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We present timing and spectral results of 2018 outburst of Cepheus X-4, observed twice by AstroSat at luminosity of $2.04 \times 10^{37}$ erg s$^{-1}$ and $1.02 \times 10^{37}$ erg s$^{-1}$ respectively. The light curves showed strong pulsation and co-related X-ray intensity variation in SXT (0.5--8.0 keV) and LAXPC (3--60 keV) energy bands. Spin-period and spin-down rate of the pulsar were determined from two observations as $65.35080\pm0.00014$ s , $(-2.10\pm0.8)\times10^{-12}$ Hz s$^{-1}$ at an epoch MJD 58301.61850 and $65.35290\pm0.00017$ s, $(-1.6\pm0.8)\times10^{-12}$ Hz s$^{-1}$ for an epoch MJD 58307.40211. Pulse-shape studies with AstroSat showed energy and intensity dependent variations. The pulsar showed an overall continuous spin-down, over 30 years at an average-rate of $(-2.455\pm0.004)\times10^{-14}$ Hz s$^{-1}$, attributed to propeller-effect in the subsonic-regime of the pulsar, in addition to variations during its outburst activities. Spectra between 0.7--55 keV energy band were well fitted by two continuum models, an absorbed compTT-model and an absorbed power-law with a Fermi-Dirac cutoff (FD-cutoff) model with a black-body. These were combined with an iron-emission line and a cyclotron absorption line. The prominent cyclotron resonance scattering features with a peak absorption energy of $30.48^{+0.33}_{-0.34}$ keV and $30.68^{+0.45}_{-0.44}$ keV for FD-cutoff-model and $30.46^{+0.32}_{-0.28}$ keV and $30.30^{+0.36}_{-0.34}$ keV for compTT-model were detected during two AstroSat observations. These when compared with earlier results, showed long term stability of its average value of $30.23 \pm 0.22$ keV. The pulsar showed pulse-phase as well as luminosity dependent variations in cyclotron-line energy and width and in plasma optical-depth of its spectral continuum., 25 pages, 18 figures, 3 tables, Accepted for Publication in The Astrophysical Journal

It has recently been shown that the persistent emission of a neutron star low-mass X-ray binary (LMXB) evolves during a thermonuclear (type-I) X-ray burst. The reason of this evolution, however, is not securely known. This uncertainty can introduce significant systematics in the neutron star radius measurement using burst spectra, particularly if an unknown but significant fraction of the burst emission, which is reprocessed, contributes to the changes in the persistent emission during the burst. Here, by analyzing individual burst data of AstroSat/LAXPC from the neutron star LMXB 4U 1728--34 in the soft state, we show that the burst emission is not significantly reprocessed by a corona covering the neutron star. Rather, our analysis suggests that the burst emission enhances the accretion disk emission, possibly by increasing the accretion rate via disk. This enhanced disk emission, which is Comptonized by a corona covering the disk, can explain an increased persistent emission observed during the burst. This finding provides an understanding of persistent emission components, and their interaction with the thermonuclear burst emission. Furthermore, since burst photons are not significantly reprocessed, non-burst and burst emissions can be reliably separated, which is required to reduce systematic uncertainties in the stellar radius measurement., 8 pages, 3 tables, 1 figure, accepted for publication in The Astrophysical Journal

We have analyzed available full-disc data from the Michelson Doppler Imager (MDI) on board SoHO using the "ring diagram" technique to determine the behavior of solar meridional flows over solar cycle 23 in the outer 2% of the solar radius. We find that the dominant component of meridional flows during solar maximum was much lower than that during the minima at the beginning of cycles 23 and 24. There were differences in the flow velocities even between the two minima. The meridional flows show a migrating pattern with higher-velocity flows migrating towards the equator as activity increases. Additionally, we find that the migrating pattern of the meridional flow matches those of sunspot butterfly diagram and the zonal flows in the shallow layers. A high latitude band in meridional flow appears around 2004, well before the current activity minimum. A Legendre polynomial decomposition of the meridional flows shows that the latitudinal pattern of the flow was also different during the maximum as compared to that during the two minima. The different components of the flow have different time-dependences, and the dependence is different at different depths., To appear in ApJ

