Comparison between Time-domain and Frequency-domain Measurement Techniques for Mantle Shear-wave Attenuation     

J. Bhattacharyya 《Pure and Applied Geophysics》1998,153(2-4):399-417

—We study the frequency- and time-domain techniques which have been used to measure shear attenuation in the mantle using long-period body waveforms. In the time-domain technique, waveform modeling is carried out and the attenuation model that best fits the data is chosen. In the frequency-domain technique, we solve for the attenuation model that best fits the spectra of the seismic waveforms. Though theoretically both these techniques are equivalent, modeling assumptions and measurement biases associated with each technique can give rise to different results. In this study, we compare these two techniques in terms of their accuracy in obtaining mantle shear attenuation. Specifically, we estimate the biases in constraining attenuation from differential SS?S and absolute S waveforms. We carry out these tests using realistic synthetic seismograms and we follow this with an analysis of recorded data to verify the results from the synthetic tests. For the SS?S waveforms, the primary biasing factors are interference with seismic phases due to mantle discontinuities and due to crustal reverberation under the SS bounce point. These factors can affect the t* measurements by up to 0.5 s in the frequency domain and more than 1.5 s in the time domain. For the S waveforms, the frequency-domain measurements are accurate to 0.3 s while the time-domain measurements can vary by more than 2.0 s from the predicted values. These errors are also manifested in the t* measurements made using teleseismically recorded waveforms and lead to comparatively larger noise levels in the time-domain measurements. Based on these results, we propose that in long-period body-wave attenuation studies, frequency-domain techniques should be the method of choice.  相似文献   

New telescopes in India     

J. C. Bhattacharyya 《Astrophysics and Space Science》1986,118(1-2):45-49

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Mass accretion by the nuclei of disk galaxies,II     

Tara Bhattacharyya  Anuradha Saha  B. Basu 《Astrophysics and Space Science》1985,113(1):33-51

Repeated explosions in the nuclei of galaxies are now accepted as observationally established phenomena. Each explosion leads to the ejection of gas from the central region of a galaxy with velocities depending on the strength of the explosive event. In the process the nucleus temporarily becomes gas-deficient. It is suggested that the mass los is replenished by the accretion of the mass which is shed by those evolved stars in the galactic bulge that possess relatively low rotational velocities. The gas to be accreted is assumed to be magnetized. In the present model, the accretion rate has been assumed to be a function of both radial distance and time. The cross-radial equation of motion has been solved to derive the expression for the rotational velocity which is found to bealmost linear with the radial distance from the centre. The radial equation has been solved to calculate the time-scale over which the nucleus accumulates sufficient mass to undergo instability and suffer explosion. The calculated time-scale range from few multiples of 10⁷ to a few multiples of 10⁸ yr. This range agrees very well with that as has been suggested on the basis of observation in the case of our own Galaxy.  相似文献   

Massive Compact Halo Objects from the relics of the cosmic quark–hadron transition     

Shibaji Banerjee  Abhijit Bhattacharyya  Sanjay K. Ghosh  Sibaji Raha  Bikash Sinha  Hiroshi Toki 《Monthly notices of the Royal Astronomical Society》2003,340(1):284-288

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Short-wavelength drift dissipative instability in the presence of field-aligned currents     

A. Bhattacharyya  G.S. Lakhina 《Planetary and Space Science》1982,30(6):581-585

Sharp density gradients coupled with field-aligned currents can give rise to short wavelength (?15 m) drift waves due to collisional effects in the F-region of the auroral ionosphere. In this wavelength range, ion-ion collisions at altitudes of 300–450 km render the ions unmagnetized and a field-aligned current can drive a drift wave, propagating almost transverse to the magnetic field, unstable due to the resistance in electron parallel motion arising from electron collisions.  相似文献   

General relativistic spectra of accretion discs around rapidly rotating neutron stars: effect of light bending   总被引：1，自引：0，他引：1  

Sudip Bhattacharyya  Dipankar Bhattacharya  Arun V. Thampan 《Monthly notices of the Royal Astronomical Society》2001,325(3):989-994

