188 research outputs found
Scheduling Under Fading and Partial Channel Information
We consider a scheduler for the downlink of a wireless channel when only
partial channel-state information is available at the scheduler. We
characterize the network stability region and provide two throughput-optimal
scheduling policies. We also derive a deterministic bound on the mean packet
delay in the network. Finally, we provide a throughput-optimal policy for the
network under QoS constraints when real-time and rate-guaranteed data traffic
may be present.Comment: 22 pages, 3 figures; Added IEEE Journal submission notic
Implications on accretion flow dynamics from spectral study of Swift~J1357.2-0933
We report a detailed spectral study of Swift~J1357.2-0933 low-mass X-ray
binary during its 2017 outburst using {\it Swift} and {\it NuSTAR}
observations. We fit the data with two component advective flow (TCAF) model
and power-law model. We observe that the source is in hard state during the
outburst, where the size of the Compton cloud changes significantly with disc
accretion rate. The typical disc accretion rate for this source is of the Eddington accretion rate . The model fitted
intermediate shock compression ratio gives an indication of the presence of
jet, which is reported in the literature in different energy bands. We also
split NuSTAR data into three equal segments and fit with the model. We check
spectral stability using color-color diagram and accretion rate ratio (ARR) vs.
intensity diagram using different segments of the light curve but do not find
any significant variation in the hardness ratio or in the accretion rate ratio.
To estimate the mass of the candidate, we use an important characteristics of
TCAF that the the model normalization always remains a constant. We found that
the mass comes out to be in the range of . From the model
fitted results, we study the disc geometry and different physical parameters of
the flow in each observation. The count rate of the source appears to decay in
a time scale of .Comment: 6 pages, 3 figures, Accepted (21/11/2018) by MNRA
Effect of r averaging on Chiral Anomaly in Lattice QCD with Wilson Fermion: Finite volume and cutoff effects
We demonstrate the effectiveness of averaging over the Wilson parameter r
(which has been proposed earlier) in removing the cutoff effects of naive
Wilson fermions in both the anomaly term and the pseudoscalar density term in
the flavor singlet axial Ward identity at O(g^2) involving slowly varying
background gauge fields. We show that it is the physical fermion contribution
which is largely influenced by the r averaging. We have studied the possible
interplay between finite size and cutoff effects by investigating in detail
naive, O(a) improved and OStm Wilson fermion cases for a range of volumes and
lattice fermion mass (am). For naive Wilson fermions r averaging is shown to
remove the effects of the interplay. We have shown that for the pseudoscalar
density term to O (g^2) the lattice result differs from the continuum result by
exhibiting considerable am dependence which appears to be a manifestation of
cutoff effects with naive Wilson fermion. The pseudoscalar density term to
O(g^2) is shown to be almost independent of am when r-averaging is performed.Comment: 14 pages, 13 figures, revise
Spectral signatures of dissipative standing shocks and mass outflow in presence of Comptonization around a black hole
Accretion flows having positive specific energy are known to produce outflows
and winds which escape to a large distance. According to Two Component
Advective Flow (TCAF) model, centrifugal pressure dominated region of the flow
just outside the black hole horizon, with or without shocks, acts as the base
of this outflow. Electrons from this region are depleted due to the wind and
consequently, energy transfer rate due to inverse Comptonization of low energy
photons are affected. Specifically, it becomes easier to cool this region and
emerging spectrum is softened. Our main goal is to show spectral softening due
to mass outflow in presence of Compton cooling. To achieve this, we modify
Rankine-Hugoniot relationships at the shock front when post-shock region
suffers mass loss due to winds and energy loss due to inverse Comptonization.
We solve two-temperature equations governing an accretion flow around a black
hole which include Coulomb exchange between protons and electrons and other
major radiative processes such as bremsstrahlung and thermal Comptonization. We
then compute emitted spectrum from this post-shock flow. We also show how
location of standing shock which forms outer boundary of centrifugal barrier
changes with cooling. With an increase in disc accretion rate ,
cooling is enhanced and we find that the shock moves in towards the black hole.
With cooling, thermal pressure is reduced, and as a result, outflow rate is
decreased. We thus directly correlate outflow rate with spectral state of the
disc.Comment: 10 pages, 6 figures, Accepted for publication in Astrophysics and
Space Scienc
Spectral study of GX 339-4 with TCAF using Swift/XRT and NuSTAR Observation
We fit spectra of galactic transient source GX~339-4 during its 2013 outburst
using Two Component Advective Flow (TCAF) solution. For the first time, we are
fitting combined NuSTAR and Swift observation with TCAF. We use TCAF to fit
0.8-9.0~keV Swift and 4-79 keV NuSTAR spectra along with the LAOR model. To fit
the data we use disk accretion rate, halo accretion rate, size of the Compton
cloud and the density jump of advective flows at this cloud boundary as model
parameters. From TCAF fitted flow parameters, and energy spectral index we
conclude that the source was in the hard state throughout this particular
outburst. The present analysis also gives some idea about the broadening of Fe
with the accretion rate. Since TCAF does not include Fe line yet,
we make use of the `LAOR model' as a phenomenological model and find an
estimate of the Kerr parameter to be for this candidate.Comment: 10 pages, 4 figures, 1 table, Accepted for Publication in
Astrophysics and Space Scienc
Spectral properties of NGC 4151 and the Estimation of black hole mass using TCAF solution
We present X-ray spectral analysis of Seyfert 1.5 Active Galactic Nuclei
(AGN) NGC~4151 using \textit{NuSTAR} observation during 2012. This is the first
attempt to fit an AGN data using the physical Two Component Advective flow
(TCAF) solution. We disentangle the continuum emission properties of the source
in the energy range to ~keV using the spectrum obtained from TCAF
model. This model was used as an additive local model directly in
{\fontfamily{qcr}\selectfont XSPEC}. Additionally, we used a power law (PL)
component, to take care of possible X-ray contribution from the jet, which is
not incorporated in the present version of TCAF. Our primary aim is to obtain
the flow properties and the mass of the central supermassive black hole from
the available archival data.
