151 research outputs found
Multiphase turbulent interstellar medium: some recent results from radio astronomy
The radio frequency 1.4 GHz transition of the atomic hydrogen is one of the
important tracers of the diffuse neutral interstellar medium. Radio
astronomical observations of this transition, using either a single dish
telescope or an array interferometer, reveal different properties of the
interstellar medium. Such observations are particularly useful to study the
multiphase nature and turbulence in the interstellar gas. Observations with
multiple radio telescopes have recently been used to study these two closely
related aspects in greater detail. Using various observational techniques, the
density and the velocity fluctuations in the Galactic interstellar medium was
found to have a Kolmogorov-like power law power spectra. The observed power law
scaling of the turbulent velocity dispersion with the length scale can be used
to derive the true temperature distribution of the medium. Observations from a
large ongoing atomic hydrogen absorption line survey have also been used to
study the distribution of gas at different temperature. The thermal steady
state model predicts that the multiphase neutral gas will exist in cold and
warm phase with temperature below 200 K and above 5000 K respectively. However,
these observations clearly show the presence of a large fraction of gas in the
intermediate unstable phase. These results raise serious doubt about the
validity of the standard model, and highlight the necessity of alternative
theoretical models. Interestingly, numerical simulations suggest that some of
the observational results can be explained consistently by including the
effects of turbulence in the models of the multiphase medium. This review
article presents a brief outline of some of the basic ideas of radio
astronomical observations and data analysis, summarizes the results from recent
observations, and discusses possible implications of the results.Comment: 20 pages, 10 figures. Invited review accepted for publication in the
Proceedings of the Indian National Science Academy. The definitive version
will be available at http://insaindia.org/journals/proceedings.ph
Turbulent power spectrum in warm and cold neutral medium using the Galactic HI 21 cm emission
Small-scale fluctuations of different tracers of the interstellar the medium
can be used to study the nature of turbulence in astrophysical scales. Of
these, the `continuum' emission traces the fluctuations integrated along the
line of sight whereas, the spectral line tracers give the information along
different velocity channels as well. Recently, Miville-Desch\^enes et al.
(2016) have measured the intensity fluctuation power spectrum of the continuum
dust emission, and found a power law behaviour with a power law index of for a region of our Galaxy. Here, we study the same region using
high-velocity resolution 21-cm emission from the diffuse neutral medium, and
estimate the power spectrum at different spectral channels. The measured 21-cm
power spectrum also follows a power law, however, we see a significant
variation in the power law index with velocity. The value of the power-law
index estimated from the integrated map for different components are quite
different which is indicative of the different nature of turbulence depending
on temperature, density and ionization fraction. We also measure the power
spectra after smoothing the 21 cm emission to velocity resolution ranging from
to , but the power spectrum remains unchanged
within the error bar. This indicates that the observed fluctuations are
dominantly due to density fluctuations, and we can only constrain the power-law
index of velocity structure function of which is consistent with
the predicted Kolmogorov turbulence and also with a
shock-dominated medium .Comment: 8 pages, 7 figures. Accepted for publication in MNRAS. The definitive
version will be available at http://mnrasl.oxfordjournals.org
Simultaneously Learning Speaker's Direction and Head Orientation from Binaural Recordings
Estimation of a speaker's direction and head orientation with binaural
recordings can be a critical piece of information in many real-world
applications with emerging `earable' devices, including smart headphones and
AR/VR headsets. However, it requires predicting the mutual head orientations of
both the speaker and the listener, which is challenging in practice. This paper
presents a system for jointly predicting speaker-listener head orientations by
leveraging inherent human voice directivity and listener's head-related
transfer function (HRTF) as perceived by the ear-mounted microphones on the
listener. We propose a convolution neural network model that, given binaural
speech recording, can predict the orientation of both speaker and listener with
respect to the line joining the two. The system builds on the core observation
that the recordings from the left and right ears are differentially affected by
the voice directivity as well as the HRTF. We also incorporate the fact that
voice is more directional at higher frequencies compared to lower frequencies
Estimating kinetic temperature from H I 21 cm absorption studies: correction for the turbulence broadening
Neutral hydrogen 21 cm transition is a useful tracer of the neutral
interstellar medium. However, inferring physical condition from the observed 21
cm absorption and/or emission spectra is often not straightforward. One
complication in estimating the temperature of the atomic gas is that the line
width may have significant contribution from non-thermal broadening. We propose
a formalism here to separate the thermal and non-thermal broadening using a
self-consistent model of turbulence broadening of the HI 21 cm absorption
components. Applying this novel method, we have estimated the spin and the
kinetic temperature of diffuse Galactic neutral hydrogen, and found that a
large fraction of gas has temperature in the unstable range. The turbulence is
found to be subsonic or transonic in nature, and the clouds seem to have a
bimodal size distribution. Assuming that the turbulence is magnetohydrodynamic
in nature, the estimated magnetic field strength is of {\mu}G order, and is
found to be uncorrelated with the HI number density.Comment: 6 pages, 10 figures. Accepted(16-Nov-2018) for publication in MNRA
Walkcompass: Finding Walking Direction Leveraging Smartphone\u27s Inertial Sensors
Determining moving direction with smartphone\u27s inertial sensors is a well known problem in the field of location service. Compass alone cannot solve this problem because smartphone\u27s compass cannot achieve high accuracy. Moreover GPS is not suitable in indoor scenario. Another well known approach is dead-reckoning but dead-reckoning needs to know phones initial orientation and over time it keeps accumulating errors and after some time the estimation becomes to noisy to use. To overcome these limitations, we propose a solution called WalkCompass which is specially designed for pedestrians keeping in mind the variation of force during normal human walk. Therefore the algorithm is inherently free from any error generated by the orientation of the phone. However, the performance of the system does not depend on the holding style or location of the phone in the body. The algorithm can work very fast to determine the direction of movement in real time and because of its low complexity the complete system can be implemented on a smartphone. WalkCompass does not need any bootstrapping and can produce results with the granularity of each step of a walk
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