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Quiver Matrix Mechanics for IIB String Theory (I): Wrapping Membranes and Emergent Dimension
In this paper we present a discrete, non-perturbative formulation for type
IIB string theory. Being a supersymmetric quiver matrix mechanics model in the
framework of M(atrix) theory, it is a generalization of our previous proposal
of compactification via orbifolding for deconstructed IIA strings. In the
continuum limit, our matrix mechanics becomes a -dimensional Yang-Mills
theory with 16 supercharges. At the discrete level, we are able to construct
explicitly the solitonic states that correspond to membranes wrapping on the
compactified torus in target space. These states have a manifestly
SL(2,\integer)-invariant spectrum with correct membrane tension, and give
rise to an emergent flat dimension when the compactified torus shrinks to
vanishing size.Comment: LaTeX 2e; 39 pages, 3 eps figures. v2: typos corrected; references
added; identification of certain membrane states added. v3: minor corrections
on membrane state
Late-Time Optical Afterglow Observations with LBT and MDM
Using the 2.4m MDM and 8.4m Large Binocular Telescope, we observed nine GRB
afterglows to systematically probe the late time behaviors of afterglows
including jet breaks, flares, and supernova bumps. In particular, the LBT
observations have typical flux limits of 25-26 mag in the Sloan r' band, which
allows us to extend the temporal baseline for measuring jet breaks by another
decade in time scale. We detected four jet breaks (including a "textbook" jet
break in GRB070125) and a fifth candidate, all of which are not detectable
without deep, late time optical observations. In the other four cases, we do
not detect the jet breaks either because of contamination from the host galaxy
light, the presence of a supernova bump, or the intrinsic faintness of the
optical afterglow. This suggests that the basic picture that GRBs are
collimated is still valid and that the apparent lack of Swift jet breaks is due
to poorly sampled afterglow light curves, particularly at late times. Besides
the jet breaks, we also detected late time flares, which could attribute to
late central engine activities, and two supernova bumps.Comment: 5 pages, 5 figures, 2008 NANJING GAMMA-RAY BURST CONFERENCE. AIP
Conference Proceedings, Volume 1065, pp. 93-97 (2008), Eds. Y.F. Huang, Z.G.
Dai, B. Zhan
Long lived central engines in Gamma Ray Bursts
The central engine of Gamma Ray Bursts may live much longer than the duration
of the prompt emission. Some evidence of it comes from the presence of strong
precursors, post-cursors, and X-ray flares in a sizable fraction of bursts.
Additional evidence comes from the fact that often the X-ray and the optical
afterglow light curves do not track one another, suggesting that they are two
different emission components. The typical "steep-flat-steep" behavior of the
X-ray light curve can be explained if the same central engine responsible for
the main prompt emission continues to be active for a long time, but with a
decreasing power. The early X-ray "afterglow" emission is then the extension of
the prompt emission, originating at approximately the same location, and is not
due to forward shocks. If the bulk Lorentz factor Gamma is decreasing in time,
the break ending the shallow phase can be explained, since at early times Gamma
is large, and we see only a fraction of the emitting area. Later, when Gamma
decreases, we see an increasing fraction of the emitting surface up to the time
when Gamma ~ 1/theta_j. This time ends the shallow phase of the X-ray light
curve. The origin of the late prompt emission can be the accretion of the
fall-back material, with an accretion rate dot M proportional to t^(-5/3). The
combination of this late prompt emission with the flux produced by the standard
forward shock can explain the great diversity of the optical and the X-ray
light curves.Comment: 6 pages, 6 figures, To appear in: 2008 Nanjing GRB Conference, AIP,
Eds. Y.F. Huang, Z.G. Dai, B. Zhan
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