Available helioseismic data now span almost the entire solar activity cycle 23 making it possible to study solar-cycle related changes of the solar rotation rate in detail. In this paper we study how the solar rotation rate, in particular, the zonal flows change with time. In addition to the zonal flows that show a well known pattern in the solar convection zone, we also study changes in the radial and latitudinal gradients of the rotation rate, particularly in the shear layer that is present in the immediate sub-surface layers of the Sun. In the case of the zonal-flow pattern, we find that the band indicating fast rotating region close to the equator seems to have bifurcated around 2005. Our investigation of the rotation-rate gradients show that the relative variation in the rotation-rate gradients is about 20% or more of their average values, which is much larger than the relative variation in the rotation rate itself. These results can be used to test predictions of various solar dynamo models., To appear in ApJ. Fig 5 has been corrected in this version

We measure differences in structure between active and quiet regions of the Sun using the frequencies of high-degree modes determined from ring-diagram analyses. We find that both the speed of sound and the adiabatic index Γ1 differ in active regions as compared with quiet regions. In the immediate subsurface layers, the sound speed is lower in active regions, but below a depth of about 7 Mm the opposite is true. A comparison of sound-speed inversion results with those for Γ1 indicates that at least a part of the differences between active and quiet regions is likely to be due to the structural and thermal perturbations caused by magnetic fields in the active region.

The low mass X-ray binary 4U 1728-24 was observed with AstroSat/LAXPC on 8th March 2016. Data from a randomly chosen one orbit of over 3 ks was analyzed for detection of rapid intensity variations. We found that the source intensity was nearly steady but towards the end of the observation a typical Type-1 burst was detected. Dynamical power spectrum of the data in the 3 to 20 keV band, reveals presence of a kHz Quasi-Periodic Oscillation (QPO) whose frequency drifted from around 815 Hz at the beginning of the observation to about 850 Hz just before the burst. The QPO is also detected in the 10 to 20 keV band, which was not obtainable by earlier RXTE observations of this source. Even for such a short observation with a drifting QPO frequency, the time-lag between the 5 to 10 and 10 to 20 keV bands can be constrained to be less than 100 microseconds. The Type-1 burst that lasted for about 20 secs had a typical profile. During the first four seconds dynamic power spectra reveal a burst oscillation whose frequency increased from 361.5 to 363.5 Hz. This is consistent with the earlier results obtained with RXTE/PCA, showing the same spin frequency of the neutron star. The present results demonstrate the capability of LAXPC instrument for detecting millisecond variability even from short observations. After RXTE ceased operation, LAXPC on AstroSat is the only instrument at present with capability of detecting kHz QPOs and other kind of rapid variations from 3 keV to 20 keV and possibly at higher energies also., Comment: Accepted for publication in Astrophysical Journal, 6 figures, five pages

The temporal variations of the rotation rate in the solar interior are studied using frequency splittings from Global Oscillations Network Group (GONG) data obtained during the period 1995-99. We find alternating latitudinal bands of faster and slower rotation which appear to move towards the equator with time - similar to the torsional oscillations seen at the solar surface. This flow pattern appears to persist to a depth of about 0.1R_sun and in this region its magnitude is well correlated with solar activity indices. We do not find any periodic or systematic changes in the rotation rate near the base of the convection zone., To appear in ApJ

Ring diagram analysis of solar oscillation power spectra obtained from MDI data is carried out to study the velocity fields in the outer part of the solar convection zone. The three dimensional power spectra are fitted to a model which has a Lorentzian profile in frequency and which includes the advection of the wave front by horizontal flows, to obtain the two components of the sub-surface flows as a function of the horizontal wave number and radial order of the oscillation modes. This information is then inverted using OLA and RLS methods to infer the variation in horizontal flow velocity with depth. The average rotation velocity at different latitudes obtained by this technique agrees reasonably with helioseismic estimates made using frequency splitting data. The shear layer just below the solar surface appears to consist of two parts with the outer part up to a depth of 4 Mm, where the velocity gradient does not show any reversal up to a latitude of 60 degrees. In the deeper part the velocity gradient shows reversal in sign around a latitude of 55 degrees. The zonal flow velocities inferred in the outermost layers appears to be similar to those obtained by other measurements. A meridional flow from equator polewards is found. It has a maximum amplitude of about 30 m/s near the surface and the amplitude is nearly constant in the outer shear layer., Comment: aastex, 12 figures, to appear in Ap.J

