arXiv subjects

A. K. Sen

Publications and source records attributed to A. K. Sen.

At least 37 records · Page 2Linked to original sources

arXiv2015

Trajectory of a light ray in Kerr field: A material medium approach

The deflection of light ray as it passes around a gravitational mass can be calculated by different methods. Such calculations are generally done by using the null geodesics under both strong field and weak field approximation. However, several authors have studied the gravitational deflection of light ray using material medium approach. For a static, non-rotating spherical mass, one can determine the deflection in Schwarzschild field, by expressing the line element in an isotropic form and calculating the refractive index to determine the trajectory of the light ray. In this paper, we draw our attention to the refractive index of light ray in Kerr field using the material medium approach. The frame dragging effects in Kerr field was considered to calculate the velocity of light ray and finally the refractive index in Kerr field geometry was determined. Hence the deflection of light ray in Kerr field was calculated, assuming far field approximation and compared the results with those calculations done earlier using Null geodesics.

gr-qc↗

arXiv2015

Gravitational Redshift in Kerr - Newman Geometry

It is well known fact that gravitational mass can alter the space time structure and gravitational redshift is one of its examples. Static electric or magnetic charge can also alter the space time structure, similar to gravitational mass, giving rise to its effect on redshift. This can also be considered as electro and magneto static redshift. Gravitational redshift has been reported by most of the authors without consideration of static electric and / or magnetic charges present in the rotating body. In the present paper, we considered the three parameters: mass, rotation parameter and charge to discuss their combined effect on redshift, for a charged rotating body by using Kerr - Newman metric. It has been found that, the presence of electrostatic and magnetostatic charge increases the value of so-called gravitational redshift. Calculations have been also done here to determine the effect of electrostatic and magnetostatic charges on the amount of redshift of a light ray emitted at various latitudes from a charged rotating body. The variation of gravitational redshift from equatorial to non- equatorial region has been calculated, for a given set of values of electrostatic and magnetostatic charges.

gr-qc↗

arXiv2015

Laboratory light scattering from regolith surface and simulation of data by Hapke model

The small atmosphereless objects of our solar system, such as asteroids, the moon are covered by layer of dust particles known as regolith, formed by meteoritic impact. The light scattering studies of such dust layer by laboratory experiment and numerical simulation are two important tools to investigate their physical properties. In the present work, the light scattered from a layer of dust particles, containing 0.3μm Al2O3 at wavelength 632.8 nm is analysed. This work has been performed by using a light scattering instrument 'ellipsometer', at the Department of Physics, Assam Universiy, Silchar, India. Through this experiment, we generated in laboratory the photometric and polarimetric phase curves of light scattered from such a layer. In order to numerically simulate this data, we used Hapke's model combined with Mie's single particle scattering properties. The perpendicular and parallel components of single particle albedo and the phase function were derived from Mie theory. By using the Hapke's model combined with Mie theory, the physical properties of the dust grain such as grain size, optical constant (n,k) and wavelength can be studied through this scheme. In literature, till today no theoretical model to represent polarisation caused due to scattering from rough surface is available, which can successfully explain the scattering process. So the main objective of this work is to develop a model which can theoretically estimate polarisation as caused due to scattering from rough surface and also to validate our model with the laboratory data generated in the present work.

astro-ph.EP↗

arXiv2014

Light defection due to a charged, rotating body

According to GTR and subsequent developments in the field, it is known that there are three factors namely mass, rotation and charge that can influence the space-time geometry. Accordingly, we discuss the effect of space-time geometry of a charged, rotating body on the motion of the light ray. We obtained the expression for equatorial defection of light due to such a body up to fourth order term. In our expression for defection angle it is clear that charge can influence the path of light ray. We used the null geodesic approach of light ray for our calculation. If we set the charge to zero our expression of bending angle gets reduced to the Kerr equatorial bending angle.If we set rotation to zero our expression reduces to Resinner-Nordstr$\ddot{o}$m defection angle and if we set both charge and rotation to zero our expression reduces to Schwarzschild bending angle. However, we get non-zero bending angle for a hypothetical massless, rotating, charged body.

gr-qc↗

arXiv2014

Imaging polarimetry of the rotating Bok globule CB67

Polarimetric observations of about 50 stars located in a close vicinity of the Bok globule CB67 having significantly nonspherical shape and rapid rotation are performed. The data obtained are compared with the available observations of this globule at radio and submillimeter wavelengths as well as some theoretical calculations. It is found that the elongation and the rotation moment of CB67 are oriented rather perpendicular to the magnetic fields, which is unusual for Bok globules and is difficult to be explained from the theoretical point of view.

