662 research outputs found
Remediation of chromium(VI) at polypyrrole-coated titanium
The application of conducting polypyrrole (PPy) and polyaniline (PAni) coated substrates in remediation of chromium, Cr(VI), is an area of considerable interest. Here, we discuss the implementation of PPy-coated titanium as a new material for the reduction of Cr(VI) to the less toxic trivalent state, Cr(III). An alkaline-peroxide based etching process was used to ensure the adhesion of the PPy coatings to the underlying titanium. The PPy films showed excellent resistance to acidic Cr(VI) solutions and remained adherent after continuous exposure to the solutions. In order to optimise the remediation process a number of experimental parameters were investigated, including the thickness of the PPy coating, the reduction potential used in pre-treatment of the PPy and the degree of solution agitation. The durability of the materials on exposure to the Cr(VI) test solutions made them suitable for repeated remediation experiments. Following several test-runs, the cleanup efficiency of the material was found to decrease slightly, however, increasing the exposure/experiment time resulted in significantly improved cleanup ability
The Flux Variability of Markarian 501 in Very High Energy Gamma Rays
The BL Lacertae object Markarian 501 was identified as a source of gamma-ray
emission at the Whipple Observatory in March 1995. Here we present a flux
variability analysis on several times-scales of the 233 hour data set
accumulated over 213 nights (from March 1995 to July 1998) with the Whipple
Observatory 10 m atmospheric Cherenkov imaging telescope. In 1995, with the
exception of a single night, the flux from Markarian 501 was constant on daily
and monthly time-scales and had an average flux of only 10% that of the Crab
Nebula, making it the weakest VHE source detected to date. In 1996, the average
flux was approximately twice the 1995 flux and showed significant
month-to-month variability. No significant day-scale variations were detected.
The average gamma-ray flux above ~350 GeV in the 1997 observing season rose to
1.4 times that of the Crab Nebula -- 14 times the 1995 discovery level --
allowing a search for variability on time-scales shorter than one day.
Significant hour-scale variability was present in the 1997 data, with the
shortest, observed on MJD 50607, having a doubling time of ~2 hours. In 1998
the average emission level decreased considerably from that of 1997 (to ~20% of
the Crab Nebula flux) but two significant flaring events were observed. Thus,
the emission from Markarian 501 shows large amplitude and rapid flux
variability at very high energies as does Markarian 421. It also shows large
mean flux level variations on year-to-year time-scales, behaviour which has not
been seen from Markarian 421 so far.Comment: 19 pages, 3 figures, to appear in ApJ, June 20, 1999, Vol. 518 #
Continuous Quantum Measurement and the Emergence of Classical Chaos
We formulate the conditions under which the dynamics of a continuously
measured quantum system becomes indistinguishable from that of the
corresponding classical system. In particular, we demonstrate that even in a
classically chaotic system the quantum state vector conditioned by the
measurement remains localized and, under these conditions, follows a trajectory
characterized by the classical Lyapunov exponent.Comment: 5 pages, multicol revte
Quantum Revivals in Periodically Driven Systems close to nonlinear resonance
We calculate the quantum revival time for a wave-packet initially well
localized in a one-dimensional potential in the presence of an external
periodic modulating field. The dependence of the revival time on various
parameters of the driven system is shown analytically. As an example of
application of our approach, we compare the analytically obtained values of the
revival time for various modulation strengths with the numerically computed
ones in the case of a driven gravitational cavity. We show that they are in
very good agreement.Comment: 14 pages, 1 figur
Limits to Quantum Gravity Effects from Observations of TeV Flares in Active Galaxies
We have used data from the TeV gamma-ray flare associated with the active
galaxy Markarian 421 observed on 15 May 1996 to place bounds on the possible
energy-dependence of the speed of light in the context of an effective quantum
gravitational energy scale. The possibility of an observable time dispersion in
high energy radiation has recently received attention in the literature, with
some suggestions that the relevant energy scale could be less than the Planck
mass and perhaps as low as 10^16 GeV. The limits derived here indicate this
energy scale to be in excess of 4x10^16 GeV at the 95% confidence level. To the
best of our knowledge, this constitutes the first convincing limit on such
phenomena in this energy regime.Comment: 6 pages, including 2 figure
TeV Observations of the Variability and Spectrum of Markarian 501
Markarian 501 is only the second extragalactic source to be detected with
high statistical certainty at TeV energies; it is similar in many ways to
Markarian 421. The Whipple Observatory gamma-ray telescope has been used to
observe the AGN Markarian 501 in 1996 and 1997, the years subsequent to its
initial detection. The apparent variability on the one-day time-scale observed
in TeV gamma rays in 1995 is confirmed and compared with the variability in
Markarian 421. Observations at X-ray and optical wavelengths from 1997 are also
presented.Comment: 4 pages, 2 figures, to appear in proceedings of 25th ICRC (Durban
Tev Observations of the Variability and Spectrum of Markarian 421
Markarian 421 was the first extragalactic source to be detected with high
statistical certainty at TeV energies. The Whipple Observatory gamma-ray
telescope has been used to observe the Active Galactic Nucleus, Markarian 421
in 1996 and 1997. The rapid variability observed in TeV gamma rays in previous
years is confirmed. Doubling times as short as 15 minutes are reported with
flux levels reaching 15 photons per minute. The TeV energy spectrum is derived
using two independent methods. The implications for the intergalactic infra-red
medium of an observed unbroken power law spectrum up to energies of 5 TeV is
discussed.Comment: 4 pages, 4 figures, to appear in proceedings of 25 ICRC (Durban
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