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IGR J17591--2342, a transient accretion-powered millisecond X-ray pulsar, was discovered during its 2018 outburst. Here, we present a timing and spectral analysis of the source using {\it AstroSat} data of the same outburst. From the timing analysis, we obtain updated values of binary orbital parameters, which reveal an average pulsar spin frequency of 527.4256984(8) Hz. The pulse profiles can be fit well with four harmonically related sinusoidal components with fractional amplitudes of fundamental and second, third, and fourth harmonics as $\sim13$\%, $\sim$6\%, $\sim$0.9\%, $\sim$0.2\%, respectively. The energy-dependent study of pulse profiles in the range of $3-20$ keV shows that the fractional amplitude of both the fundamental and first overtone is consistent with being constant across the considered energy band. Besides, a decaying trend has been observed for both the fundamental and first overtone in the phase-delay versus energy relation resulting in soft X-ray (2.8-3.3 keV) phase lags of $\sim$0.05 and $\sim$0.13 with respect to $\leq 15$ keV photons, for the fundamental and first overtone, respectively. The combined spectra from the Large Area X-ray Proportional Counters and the Soft X-ray Telescope aboard {\it AstroSat} in the $1-18$ keV range can be fit well with an absorbed model consisting of a Comptonization, a blackbody and a Gaussian emission line component yielding as best-fit parameters a blackbody seed photon temperature $kT_{\rm bb}$ $\sim 0.95 \pm 0.03$ keV, and an electron temperature $kT_{\rm e}$ $\sim 1.54 \pm0.03$ keV. The spectral aspects suggest the scattering of photons from the accretion disc or the neutron star's surface., Comment: Accepted in MNRAS on November 14, 2024

We report the first detection of evolving Low-Frequency Quasi-periodic Oscillation (LFQPO) frequencies in hard X-rays upto $100$ keV with $AstroSat/LAXPC$ during `unusual' outburst phase of Swift J1727.8-1613 in hard-intermediate state (HIMS). The observed LFQPO in $20 - 100$ keV has a centroid $\nu_{_{\rm QPO}}=1.43$ Hz, a coherence factor $Q= 7.14$ and an amplitude ${\rm rms_{_{\rm QPO}}} = 10.95\%$ with significance $\sigma = 5.46$. Type-C QPOs ($1.09-2.6$ Hz) are found to evolve monotonically during HIMS of the outburst with clear detection in hard X-rays ($80 - 100$ keV), where ${\rm rms_{_{\rm QPO}}}$ decreases ($\sim 12-3\%$) with energy. Further, $\nu_{_{\rm QPO}}$ is seen to correlate (anti-correlate) with low (high) energy flux in $2-20$ keV ($15-50$ keV). Wide-band ($0.7 - 40$ keV) energy spectrum of $NICER/XTI$ and $AstroSat/LAXPC$ is satisfactorily described by the `dominant' thermal Comptonization contribution ($\sim 88\%$) in presence of a `weak' signature of disk emissions ($kT_{\rm in} \sim 0.36$ keV) indicating the harder spectral distribution. Considering source mass $M_{\rm BH}=10M_\odot$ and distance $1.5 < {\rm d~(kpc)} < 5$, the unabsorbed bolometric luminosity is estimated as $\sim 0.03-0.92\%L_{\rm Edd}$. Finally, we discuss the implications of our findings in the context of accretion dynamics around black hole X-ray binaries., Comment: 10 pages, 5 figures, 3 tables, accepted for publication in MNRAS

We present an improved background model for the Large Area X-ray Proportional Counter (LAXPC) detectors on-board AstroSat. Because of the large collecting area and high pressure, the LAXPC instrument has a large background count rate, which varies during the orbit. Apart from the variation with latitude and longitude during the orbit there is a prominent quasi-diurnal variation which has not been modelled earlier. Using over 5 years of background observations, we determined the period of the quasi-diurnal variation to be 84495 s and using this period, it is possible to account for the variation and also identify time intervals where the fit is not good. These lead to a significant improvement in the background model. The quasi-diurnal variation can be ascribed to the changes in charged particle flux in the near Earth orbit., Comment: Accepted for publication in ApJS

