The evolution of the magnetic fields of neutron stars     

Dipankar Bhattacharya 《Journal of Astrophysics and Astronomy》1995,16(2):217-232

Observational evidence, and theoretical models of the magnetic field evolution of neutron stars is discussed. Observational data indicates that the magnetic field of a neutron star decays significantly only if it has been a member of a close interacting binary. Theoretically, the magnetic field evolution has been related to the processing of a neutron star in a binary system through the spin evolution of the neutron star, and also through the accretion of matter on the neutron star surface. I describe two specific models, one in which magnetic flux is expelled from the superconducting core during spin-down, via a copuling between Abrikosov fluxoids and Onsager-Feynman vortices; and another in which the compression and heating of the stellar crust by the accreted mass drastically reduces the ohmic decay time scale of a magnetic field configuration confined entirely to the crust. General remarks about the behaviour of the crustal field under ohmic diffusion are also made.  相似文献   

A multi-layered backpropagation artificial neural network algorithm for UV spectral classification     

Soma Mukherjee  Ujjwal Bhattacharya  S. K. Parui  Ranjan Gupta  Ravi K. Gulati 《Astrophysics and Space Science》1996,239(2):361-373

In this paper we present an application of an artificial neural network model based on a multi-layered backpropagation algorithm for spectral classification of UV data from the International Ultraviolet Explorer (IUE) low dispersion spectra reference atlas. The model used is similar to that of von Hippel et al. (1994), and is found to reduce the classification error as compared to the recently reported results on the same data set (Gulati et al. 1994b). The improved version of the network is much simpler in structure and the training time is reduced by a factor of almost 20. Such networks will prove very useful in efficient classification of large databases Subject headings: neural networks, stellar spectra, classification  相似文献   

Rayleigh wave dispersion equation for a layered spherical earth with exponential function solutions in each shell     

Swarn Arora  S. N. Bhattacharya  M. L. Gogna 《Pure and Applied Geophysics》1996,147(3):515-536

We consider the second-order differential equations ofP-SV motion in an isotropic elastic medium with spherical coordinates. We assume that in the medium Lamé's parameters , r ^p and compressional and shear-wave velocities , r, wherer is radial distance. With this regular heterogeneity both the radial functions appearing in displacement components satisfy a fourth-order differential equation which provides solutions in terms of exponential functions. We then consider a layered spherical earth in which each layer has heterogeneity as specified above. The dispersion equation of the Rayleigh wave is obtained using the Thomson-Haskel method. Due to exponential function solutions in each layer, the dispersion equation has similar simplicity, as in a flat-layered earth. The dispersion equation is further simplified, whenp=–2. We obtain numerical results which agree with results obtained by other methods.  相似文献   

Long-period fading in atmospherics during severe meteorological activity and associated solar geophysical phenomena at low latitudes     

A. B. Bhattacharya  S. K. Kar  M. K. Chatterjee  R. Bhattacharya 《Annales Geophysicae》1998,16(2):183-188

The records of VLF atmospherics over Calcutta and then over Kalyani (West Bengal) during the torrential rainfall, caused by violent monsoon and post-monsoon depressions, exhibit distinct long-period fadings both at day and night. Interesting results obtained from an analysis of round-the-clock atmospherics data and associated meteorological parameters are reported in this paper. A possible correlation between the severe meteorological activity with the solar geophysical phenomena was studied. The results are indicative of an interesting sequence of solar-terrestrial events. A tentative conclusion is reached, suggesting an origin of the fading from atmospheric gravity waves generated in the centre of activity of the depressions concerned.Currently at: Department of Physics, Serampore College, Serampore, 712 201 West Bengal  相似文献   

Field and petrographic aspects of the iron ore mineralizations of Gandhamardan hill,Keonjhor, Orissa and their genetic significance     

