Spectral fit of Cygnus X-1 in high energy— a self-consistent study     

Samir Mandal  Sandip K. Chakrabarti 《Astrophysics and Space Science》2007,309(1-4):305-308

We fit the spectra of Cyg X-1 using two component advective flows with Keplerian accretion disks on the equatorial plane surrounded by sub-Keplerian disks when standing shocks are present. The soft photons generated by the bremsstrahlung and synchrotron processes in the sub-Keplerian flow, as well as the multi-colour black body emission from the Keplerian disk are Comptonized by the thermal and non-thermal electrons. By varying Keplerian and sub-Keplerian rates we are able to reproduce the observed soft and hard states as far as X-ray region is concerned and ‘low γ-ray intensity’ and ‘high γ-ray intensity’ states as far as the soft γ-ray region is concerned. We also find two pivotal points where the spectra intersect as is observed in Cyg X-1.   相似文献   

Accretion flow behaviour during the evolution of the quasi-periodic oscillation frequency of XTE J1550−564 in 1998 outburst     

Sandip K. Chakrabarti  Broja G. Dutta  P. S. Pal 《Monthly notices of the Royal Astronomical Society》2009,394(3):1463-1468

Low and intermediate frequency quasi-periodic oscillations (QPOs) are thought to be due to oscillations of Comptonizing regions or hot regions embedded in Keplerian discs. Observational evidence of evolutions of QPOs would therefore be very important as they throw lights on the dynamics of the hotter region. Our aim is to find systems in which there is a well-defined correlation among the frequencies of the QPOs over a range of time so as to understand the physical picture. In this paper, we concentrate on the archival data of XTE J1550−564 obtained during 1998 outburst, and study the systematic drifts during the rising phase from the 1998 September 7 to the 1998 September 19, when the QPO frequency increased monotonically from 81 mHz to 13.1 Hz. Immediately after that, QPO frequency started to decrease and on the 1998 September 26, the QPO frequency became 2.62 Hz. After that, its value remained almost constant. This frequency drift can be modelled satisfactorily with a propagatory oscillating shock solution where the post-shock region behaves as the Comptonized region. Comparing with the nature of a more recent 2005 outburst of another black hole candidate GRO 1655−40, where QPOs disappeared at the end of the rising phase, we conjecture that this so-called 'outburst' may not be a full-fledged outburst.  相似文献   

Fresnel zone plate telescopes for X-ray imaging I: experiments with a quasi-parallel beam     

Sandip K. Chakrabarti  S. Palit  D. Debnath  A. Nandi  V. Yadav  Ritabrata Sarkar 《Experimental Astronomy》2009,24(1-3):109-126

Combination of Fresnel Zone Plates (FZP) can make an excellent telescope for imaging in X-rays. We present here the results of our experiments with several pairs of tungsten made Fresnel Zone plates in presence of an X-ray source kept at a distance of about 45 ft. The quasi-parallel beam allowed us to study sources placed on the axis as well as off the axis of the telescope. We present theoretical study of the fringe patterns produced by the zone plates in presence of a quasi-parallel source. We compare the patterns obtained from experiments with those obtained by our Monte-Carlo simulations. The images are also reconstructed by deconvolution from both the patterns. We compare the performance of such a telescope with other X-ray imaging devices used in space-astronomy.  相似文献   

Letter to the editor     

A. K. Chopra  P. Chakrabarti 《地震工程与结构动力学》1973,2(1):103-104

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Earthquake analysis of gravity dams including hydrodynamic interaction     

P. Chakrabarti  Anil K. Chopra 《地震工程与结构动力学》1973,2(2):143-160

A general procedure for analysis of the response of gravity dams, including hydrodynamic interaction and compressibility of water, to the transverse horizontal and vertical components of earthquake ground motion is presented. The problem is reduced to one in two dimensions considering the transverse vibration of a monolith of a dam, and the material behaviour is assumed to be linearly elastic The complete system is considered as composed of two substructures—the dam, represented as a finite element system, and the reservoir, as a continuum of infinite length in the upstream direction governed by the wave equation. The structural displacements of the dam (including effects of water) are expressed as a linear combination of the modes of vibration of the dam with the reservoir empty. The effectiveness of this analytical formulation lies in its being able to produce excellent results by considering only the first few modes. The complex frequency response for the modal displacements are obtained first. The responses to arbitrary ground motion are subsequently obtained with the aid of the Fast Fourier Transform algorithm An example analysis is presented to illustrate results obtained from this method. It is concluded that the method is very effective and efficient and is capable of producing results to any desired degree of accuracy by including the necessary number of modes of vibration of the dam.  相似文献   

