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Astrophysics: Compact
Objects
Overview
Faculty in this Area of Research:
Astrophysics Areas of Research
Overview
MIT has long been a leader in the study of the densest objects
in the universe—white dwarfs, neutron stars, and black holes,
known collectively as "compact objects." When matter accretes
onto a compact object, typically from an orbiting companion star,
viscous heating in the accretion disk releases large amounts of
energy that ultimately emerges mainly in X-rays. MIT X-ray astronomers
(Hale
Bradt, Claude
Canizares, Deepto
Chakrabarty, George
Clark, Walter
Lewin, and Saul
Rappaport) have long been leaders in the study of accreting
compact objects.
The study of compact objects probes physics at extreme conditions
of density, temperature, and magnetic fields. The mass-radius relation
for neutron stars probes the equation of state at supernuclear densities
and may reveal quark matter (as first studied by Edward
Farhi and Robert
Jaffe in the Center for Theoretical
Physics) in one of the color superconducting phases (pioneered
by Krishna
Rajagopal and Frank
Wilczek). Accurate neutron star masses can be measured for some
binaries especially those including radio pulsars; measuring radii
is more difficult but may be possible through studies of gravitational
redshifts, neutron star cooling or the dynamics of gas near the
innermost stable circular orbit predicted by general relativity.
Important stellar evolution questions are being addressed concerning
the evolutionary pathways to each of the endpoints for compact objects
(Paul
Joss and Saul
Rappaport). Binary star systems can undergo complex mass transfer
evolutionary phases that have been studied extensively at MIT. In
particular, considerable insight has been gained into how close
binary systems containing compact objects are formed from primordial
binaries in the Galaxy and via dynamical capture processes in globular
star clusters. Once an accreting compact binary forms, many questions
remain about the accretion process itself. For example, largely
through observational work conducted with the Rossi X-ray Timing
Explorer Satellite, for which MIT provided two instruments under
the leadership of Hale
Bradt, astronomers have found that accreting neutron stars often
flicker quasi-periodically at frequencies ranging from a few Hz
to more than one kilohertz. The cause of this flickering is poorly
understood but may involve effects of strong field gravity in the
accretion disk or oscillations of the neutron star.
Cosmic gamma ray bursts are important for their own intrinsic physics
as well as for providing a probe of cosmology. We still do not know
the nature of the tremendous explosions that in about one minute
release a few percent of a solar mass of rest energy in the form
of gamma rays. However, several clues point to an association with
the explosions of massive stars, and current models assume that
a gamma ray burst is triggered by the formation of a black hole.
MIT researchers in the Center for
Space Research are leaders in the discovery and study of gamma
ray bursts.
Compact objects offer the ultimate strong-field tests of general
relativity through the gravitational radiation emitted when black
holes form. MIT and Caltech are the two lead institutions in LIGO
(Laser Interferometer Gravitational Wave Observatory), a major NSF-funded
project to detect and measure gravitational radiation. MIT researchers
play key roles in developing instrumentation (Nergis
Mavalvala and Rainer
Weiss) and data analysis strategies (Erik
Katsavounidis) for LIGO. The second-generation LIGO effort under
planning for later this decade should increase the sensitivity of
LIGO enough for regular astronomical observations of black hole
formation through the coalescence of binary neutron stars. Observation
of these signals would offer a wealth of information about neutron
stars and provide the most stringent tests of general relativity.
Theoretical work is being conducted by Scott
Hughes in preparation for a future space-based mission, LISA
(Laser Interferometer Space Antenna).
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