We use helioseismic data obtained over two solar cycles to determine whether there are changes in the near-surface shear layer (NSSL). We examine this by determining the radial gradient of the solar rotation rate. The radial gradient itself shows a solar-cycle dependence, and the changes are more pronounced in the active latitudes than at adjoining higher latitudes; results at the highest latitudes (greater than about70 degrees) are unreliable. The pattern changes with depth, even within the NSSL. We find that the near-surface shear layer is deeper at lower latitudes than at high latitudes and that the extent of the layer also shows a small solar-cycle related change., Comment: Accepted for publication in ApJ

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., Comment: 25 pages, 18 figures, 3 tables, Accepted for Publication in The Astrophysical Journal

The Large Area X-ray Proportional Counter (LAXPC) instrument on-board AstroSat has three nominally identical detectors for timing and spectral studies in the energy range of 3--80 keV. The performance of these detectors during the five years after the launch of AstroSat is described. Currently, only one of the detector is working nominally. The variation in pressure, energy resolution, gain and background with time are discussed. The capabilities and limitations of the instrument are described. A brief account of available analysis software is also provided., Comment: Accepted for publication in JAA

IGR J17091--3624 is a transient galactic black hole which has a distinct quasi-periodic variability known as `heartbeat', similar to the one observed in GRS 1915+105. In this paper, we report the results of $\sim 125$ ks \textit{AstroSat} observations of this source during the 2016 outburst. For the first time a double peaked QPO (DPQ) is detected in a few time segments of this source with a difference of $\delta f ~\sim12$ mHz between the two peaks. The nature of the DPQ was studied based on hardness ratios and using the static as well as the dynamic power spectrum. Additionally, a low frequency (25--48 mHz) `heartbeat' single peak QPO (SPQ) was observed at different intervals of time along with harmonics ($50-95$ mHz). Broadband spectra in the range $0.7-23$ keV, obtained with \textit{SXT} and \textit{LAXPC}, could be fitted well with combination of a thermal Comptonisation and a multicolour disc component model. During \textit{AstroSat} observation, the source was in the Soft-Intermediate State (SIMS) as observed with \textit{Swift/XRT}. We present a comparative study of the `heartbeat' state variability in IGR J17091--3624 with GRS 1915+105. Significant difference in the timing properties is observed although spectral parameters ($\Gamma\sim2.1-2.4$ and $T_\mathrm{max}\sim0.6-0.8$ keV) in the broad energy band remain similar. Spectral properties of segments exhibiting SPQ and DPQ are further studied using simple phase resolved spectroscopy which does not show a significant difference. Based on the model parameters, we obtain the maximum ratio of mass accretion rate in GRS 1915+105 to that in IGR J17091--3624 as $\sim25:1$. We discuss the implications of our findings and comment on the physical origin of these exotic variabilities., Comment: 17 pages, 13 Figures, 6 Tables; Accepted for publication in MNRAS

