86,618 research outputs found

### On the relationship of gravitational constants in KK reduction

In this short note, we try to clarify a seemly trivial but often confusing
question in relating a higher-dimensional physical gravitational constant to
its lower-dimensional correspondence in Kaluza-Klein reduction. In particular,
we re-derive the low-energy M-theory gravitational constant in terms of type
IIA string coupling $g_s$ and constant $\alpha'$ through the metric relation
between the two theories.Comment: Final version published in JHE

### (1 + p)-Dimensional Open D(p - 2) Brane Theories

The dynamics of a Dp brane can be described either by an open string ending
on this brane or by an open D(p - 2) brane ending on the same Dp brane. The
ends of the open string couple to a Dp brane worldvolume gauge field while the
boundary of the open D(p - 2) brane couples to a (p - 2)-form worldvolume
potential whose field strength is Poincare dual to that of the gauge field on
the Dp-brane worldvolume. With this in mind, we find that the Poincare dual of
the fixed rank-2 magnetic field used in defining a (1 + p)-dimensional
noncommutative Yang-Mills (NCYM) gives precisely a near-critical electric field
for the open D(p - 2) brane. We therefore find (1 + p)-dimensional open D(p -
2) brane theories along the same line as for obtaining noncommutative open
string theories (NCOS), OM theory and open Dp brane theories (ODp) from NS5
brane. Similarly, the Poincare dual of the near-critical electric field used in
defining a (1 + p)-dimensional NCOS gives a fixed magnetic-like field. This
field along with the same bulk field scalings defines a (1 + p)-dimensional
noncommutative field theory. In the same spirit, we can have various (1 +
5)-dimensional noncommutative field theories resulting from the existence of
ODp if the description of open D(4 - p) brane ending on the NS5 brane is
insisted.Comment: 35 pages, references added and discussion on decoupled field theories
refine

### Some aspects of interaction amplitudes of D branes carrying worldvolume fluxes

We report a systematic study of the stringy interaction between two sets of
Dp branes placed parallel at a separation in the presence of two worldvolume
fluxes for each set. We focus in this paper on that the two fluxes on one set
have the same structure as those on the other set but they in general differ in
values, which can be both electric or both magnetic or one electric and one
magnetic. We compute the respective stringy interaction amplitude and find that
the presence of electric fluxes gives rise to the open string pair production
while that of magnetic ones to the open string tachyon mode. The interplay of
these two leads to the open string pair production enhancement in certain cases
when one flux is electric and the other is magnetic. In particular, we find
that this enhancement occurs even when the electric flux and the magnetic one
share one common field strength index which is impossible in the one-flux case
studied previously by the present author and his collaborator in [17]. This
type of enhancement may have realistic physical applications, say, as a means
to explore the existence of extra dimensions.Comment: 49 pages, improved discussion on the matrix element evaluations in
section 2, a few footnotes added, typos correcte

### GRB 060206: hints of precession of the central engine?

Aims. The high-redshift (z=4.048) gamma-ray burst GRB 060206 showed unusual behavior, with a significant rebrightening by a factor of ~4 at about 3000 s after the burst. We argue that this rebrightening implies that the central engine became active again after the main burst produced by the first ejecta, then drove another more collimated jet-like ejecta with a larger viewing angle. The two ejecta both interacted with the ambient medium, giving rise to forward shocks that propagated into the ambient medium and reverse shocks that penetrated into the ejecta. The total emission was a combination of the emissions from the reverse- and forward- shocked regions. We discuss how this combined emission accounts for the observed rebrightening.
Methods. We apply numerical models to calculate the light curves from the shocked regions, which include a forward shock originating in the first ejecta and a forward-reverse shock for the second ejecta.
Results. We find evidence that the central engine became active again 2000 s after the main burst. The combined emission produced by interactions of these two ejecta with the ambient medium can describe the properties of the afterglow of this burst. We argue that the rapid rise in brightness at ~3000 s in the afterglow is due to the off-axis emission from the second ejecta. The precession of the torus or accretion disk of the central engine is a natural explanation for the departure of the second ejecta from the line of sight

### (m,n)-String-Like Dp-Brane Bound States

An (m, n)-string bound state (with m, n relatively prime integers) in type
IIB string theory can be interpreted from the D-string worldsheet point of view
as n D-strings carrying m units of quantized electric flux or quantized
electric field. We argue, from the D-brane worldvolume point of view, that
similar Dp-brane bound states should also exist for $2 \le p \le 8$ in both
type IIA (when $p$ is even) and type IIB (when $p$ is odd) string theories. As
in $p = 1$ case, these bound states can each be interpreted as $n$ Dp-branes
carrying m units of quantized constant electric field. In particular, they all
preserve one half of the spacetime supersymmetries.Comment: 16 pages, Revtex, typos corrected and references adde

### GRB 060206: Evidence of Precession of Central Engine

The high-redshift (z = 4.048) gamma-ray burst GRB 060206 showed unusual behavior, with a significant re-brightening about 3000 s after the burst. We assume that the central engine became active again 2000 s after the main burst and drove another more collimated off-axis jet. The two jets both interacted with the ambient medium and contributed to the whole emission. We numerically fit this optical afterglow from the two jets using the forward-shock model and the forward-reverse shock model. Combining with the zero time effect, we suggest that the fast rise at ~3000 s in the afterglow was due to the off-axis emission from the second jet. The precession of the torus or accretion disk of the gamma ray burst engine is the natural explanation for the symmetry axes of these two jets not to lie on the same line

### ((F, D1), D3) Bound State and Its T-dual Daughters

In the previous paper [hep-th/9904129], we constructed a general explicit BPS
solution for (F, D3) non-threshold bound state. By using the SL(2,Z) symmetry
of type IIB string theory, we here construct from (F, D3) a more general BPS
configuration for a D3 brane with certain units of quantized 5-form flux and an
infinite number of parallel (F, D1)-strings. We study its decoupling limit and
find that given Maldacena's $AdS_5/CFT_4$ correspondence with respect to simple
D3 branes and with the usual string coupling, we should have a similar
correspondence with respect to this bound state but now with an effective
string coupling. We discuss possible descendants of this bound state by
T-dualities along its longitudinal or transverse directions. In particular, we
present explicit configurations for ((F, Dp), D(p + 2)) bound states for $2 \le
p \le 5$. All these configurations preserve one half of the spacetime
supersymmetries.Comment: 27 pages; Improved discussion on AdS/CFT correspondence, discussion
on extra bound states added, and more references adde

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