It has long been argued that better timing precision allowed by satellites like Rossi X‐ray Timing Explorer (RXTE) will allow us to measure the orbital eccentricity and the angle of periastron of some of the bright persistent high‐mass X‐ray binaries (HMXBs) and hence a possible measurement of apsidal motion in these system. Measuring the rate of apsidal motion allows one to estimate the apsidal motion constant of the mass losing companion star and hence allows for the direct testing of the stellar structure models for these giant stars present in the HMXBs. In the present paper, we use the archival RXTE data of two bright persistent sources, namely Cen X‐3 and SMC X‐1, to measure the very small orbital eccentricity and the angle of periastron. We find that the small variations in the pulse profiles of these sources, rather than the intrinsic time resolution provided by RXTE, limit the accuracy with which we can measure arrival time of the pulses from these sources. This influences the accuracy with which one can measure the orbital parameters, especially the very small eccentricity and the angle of periastron in these sources. The observations of SMC X‐1 in the year 2000 were taken during the high‐flux state of the source and we could determine the orbital eccentricity and ω using this data set.

We review the pulse profile evolution of the unique accretion powered Xray pulsar 4U 1626-67 over the last 40 years since its discovery. This pulsar showed two distinct eras of steady spin-up separated by a steady spin-down episode for about 18 years. In the present work, using data from different observatories active during each phase of spin-up and spin-down we establish a clear correlation between the accretion torque acting on this pulsar and its pulse profile. The energy resolved pulse profiles are identical in both the spin-up eras and quite different in the spin-down era, especially in the low energy band. This correlation, along with the already known feature of strong Quasi Periodic Oscillations (QPO) that was present only in the spin-down era, clearly establish two different accretion modes onto the neutron star which produce different pulse profiles and only one of which produces the QPOs.

We report a systematic strengthening of the local solar surface or fundamental f -mode 1-2 days prior to the emergence of an active region (AR) in the same (corotating) location. Except for a possibly related increase in the kurtosis of the magnetic field, no indication can be seen in the magnetograms at that time. Our study is motivated by earlier numerical findings of Singh et al. (2014) which showed that, in the presence of a nonuniform magnetic field that is concentrated a few scale heights below the surface, the f -mode fans out in the diagnostic kω diagram at high wavenumbers. Here we explore this possibility using data from the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory and show for six isolated ARs, 11130, 11158, 11242, 11105, 11072, and 11768, that at large latitudinal wavenumbers (corresponding to horizontal scales of around 3000 km), the f -mode displays strengthening about two days prior to AR formation and thus provides a new precursor for AR formation. Furthermore, we study two ARs, 12051 and 11678, apart from a magnetically quiet patch lying next to AR 12529, to demonstrate the challenges in extracting such a precursor signal when a newly forming AR emerges in a patch that lies in close proximity of one or several already existing ARs which are expected to pollute neighboring patches. We then discuss plausible procedures for extracting precursor signals from regions with crowded environments. The idea that the f -mode is perturbed days before any visible magnetic activity occurs at the surface can be important in constraining dynamo models aimed at understanding the global magnetic activity of the Sun.

We report here discovery of the existence of two different accretion modes in the high mass X-ray binary pulsar Cen X-3 during its high states. The multiband X-ray light curves of Cen X-3 lasting for more than 3400 days obtained with the All Sky Monitor (ASM) onboard the Rossi X-ray Timing Explorer (RXTE) shows many episodes of high and low X-ray intensities. The high intensity phases last between a few to upto 110 days and the separation between two high intensity phases also varies widely. One remarkable feature deduced from the RXTE-ASM light curves is that during these high intensity phases, Cen X-3 manifests in two very distinct spectral states. When the source makes a transition from the low intensity phase to the high intensity phase, it adopts one of these two spectral states and during the entire high intensity phase remains in that particular spectral state. During December 2000 to April 2004, all the high intensity episodes showed a hardness ratio which is significantly larger than the same during all the high states prior to and subsequent to this period. It is also found that most of the soft outbursts reach a nearly constant peak flux in the 5-12 keV band. For comparison, similar analysis was carried out on the long term X-ray light curves of three other X-ray binary pulsars Her X-1, Vela X-1, and SMC X-1. Results obtained with these sources are also presented here and we found that none of the other sources show such a behaviour. From these observations, we suggest that Cen X-3 has two different accretion modes and in the course of nine years it has exhibited two switch overs between these.