The Galactic microquasar GRS 1915+105 shows rich variability which is categorized into different classes. In this paper, we report temporal and spectral analysis of GRS 1915+105 to study the properties of the accretion flow when the light curve is showing $\theta$ class variability. For this purpose, we use the LAXPC data from the Target of Opportunity observations of India's first multi-wavelength astronomy satellite AstroSat. The $\theta$ class is marked by the recurrent appearance of U-shaped regions in the light curve, where the photon count rate first decreases rapidly and then increases slowly. For our analysis, we use U-shaped regions of first two orbits (02345 and 02346) on 2016 March 04. In both of the cases, the dynamic Power Density Spectra (PDS) showed significant power at around $4-5$ Hz, suggesting the presence of a low-frequency Quasi-Periodic Oscillation (QPO) around that frequency interval. The QPO frequency is found to increase with time when the enhancement of the energy flux also takes place. From the evolution of the spectra, we determine the evolution of the accretion flow parameters in both of these observations. Fitting the spectra with the transonic flow solution based Two-Component Advective Flow (TCAF) model in $4-25$ keV energy band shows that the Keplerian disk accretion rate rises with the rise in the radiation intensity, while the location of the centrifugal pressure driven shock front decreases. In both these data, a gradual increment of power-law photon index with intensity is observed suggesting the progressive softening of the source., Comment: 16 pages, 6 figures, 2 tables. Accepted in the Astrophysical Journal on May 5, 2021

We present an in-depth spectral and timing analysis of the Black Hole binary 4U 1630-472 during 2016 and 2018 outbursts as observed by \textit{AstroSat} and \textit{MAXI}. The extensive coverage of the outbursts with \textit{MAXI} is used to obtain the Hardness Intensity Diagram (HID). The source follows a `c'-shaped profile in agreement with earlier findings. Based on the HIDs of previous outbursts, we attempt to track the evolution of the source during a `super'-outburst and `mini'-outbursts. We model the broadband energy spectra ($0.7-20.0$ keV) of \textit{AstroSat} observations of both outbursts using phenomenological and physical models. No Keplerian disc signature is observed at the beginning of 2016 outburst. However, the disc appears within a few hours after which it remains prominent with temperature ($T_{in}$) $\sim$ 1.3 keV and increase in photon index ($\Gamma$) from 1.8 to 2.0, whereas the source was at a disc dominant state throughout the \textit{AstroSat} campaign of 2018 outburst. Based on the HIDs and spectral properties, we classify the outbursts into three different states - the `canonical' hard and soft states along with an intermediate state. Evolution of rms along different states is seen although no Quasi-periodic Oscillations (QPOs) are detected. We fit the observed spectra using a dynamical accretion model and estimate the accretion parameters. Mass of the black hole is estimated using inner disc radius, bolometric luminosity and two component flow model to be $3-9$ $M_{\odot}$. Finally, we discuss the possible implications of our findings., Comment: 16 pages, 11 figures, Accepted for publication in MNRAS, typos fixed and figures updated

We present results from AstroSat observation of the recent outburst of GRO J2058+42, an X-ray pulsar in a Be-binary system. The source was observed on April 10, 2019 by LAXPC and SXT instruments on AstroSat during its declining phase of the latest giant outburst. Light curves showed a strong pulsation of the pulsar with a period of 194.2201 \pm 0.0016 s, and a spin-up rate of (1.65\pm0.06)\times10^{-11} Hz s^{-1}. Intermittent flaring was detected in light curves between 3--80 keV energy band with increase in intensity by up to 1.8 times its average intensity. Pulse profiles obtained between 3--80 keV energy band of the pulsar showed strong dependence on energy. A broad peak was observed in the power density spectrum of the source consistently during AstroSat observations with its peak oscillation frequency of 0.090 Hz along with its higher harmonics, which may be due to quasi-periodic oscillations, a commonly observed phenomenon in transient X-ray pulsars, during their outburst. AstroSat observation also detected cyclotron absorption features in its spectrum corresponding to (9.7--14.4) keV, (19.3--23.8) keV and (37.8--43.1) keV. The pulse phase resolved spectroscopy of the source showed phase dependent variation in its energy and relative strength of these features. The spectrum was well fitted with an absorbed black-body, a Fermi Dirac cut-off model and alternatively with an absorbed CompTT model. Both these models were combined with a Fe-line and three Gaussian absorption lines to account for observed cyclotron resonance scattering features in the spectrum., Comment: 22 pages, 22 figures, 3 tables, Accepted for publication in Astrophysical Journal