Petrochemistry of the lavas from proterozoic dalma volcanic belt,Singhbhum, eastern India     

Dr. Mihir K. Bose  Dr. Manas K. Chakrabarti  Dr. A. D. Saunders 《International Journal of Earth Sciences》1989,78(2):633-648

The Dalma volcanic belt of the Singhbhum Precambrian terrain in eastern India is developed along the median zone of a linear basin flanked by Archaean cratonic basement to the south. The lavas, in a low grade metamorphic environment, preserve a strong compositional bimodality, with highly magnesian picritic volcanics developed at the base and low-K basalt flows above, constituting the bulk of the lava pile. The ultrabasic lavas have low concentrations of immobile incompatible elements, ratios of which are probably controlled by the source character. Dalma mafic flows are closely comparable in geochemical character to modern day mid-oceanic ridge basalts (MORB) with a dominance of light-REE-depleted basalts and ferrobasalts. However, in terms of certain element ratios (Th/Ta), deviation from MORB characteristics toward island arc tholeiites is apparent. In this respect the Dalma basalts appear to have a modified MORB composition and the closest chemical analogy may be basalts from back-arc basins, i. e. a supra-subduction zone environment. The bimodality of ultrabasic and mafic lavas is reminiscent of Archaean komatiitic provinces. The geochemical signature of the Dalma lavas, and the geological framework of the terrain, strongly point to a marginal basin domain developed in this crustal segment during the Proterozoic.

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Zusammenfassung Der Vulkangürtel von Dalma liegt in der präkambrischen Region Singhbhum in Ostindien. Aufgeschlossen ist der Dalma-Gürtel im mittleren Bereich eines geradlinigen Bekkens, das im Süden durch Basement des Archaikums begrenzt wird. Die Laven befinden sich in einer geringmetamorphen Umgebung und weisen eine bimodale Zusammensetzung auf. An der Basis sind Magnesium-reiche und darüber Kalium-arme Basalte entwickelt, die repräsentativ für den Durchschnitt der Laven sind. Die ultrabasischen Laven enthalten niedrige Konzentrationen an immobilen nicht kompatiblen Elementen, deren Verhältnis wahrscheinlich von den Eigenschaften der Lavaquelle gesteuert wird. Der geochemische Charakter der mafischen Dalma-Vulkanite entspricht weitgehend den heutigen Basalten der ozeanischen Rücken (MORB). Dabei dominieren leicht abgereicherte REE-Basalte sowie eisenreiche Basalte. Vergleicht man allerdings Elementverhältnisse wie das von Th/Ta, wird eine Abweichung von dem typischen MORB-Charakter hin zu Inselbögen-Tholeiten deutlich. Berücksichtigt man diese Beobachtungen, scheinen die Dalma-Basalte eine modifizierte MORB-Zusammensetzung zu haben, die in ihrem Chemismus am ehesten den Basalten von Back-Arc-Becken, z. B. in einem Gebiet einer Supra-Subduktionszone, zu entsprechen. Die Bimodalität der ultrabasischen und mafischen Laven erinnert an archaische komatiitische Regionen. Die geochemische Zusammensetzung der Dalma-Laven sowie der geologische Rahmen des Gebietes deuten auf ein randliches Becken hin, das sich während des Proterozoikums entwickelt haben könnte.

