499 research outputs found
Dust properties from GALEX observations of a UV halo around Spica
GALEX has detected ultraviolet halos extending as far as 5 around
four bright stars (Murthy et al. (2011)). These halos are produced by
scattering of starlight by dust grains in thin foreground clouds that are not
physically associated with the star. Assuming a simple model consisting of a
single layer of dust in front of the star, Murthy et al.(2011) have been able
to model these halo intensities and constrain the value of the phase function
asymmetry factor of the scattering grains in the FUV and NUV. However due
to the uncertainty in the dust geometry they could not constrain the albedo. In
this work we have tried to constrain the optical constants and dust geometry by
modeling the UV halo of Spica. Since the halo emission is not symmetric, we
have modeled the Northern and Southern parts of the halo separately. To the
North of Spica, the best-fit albedo is 0.260.1 and is 0.580.11 in
the FUV at the 90% confidence level. The corresponding limits on the distance
and optical depth () of the dust sheet is 3.651.05 pc and
0.0470.006 respectively. However, owing to a complicated dust distribution
to the South of Spica, we were unable to uniquely constrain the dust parameters
in that region. Nevertheless, by assuming the optical constants of the Northern
region and assuming a denser medium, we were able to constrain the distance of
the dust to 9.51.5 pc and the corresponding to 0.040.01.Comment: 4 pages, accepted for publication in Earth, Planets and Spac
X-ray ejecta kinematics of the Galactic core-collapse supernova remnant G292.0+1.8
We report on the results from the analysis of our 114 ks Chandra HETGS
observation of the Galactic core-collapse supernova remnant G292.0+1.8. To
probe the 3D structure of the clumpy X-ray emitting ejecta material in this
remnant, we measured Doppler shifts in emission lines from metal-rich ejecta
knots projected at different radial distances from the expansion center. We
estimate radial velocities of ejecta knots in the range of -2300 <~ v_r <~ 1400
km s^-1. The distribution of ejecta knots in velocity vs. projected-radius
space suggests an expanding ejecta shell with a projected angular thickness of
~90" (corresponding to ~3 pc at d = 6 kpc). Based on this geometrical
distribution of the ejecta knots, we estimate the location of the reverse shock
approximately at the distance of ~4 pc from the center of the supernova
remnant, putting it in close proximity to the outer boundary of the radio
pulsar wind nebula. Based on our observed remnant dynamics and the standard
explosion energy of 10^51 erg, we estimate the total ejecta mass to be <~ 8
M_sun, and we propose an upper limit of <~ 35 M_sun on the progenitor's mass.Comment: 5 figures, accepted by Ap
Hidden Markov model technique for dynamic spectrum access
Dynamic spectrum access is a paradigm used to access the spectrum dynamically. A hidden Markov model (HMM) is one in which you observe a sequence of emissions, but do not know the sequence of states the model went through to generate the emissions. Analysis of hidden Markov models seeks to recover the sequence of states from the observed data. In this paper, we estimate the occupancy state of channels using hidden Markov process. Using Viterbi algorithm, we generate the most likely states and compare it with the channel states. We generated two HMMs, one slowly changing and another more dynamic and compare their performance. Using the Baum-Welch algorithm and maximum likelihood algorithm we calculated the estimated transition and emission matrix, and then we compare the estimated states prediction performance of both the methods using stationary distribution of average estimated transition matrix calculated by both the methods
BASIC CONCEPT OF YOGA AND ITS HEALTH BENEFITS- A SHORT REVIEW
Yoga is a group of physical, mental, and spiritual practices which has its root in ancient India. Yoga is a complete science by itself being a holistic and very scientific background. These days everyone is suffering from some sort of lifestyle disorders, anxiety, stress, depression etc. and almost all streams of medicines have their limitations for these diseases. Ancient Acharyas have given the world a gift called Yoga which is proving extremely beneficial from children to adults to get relief from life style disorders. Yoga science is magnificently described in many ancient treatises like Vedas, Bhagavdgita, Yogavashishta, Patanjal Yogasutra, Gherand Samhita. Yoga is accomplished by Asthanga known as Yama, Niyama, Asana, Pranayama, Pratyahara, Dharana, Dhyana and Samadhi. Now-a-days, millions and millions of people across the globe have benefitted by the practice of Yoga which has been preserved and promoted by the great eminent Yoga Masters from ancient time to this date
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