A key component of solar interior dynamics is the meridional circulation (MC), whose poleward component in the surface layers has been well observed. Time-distance helioseismic studies of the deep structure of MC, however, have yielded conflicting inferences. Here, following a summary of existing results we show how a large center-to-limb systematics (CLS) in the measured travel times of acoustic waves affect the inferences through an analysis of frequency dependence of CLS, using data from the Helioseismic and Doppler Imager (HMI) onboard Solar Dynamics Observatory (SDO). Our results point to the residual systematics in travel times as a major cause of differing inferences on the deep structure of MC., Comment: 6 pages, 3 figures, to appear in the Springer series Astrophysics and Space Science Proceedings of "Dynamics of the Sun & Stars: Honoring the Life & Work of Michael Thompson" (2020)

Over the course of its history, the Milky Way has ingested multiple smaller satellite galaxies. While these accreted stellar populations can be forensically identified as kinematically distinct structures within the Galaxy, it is difficult in general to precisely date the age at which any one merger occurred. Recent results have revealed a population of stars that were accreted via the collision of a dwarf galaxy, called \textit{Gaia}-Enceladus, leading to a substantial pollution of the chemical and dynamical properties of the Milky Way. Here, we identify the very bright, naked-eye star $\nu$\,Indi as a probe of the age of the early in situ population of the Galaxy. We combine asteroseismic, spectroscopic, astrometric, and kinematic observations to show that this metal-poor, alpha-element-rich star was an indigenous member of the halo, and we measure its age to be $11.0 \pm 0.7$ (stat) $\pm 0.8$ (sys)$\,\rm Gyr$. The star bears hallmarks consistent with it having been kinematically heated by the \textit{Gaia}-Enceladus collision. Its age implies that the earliest the merger could have begun was 11.6 and 13.2 Gyr ago at 68 and 95% confidence, respectively. Input from computations based on hierarchical cosmological models tightens (i.e. reduces) slightly the above limits., Comment: Accepted for publication as a Letter in Nature Astronomy (26 pages, 7 figures, including main article and methods section)

The Transiting Exoplanet Survey Satellite (TESS) is performing a near all-sky survey for planets that transit bright stars. In addition, its excellent photometric precision enables asteroseismology of solar-type and red-giant stars, which exhibit convection-driven, solar-like oscillations. Simulations predict that TESS will detect solar-like oscillations in nearly 100 stars already known to host planets. In this paper, we present an asteroseismic analysis of the known red-giant host stars HD 212771 and HD 203949, both systems having a long-period planet detected through radial velocities. These are the first detections of oscillations in previously known exoplanet-host stars by TESS, further showcasing the mission's potential to conduct asteroseismology of red-giant stars. We estimate the fundamental properties of both stars through a grid-based modeling approach that uses global asteroseismic parameters as input. We discuss the evolutionary state of HD 203949 in depth and note the large discrepancy between its asteroseismic mass ($M_\ast = 1.23 \pm 0.15\,{\rm M}_\odot$ if on the red-giant branch or $M_\ast = 1.00 \pm 0.16\,{\rm M}_\odot$ if in the clump) and the mass quoted in the discovery paper ($M_\ast = 2.1 \pm 0.1\,{\rm M}_\odot$), implying a change $>30\,\%$ in the planet's mass. Assuming HD 203949 to be in the clump, we investigate the planet's past orbital evolution and discuss how it could have avoided engulfment at the tip of the red-giant branch. Finally, HD 212771 was observed by K2 during its Campaign 3, thus allowing for a preliminary comparison of the asteroseismic performances of TESS and K2. We estimate the ratio of the observed oscillation amplitudes for this star to be $A_{\rm max}^{\rm TESS}/A_{\rm max}^{\rm K2} = 0.75 \pm 0.14$, consistent with the expected ratio of $\sim0.85$ due to the redder bandpass of TESS., Comment: Accepted for publication in ApJ; 17 pages, 4 figures, 3 tables