The splitting of the frequencies of the global resonant acoustic modes of the Sun by large-scale flows and rotation permits study of the variation of angular velocity Ω with both radius and latitude within the turbulent convection zone and the deeper radiative interior. The nearly uninterrupted Doppler imaging observations, provided by the Solar Oscillations Investigation (SOI) using the Michelson Doppler Imager (MDI) on the Solar and Heliospheric Observatory (SOHO) spacecraft positioned at the L1 Lagrangian point in continuous sunlight, yield oscillation power spectra with very high signal-to-noise ratios that allow frequency splittings to be determined with exceptional accuracy. This paper reports on joint helioseismic analyses of solar rotation in the convection zone and in the outer part of the radiative core. Inversions have been obtained for a medium-l mode set (involving modes of angular degree l extending to about 250) obtained from the first 144 day interval of SOI-MDI observations in 1996. Drawing inferences about the solar internal rotation from the splitting data is a subtle process. By applying more than one inversion technique to the data, we get some indication of what are the more robust and less robust features of our inversion solutions. Here we have used seven different inversion methods. To test the reliability and sensitivity of these methods, we have performed a set of controlled experiments utilizing artificial data. This gives us some confidence in the inferences we can draw from the real solar data. The inversions of SOI-MDI data have confirmed that the decrease of Ω with latitude seen at the surface extends with little radial variation through much of the convection zone, at the base of which is an adjustment layer, called the tachocline, leading to nearly uniform rotation deeper in the radiative interior. A prominent rotational shearing layer in which Ω increases just below the surface is discernible at low to mid latitudes. Using the new data, we have also been able to study the solar rotation closer to the poles than has been achieved in previous investigations. The data have revealed that the angular velocity is distinctly lower at high latitudes than the values previously extrapolated from measurements at lower latitudes based on surface Doppler observations and helioseismology. Furthermore, we have found some evidence near latitudes of 75° of a submerged polar jet which is rotating more rapidly than its immediate surroundings. Superposed on the relatively smooth latitudinal variation in Ω are alternating zonal bands of slightly faster and slower rotation, each extending some 10° to 15° in latitude. These relatively weak banded flows have been followed by inversion to a depth of about 5% of the solar radius and appear to coincide with the evolving pattern of torsional oscillations reported from earlier surface Doppler studies.

We present and discuss results from time-distance helioseismic measurements of meridional circulation in the solar convection zone using 4 years of Doppler velocity observations by the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO). Using an in-built mass conservation constraint in terms of the stream function we invert helioseismic travel times to infer meridional circulation in the solar convection zone. We find that the return flow that closes the meridional circulation is possibly beneath the depth of $0.77 R_{\odot}$. We discuss the significance of this result in relation to other helioseismic inferences published recently and possible reasons for the differences in the results. Our results show clearly the pitfalls involved in the measurements of material flows in the deep solar interior given the current limits on signal-to-noise and our limited understanding of systematics in the data. We also discuss the implications of our results for the dynamics of solar interior and popular solar dynamo models., 8 pages (ApJ style), 5 figures, accepted for publication in the Astrophysical Journal

16 Cyg A and B are among the brightest stars observed by Kepler. What makes these stars more interesting is that they are solar analogs. 16 Cyg A and B exhibit solar-like oscillations. In this work we use oscillation frequencies obtained using 2.5 years of Kepler data to determine the current helium abundance of these stars. For this we use the fact that the helium ionization zone leaves a signature on the oscillation frequencies and that this signature can be calibrated to determine the helium abundance of that layer. By calibrating the signature of the helium ionization zone against models of known helium abundance, the helium abundance in the envelope of 16 Cyg A is found to lie in the range 0.231 to 0.251 and that of 16 Cyg B lies in the range 0.218 to 0.266., Comment: Accepted for publication in ApJ

Rotation is expected to have an important influence on the structure and the evolution of stars. However, the mechanisms of angular momentum transport in stars remain theoretically uncertain and very complex to take into account in stellar models. To achieve a better understanding of these processes, we desperately need observational constraints on the internal rotation of stars, which until very recently were restricted to the Sun. In this paper, we report the detection of mixed modes - i.e. modes that behave both as g modes in the core and as p modes in the envelope - in the spectrum of the early red giant KIC7341231, which was observed during one year with the Kepler spacecraft. By performing an analysis of the oscillation spectrum of the star, we show that its non-radial modes are clearly split by stellar rotation and we are able to determine precisely the rotational splittings of 18 modes. We then find a stellar model that reproduces very well the observed atmospheric and seismic properties of the star. We use this model to perform inversions of the internal rotation profile of the star, which enables us to show that the core of the star is rotating at least five times faster than the envelope. This will shed new light on the processes of transport of angular momentum in stars. In particular, this result can be used to place constraints on the angular momentum coupling between the core and the envelope of early red giants, which could help us discriminate between the theories that have been proposed over the last decades., Comment: Accepted in ApJ, 39 pages, 16 figures

