5 research outputs found
VLBI and Archival VLA and WSRT Observations of the GRB 030329 Radio Afterglow
We present VLBI and archival Karl G. Jansky Very Large Array (VLA) and
Westerbork Synthesis Radio Telescope (WSRT) observations of the radio afterglow
from the gamma-ray burst (GRB) of 2003 March 29 (GRB 030329) taken between 672
and 2032 days after the burst. The EVLA and WSRT data suggest a simple power
law decay in the flux at 5 GHz, with no clear signature of any rebrightening
from the counter jet. We report an unresolved source at day 2032 of size
mas, which we use in conjunction with the expansion rate of the
burst to argue for the presence of a uniform, ISM-like circumburst medium.
We develop a semi-analytic method to model gamma-ray burst afterglows, and
apply it to the 5 GHz light curve to perform burst calorimetry. A limit of mas yr is placed on the proper motion, supporting the standard
afterglow model for gamma-ray bursts.Comment: 24 pages, 5 figure
Gamma-Ray Bursts in Circumstellar Shells: A Possible Explanation for Flares
It is now generally accepted that long-duration gamma ray bursts (GRBs) are
due to the collapse of massive rotating stars. The precise collapse process
itself, however, is not yet fully understood. Strong winds, outbursts, and
intense ionizing UV radiation from single stars or strongly interacting
binaries are expected to destroy the molecular cloud cores that give birth to
them and create highly complex circumburst environments for the explosion. Such
environments might imprint features on GRB light curves that uniquely identify
the nature of the progenitor and its collapse. We have performed numerical
simulations of realistic environments for a variety of long-duration GRB
progenitors with ZEUS-MP, and have developed an analytical method for
calculating GRB light curves in these profiles. Though a full,
three-dimensional, relativistic magnetohydrodynamical computational model is
required to precisely describe the light curve from a GRB in complex
environments, our method can provide a qualitative understanding of these
phenomena. We find that, in the context of the standard afterglow model,
massive shells around GRBs produce strong signatures in their light curves, and
that this can distinguish them from those occurring in uniform media or steady
winds. These features can constrain the mass of the shell and the properties of
the wind before and after the ejection. Moreover, the interaction of the GRB
with the circumburst shell is seen to produce features that are consistent with
observed X-ray flares that are often attributed to delayed energy injection by
the central engine. Our algorithm for computing light curves is also applicable
to GRBs in a variety of environments such as those in high-redshift
cosmological halos or protogalaxies, both of which will soon be targets of
future surveys such as JANUS or Lobster.Comment: 12 pages, 5 figures, Accepted by Ap
Managerial education of naval officers
In the Navy almost all officers, whether junior or senior,
hold positions as managers. The economy and efficiency of the
Navy's operations, therefore, depends on the ability of it':
managers to plan, organize, staff, direct and control the
organization.
A review is made of what noted authorities on the subject of
management have said about the universality of management
principles and the need to develop managers through education
and training. Also, a review is made of the educational background
of officer candidates as to the amount of management
development they have received.
From these reviews, an attempt is made to show that although
naval officers are well educated they are lacking in one important
area. This is the area of management development, which is
necessary if the Navy is to obtain the maximum performance from
its officers.
A comparison is also made of the Navy' s manager development
program with those of industry and other services. From
these are drawn conclusions and recommendations to assist in
the managerial development of today's naval officers.http://www.archive.org/details/managerialeducat00leamLieutenant Commander, United States NavyLieutenant Commander, United States NavyApproved for public release; distribution is unlimited