H. N. Bhattacharya  Kaushik K. Ghosh 《Journal of the Geological Society of India》2012,79(5):497-504

Banded iron formations of the Iron Ore Group (Archean greenstone belts) of Jharkhand-Orissa region, India host a good number of large iron ore deposits (Fe wt %> 62). Iron ore mineralization of Gandhamardan hill is one of them where iron ores occur in two stratigraphic horizons. One is strictly confined within banded iron formation (stratabound mineralization) with irregular geometry, and show fracture filling and replacement vein-type mineralization along the fringes of hard massive ores of the core. This type of mineralization is exposed along the western slope of the hill. Hard massive and laminated ores dominate this mineralization. The other type occurs as low dipping sheet like body above banded iron formation and covered by laterites forming the top of the hill. Flaky ores dominate this mineralization with formation of hard goethitic crust near the top. Both the mineralizations contain mineralized banded iron formation corestones surrounded by hard massive or flaky iron ores. Hard massive ores are entirely represented by martite-microplaty hematite mineralogy. Hard laminated ores contain microplaty hematite and few martite grains representing early magnetites of the banded iron formation. Flaky ores are high porosity ores produced by leaching of silica, martite and microplaty hematite. Hard goethitic ores are developed due to replacement of martite and microplaty hematite or precipitation of goethite in the pore spaces.  相似文献   

Instruments of RT-2 experiment onboard CORONAS-PHOTON and their test and evaluation III: Coded Aperture Mask and Fresnel Zone Plates in RT-2/CZT payload     

Anuj Nandi  S. Palit  D. Debnath  Sandip K. Chakrabarti  T. B. Kotoch  R. Sarkar  Vipin K. Yadav  V. Girish  A. R. Rao  D. Bhattacharya 《Experimental Astronomy》2011,29(1-2):55-84

Imaging in hard X-rays of any astrophysical source with high angular resolution is a challenging job. Shadow-casting technique is one of the most viable options for imaging in hard X-rays. We have used two different types of shadow-casters, namely, Coded Aperture Mask (CAM) and Fresnel Zone Plate (FZP) pair and two types of pixellated solid-state detectors, namely, CZT and CMOS in RT-2/CZT payload, the hard X-ray imaging instrument onboard the CORONAS-PHOTON satellite. In this paper, we present the results of simulations with different combinations of coders (CAM & FZP) and detectors that are employed in the RT-2/CZT payload. We discuss the possibility of detecting transient Solar flares with good angular resolution for various combinations. Simulated results are compared with laboratory experiments to verify the consistency of the designed configuration.  相似文献   

Molecular fossils in Cretaceous condensate from western India     

Sharmila Bhattacharya  Suryendu Dutta  Ratul Dutta 《Journal of Earth System Science》2014,123(5):923-934

The present study reports the biomarker distribution of condensate belonging to the early Cretaceous time frame using gas chromatography–mass spectrometry (GC–MS). The early Cretaceous palaeoenvironment was inscribed into these molecular fossils which reflected the source and conditions of deposition of the condensate. The saturate fraction of the condensate is characterized by normal alkanes ranging from n-C₉ to n-C₂₉ (CPI-1.13), cycloalkanes and C₁₄ and C₁₅ sesquiterpanes. The aromatic fraction comprises of naphthalene, phenanthrene, their methylated derivatives and cyclohexylbenzenes. Isohexylalkylnaphthalenes, a product of rearrangement process of terpenoids, is detected in the condensate. Several aromatic sesquiterpenoids and diterpenoids have been recorded. Dihydro-ar-curcumene, cadalene and ionene form the assemblage of sesquiterpenoids which are indicative of higher plant input. Aromatic diterpenoid fraction comprises of simonellite and retene. These compounds are also indicative of higher plants, particularly conifer source which had been a predominant flora during the Cretaceous time.  相似文献   

Oxygen isotope analysis of bone and tooth enamel phosphate from paleogene sediments: Experimental techniques and initial results     

M. K. Bera  Kasturi Bhattacharya  A. Sarkar  A. Samanta  Kishor Kumar  Ashok Sahni 《Journal of the Geological Society of India》2010,76(3):275-282

