148,431 research outputs found
Fundamental constraints on particle tracking with optical tweezers
A general quantum limit to the sensitivity of particle position measurements
is derived following the simple principle of the Heisenberg microscope. The
value of this limit is calculated for particles in the Rayleigh and Mie
scattering regimes, and with parameters which are relevant to optical tweezers
experiments. The minimum power required to observe the zero-point motion of a
levitating bead is also calculated, with the optimal particle diameter always
smaller than the wavelength. We show that recent optical tweezers experiments
are within two orders of magnitude of quantum limited sensitivity, suggesting
that quantum optical resources may soon play an important role in high
sensitivity tracking applications
IVOA Recommendation: SAMP - Simple Application Messaging Protocol Version 1.3
SAMP is a messaging protocol that enables astronomy software tools to
interoperate and communicate.
IVOA members have recognised that building a monolithic tool that attempts to
fulfil all the requirements of all users is impractical, and it is a better use
of our limited resources to enable individual tools to work together better.
One element of this is defining common file formats for the exchange of data
between different applications. Another important component is a messaging
system that enables the applications to share data and take advantage of each
other's functionality. SAMP builds on the success of a prior messaging
protocol, PLASTIC, which has been in use since 2006 in over a dozen astronomy
applications and has proven popular with users and developers. It is also
intended to form a framework for more general messaging requirements
Cut-off Characterisation of Energy Spectra of Bright Fermi Sources: Current instrument limits and future possibilities
In this paper some of the brightest GeV sources observed by the Fermi-LAT
were analysed, focusing on their spectral cut-off region. The sources chosen
for this investigation were the brightest blazar flares of 3C~454.3 and 3C~279
and the Vela pulsar with a reanalysis with the latest Fermi-LAT software. For
the study of the spectral cut-off we first explored the Vela pulsar spectrum,
whose statistics in the time interval of the 3FGL catalog allowed strong
constraints to be obtained on the parameters. We subsequently performed a new
analysis of the flaring blazar SEDs. For these sources we obtained constraints
on the cut-off parameters under the assumption that their underlying spectral
distribution is described by a power-law with a stretched exponential cut-off.
We then highlighted the significant potential improvements on such constraints
by observations with next generation ground based Cherenkov telescopes,
represented in our study by the Cherenkov Telescope Array (CTA). Adopting
currently available simulations for this future observatory, we demonstrate the
considerable improvement in cut-off constraints achievable by observations with
this new instrument when compared with that achievable by satellite
observations.Comment: total number of pages 24, including 6 pages of references. Accepted
by Astroparticle Physic
Measuring the correlation length of intergalactic magnetic fields from observations of gamma-ray induced cascades
Context. The imaging and timing properties of {\gamma}-ray emission from
electromagnetic cascades initiated by very-high-energy (VHE) {\gamma}-rays in
the intergalactic medium depend on the strength B and correlation length
{\lambda}B of intergalactic magnetic fields (IGMF). Aims. We study the
possibility of measuring both B and {\lambda}B via observations of the cascade
emission with {\gamma}-ray telescopes. Methods. For each measurement method, we
find two characteristics of the cascade signal, which are sensitive to the IGMF
B and {\lambda}B values in different combinations. For the case of IGMF
measurement using the observation of extended emission around extragalactic VHE
{\gamma}-ray sources, the two characteristics are the slope of the surface
brightness profile and the overall size of the cascade source. For the case of
IGMF measurement from the time delayed emission, these two characteristics are
the initial slope of the cascade emission light curve and the overall duration
of the cascade signal. Results. We show that measurement of the slope of the
cascade induced extended emission and/or light curve can both potentially
provide measure of the IGMF correlation length, provided it lies within the
range 10 kpc< {\lambda}B <1 Mpc. For correlation lengths outside this range,
gamma-ray observations can provide upper or lower bound on {\lambda}B. The
latter of the two methods holds great promise in the near future for providing
a measurement/constraint using measurements from present/next-generation
{\gamma}-ray-telescopes. Conclusions. Measurement of the IGMF correlation
length will provide an important constraint on its origin. In particular, it
will enable to distinguish between an IGMF of galactic wind origin from an IGMF
of cosmological origin.Comment: 5 pages, 3 figure
Dispersion of biased swimming microorganisms in a fluid flowing through a tube
Classical Taylor-Aris dispersion theory is extended to describe the transport
of suspensions of self-propelled dipolar cells in a tubular flow. General
expressions for the mean drift and effective diffusivity are determined exactly
in terms of axial moments, and compared with an approximation a la Taylor. As
in the Taylor-Aris case, the skewness of a finite distribution of biased
swimming cells vanishes at long times. The general expressions can be applied
to particular models of swimming microorganisms, and thus be used to predict
swimming drift and diffusion in tubular bioreactors, and to elucidate competing
unbounded swimming drift and diffusion descriptions. Here, specific examples
are presented for gyrotactic swimming algae.Comment: 20 pages, 4 figures. Published version available at
http://rspa.royalsocietypublishing.org/content/early/2010/02/09/rspa.2009.0606.short?rss=
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