Class Transitions and Two Component Accretion Flow in GRS 1915+105

The light curve of the galactic micro-quasar GRS 1915+105 changes in at least thirteen different ways which are called classes. We present examples of the transitions from one class to another as observed by the IXAE instrument aboard the Indian Satellite IRS-P3. We find that the transitions are associated with changes in photon counts over a time-scale of only a few hours and they take place through unknown classes. Assuming that the transitions are caused by variation of the accretion rates, this implies that a significant fraction of the matter must be nearly freely falling in order to have such dramatic changes in such a short time.Comment: 9 pages, 6 figures, Astronomy and Astrophys. (in press

X-ray properties of the microquasar GRS 1915+105 during a variability class transition

We present a detailed X-ray study of the microquasar GRS 1915+105 during a variability class transition observed in 2000 June with the PPCs of the Indian X-ray Astronomy Experiment. We supplement this observation with data from the RXTE archives. The source made a transition from a steady low-hard state to a regular oscillatory behaviour in the light curve known as bursts or class `rho' (Belloni et al. 2000) between 2000 May 11 and 17 and reverted back to the low-hard state on 2000 June 27. A gradual change in the burst recurrence time from about 75 s to about 40 s was observed which then increased to about 120 s during the ~ 40 days of class `rho'. The regular bursts disappeared from the X-ray light curves and the class transition was observed to occur within 1.5 hours on 2000 June 27 with the PPCs. A correlation is found between the observed QPO frequency at 5-8 Hz in the quiescent phase and the average X-ray intensity of the source during the class `rho'. We notice a strong similarity between the properties of the source during the class `rho' and those during the oscillatory phase of the observations of class `alpha'. From the timing and spectral analysis, it is found that the observed properties of the source over tens of days during the class `rho' are identical to those over a time scale of a few hundreds of seconds in the class `alpha'. Examining the light curves from the beginning of the RXTE/PCA and RXTE/ASM observations, it is found that the change of state from radio-quiet low-hard state to high state occurs through the X-ray classes `rho' and `alpha' which appear together during the state transition. It is further inferred that the source switches from low-hard state to the class `rho' through the intermediate class `alpha'.Comment: 10 pages with 9 figures, LaTex. To be appeared in MNRA

Fatigue damage growth mechanisms in continuous fiber reinforced titanium matrix composites

The role of fiber/matrix interface strength, residual thermal stresses, and fiber and matrix properties on fatigue damage accumulation in continuous fiber metal matrix composites (MMC) is discussed. Results from titanium matrix silicon carbide fiber composites is the primary topic of discussion. Results were obtained from both notched and unnotched specimens at room and elevated temperatures. The stress in the 0 deg fibers was identified as the controlling factor in fatigue life. Fatigue of the notched specimens indicated that cracks can grow in the matrix materials without breaking fibers

On the origin of the various types of radio emission in GRS 1915+105

We investigate the association between the radio ``plateau'' states and the large superluminal flares in GRS 1915+105 and propose a qualitative scenario to explain this association. We identify several candidate superluminal flare events from available monitoring data on this source and analyze the contemporaneous RXTE pointed observations. We detect a strong correlation between the average X-ray flux during the ``plateau'' state and the total energy emitted in radio during the subsequent radio flare. We find that the sequence of events is similar for all large radio flares with a fast rise and exponential decay morphology. Based on these results, we propose a qualitative scenario in which the separating ejecta during the superluminal flares are observed due to the interaction of the matter blob ejected during the X-ray soft dips, with the steady jet already established during the ``plateau'' state. This picture can explain all types of radio emission observed from this source in terms of its X-ray emission characteristics.Comment: Corrected typo in the author names, contents unchanged, accepted in Ap

Fast Transition between High-soft and Low-soft States in GRS 1915+105: Evidence for a Critically Viscous Accretion Flow

We present the results of a detailed analysis of RXTE observations of class $\omega$ which show an unusual state transition between high-soft and low-soft states in the 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 occasions) when hard X-ray and radio flux were very low. The X-ray light curve and color-color 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 class $\omega$ and the PDS is a power law in 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 these 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 class $\omega$ 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.Comment: Replaced with correct figures, Jour. of Astrophysics and Astronomy (accepted

Evidence of Class Transitions in GRS 1915+105 from IXAE Data

GRS 1915+105 shows at least twelve distinct classes of light curves. By analysing the data obtained from Indian X-ray Astronomy Experiment (IXAE) instrument aboard IRS-P3 satellite, we show that in at least two days, transitions between one class to another were observed. In these days the so-called $\kappa$ class went to $\rho$ class and $\chi$ class went to $\rho$ class. In the frequency-time plane such transitions exhibited change in quasi-periodic oscillation (QPO) frequency. We could detect that low-frequency QPOs can occur in anticipation of a class transition several hundred minutes before the actual transitionComment: 14 pages, 3 figures, Astrophysical Journa