We use helioseismic data from ground and space-based instruments to analyze how solar rotation has changed since the beginning of solar Cycle 23 with emphasis on studying the differences between Cycles 23 and 24. We find that the nature of solar rotation is indeed different for the two cycles. While the changes in the latitudinally independent component follows solar-cycle indices, some of the other components have a more complicated behavior. There is a substantial change in the behavior of the solar zonal flows and their spatial gradients too. While the zonal flows are in general weaker in Cycle 24 than those in Cycle 23, there are clear signs of the emergence of Cycle 25. We have also investigated the properties of the solar tachocline, in particular, its position, width, and the change (or jump) in the rotation rate across it. We find significant temporal variation in the change of the rotation rate across the tachocline. We also find that the changes in solar Cycle 24 were very different from those of Cycle 23. We do not find any statistically significant change in the position or the width of the tachocline., Comment: Accepted for publication in ApJ

In this letter, we report the discovery of 24 new super Li-rich (A(Li) $\ge$ 3.2) giants of He-core burning phase at red clump region. Results are based on systematic search of a large sample of about 12,500 giants common to the LAMOST spectroscopic and Kepler time resolved photometric surveys. The two key parameters derived from Kepler data; average period spacing ($\Delta p$) between $l=1$ mixed gravity dominated g-modes and average large frequency separation ($\Delta \nu$) $l=0$ acoustic p-modes, suggest all the Li-rich giants are in He-core burning phase. This is the first unbiased survey subjected to a robust technique of asteroseismic analysis to unambiguously determine evolutionary phase of Li-rich giants. The results provide a strong evidence that Li enhancement phenomenon is associated with giants of He-core burning phase, post He-flash, rather than any other phase on RGB with inert He-core surrounded by H-burning shell., Comment: 8 pages, 4 figures, Accepted in ApJL

We report the results from \textit{AstroSat} observations of the transient Galactic black hole X-ray binary MAXI J1535-571 during its hard-intermediate state of the 2017 outburst. We systematically study the individual and joint spectra from two simultaneously observing \textit{AstroSat} X-ray instruments, and probe and measure a number of parameter values of accretion disc, corona and reflection from the disc in the system using models with generally increasing complexities. Using our broadband ($1.3-70$ keV) X-ray spectrum, we clearly show that a soft X-ray instrument, which works below $\sim 10-12$ keV, alone cannot correctly characterize the Comptonizing component from the corona, thus highlighting the importance of broadband spectral analysis. By fitting the reflection spectrum with the latest version of the \textsc{relxill} family of relativistic reflection models, we constrain the black hole's dimensionless spin parameter to be $0.67^{+0.16}_{-0.04}$. We also jointly use the reflection spectral component (\textsc{relxill}) and a general relativistic thin disc component (\texttt{Kerrbb}), and estimate the black hole's mass and distance to be $10.39_{-0.62}^{+0.61} M_{\odot}$ and $5.4_{-1.1}^{+1.8}$ kpc respectively., Comment: Accepted for publication in MNRAS

The Be X-ray Binary 4U 0115+63 was observed by Large Area X-ray Proportional Counter (LAXPC) instrument on AstroSat on 2015 October 24 during the peak of a giant Type II outburst. Prominent intensity oscillations at ~ 1 and ~ 2 mHz frequency were detected during the outburst. Nuclear Spectroscopic Telescope Array (NuSTAR) observations made during the same outburst also show mHz quasi periodic oscillations (QPOs). Details of the oscillations and their characteristics deduced from LAXPC/AstroSat and NuSTAR observations are reported in this paper. Analysis of the archival Rossi X-ray Timing Explorer (RXTE)/Proportional Counter Array (PCA) data during 2001-11 also show presence of mHz QPOs during some of the outbursts and details of these QPOs are also reported. Possible models to explain the origin of the mHz oscillations are examined. Similar QPOs, albeit at higher frequencies, have been reported from other neutron star and black hole sources and both may have a common origin. Current models to explain the instability in the inner accretion disk causing the intense oscillations are discussed., Comment: Accepted for publication in ApJ

