4,061 research outputs found
Tachyon cosmology with non-vanishing minimum potential: a unified model
We investigate the tachyon condensation process in the effective theory with
non-vanishing minimum potential and its implications to cosmology. It is shown
that the tachyon condensation on an unstable three-brane described by this
modified tachyon field theory leads to lower-dimensional branes (defects)
forming within a stable three-brane. Thus, in the cosmological background, we
can get well-behaved tachyon matter after tachyon inflation, (partially)
avoiding difficulties encountered in the original tachyon cosmological models.
This feature also implies that the tachyon inflated and reheated universe is
followed by a Chaplygin gas dark matter and dark energy universe. Hence, such
an unstable three-brane behaves quite like our universe, reproducing the key
features of the whole evolutionary history of the universe and providing a
unified description of inflaton, dark matter and dark energy in a very simple
single-scalar field model.Comment: 18 p
A Possible Cepheid-Like Luminosity Estimator for the Long Gamma-Ray Bursts
We present a possible Cepheid-like luminosity estimator for the long
gamma-ray bursts based on the variability of their light curves. To construct
the luminosity estimator, we use CGRO/BATSE data for 13 bursts, Wind/KONUS data
for 5 bursts, Ulysses/GRB data for 1 burst, and NEAR/XGRS data for 1 burst.
Spectroscopic redshifts, peak fluxes, and high resolution light curves are
available for 11 of these bursts; partial information is available for the
remaining 9 bursts. We find that the isotropic-equivalent luminosities L of
these bursts positively correlate with a rigorously-constructed measure V of
the variability of their light curves. We fit a model to these data that
accommodates both intrinsic scatter (statistical variance) and extrinsic
scatter (sample variance). If one excludes GRB 980425 from the fit on the
grounds that its association with SN 1998bw at a redshift of z = 0.0085 is not
secure, the luminosity estimator spans approx. 2.5 orders of magnitude in L,
and the slope of the correlation between L and V is positive with a probability
of 1 - 1.4 x 10^-4 (3.8 sigma). Although GRB 980425 is excluded from this fit,
its L and V values are consistent with the fitted model, which suggests that
GRB 980425 may well be associated with SN 1998bw, and that GRB 980425 and the
cosmological bursts may share a common physical origin. If one includes GRB
980425 in the fit, the luminosity estimator spans approx. 6.3 orders of
magnitude in L, and the slope of the correlation is positive with a probability
of 1 - 9.3 x 10^-7 (4.9 sigma). Independently of whether or not GRB 980425
should be included in the fit, its light curve is unique in that it is much
less variable than the other approx. 17 light curves in our sample for which
the signal-to-noise is reasonably good.Comment: Accepted to The Astrophysical Journal, 31 pages, 13 figures, LaTe
TGRS Observations of Positron Annihilation in Classical Novae
The TGRS experiment on board the Wind spacecraft has many advantages as a sky
monitor --- broad field of view (~2 pi) centered on the south ecliptic pole),
long life (1994-present), and stable low background and continuous coverage due
to Wind's high altitude high eccentricity orbit. The Ge detector has sufficient
energy resolution (3-4 keV at 511 keV) to resolve a cosmic positron
annihilation line from the strong background annihilation line from beta-decays
induced by cosmic ray impacts on the instrument, if the cosmic line is
Doppler-shifted by this amount. Such lines (blueshifted) are predicted from
nucleosynthesis in classical novae. We have searched the entire TGRS database
for 1995-1997 for this line, with negative results. In principle such a search
could yield an unbiased upper limit on the highly-uncertain Galactic nova rate.
