Anti-correlated hard X-ray time lags in Galactic black hole sources

We investigate the accretion disk geometry in Galactic black hole sources by measuring the time delay between soft and hard X-ray emissions. Similar to the recent discoveries of anti-correlated hard X-ray time lags in Cyg X-3 and GRS 1915+105, we find that the hard X-rays are anti-correlated with soft X-rays with a significant lag in another source: XTE J1550-564. We also find the existence of pivoting in the model independent X-ray spectrum during these observations. We investigate time-resolved X-ray spectral parameters and find that the variation in these parameters is consistent with the idea of a truncated accretion disk. The QPO frequency, which is a measure of the size of truncated accretion disk, too changes indicating that the geometric size of the hard X-ray emitting region changes along with the spectral pivoting and soft X-ray flux. Similar kind of delay is also noticed in 4U 1630-47.Comment: 14 pages, 7 figures, accepted for publication in Ap

Observational evidence for mass ejection during soft X-ray dips in GRS1915+105

We investigate the connection between the X-ray and radio properties of the Galactic microquasar GRS1915+105, by analyzing the X-ray data observed with RXTE, during the presence of a huge radio flare (~450 mJy). The X-ray lightcurve shows two dips of ~100 second duration. Detailed time resolved spectral analysis shows the existence of three spectral components: a multicolor disk-blackbody, a Comptonized component due to hot plasma and a power-law. We find that the Comptonized component is very weak during the dip. This is further confirmed by the PHA ratio of the raw data and ratio of the lightcurves in different energy bands. These results, combined with the fact that the 0.5 -- 10 Hz QPO disappears during the dip and that the Comptonized component is responsible for the QPO lead to the conclusion that during the dips the matter emitting Comptonized spectrum is ejected away. This establishes a direct connection between the X-ray and radio properties of the source.Comment: Replaced with some minor changes, corrected typos. Added Journal Re

A unified data flow model for fault tolerant computers

The Dataflow Simulation System (DFSS) at USL was used as the medium on which a functional simulaton of sIFT was produced. DFSS is written in PL/I and is supported by MULTICS. Within the simulation, all the interprocessor communication, fault simulation, system state data, and monitoring were implemented in dataflow and supported directly by DFSS. The actual processor level computation was carried out by the SIFT code in PASCAL. The interface between DFSS in PL/I and the SIFT code in PASCAL was supported under a mechanism in DFSS called a Node Realization Module (NRM)

Energy-dependent time lags in the Seyfert 1 galaxy NGC 4593

We investigate the energy-time lag dependence of the source NGC 4593 using XMM-Newton/EPIC pn data. We found that the time lag dependency is linear in nature with respect to the logarithm of different energy bands. We also investigate the frequency-dependent time lags and identify that at some frequency range (5 x 10-5 Hz to 2 x 10-4 Hz) the X-ray emission is highly coherent, mildly frequency dependent, and very strongly energy dependent. These observations can be explained in the framework of the thermal Comptonization process, and they indicate a truncated accretion disk very close to the black hole. We discuss the plausible spectral state to explain the phenomenon and conclude that the observed properties bear a close resemblance to the intermediate state or the steep power-law state, found in galactic black hole sources

CSPOB-Continuous Spectrophotometry of Black Holes

The goal of a small and dedicated satellite called the "Continuous Spectro-Photometry of Black Holes" or CSPOB is to provide the essential tool for the theoretical understanding of the hydrodynamic and magneto-hydrodynamic flows around black holes. In its life time of about three to four years, only a half a dozen black holes will be observed continuously with a pair of CSPOBs. Changes in the spectral and temporal variability properties of the high-energy emission would be caught as they happen. Several important questions are expected to be answered and many puzzles would be sorted out with this mission.Comment: 4 Pages, 3 Figures, Proceeding of the 2nd Kolkata Conference on "Observational Evidence for the Black Holes in the Universe", Published in AIP, 200

Finite element modelling of natural vibration problems

Finite element models of Modified Rayleigh-Ritz, Galerkin, Least Square, Hybrid (Pian's) and Collocation methods are presented for natural vibration problems. A comparative assessment is made of these methods with Rayleigh-Ritz finite element models. This study brings out that the Galerkin model is relatively superior. A method of improving the accuracy in the estimation of eigenvalues is also included

Constant amplitude and post-overload fatigue crack growth behavior in PM aluminum alloy AA 8009

A recently developed, rapidly solidified, powder metallurgy, dispersion strengthened aluminum alloy, AA 8009, was fatigue tested at room temperature in lab air. Constant amplitude/constant delta kappa and single spike overload conditions were examined. High fatigue crack growth rates and low crack closure levels compared to typical ingot metallurgy aluminum alloys were observed. It was proposed that minimal crack roughness, crack path deflection, and limited slip reversibility, resulting from ultra-fine microstructure, were responsible for the relatively poor da/dN-delta kappa performance of AA 8009 as compared to that of typical IM aluminum alloys