Our best estimate of the average mass obtained from spectral fits of three
observations, is . This is
consistent with earlier estimations in the literature such as reverberation
mapping, gas kinematics and stellar dynamics around black holes. We also
discuss the accretion dynamics and the flow geometry on the basis of model
fitted physical parameters. Model fitted disk accretion rate is found to be
lower than the low angular momentum halo accretion rate, indicating that the
source was in a hard state during the observation.Comment: 15 pages, 5 figures, Accepted for publicatio
Chiral Anomaly in Lattice QCD with Twisted Mass Wilson Fermion
The flavour singlet axial Ward identity with Osterwalder-Seiler twisted mass
Wilson fermion action is studied on a finite lattice, with finite fermion mass
and the Wilson parameter r up to 1. Approach to the infinite volume chiral
limit and emergence of the anomaly is significantly better than that obtained
with O(a) and O(a^2) improved fermion actions. We have shown explicitly that up
to O(g^2), parity violating terms cancel in the Ward identity even at finite
volume and finite lattice spacing.Comment: Typos correcte
Inference on accretion flow dynamics using TCAF solution from the analysis of spectral evolution of H 1743-322 during 2010 outburst
We study accretion flow dynamics of Galactic transient black hole candidate
(BHC) H 1743-322 during its 2010 outburst by analyzing spectral data using Two
Component (Keplerian and sub-Keplerian) Advective Flow (TCAF) solution, after
its inclusion in XSPEC as a local model. We compare our TCAF solution fitted
results with combined disk black body and power-law model fitted results and
find a similar smooth variation of thermal (Keplerian or disk black body) and
non-thermal (power-law or sub-Keplerian) fluxes/rates in two types of model
fits. For a spectral analysis, 2.5-25 keV spectral data from RXTE PCA
instrument are used. From the TCAF solution fit, accretion flow parameters,
such as Keplerian rate, sub-Keplerian rate, location of centrifugal pressure
supported shock and strength of the shock are extracted, thus providing a
deeper understanding of accretion process and properties of accretion disks
around BHC H 1743-322 during its X-ray outburst. Based on the halo to disk
accretion rate ratio (ARR), shock properties, accretion rates and nature of
quasi-periodic oscillations (QPOs, if observed) entire outburst is classified
into four different spectral states, such as, hard, hard-intermediate,
soft-intermediate, and soft. From time variation of intrinsic flow parameters
it appears that their evolutions in decline phase do not retrace path of rising
phase. Since our current model does not include magnetic fields, spectral
turnover at energies beyond 500-600 keV cannot be explained.Comment: 7 pages, 4 figures, 1 table; accepted for publication in Ap
Characterization of GX 339-4 outburst of 2010-11: analysis by xspec using two component advective flow model
We study spectral properties of GX 339-4 during its 2010-11 outburst with Two
Component Advective Flow (TCAF) model after its inclusion in XSPEC as a table
model. We compare results fitted by TCAF model with combined disk black body
and power-law model. For a spectral fit, we use 2.5-25 keV spectral data of the
PCA instrument onboard RXTE satellite. From our fit, accretion flow parameters
such as Keplerian (disk) rate, sub-Keplerian (halo) rate, location and strength
of shock are extracted. We quantify how the disk and the halo rates vary during
the entire outburst. We study how the halo to disk accretion rate ratio (ARR),
quasi-periodic oscillations (QPOs), shock locations and its strength vary when
the system passes through hard, hard-intermediate, soft-intermediate, and soft
states. We find pieces of evidence of monotonically increasing and decreasing
nature of QPO frequencies depending on the variation of ARR during rising and
declining phases. Interestingly, on days of transition from hard state to
hard-intermediate spectral state (during the rising phase) or vice-versa
(during decline phase), ARR is observed to be locally maximum. Non-constancy of
ARR while obtaining reasonable fits points to the presence of two independent
components in the flow.Comment: 12 Pages, 4 Figures, 1 Table, and 1 Appendix (Table
Many avatars of the Wilson fermion: A perturbative analysis
We explore different branches of the fermion doublers with Wilson fermion in
perturbation theory, in the context of additive mass renormalization and chiral
anomaly, and show that by appropriately averaging over suitably chosen branches
one can reduce cut-off artifacts. Comparing the central branch with all other
branches, we find that the central branch, among all the avatars of the Wilson
fermion, is the most suitable candidate for exploring near conformal lattice
field theories.Comment: 10 pages, 5 figures, one reference added, accepted for publication in
JHE
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