astro-ph.GA↗

arXiv2012

Imaging polarimetry of the Bok globule CB56

The measurement of polarization of the background stars in the region of Bok globules is important to study the magnetic field geometry and dust grain characteristics in the globule. These parameters are important for the formation and evolution of dark clouds. We made polarimetric observations of Bok globule CB56 in the R-filter from the 2-metre telescope at IUCAA Girawali Observatory (IGO). The observations were carried out on 2011 March 4th and 5th. The CCD images obtained from the instrument (IFOSC) were analyzed, to produce the polarization map of the Bok globule CB56.

astro-ph.IM↗

arXiv2012

High-Resolution Near-Infrared Polarimetry of a Circumstellar Disk around UX Tau A

We present H-band polarimetric imagery of UX Tau A taken with HiCIAO/AO188 on the Subaru Telescope. UX Tau A has been classified as a pre-transitional disk object, with a gap structure separating its inner and outer disks. Our imagery taken with the 0.15 (21 AU) radius coronagraphic mask has revealed a strongly polarized circumstellar disk surrounding UX Tau A which extends to 120 AU, at a spatial resolution of 0.1 (14 AU). It is inclined by 46 \pm 2 degree as the west side is nearest. Although SED modeling and sub-millimeter imagery suggested the presence of a gap in the disk, with the inner edge of the outer disk estimated to be located at 25 - 30 AU, we detect no evidence of a gap at the limit of our inner working angle (23 AU) at the near-infrared wavelength. We attribute the observed strong polarization (up to 66 %) to light scattering by dust grains in the disk. However, neither polarization models of the circumstellar disk based on Rayleigh scattering nor Mie scattering approximations were consistent with the observed azimuthal profile of the polarization degrees of the disk. Instead, a geometric optics model of the disk with nonspherical grains with the radii of 30 micron meter is consistent with the observed profile. We suggest that the dust grains have experienced frequent collisional coagulations and have grown in the circumstellar disk of UX Tau A.

astro-ph.SR↗

arXiv2011

Modelling of laboratory data of bi-directional reflectance of regolith surface containing Alumina

Bidirectional reflectance of a surface is defined as the ratio of the scattered radiation at the detector to the incident irradiance as a function of geometry. The accurate knowledge of the bidirectional reflection function (BRF) of layers composed of discrete, randomly positioned scattering particles is very essential for many remote sensing, engineering, biophysical applications and in different areas of Astrophysics. The computations of BRF's for plane parallel particulate layers are usually reduced to solve the radiative transfer equation (RTE) by the existing techniques. In this work we present our laboratory data on bidirectional reflectance versus phase angle for two sample sizes of 0.3 and 1 $μm$ of Alumina for the He-Ne laser at 632.8 nm (red) and 543.5nm(green) wavelength. The nature of the phase curves of the asteroids depends on the parameters like- particle size, composition, porosity, roughness etc. In our present work we analyse the data which are being generated using single scattering phase function i.e. Mie theory considering particles to be compact sphere. The well known Hapke formula will be considered along with different particle phase function such as Mie and Henyey Greenstein etc to model the laboratory data obtained at the asteroid laboratory of Assam University.

astro-ph.IM↗

arXiv2011

Modelling polarization properties of comet 1P/Halley using a mixture of compact and aggregate particles

Recently, the result obtained from `Stardust' mission suggests that the overall ratio of compact to aggregate particles is 65:35 (or 13:7) for Comet 81P/Wild 2 (Burchell et al. 2008). In the present work, we propose a model which considers cometary dust as a mixture of compact and aggregate particles, with composition of silicate and organic. We consider compact particles as spheroidal particles and aggregates as BCCA and BAM2 aggregate with some size distribution. For modeling Comet 1P/ Halley, the power-law size distribution n(a)= a^{-2.6}, for both compact and aggregate particles is taken. We take a mixture of BAM2 and BCCA aggregates with a lower and upper cutoff size around 0.20$μm$ and 1$μm$. We also take a mixture of prolate, spherical and oblate compact particles with axial ratio (E) from 0.8 to 1.2 where a lower and upper cutoff size around 0.1$μm$ and 10$μm$ are taken. Using T-matrix code, the average simulated polarization curves are generated which can best fit the observed polarization data at the four wavelengths $λ$ = 0.365$μm$, 0.485$μm$, 0.670$μm$ and 0.684$μm$. The suitable mixing percentage of aggregates emerging out from the present modeling corresponds to 50% BAM2 and 50% BCCA particles and silicate to organic mixing percentage corresponds to 78% silicate and 22% organic in terms of volume. The present model successfully reproduces the observed polarization data, especially the negative branch, more effectively as compared to other work done in the past. It is found that among the aggregates, the BAM2 aggregate plays a major role, in deciding the cross-over angle and depth of negative polarization branch.