The observed frequencies of the solar f-mode show systematic shifts from the (parabolic) dispersion relation characteristic of a pure surface mode, owing to the perturbing effects of such phenomena in the outer layers of the Sun as velocity fields, magnetic fields, and temperature gradients/discontinuities. These frequency shifts thus provide a good diagnostic probe of these phenomena. In this paper, we wish to focus our attention on the possible influence of shearing velocity fields on the f-mode frequencies. We show here that velocity fields of a wide class leave a signature on the frequency-shift profile which is consistent with observations, including the characteristic change of sign (crossover) in ?? at a spherical harmonic degree of approximately 800. We demonstrate that quantitative fits to the observed frequency shifts are possible with parameters which are characteristic of the underlying motions of the solar convection zone. However, the simultaneous effects of other phenomena, e.g., chromospheric magnetic fields, should also be taken into account.

Variations in the adiabatic index of stellar material in the second helium ionization zone enable one to infer the helium abundance in the solar envelope, using the observed frequencies of solar oscillations. Three techniques based on the differential asymptotic method for sound speed inversion are considered. With the help of the signature of helium abundance on various tracers of ionization, it is possible to estimate the helium abundance. Using several test models, the systematic errors in these techniques are estimated. All these techniques are found to be sensitive to the equation of state. The magnetohydrodynamic (MHD) equation of state is found to be close to that of solar material. Using reference models employing MHD equation of state we find the solar helium abundance Y=0.252±0.003.

The extent of overshoot from stellar convection zones into the adjoining stable layers has been recognized to have a nonnegligible influence on evolutionary tracks of stars. Recently, Stothers & Chin (1992) have carried out a detailed model-independent analysis of substantial body of observational data to conclude that the maximum permissible overshoot is 0.2 times the local pressure scale height. In the present work a realistic solar convection zone model is constructed by employing a nonlocal equation for the velocity of convective elements and by including dissipative effects in the calculations. The convection model approach and the analysis of linear eigenmodes are combined to estimate the penetration depths below the base of the convection zone and into the overlying solar atmosphere. It is demonstrated that for an arbitrary extent of overshoot into the underlying stable region, it may not be possible to find a combination of linear modes capable of reproducing the model convective flux profile over the overshoot layers. The acceptable overshoot distance below the base of the convection zone turns out to be ≤0.2H p or less with a probable value of 0.1H p , which appears to be consistent with helioseismological data.

Un modele pour les zones de convection stellaire base sur les modes convectifs lineaires utilisant une theorie de longueur de melange non locale est developpe pour etudier les asymetries des raies spectrales resultant des mouvements convectifs dans la region photospherique stellaire. Les profils synthetiques des raies spectrales sont obtenus par sommation des profils localement symetriques sur le disque stellaire suivant la vitesse Doppler locale et les fluctuations d'intensite. Les modeles de zone de convection sont construits pour 4 etoiles: le Soleil, α Cen A, Arcturus et Procyon

The observed thermodynamic distribution function f(N) for galaxies places significant constraints on the amount of structured dark matter in the universe. The simplest models of cold dark matter require the cosmological density parameter Omega(0) of less than about 0.4. Biased galaxy formation in more complicated models must have specific forms which depend on the amount and structure of dark matter in the model. 20 references.

The structure of the solar convection zone calculated according to the mixing length theory is found to be consistent with the transport of convective flux by a linear superposition of statistically independent unstable convective modes, provided the effects of turbulent conductivity and viscosity are taken into account. The resultant vertical velocity is in reasonable agreement with observed granular velocity in the atmosphere, but the horizontal velocity turns out to be too large for low l modes.

The nature of convection in the envelopes of red giants is investigated, and the linear convective modes are computed to demonstrate the consistency of the mixing length theory. The mixing length at a given depth is tentatively identified with an equivalent width of the luminosity profile of the convective eigenmodes. The dominant convective element turns out to be comparable in size to the radius of the star, and this could account for the observed irregular variations in red giants.