We present computed spectra, as seen by a distant observer, from the accretion disc around a rapidly rotating neutron star. Our calculations are carried out in a fully general relativistic framework, with an exact treatment of rotation. We take into account the Doppler shift, gravitational redshift and light-bending effects in order to compute the observed spectrum. We find that light bending significantly modifies the high-energy part of the spectrum. Computed spectra for slowly rotating neutron stars are also presented. These results would be important for modelling the observed X-ray spectra of low-mass X-ray binaries containing fast-spinning neutron stars.  相似文献   

A note on the post-explosion gas depletion in the central region of normal spiral galaxies     

T. Bhattacharyya 《Astrophysics and Space Science》1980,68(2):459-467

The problem of post-explosive gas depletion in the central region of normal spiral galaxies like our own has been investigated. It has been calculated that with a plausible density law and with some restriction on gas temperature, a sufficient quantity of gas will be depleted from the central region over a time-scale ranging from a few times 10⁷ to a few times 10⁸ years, the exact time depending on the particular density law. Such a time-scale has been suggested by many authors as the period for one phase of activity in the nuclei of these galaxies. A similar time-scale has also been proposed by several authors as that for the formation and destruction of the spiral patterns of these galaxies.  相似文献   

In memoriam     

J. C. Bhattacharyya 《Astrophysics and Space Science》1983,91(1):3-4

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Fast Transients with the Square Kilometre Array and its Pathfinders: An Indian Perspective     

Yashwant Gupta  Poonam Chandra  Manjari Bagchi  Niruj M. Ramanujam  Yogesh Maan  Avinash A. Deshpande  Siddhartha Bhattacharyya 《Journal of Astrophysics and Astronomy》2016,37(4):37

In the rapidly developing field of study of the transient sky, fast radio transients are perhaps the most exciting objects of scrutiny at present. The SKA, with its wide field-of-view and significant improvement in sensitivity over existing facilities, is expected to detect a plethora of fast transients which, in addition to help resolve the mysteries surrounding their nature and origin, will also lead to other interesting applications in astrophysics. We explore some of these possibilities here, and also emphasize the current status and future plans of the Indian community working in this area, in the context of ongoing work and extension of this to the SKA.  相似文献   

Physico-chemical conditions of four calc-alkaline granitoid plutons of Chhotanagpur Gneissic Complex,eastern India: Tectonic implications     

B Goswami  P Roy  A Basak  S Das  C Bhattacharyya 《Journal of Earth System Science》2018,127(8):120

Petrography and mineralogy of four calc-alkaline granitoid plutons Agarpur, Sindurpur, Raghunathpur and Sarpahari located from west to east of northern Purulia of Chhotanagpur Gneissic Complex, eastern India, are investigated. The plutons, as a whole, are composed of varying proportions of Qtz–Pl–Kfs–Bt–Hbl±Px–Ttn–Mag–Ap–Zrn±Ep. The composition of biotite is consistent with those of calc-alkaline granitoids. Hornblende–plagioclase thermometry, aluminium-in-hornblende barometry and the assemblage sphene–magnetite–quartz were used to determine the P, T and \(f_{\mathrm{O}_2}\) during the crystallisation of the parent magmas in different plutons. The plutons are crystallised under varying pressures (6.2–2.4 kbar) and a wide range of temperatures (896–\(718{^{\circ }}\hbox {C}\)) from highly oxidised magmas (log \(f_{\mathrm{O}_2}\) \(-11.2\) to \(-15.4\) bar). The water content of the magma of different plutons varied from 5.0 to 6.5 wt%, consistent with the calc-alkaline nature of the magma. Calc-alkaline nature, high oxygen fugacity and high \(\hbox {H}_{2}\hbox {O}_{{\mathrm{melt}}}\) suggest that these plutons were emplaced in subduction zone environment. The depths of emplacement of these plutons seem to increase from west to east. Petrologic compositions of these granitoids continuously change from enderbite (opx-tonalite: Sarpahari) in the east to monzogranite (Raghunathpur) to syenogranite (Sindurpur) to alkali feldspar granite (Agarpur) in the west. The water contents of the parental magmas of different plutons also increase systematically from east to west. No substantial increase in the depth of emplacement is found in these plutons lying south and north of the major shear zone passing through the study area suggesting the strike-slip nature of the east–west shear zone.  相似文献