astro-ph.EP↗

arXiv2010

Towards a more exact value of deflection of light due to static gravitational mass

The deflection of a ray of light passing close to a gravitational mass, is generally calculated from the null geodesic which the light ray (photon) follows. However, there is an alternate approach, where the effect of gravitation on the ray of light is estimated by considering the ray to be passing through a material medium. Calculations have been done in this paper, following the later approach, to estimate the amount of deflection due to a static non-rotating mass. The refractive index of such a material medium has been calculated in a more rigorous manner in the present work and the final value of amount of deflection calculated here is claimed to be more accurate than all other values resulting from previous calculations.

gr-qc↗

arXiv2010

Aggregate dust model to study the polarization properties of comet C/1996 B2 Hyakutake

In our present study, the observed linear polarization data of comet Hyakutake are studied at wavelengths $λ=0.365μm$, $λ=0.485μm$ and 0.684$μm$ through simulations using Ballistic Particle-Cluster Aggregate and Ballistic Cluster-Cluster Aggregate aggregates of 128 spherical monomers. We first investigated that the size parameter of the monomer, $x$ $\sim$ 1.56 -- 1.70, turned out to be most suitable which provides the best fits to the observed dust scattering properties at three wavelengths $λ= 0.365$$μm$, 0.485$μm$ and 0.684$μm$. Thus the effective radius of the aggregate (r) lies in the range $0.45 μm \le r \le 0.49 μm$ at $λ= 0.365$$μm$; $ 0.60 μm \le r \le 0.66 μm$ at $λ= 0.485$$μm$ and $0.88 μm \le r \le 0.94 μm$ at $λ= 0.684$$μm$. Now using superposition \textsc{t-matrix} code and the power-law size distribution, $n(r) \sim r^{-3}$, the best-fitting values of complex refractive indices are calculated which can best fit the observed polarization data at the above three wavelengths. The best-fitting complex refractive indices $(n,k)$ are found to be (1.745, 0.095) at $λ= 0.365 $ $μm$, (1.743, 0.100) at $λ= 0.485 $ $μm$ and (1.695, 0.100) at $λ= 0.684$ $μm$. The refractive indices coming out from the present analysis correspond to mixture of both silicates and organics, which are in good agreement with the \textit{in situ} measurement of comets by different spacecraft.

astro-ph.EP↗

arXiv2009

Optical and submillimetre observations of Bok globules -- tracing the magnetic field from low to high density

We present optical and submillimetre polarimetry data of the Bok globule CB3 and optical polarimetry data of the Bok globule CB246. We use each set of polarimetry data to infer the B-field orientation in each of the clouds. The optical data can only be used in the low density, low extinction edge regions of clouds. The submillimetre data can only be used in the high column-density, central regions of the clouds. It has previously been found that near-infrared polarisation mapping of background stars does not accurately trace the magnetic field in dense cloud regions. This may be due to a lack of aligned grains in dense regions. We test this by comparing the field orientations measured by our two independent polarimetry methods. We find that the field orientation deduced from the optical data matches up well with the orientation estimated from the submillimetre data. We therefore claim that both methods are accurately tracing the same magnetic field in CB3. Hence, in this case, there must be significant numbers of aligned dust grains in the high density region, and they do indeed trace the magnetic field in the submillimetre. We find an offset of 40$\pm$14 degrees between the magnetic field orientation and the short axis of the globule. This is consistent with the mean value of 31$\pm$3 degrees found in our previous work on prestellar cores, even though CB3 is a protostellar core. Taken together, the six prestellar cores that we have now studied in this way show a mean offset between magnetic field orientation and core short axis of $\sim30\pm$3 degrees, in apparent contradiction with some models of magnetically dominated star formation.

astro-ph.GA↗

arXiv2009

New Possibilities for Obtaining a Steep Nonlinear Current-Voltage Characteristics in some Semiconductor Structures

Electronic processes in a semiconductor system consisting of some Resonant Tunnelling Structures, built in the depletion region of a Schottky barrier, are investigated. It is shown that the Schottky barrier can block or unblock the resonant tunnelling current effectively. Tunnelling processes do reveal the coherent character. Sharp nonlinear current-voltage characteristics are observed on both of the forward and the reverse branches.