Oxygen isotope composition (δ¹⁸O) of fossil bone and tooth enamel phosphate (bioapatite) is an important tool for estimating the isotopic composition of past environmental water. Lack of analytical facility was a hindrance for studying such bioapatites in spite of large number of fossil materials reported from various geological ages in India. We have established in our laboratory, based on available methods, the chemical procedure for extraction of very small amount (<1000 μg) PO₄ ⁻³ from bioapatite and on-line mass spectrometric measurement of its δ¹⁸O composition by high temperature (∼1450°C) pyrolysis. The achieved precision is ∼± 0.3 ‰ similar to obtained elsewhere, with interlaboratory calibration showing excellent agreement of standard phosphates. Inferred δ¹⁸O values of environmental water, based on the analysis of teeth and bones of sharks, fish and terrestrial mammals from the Paleogene successions of the northwest sub-Himalayan and the Peninsular India show strong correspondence with animal habitats. The freshwater δ¹⁸O values are much depleted having range similar to modern monsoon precipitation. However, owing to our small dataset it is not possible at this stage to infer about the existence of monsoon over the Indian sub-continent during the Eocene-Oligocene time.  相似文献   

亚致死剂量铜对文昌鱼和玫瑰无须钯的组织病理学影响   总被引：1，自引：0，他引：1  

韩莉  郭斌  Haimanti.  Bhattacharya 《中国海洋大学学报(自然科学版)》2008,38(1):95-102,22

采用半静水实验法测试研究了重金属Cu^2＋对文昌鱼（Branchiostoma belcher）和玫瑰无须喜巴（Puntius conchonius）的急性致死毒性和亚急性毒性。实验结果表明，Cu^2＋对文昌鱼和玫瑰无须喜巴24，48，72，96h的半致死浓度分别为0．561，0．373，0．261，0．211mg／L及0．976，0．783，0．562，0．473mg／L，其安全质量浓度分别为0．021和0．047mg／L。亚急性毒性实验中，文昌鱼和玫瑰无须喜巴分别用浓度为20％和30％的96h LC50的Cu^2＋处理。不同器官的组织病理学观察结果表明，过量Cu^2＋对玫瑰无须喜巴的鳃、肝脏、肾脏等器官产生毒害，对文昌鱼鳃和肝盲囊细胞也产生了严重的毒害，表现为细胞空泡化等明显的结构破坏。实验结果显示文昌鱼的肝盲囊和脊椎动物的肝脏一样，也是受过量铜显著毒害的主要靶器官器官。因此，文昌鱼的肝盲囊至少在经受过量Cu^2＋毒害方面与脊椎动物肝脏具有相似性，从生理功能方面为类文昌鱼祖先的盲囊（digestive caecum）是脊椎动物肝脏的前体这一假说提供了资料。  相似文献   

Prospects of hard X-ray polarimetry with Astrosat-CZTI     

T. Chattopadhyay  S. V. Vadawale  A. R. Rao  S. Sreekumar  D. Bhattacharya 《Experimental Astronomy》2014,37(3):555-577

Astrosat is the first Indian satellite mission dedicated for astronomical studies. It is planned for launch during 2014 and will have five instruments for multi-wavelength observations from optical to hard X-rays. Cadmium Zing Telluride Imager (CZTI) is one of the five instruments aiming for simultaneous X-ray spectroscopy and imaging in the energy range of 10 keV to 100 keV (along with all sky photometric capability unto 250 keV). It is based on pixilated CZT detector array with total geometric area of 1024 cm². It will have two-dimensional coded mask for medium resolution X-ray imaging. The CZT detector plane will be realized using CZT detector modules having integrated readout electronics. Each CZT detector module consists of 4 cm × 4 cm CZT with thickness of 5 mm which is further pixilated into 16 × 16 array of pixels. Thus each pixel has size of 2.5 mm × 2.5 mm and thickness of 5 mm. Such pixilated detector plane can in principle be used for hard X-ray polarization measurements based on the principle of Compton scattering by measuring azimuthal distribution of simultaneous events in two adjacent pixels. We have carried out detailed Geant4 simulations for estimating polarimetric capabilities of CZTI detector plane. The results indicate that events in the energy range of 100 keV to 250 keV, where the 5 mm thick CZT detector has significant detection efficiency, can be used for polarimetric studies. Our simulation results indicate the minimum detectable polarization (MDP) at the level of ～ 10% can be achieved for bright Crab like X-ray sources with exposure time of ～500 ks. We also carried out preliminary experiments to verify the results from our simulations. Here we present detailed method and results of our simulations as well as preliminary results from the experimental verification of polarimetric capabilities of CZT detector modules used in Astrosat CZTI.  相似文献