We present an X-ray spectral and timing analysis of 4U 1543À47 during its 2002 outburst based on 49 pointed observations obtained using the Rossi X-Ray Timing Explorer. The outburst reached a peak intensity of 4.2 crab in the 2-12 keV band and declined by a factor of 32 throughout the month-long observation. A 21:9 AE 0:6 mJy radio flare was detected at 1026.75 MHz two days before the X-ray maximum; the radio source was also detected late in the outburst, after the X-ray source entered the ''hard'' state. The X-ray light curve exhibits the classic shape of a rapid rise and an exponential decay. The spectrum is soft and dominated by emission from the accretion disk. The continuum is fitted with a multicolor disk blackbody (kT max ¼ 1:04 keV) and a power law (À $ 2:7). Midway through the decay phase, a strong low-frequency quasi-periodic oscillation ( ¼ 7:3 8:1 Hz) was present for several days. The spectra feature a broad Fe K line that is asymmetric, suggesting that the line is due to relativistic broadening rather than Comptonization. Relativistic Laor models provide much better fits to the line than nonrelativistic Gaussian models, particularly near the beginning and end of our observations. The line fits yield estimates for the inner disk radius that are within 6R g ; this result and additional evidence indicates that this black hole may have a nonzero angular momentum.

We present results from a study of X‐shaped sources based on observations using the Giant Metrewave Radio Telescope (GMRT). These observations were motivated by our low‐frequency study of 3C 223.1, an X‐shaped radio source, which showed that the wings (or low surface brightness jets) have flatter spectral indices than that of the active lobes (or high surface brightness jets), a result not easily explained by most models. We have now obtained GMRT data at 240 and 610 MHz for almost all the known X‐shaped radio sources and have studied the distribution of the spectral indices across the sources. While the radio morphologies of all the sources at 240 and 610 MHz show the characteristic X‐shape, the spectral characteristics of the X‐shaped radio sources seem to fall into three categories, namely, sources in which (i) the wings have flatter spectral indices than the active lobes have, (ii) the wings and the active lobes have comparable spectral indices, and (iii) the wings have steeper spectral indices than the active lobes have. We discuss the implications of the new observational results on the various formation models that have been proposed for X‐shaped sources.

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. To investigate the properties of the source during a superluminal flare, we present GMRT observations during a radio flare which turned out to be a pre-plateau flare as shown by the contemporaneous Ryle telescope observations. A major superluminal ejection was observed at the end of this "plateau" state (Dhawan et al. 2003), associated with highly variable X-ray emission showing X-ray soft dips. This episode, thus has all the three types of radio emission: a pre-plateau flare, a "plateau" state and superluminal jets. We analyze all the available RXTE-PCA data during this episode and show that: (1) the pre-flare "plateau" state consists of a three-component X-ray spectra which includes a multicolor disk-blackbody, a Comptonized component and a power-law and (2) the Compton cloud, which is responsible for the Comptonizing component, is ejected away during the X-ray soft dips. We investigate all the available monitoring data on this source and identify several candidate superluminal flare events 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.

We report long-term scintillation observations of 18 pulsars in the dispersion measure range 3 − 35 pc cm −3 carried out from 1993 January to 1995 August using the Ooty Radio Telescope at 327 MHz. These observations were made with the aim of studying refractive effects in pulsar scintillation, and obtaining reliable estimates of diffractive and refractive scintillation properties. Dynamic scintillation spectra of pulsars were regularly monitored at 10 − 90 epochs spanning 100 − 1000 days. Significant changes are observed in the dynamic spectra over time scales as short as a few days. Large-amplitude fluctuations are observed in quantities such as decorrelation bandwidth, scintillation time scale, drift rate, and flux density. Several pulsars show organized features such as drifting bands in a highly pronounced manner. For some pulsars, gradual and systematic variations are seen in the drift rate of patterns which undergo several sign reversals during the observing time spans. Anomalous behaviour such as persistent drifts lasting over many months are seen for PSRs B0834+06 and B1919+21. Four pulsars were studied for 2 − 4 well separated observing sessions, each lasting over ∼ 100 days. In some cases, significant variations are seen in the average scintillation properties and/or flux densities between successive observing sessions. From our data, we have been able to obtain more accurate and reliable estimates of scintillation properties and flux densities than those from the earlier observations, by averaging out the fluctuations due to refractive scintillation effects. These measurements are used to derive parameters such as the strength of scattering and scintillation speeds. The scintillation speed estimates are found to be reasonably good indicators of proper motion speeds of pulsars. The present measurements are compared with earlier measurements and the long-term stability of scintillation properties and flux densities is discussed.