728 research outputs found
High Energy Emission Processes in OJ 287 during 2009 Flare
The broadband spectrum of a BL Lac object, OJ 287, from radio to
-rays obtained during a major -ray flare detected by
\emph{Fermi} in 2009 are studied to understand the high energy emission
mechanism during this episode. Using a simple one-zone leptonic model,
incorporating synchrotron and inverse Compton emission processes, we show that
the explanation of high energy emission from X-rays to -rays, by
considering a single emission mechanism, namely, synchrotron self-Compton (SSC)
or external Compton (EC) requires unlikely physical conditions. However, a
combination of both SSC and EC mechanisms can reproduce the observed high
energy spectrum satisfactorily. Using these emission mechanisms we extract the
physical parameters governing the source and its environment. Our study
suggests that the emission region of OJ 287 is surrounded by a warm infrared
(IR) emitting region of . Assuming this region as a spherical
cloud illuminated by an accretion disk, we obtain the location of the emission
region to be . This supports the claim that the -ray
emission from OJ 287 during the 2009 flare arises from a location far away from
the central engine as deduced from millimeter-gamma ray correlation study and
very long baseline array images.Comment: 22 pages, 7 figures, 1 table, accepted for publication in MNRA
Brightest Fermi-LAT Flares of PKS 1222+216: Implications on Emission and Acceleration Processes
We present a high time resolution study of the two brightest -ray
outbursts from a blazar PKS 1222+216 observed by the \textit{Fermi} Large Area
Telescope (LAT) in 2010. The -ray light-curves obtained in four
different energy bands: 0.1--3, 0.1--0.3, 0.3--1 and 1--3 GeV, with time bin of
6 hr, show asymmetric profiles with a similar rise time in all the bands but a
rapid decline during the April flare and a gradual one during the June. The
light-curves during the April flare show days long plateau in 0.1--0.3
GeV emission, erratic variations in 0.3--1 GeV emission, and a daily recurring
feature in 1--3 GeV emission until the rapid rise and decline within a day. The
June flare shows a monotonic rise until the peak, followed by a gradual decline
powered mainly by the multi-peak 0.1--0.3 GeV emission. The peak fluxes during
both the flares are similar except in the 1--3 GeV band in April which is twice
the corresponding flux during the June flare. Hardness ratios during the April
flare indicate spectral hardening in the rising phase followed by softening
during the decay. We attribute this behavior to the development of a shock
associated with an increase in acceleration efficiency followed by its decay
leading to spectral softening. The June flare suggests hardening during the
rise followed by a complicated energy dependent behavior during the decay.
Observed features during the June flare favor multiple emission regions while
the overall flaring episode can be related to jet dynamics.Comment: 17 pages, 9 figures, 4 tables, accepted for publication in Ap
Scale Invariance as a Solution to the Cosmological Constant Problem
We show that scale invariance provides a solution to the fine tuning problem
of the cosmological constant. We construct a generalization of the standard
model of particle physics which displays exact quantum scale invariance. The
matter action is invariant under global scale transformations in arbitrary
dimensions. However the gravitational action breaks scale invariance
explicitly. The scale symmetry is broken spontaneously in the matter sector of
the theory. We show that the contribution to the vacuum energy and hence the
cosmological constant is identically zero from the matter sector within the
full quantum theory. However the gravitational sector may give non-zero
contributions to the cosmological constant at loop orders. No fine tuning may
be required at loop orders since the matter sector gives zero contribution to
the cosmological constant. We also show that we do not require full scale
invariance in order to constrain the vacuum energy from the matter sector. We
only require invariance under pseudoscale transformations. Using this idea and
motivated by the concept of unimodular gravity we propose an alternative model.
In this case also we show that matter sector gives exactly zero contribution to
the vacuum energy.Comment: 12 pages, no figure
Multi-wavelength Temporal Variability of the Blazar 3C 454.3 during 2014 Activity Phase
We present a multi-wavelength temporal analysis of the blazar 3C 454.3 during
the high -ray active period from May-December, 2014. Except for X-rays,
the period is well sampled at near-infrared (NIR)-optical by the \emph{SMARTS}
facility and the source is detected continuously on daily timescale in the
\emph{Fermi}-LAT -ray band. The source exhibits diverse levels of
variability with many flaring/active states in the continuously sampled
-ray light curve which are also reflected in the NIR-optical light
curves and the sparsely sampled X-ray light curve by the \emph{Swift}-XRT.
Multi-band correlation analysis of this continuous segment during different
activity periods shows a change of state from no lags between IR and
-ray, optical and -ray, and IR and optical to a state where
-ray lags the IR/optical by 3 days. The results are consistent
with the previous studies of the same during various -ray flaring and
active episodes of the source. This consistency, in turn, suggests an extended
localized emission region with almost similar conditions during various
-ray activity states. On the other hand, the delay of -ray with
respect to IR/optical and a trend similar to IR/optical in X-rays along with
strong broadband correlations favor magnetic field related origin with X-ray
and -ray being inverse Comptonized of IR/optical photons and external
radiation field, respectively.Comment: 15 pages, 5 figures, 1 table, MNRAS accepte
Cosmological Perturbation Analysis in a Scale Invariant Model of Gravity
We consider a model for gravity that is invariant under global scale
transformations. It includes one extra real scalar field coupled non-minimally
to the gravity fields. In this model all the dimensionful parameters like the
gravitational constant and the cosmological constant etc. are generated by a
solution of the classical equations of motion which breaks scale invariance. In
this paper we demonstrate the stability of such a solution against small
perturbations in a flat FRW background by making a perturbative expansion
around this solution and solving the resulting equations linear in the
perturbations.Comment: 9 pages, 1 figur
Arsenic removal from water through adsorption-A Review
Arsenic is a commonly occurring toxic metal in natural systems. It is ahuman carcinogen in water over a wide range of pH values, having harmful effects on both human health andenvironment, even at low concentration. Because ofthis effect, the World Health Organization (WHO) and the US Environmental Protection Agency (USEPA) set the arsenic standard for drinking water at .010 ppm to protect consumers served by public water systems.Occurrence of arsenic contaminated water is reported in several countries like USA, China, Bangladesh, Mexico, Argentina, Poland, Canada, Hungary, Japan and India. The largest population at risk is in Bangladesh followed by India (West Bengal).There is no effective treatment for arsenic toxicity. Onlythe removal of arsenic from water can prevent the toxicity. A great deal of research over recent decades has been done to lower the concentration of arsenic in drinking water and still there is a need to develop low cost viable techniques. Existing major arsenic removal technologies include oxidation, adsorption, precipitation, coagulation andmembrane separation. This paper presents the review of current status of research in the area of arsenic removal from contaminated water and comparison of all technologies available with more emphasis on adsorption
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