact object in the G296.5+10.0
supernova remnant, has been proposed to be an "anti-magnetar" - a young neutron
star born with a weak dipole field. Accretion, possibly of supernova fallback
material, has also been invoked to explain a large surface temperature
anisotropy as well as the generation of peculiar cyclotron absorption features
superimposed to its thermal spectrum. Interestingly enough, a faint
optical/infrared source was proposed as a possible counterpart to 1E
1207.4-5209, but later questioned, based on coarse positional coincidence.
Considering the large offset of 1E 1207.4-5209 with respect to the center of
its host supernova remnant, the source should move at ~70 mas/yr. Thus, we
tested the association by measuring the proper motion of the proposed optical
counterpart. Using HST observations spanning 3.75 years, we computed a 3 sigma
upper limit of 7 mas/yr. Absolute astrometry on the same HST data set also
places the optical source significantly off the 99% confidence Chandra
position. This allows us to safely rule out the association. Using the HST data
set, coupled to ground-based observations collected at the ESO/VLT, we set the
deepest limits ever obtained to the optical/infrared emission from 1E
1207.4-5209. By combining such limits to the constraints derived from X-ray
timing, we rule out accretion as the source of the thermal anisotropy of the
neutron star.Comment: 8 pages, 3 figures. Accepted for publication in Astronomy &
Astrophysic
Co-opetition as the new path to innovation? Negotiating strategic change through user-centred design approaches:
Archaeological and biological examination of “The Mystery Wreck” (8MO143) off Vaca Key, Monroe County, Florida: A Report Submitted to the Florida Keys National Marine Sanctuary in Fulfillment of a NOAA Maritime Heritage Program Mini-grant
During the summer of 2004, the Florida Bureau of Archaeological Research Underwater Archaeology team undertook a project to relocate, assess, and record thirteen of the shipwrecks of the 1733 Spanish Plate Fleet in the Florida Keys. One source of background information that they used was a commercially available videotape entitled “Galleon Hunter,” produced by Don Ferguson. Aside from the 1733 wrecksites, the video features another site, locally known as “the Mystery Galleon,” that was shown to Ferguson by local diver Stefan Sykora. Using location numbers supplied in Ferguson’s video, Roger Smith, Della Scott-Ireton, and Dave McCampbell relocated the site in Hawk Channel, off the city of Marathon. Later, the site further was examined by Smith, Jennifer McKinnon, and Jason Raupp, who made initial sketches, still photos, and video recordings.ReportSubmitte