Spectral Properties of Black Holes in Gamma Rays

Black holes are the most compact objects in the universe. Therefore, matter accreting onto them is likely to radiate photons of energy comparable to very high gravitational potential energy. We discuss the nature of the emitted radiation in X-rays and gamma-rays from black hole candidates. We present theoretical solutions, which comprise both Keplerian and sub-Keplerian components and suggest that shocks in accretion and outflows may play a major role in producing these spectra.     

Detection of ionospheric perturbation due to a soft gamma ray repeater SGR J1550-5418 by very low frequency radio waves

The properties of cylindrical and spherical dust acoustic (DA) solitary and shock waves in an unmagnetized electron depleted dusty plasma consisting of inertial dust fluid and ions featuring Tsallis statistics are investigated by employing the reductive perturbation technique. A Korteweg-de Vries Burgers (KdVB) equation is derived and its numerical solution is obtained. The effects of ion nonextensivity and dust kinematic viscosity on the basic features of DA solitary and shock waves are discussed in nonplanar geometry. It is found that nonextensive nonplanar DA waves behave quite differently from their one-dimensional planar counterpart.     

Unique Observation of a Solar Flare by Lunar Occultation During the 2010 Annular Solar Eclipse Through Ionospheric Disturbances of VLF Signals

Very Low Frequency (VLF) radio waves propagate through the Earth-ionosphere waveguide. Irregularities caused by excess or deficient extreme ultra-violet and X-rays, which otherwise sustain the ionosphere, change the waveguide properties and hence the signals are modified. We report the results of monitoring of the NWC transmitter (19.8?kHz) by a receiver placed at Khukurdaha (22°27′N, 87°45′E) during the partial solar eclipse (75?%) of 15th January, 2010. The propagation path from the transmitter to the receiver crosses the annular eclipse belt. We got a clear depression in the data during the period of the eclipse. Most interestingly, there was also a X-ray flaring activity in the sun on that day which reached its peak (C-type) right after the time when the eclipse reached its maximum. We saw the effects of the occultation of this flare in our VLF signal since a part of the X-ray active region was clearly blocked by the moon. We quantitatively compared by using analogies with previous observations and found best fitting parameters for the time when the flare was occulted. We then reconstructed the VLF signal in the absence of the occulted flare. To our knowledge, this is the first such incident where the solar flare was observed through lunar occultation and that too during a partial eclipse.     

Seismic fragility of base-isolated water storage tanks under non-stationary earthquakes

Seismic fragility curves for fixed-base and base-isolated liquid storage tanks are developed under non-stationary earthquakes, and their seismic performance are compared. The correlation between different earthquake intensity measure (IM) parameters and peak response quantities of the base-isolated liquid storage tanks are investigated. The failure criteria are chosen based on (1) the elastic buckling strength of the tank wall, which is defined in terms of critical base shear and critical overturning moment, and (2) in terms of the critical isolation displacement. The uncertainty involved is considered in the earthquake characteristics. Non-stationary earthquake ground motions are generated using Monte Carlo (MC) simulation. Influence of the isolator characteristic parameters and modeling approaches on the seismic fragility of the base-isolated liquid storage tanks is also investigated. Peak ground acceleration is found to be the well correlated IM parameter with the peak response quantities of the base-isolated liquid storage tanks. Substantial decrease in the seismic fragility of the base-isolated liquid storage tanks is observed as compared to the fixed-base tanks. Significant influence of the isolator characteristic parameters on the seismic fragility of the base-isolated liquid storage tanks are reported in the present study.     