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Résumé La ceinture volcanique de Dalma, dans le Précambrien de Singhbhum (Inde Orientale) est située dans la partie médiane d'un bassin linéaire bordé au Sud par un socle cratonique archéen. Les laves se trouvent dans un environnement de faible degré de métamorphisme et présentent une composition nettement bimodale: volcanites picritiques très magnésiennes à la base, surmontées de coulées basaltiques pauvres en K, qui constituent l'essentiel de l'ensemble. Les laves ultrabasiques montrent de faibles concentrations en éléments immobiles incompatibles dont les rapports sont probablement déterminés par le caractère de la source. Les coulées basiques de Dalma sont très comparables, dans leurs propriétés géochimiques, aux basaltes récents des rides océaniques (MORB), avec une dominante de basaltes pauvres en terres rares légères et de ferro-basaltes. Toutefois, les rapport de certains éléments (Th/Ta) font apparaître, par rapport au MORB, une déviation vers les tholéiites d'arcs insulaires. A ce point de vue, les basaltes de Dalma montrent une composition de MORB modifée et une analogie avec les basaltes de bassins d'arrière-arc, c'est-à-dire en situation de supra-subduction. La bimodalité laves ultrabasiques/laves basiques est une réminiscence des provinces komatiitiques archéennes. Le signalement géochimiques des laves de Dalma et leur situation géologique plaident en faveur d'un domaine de bassin marginal développé dans ce segment central au cours du Protérozoïque.

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On the source of quasi-periodic oscillations of the black hole candidate GRS 1915+105: some new observations and their interpretation     

Anuj Nandi  Sivakumar G. Manickam  A. R. Rao  Sandip K. Chakrabarti 《Monthly notices of the Royal Astronomical Society》2001,324(1):267-272

A few classes of the light curve of the black hole candidate GRS 1915+105 have been analysed in detail. We discover that unlike the previous findings, quasi-periodic oscillations (QPOs) occasionally occur even in the so-called 'On' or softer states. Such findings may require a revision of the accretion/wind scenario of the black hole candidates. We conjecture that considerable winds that are produced in 'Off' states cool down as a result of Comptonization and fall back to the disc, creating an excess accretion rate and producing the so-called 'On' state. After the drainage of the excess matter, the disc goes back to the 'Off' state. Our findings strengthen the shock oscillation model for QPOs.  相似文献   

Radiatively driven electron–positron jets from two-component accretion flows     

Indranil Chattopadhyay  Santabrata Das  Sandip K. Chakrabarti 《Monthly notices of the Royal Astronomical Society》2004,348(3):846-856

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Fast transition between high-soft and low-soft states in GRS 1915 + 105: Evidence for a critically viscous accretion flow     

S. Naik  A. R. Rao  Sandip K. Chakrabarti 《Journal of Astrophysics and Astronomy》2002,23(3-4):213-233

We present the results of a detailed analysis of RXTE observations of classω (Klein-Woltet al. 2002) which show an unusual state transition between high-soft and low-soft states in the Galactic microquasar GRS 1915 + 105. Out of about 600 pointed RXTE observations, the source was found to exhibit such state transition only on 16 occasions. An examination of the RXTE/ASM data in conjunction with the pointed observations reveals that these events appeared as a series of quasi-regular dips in two stretches of long duration (about 20 days during each occasion) when hard X-ray and radio flux were very low. The X-ray light curve and colour-colour diagram of the source during these observations are found to be different from any reported so far. The duration of these dips is found to be of the order of a few tens of seconds with a repetition time of a few hundred seconds. The transition between these dips and non-dips which differ in intensity by a factor of ∼ 3.5, is observed to be very fast (∼ a few seconds). It is observed that the low-frequency narrow QPOs are absent in the power density spectrum (PDS) of the dip and non-dip regions of classω and the PDS is a power law in the 0.1–10 Hz frequency range. There is a remarkable similarity in the spectral and timing properties of the source during the dip and non-dip regions in this set of observations. These properties of the source are distinctly different from those seen in the observations of other classes. This indicates that the basic accretion disk structure during both dip and non-dip regions of classω is similar, but differ only in intensity. To explain these observations, we invoke a model in which the viscosity is very close to critical viscosity and the shock wave is weak or absent.  相似文献   

Interaction of accretion shocks with winds     

Kinsuk Acharya  Sandip K. Chakrabarti  D. Molteni 《Journal of Astrophysics and Astronomy》2002,23(1-2):155-164

Accretion shocks are known to oscillate in presence of cooling processes in the disk. This oscillation may also cause quasi-periodic oscillations of black holes. In the presence of strong winds, these shocks have oscillations in vertical direction as well. We show examples of shock oscillations under the influence of both the effects. When the shocks are absent and the flow is cooler, the wind becomes weaker and the vertical oscillation becomes negligible.  相似文献