CfA Theory Seminars

[Kentaro Nagamine <knagamin cfa harvard edu>]

Joint Cosmology Seminar of MIT/Tufts/Harvard

Tue, Oct 7

Donald Lamb (University of Chicago)

Some Highlights of Scientific Results from the Flash Center

The Center for Astrophysical Thermonuclear Flashes at the University of
Chicago -- a collaboration between scientists located primarily at
Chicago and at Argonne National Laboratory -- studies the long-standing
problem of thermonuclear flashes on the surfaces of compact stars such
as neutron stars and white dwarf stars, and in the interior of white
dwarf stars.  A key objective of the Center is to build a
new-generation astrophysical simulation code for computing problems
relating to X-ray bursts, novae, and Type Ia supernovae.  The
capabilities of the code are extensive; the code can treat problems in
astrophysics ranging from star formation in molecular clouds to
large-scale structure formation in cosmology.  In this talk I will
highlight a number of recent scientific results obtained by researchers
at the Center using the Flash code and new modules that are being
developed for the code.  These include: (1) 1-D relativistic
calculations of accretion onto neutron stars; (2) 2-D semi-relativistic
MHD calculations of accretion onto magnetic white dwarfs; (3) 2-D and
3-D simulations of wind-wave mixing in the envelope of pre-novae white
dwarfs, which have led to a new physical picture of nova outbursts; (4)
flame modeling within the context of Type Ia supernovae; and (5) the
first 3-D simulations of the deflagration phase of Type Ia supernovae
for an entire white dwarf star.

The seminar will be held in the Gilman room in Agassiz house, Radcliffe
Institute (at the corner of Mason and Garden St.; 5 min walk from Harvard
Sq. T-station) at 2:30pm.

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Wed, Oct 8

Ehud Nakar (Hebrew University)

Highly polarized gamma-rays in Gamma-Ray Bursts

HESSI detected a completely unexpected high polarization from GRB 021206.
This observation prompted several claims that this proves that the
magnetic field within the emitting region is uniform and therefore the
relativistic outflow  in GRBs must be electromagnetic (Poynting flux
dominated).
We compare two possible scenarios for the producing of high level
polarization within the prompt emission of a GRB: synchrotron
emission from a relativistic jet with a uniform magnetic field and
synchrotron emission from a jet with a random magnetic field.
Somewhat surprisingly we find that both scenarios can produce
a comparable level of polarization (~ 40 - 50%).
A significant polarization arise from a random field  provided that the
observer is located within 1/Gamma orientation from a narrow
(theta_j ~ 1/Gamma) jet. We show that this is "natural" within
the specific context of GRB 021206.

The seminar will be held in Pratt Conference Room (G04) at CfA
(60 Garden St.) at 2pm.

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