Optimum cross section index (OCI): a new approach for identification of an optimum channel—a case study of the Ichamati River,India

The Ichamati River drains the east and south side of the North 24 Parganas district which belongs to the Ganga-Brahmaputra Delta (GBD), covered by deep Quaternary sediments produced under tropical monsoon climate in India. The district is densely populated. The river has huge impact on its population. In this paper, particular attention has been paid to a new method, named optimum cross section index (OCI), which is a numerical representation of a river cross section with respect to its optimum condition. To establish the validity and reliability of OCI, as circumstantial evidences, we have measured and analyzed the geometric parameters, such as width, depth, hydraulic radius, wetted perimeter, and bank profile at 22 points, and applied the areal difference asymmetric index (ADAI). The paper considers that the changing characters of the geometric form of the cross sections and ADAI stand for the OCI. All the techniques reveal that (1) the area of channel decreases abruptly upstream downward, (2) the upstream channel is more symmetrical than the lower reach, and (3) the area of optimum channel gradually increases upstream downward. Results of the study show that the river is going to be deteriorating from upstream downward. OCI has been constructed based on width and depth, which are relatively easily available data. So, this model may be well assessable to identify the optimum channel for a river management.     

Estimation of Sea Ice Freeboard from SARAL/AltiKa Data

An attempt has been made to derive sea ice freeboard from Ka-band Altimeter (SARAL/AltiKa) over Arctic region for 15 March–15 April 2013 (spring) and 15 September–15 October 2013 (autumn). A waveform template matching technique is employed for classification of leads and floe pixels. The estimated sea ice freeboards were found in close agreement with “Operation IceBridge quick look” freeboards (RMSD = 0.30 m). The differences between the two freeboards were largely due to snow layer over sea ice (R = 0.8). The estimated freeboards were of the order of 0.08–0.15 m during the two seasons.     

A proposed method for measurement of cosmic-ray mass composition based on geomagnetic spectroscopy

The effect of the geomagnetic Lorentz force on the muon component of extensive air shower (EAS) has been studied in a Monte Carlo generated simulated data sample. This geomagnetic field affects the paths of muons in an EAS, causing a local contrast or polar asymmetry in the abundance of positive and negative muons about the shower axis. The asymmetry can be approximately expressed as a function of transverse separation between the positive and negative muons barycentric positions in the EAS through opposite quadrants across the shower core in the shower front plane. In the present study, it is found that the transverse muon barycenter separation and its maximum value obtained from the polar variation of the parameter are higher for iron primaries than protons for highly inclined showers. Hence, in principle, these parameters can be exploited to the measurement of primary cosmic-ray mass composition. Possibility of practical realization of the proposed method in a real experiment is briefly discussed.     

Feasibility of spectro-photometry in X-rays (SPHINX) from the moon

Doing space Astronomy on lunar surface has several advantages. We present here feasibility of an All Sky Monitoring Payload for Spectro-photometry in X-rays (SPHINX) which can be placed on a lander on the moon or in a space craft orbiting around the moon. The Si–PIN photo-diodes and CdTe crystals are used to detect solar flares, bright gamma bursts, soft gamma-ray repeaters from space and also X-ray fluorescence (XRF) from lunar surface. We present the complete Geant4 simulation to study the feasibility of such an instrument in presence of Cosmic Diffused X-Ray Background (CDXRB). We find that the signal to noise ratio is sufficient for moderate to bright GRBs (above 5 keV), for the quiet sun (up to 100 keV), solar flares, soft gamma-ray repeaters, X-ray Fluorescence (XRF) of lunar surface etc. This is a low-cost system which is capable of performing multiple tasks while stationed at the natural satellite of our planet.     

Fresnel zone plate telescopes for X-ray imaging II: numerical simulations with parallel and diverging beams

We present the results of simulations of shadows cast by a zone plate telescope which may have one to four pairs of zone plates. From the shadows we reconstruct the images under various circumstances. We discuss physical basis of the resolution of the telescope and demonstrate this by our simulations. We allow the source to be at a finite distance (diverging beam) as well as at an infinite distance (parallel beam) and show that the resolution is worsened when the source is nearby. By reconstructing the zone plates in a way that both the zone plates subtend the same solid angles at the source, we obtain back high resolution even for sources at a finite distance. We present simulated results for the observation of the galactic center and show that the sources of varying intensities may be reconstructed with accuracy. Results of these simulations would be of immense use in interpreting the X-ray images from recently launched CORONAS-PHOTON satellite.