We present the discovery of TOI-197.01, the first transiting planet identified by the Transiting Exoplanet Survey Satellite (TESS) for which asteroseismology of the host star is possible. TOI-197 (HIP116158) is a bright (V=8.2 mag), spectroscopically classified subgiant which oscillates with an average frequency of about 430 muHz and displays a clear signature of mixed modes. The oscillation amplitude confirms that the redder TESS bandpass compared to Kepler has a small effect on the oscillations, supporting the expected yield of thousands of solar-like oscillators with TESS 2-minute cadence observations. Asteroseismic modeling yields a robust determination of the host star radius (2.943+/-0.064 Rsun), mass (1.212 +/- 0.074 Msun) and age (4.9+/-1.1 Gyr), and demonstrates that it has just started ascending the red-giant branch. Combining asteroseismology with transit modeling and radial-velocity observations, we show that the planet is a "hot Saturn" (9.17+/-0.33 Rearth) with an orbital period of ~14.3 days, irradiance of 343+/-24 Fearth, moderate mass (60.5 +/- 5.7 Mearth) and density (0.431+/-0.062 gcc). The properties of TOI-197.01 show that the host-star metallicity - planet mass correlation found in sub-Saturns (4-8 Rearth) does not extend to larger radii, indicating that planets in the transition between sub-Saturns and Jupiters follow a relatively narrow range of densities. With a density measured to ~15%, TOI-197.01 is one of the best characterized Saturn-sized planets to date, augmenting the small number of known transiting planets around evolved stars and demonstrating the power of TESS to characterize exoplanets and their host stars using asteroseismology., Comment: 12 pages (excluding author list and references), 9 figures, 4 tables, accepted for publication in AJ. An electronic version of Table 3 is available as an ancillary file (sidebar on the right)

We present results from an observation of the Low Mass X-ray Binary 4U 1636-536 obtained with the LAXPC instrument aboard AstroSat. The observations of 4U 1636-536 made during the performance verification phase of AstroSat showed seven thermonuclear X-ray bursts in a total exposure of ~ 65 ks over a period of about two consecutive days. Moreover, the light curve of 4U 1636-536 revealed the presence of a rare triplet of X-ray bursts, having a wait time of about 5.5 minutes between second and the third bursts. We also present results from time-resolved spectroscopy performed during these seven X-ray bursts. In addition, we have also detected a transient Quasi-periodic oscillation (QPO) at ~ 5 Hz. However, we did not find any evidence of kilo-hertz QPOs and/or X-ray burst oscillations, perhaps due to the hard spectral state of the source during this observation., Comment: Accepted for publication in MNRAS

Solar torsional oscillations are migrating bands of slower- and faster-than-average rotation, which are strongly related to the Sun's magnetic cycle. We perform a long-term study (16 yr) of hemispherical asymmetry in solar torsional oscillation velocity using helioseismic data for the first time. We study the north-south asymmetry in the velocity using the zonal flow velocities obtained by ring diagram analysis of the Global Oscillation Network Group (GONG) Doppler images. We find significant hemispherical asymmetry in the torsional oscillation velocity and explore its variation with respect to depth, time, and latitude. We also calculate the hemispherical asymmetry in the surface velocity measurements from the Mount Wilson Observatory and the zonal flow velocities obtained from the Helioseismic and Magnetic Imager ring diagram pipeline. These asymmetries are found to be consistent with the asymmetry obtained from GONG observations. We show that the asymmetry in near-surface torsional oscillation velocity is correlated with the asymmetry in magnetic flux and sunspot number at the solar surface, with the velocity asymmetry preceding the flux and sunspot number asymmetries. We speculate that the asymmetry in torsional oscillation velocity may help in predicting the hemispherical asymmetry in sunspot cycles., Comment: Accepted for publication in The Astrophysical Journal

