15,166 research outputs found
Completely dark galaxies: their existence, properties, and strategies for finding them
There are a number of theoretical and observational hints that large numbers
of low-mass galaxies composed entirely of dark matter exist in the field. The
theoretical considerations follow from the prediction of cold dark matter
theory that there exist many low-mass galaxies for every massive one. The
observational considerations follow from the observed paucity of these low-mass
galaxies in the field but not in dense clusters of galaxies; this suggests that
the lack of small galaxies in the field is due to the inhibition of star
formation in the galaxies as opposed to the fact that their small dark matter
halos do not exist. In this work we outline the likely properties of low-mass
dark galaxies, and describe observational strategies for finding them, and
where in the sky to search. The results are presented as a function of the
global properties of dark matter, in particular the presence or absence of a
substantial baryonic dark matter component. If the dark matter is purely cold
and has a Navarro, Frenk and White density profile, directly detecting dark
galaxies will only be feasible with present technology if the galaxy has a
maximum velocity dispersion in excess of 70 km/s, in which case the dark
galaxies could strongly lens background objects. This is much higher than the
maximum velocity dispersions in most dwarf galaxies. If the dark matter in
galaxy halos has a baryonic component close to the cosmic ratio, the
possibility of directly detecting dark galaxies is much more realistic; the
optimal method of detection will depend on the nature of the dark matter. A
number of more indirect methods are also discussed.Comment: 12 pages, 4 figures, MNRAS in pres
Dyes removal from water using low cost absorbents
In this study, the removal capacity of low cost adsorbents during the adsorption of Methylene Blue (MB) and Congo Red (CR) at different concentrations (50 and 100mgâąL-1) was evaluated. These adsorbents were produced from wood wastes (cedar and teak) by chemical activation (ZnCl2). Both studied materials, Activated Cedar (AC) and activated teak (AT) showed a good fit of their experimental data to the pseudo second order kinetic model and Langmuir isotherms. The maximum adsorption capacities for AC were 2000.0 and 444.4mgâąg-1 for MB and CR, respectively, while for AT, maximum adsorption capacities of 1052.6 and 86.4mgâąg-1 were found for MB and CR, respectively. © Published under licence by IOP Publishing Ltd
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
Colombia: Seeking Prosperity Through Peace
PreprintThe recent global transition to a more diffuse distribution of economic power points to a shift in the balance of global growth from rich to low- and middle- income economies. Colombia may be a prime example as its recent rapid per capita income growth of 10.2% on average since 1999 points to the potential for Colombia's convergence to the ranks of rich countries. However, Colombia's economic growth has been constrained by over 40 years of a costly and ineffective drug war policy that has failed. The illicit activity of drug production and trafficking grosses approximately 20 billion a year; it does not enter into the GDP accounting. In addition, the FARC (Revolutionary Armed Forces of Colombia) has stifled Colombia's drive towards economic prosperity. Barring the social and political impasse, the economy would flourish
Polar phonons and intrinsic dielectric response of the ferromagnetic insulating spinel CdCrS from first principles
We have studied the dielectric properties of the ferromagnetic spinel
CdCrS from first principles. Zone-center phonons and Born effective
charges were calculated by frozen-phonon and Berry phase techniques within
LSDA+U. We find that all infrared-active phonons are quite stable within the
cubic space group. The calculated static dielectric constant agrees well with
previous measurements. These results suggest that the recently observed
anomalous dielectric behavior in CdCrS is not due to the softening of a
polar mode. We suggest further experiments to clarify this point
Gamma-Ray Bursts in the Swift Era
With its rapid-response capability and multiwavelength complement of
instruments, the Swift satellite has transformed our physical understanding of
gamma-ray bursts (GRBs). Providing high-quality observations of hundreds of
bursts, and facilitating a wide range of follow-up observations within seconds
of each event, Swift has revealed an unforeseen richness in observed burst
properties, shed light on the nature of short-duration bursts, and helped
realize the promise of GRBs as probes of the processes and environments of star
formation out to the earliest cosmic epochs. These advances have opened new
perspectives on the nature and properties of burst central engines,
interactions with the burst environment from microparsec to gigaparsec scales,
and the possibilities for non-photonic signatures. Our understanding of these
extreme cosmic sources has thus advanced substantially; yet more than 40 years
after their discovery, GRBs continue to present major challenges on both
observational and theoretical fronts.Comment: 67 pages, 16 figures; ARAA, 2009;
http://arjournals.annualreviews.org/toc/astro/47/
Spin self-rephasing and very long coherence times in a trapped atomic ensemble
We perform Ramsey spectroscopy on the ground state of ultra-cold 87Rb atoms
magnetically trapped on a chip in the Knudsen regime. Field inhomogeneities
over the sample should limit the 1/e contrast decay time to about 3 s, while
decay times of 58 s are actually observed. We explain this surprising result by
a spin self-rephasing mechanism induced by the identical spin rotation effect
originating from particle indistinguishability. We propose a theory of this
synchronization mechanism and obtain good agreement with the experimental
observations. The effect is general and susceptible to appear in other physical
systems.Comment: Revised version; improved description of the theoretical treatmen
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GRB 180620A: Evidence for Late-time Energy Injection
The early optical emission of gamma-ray bursts (GRBs) gives an opportunity to understand the central engine and first stages of these events. About 30% of GRBs present flares whose origin is still a subject of discussion. We present optical photometry of GRB 180620A with the COATLI telescope and RATIR instrument. COATLI started to observe from the end of prompt emission at T + 39.3 s and RATIR from T + 121.4 s. We supplement the optical data with the X-ray light curve from Swift/XRT. We observe an optical flare from T + 110 s to T + 550 s, with a temporal index decay α O,decay = 1.32 ± 0.01, and Ît/t = 1.63, which we interpret as the signature of a reverse shock component. After the initial normal decay the light curves show a long plateau from T + 500 s to T + 7800 s in both X-rays and the optical before decaying again after an achromatic jet break at T + 7800 s. Fluctuations are seen during the plateau phase in the optical. Adding to the complexity of GRB afterglows, the plateau phase (typically associated with the coasting phase of the jet) is seen in this object after the "normal" decay phase (associated with the deceleration phase of the jet), and the jet break phase occurs directly after the plateau. We suggest that this sequence of events can be explained by a rapid deceleration of the jet with t d âČ 40 s due to the high density of the environment (â100 cm-3) followed by reactivation of the central engine, which causes the flare and powers the plateau phase
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