cond-mat.mes-hall↗

arXiv2007

Study of grain compositions in comet Levy 1990XX

In the present work, the non-spherical dust grain characteristics of comet Levy 1990XX with different silicate grain compositions (both pyroxene and olivine) are studied using the T-matrix method. Considering amorphous pyroxene grain (Mg$_x$Fe$_{1-x}$SiO$_3$) with $x$ (= 0.4, 0.5, 0.6, 0.7, 0.8, 0.95, 1) and amorphous olivine grain (Mg$_{2y}$Fe$_{2-2y}$SiO$_4$) with $y$ (= 0.4, 0.5) (Dorschner et al. 1995), the observed polarization data of comet Levy1990XX are analyzed using the T-matrix code at $λ$ = 0.485 $μm$.

astro-ph↗

arXiv2007

Polarimetric studies of comet Hale-Bopp

In the present work, the non-spherical dust grain characteristics of comet Hale-Bopp are studied using the T-matrix method and the modified power law distribution function. It is found that the observed data fits very well with the power index ($α) = - 3$. The best fit values of complex refractive index $(n, k)$ and the aspect ratio (E) at $α= - 3$ are calculated to be $(1.382, 0.035, 0.936)$ and $(1.379, 0.041,0.936)$ at $λ= 0.485 μm$ and $0.684μm$ respectively. Kerola & Larson (K-L) analysed the same comet using the T-matrix method and the power law distribution function ($α= -3$), and found that the prolate grains can explain the observed polarization in a more satisfactory manner as compared to the other shapes. But their analysis could not reproduce the negative polarization branch beyond scattering angle $157^0$. However, the results obtained from the present work successfully generate the expected negative polarization curve beyond $157^0$ and the fitting in this case is much better than K-L's work. So it is concluded from the present study that the use of modified power law distribution function (with $α= - 3$) can fit the observed data in a better way, as compared to the power law distribution function used by previous authors.

astro-ph↗

arXiv2006

Combustion Process in a Spark Ignition Engine: Analysis of Cyclic Maximum Pressure and Peak Pressure Angle

In this paper we analyze the cycle-to-cycle variations of maximum pressure $p_{max}$ and peak pressure angle $α_{pmax}$ in a four-cylinder spark ignition engine. We examine the experimental time series of $p_{max}$ and $α_{pmax}$ for three different spark advance angles. Using standard statistical techniques such as return maps and histograms we show that depending on the spark advance angle, there are significant differences in the fluctuations of $p_{max}$ and $α_{pmax}$. We also calculate the multiscale entropy of the various time series to estimate the effect of randomness in these fluctuations. Finally, we explain how the information on both $p_{max}$ and $α_{pmax}$ can be used to develop optimal strategies for controlling the combustion process and improving engine performance.

nlin.CD↗

arXiv2005

An analysis of the distribution of background star polarization in dark clouds

The polarization observed for stars background to dark clouds (Bok Globules) is often used as diagnostic to study the ongoing star formation processes in these clouds. Such polarization in the optical have been reported for eight nearby clouds CB3, CB25, CB39, CB52, CB54, CB58, CB62 and CB246 in one of our previous work (Sen et. al. 2000). With a view to understand the origin of this polarization, the the present work attempts are made to look for any possible relation between this observed polarization and other physical parameters in the cloud (like temperature, turbulence etc.). The observed polarization does not seem to be clearly related to the dust and gas temperatures (T_d and T_g) in the cloud as expected from the Davis-Greenstein grain alignment mechanism (Davis & Greenstein, 1952). However, the average observed polarization p_av appears to be related to the turbulence delta V (measured by 12CO line width) by the mathematical relation p_av=2.95exp(-0.24delta V). The possible realtion between the direction of polarization vector and other physical parameters are also discussed. For this analysis, in addition to the data on above eight clouds, the data on CB4 (Kane et. al., 1995) are also included for comparison.

astro-ph↗

arXiv1998

Nonlinearity effect on 1D periodic and disordered lattices

The Kronig-Penney model is used to Study the effect of nonlinear interaction on the transmissive properties of both ordered and disordered chains. In the ordered case, the nonlinearity can either localize or delocalize the electronic states depending on both its sign and strength but there is a critical strength above which all the states are localized. In the disordered case, however, we found that the transmission decays as $T\sim L^{-γ}$ around the band edge of the corresponding periodic system. The exponent $γ$ is independent of the strength of the nonlinearity in the case of disordered barrier potentials, while it varies with the strength for mixed potentials.

cond-mat.dis-nn↗