We carefully examined the times around the known nova events during this
period, also with negative results. The upper limit on the nova line flux in a
6-hr interval is typically <3.8 E-3 photon/(cm2 s) at 4.6 sigma. We performed
the same analysis for times around the outburst of Nova Vel 1999, obtaining a
worse limit due to recent degradation of the detector response caused by cosmic
ray induced damage.Comment: 5 pp. inc. 3 figs. Proc. 5th Compton Symposium (AIP Conf. Series),
ed. M. McConnell, in pres
Exciting dark matter in the galactic center
We reconsider the proposal of excited dark matter (DM) as an explanation for
excess 511 keV gamma rays from positrons in the galactic center. We
quantitatively compute the cross section for DM annihilation to nearby excited
states, mediated by exchange of a new light gauge boson with off-diagonal
couplings to the DM states. In models where both excited states must be heavy
enough to decay into e^+ e^- and the ground state, the predicted rate of
positron production is never large enough to agree with observations, unless
one makes extreme assumptions about the local circular velocity in the Milky
Way, or alternatively if there exists a metastable population of DM states
which can be excited through a mass gap of less than 650 keV, before decaying
into electrons and positrons.Comment: Dedicated to the memory of Lev Kofman; 16 pages, 9 figures; v3 added
refs, minor changes, accepted to PR
The gamma-ray spectrum of Centaurus A: A high-resolution observation between 70 keV and 8 MeV
The NASA/Goddard Space Flight Center Low Energy Gamma ray Spectrometer (LEGS) observed the nearby active nucleus galaxy Centaurus A (NGC 5128) during a balloon flight on 1981 November 19. There is no evidence of a break in the spectrum or of any line features. The 1.6 MeV limit is a factor of 8 lower than the 1974 line flux, indicating that, if the 1974 feature was real, and, if it was narrow, then the line intensity decreased significantly between 1974 and 1981. The lack of observed annihilation radiation from Cen A, combined with the temporal variations that are seen in the X-ray and gamma-ray intensities, constrain the size of the emission region to be between 10 to the 13th power and 5 x 10 to the 17th power cm
Unusual Burst Emission from the New Soft Gamma Repeater SGR1627-41
In June-July,1998 the Konus-Wind burst spectrometer observed a series of
bursts from the new soft gamma repeater SGR1627-41. Time histories and energy
spectra of the bursts have been studied, revealing fluences and peak fluxes in
the ranges of 3x10^{-7} - 7.5x10^{-6} erg cm^{-2} and 10^{-5} - 10^{-4}erg
cm^{-2}/s respectively. One event, 18 June 6153.5sUT stands out dramatically
from this series. Its fluence is ~7x10^{-4} erg cm^{-2} and peak flux
~2x10^{-2} erg cm^{-2}/s. These values from a source at a distance of 5.8 kpc
yield an energy output of ~3x10^{42}erg and maximum luminosity of ~8x10^{43}
erg/s, similar to the values for the famous March 5, 1979 and August27,1998
events. In terms of energy, this event is another giant outburst seen in a
third SGR! However, this very energetic burst differs significantly from the
other giant outbursts. It exhibits no separate initial pulse with a fast rise
time, no extended tail, and no pulsations. It is rather similar to ordinary
repeated bursts but is a few hundred times stronger in intensity. According to
the magnetar model by Thompson and Duncan (1995) such a burst may be initiated
by a strong starquake when a crust fracture propagates over the whole surface
of a neutron star.Comment: 7 pages, 5 figures. To be appeared in ApJ
Cosmological Expansion in the Presence of an Extra Dimension
It has recently been pointed out that global solutions of Einstein's
equations for a 3-brane universe embedded in 4 spatial dimensions give rise to
a Friedmann equation of the form H ~ rho on the brane, instead of the usual H ~
(rho)^{1/2}, which is inconsistent with cosmological observations. We remedy
this problem by adding cosmological constants to the brane and the bulk, as in
the recent scenario of Randall and Sundrum. Our observation allows for normal
expansion during nucleosynthesis, but faster than normal expansion in the very
early universe, which could be helpful for electroweak baryogenesis, for
example.Comment: 4pp, latex, 1 figure; added and corrected references; revised
incorrect argument about sign of action on brane; final version to be
published in PR
The world is not enough
We show that the 5-dimensional model introduced by Randall and Sundrum is
(half of) a wormhole, and that this is a general result in models of the RS
type. We also discuss the gravitational trapping of a scalar particle in 5-d
spacetimes. Finally, we present a simple model of brane-world cosmology in
which the background is a static anti-de Sitter manifold, and the location of
the two 3-branes is determined by the technique of ``surgical grafting''.Comment: To be published in Phys. Rev. D with the title: ``Wormhole-surgery
and cosmology on the brane: The world is not enough''. This revised version
includes a discussion on the mechanism of gravitational trapping for a scalar
particle which will not be published in the journa
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