Accurate inference of solar meridional flow is of crucial importance for the understanding of solar dynamo process. Wave travel times, as measured on the surface, will change if the waves encounter perturbations e.g. in the sound speed or flows, as they propagate through the solar interior. Using functions called sensitivity kernels, we may image the underlying anomalies that cause measured shifts in travel times. The inference of large-scale structures e.g meridional circulation requires computing sensitivity kernels in spherical geometry. Mandal et al. (2017) have computed such spherical kernels in the limit of the first-Born approximation. In this work, we perform an inversion for meridional circulation using travel-time measurements obtained from 6 years of SDO/HMI data and those sensitivity kernels. We enforce mass conservation by inverting for a stream function. The number of free parameters is reduced by projecting the solution on to cubic B-splines in radius and derivatives of the Legendre-polynomial basis in latitude, thereby improving the condition number of the inverse problem. We validate our approach for synthetic observations before performing the actual inversion. The inversion suggests a single-cell profile with the return-flow occurring at depths below 0.78 $R_\odot$., Comment: 24 pages, 8 figures, accepted for publication in ApJ

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., Comment: 8 pages, 3 tables, 1 figure, accepted for publication in The Astrophysical Journal

Temporal variations in the Suns internal velocity field with a periodicity of about 11 years have been observed over the last four decades. The period of these torsional oscillations and their latitudinal propagation roughly coincides with the period and equatorward propagation of sunspots which originate from a magnetohydrodynamic dynamo mechanism operating in the Suns interior. While the solar differential rotation plays an important role in this dynamo mechanism by inducting the toroidal component of magnetic field, the impact of torsional oscillations on the dynamo mechanism and hence the solar cycle is not well understood. Here, we include the observed torsional oscillations into a flux transport dynamo model of the solar cycle to investigate their effect. We find that the overall amplitude of the solar cycle does not change significantly on inclusion of torsional oscillations. However, all the characteristics of the Waldmeier effect in the sunspot cycle are qualitatively reproduced by varying only the amplitude of torsional oscillations. The Waldmeier effect, first noted in 1935, includes the important characteristic that the amplitude of sunspot cycles is anti-correlated to their rise time; cycles with high initial rise rate tend to be stronger. This has implications for solar cycle predictions. Our results suggest that the Waldmeier effect could be a plausible outcome of cycle to cycle modulation of torsional oscillations and provides a physical basis for sunspot cycle forecasts based on torsional oscillation observations. We also provide a theoretical explanation based on the magnetic induction equation thereby connecting two apparently disparate phenomena., Comment: 9 pages, 8 figures, submitted to Astrophysical Journal

We present here results from the X-ray timing and spectral analysis of the X-ray binary Cyg X-3 using observations from Large Area X-ray Proportional Counter (LAXPC) on-board AstroSat. Consecutive lightcurves observed over a period of one year show the binary orbital period of 17253.56 +/- 0.19 sec. Another low-amplitude, slow periodicity of the order of 35.8 +/- 1.4 days is observed which may be due to the orbital precession as suggested earlier by Molteni et al. (1980). During the rising binary phase, power density spectra from different observations during flaring hard X-ray state show quasi-periodic oscillations (QPOs) at ~5-8 mHz, ~12-14 mHz, ~18-24 mHz frequencies at the minimum confidence of 99%. However, during the consecutive binary decay phase, no QPO is detected up to 2-sigma significance. Energy-dependent time-lag spectra show soft lag (soft photons lag hard photons) at the mHz QPO frequency and the fractional rms of the QPO increases with the photon energy. During the binary motion, the observation of mHz QPOs during the rising phase of the flaring hard state may be linked to the increase in the supply of the accreting material in the disk and corona via stellar wind from the companion star. During the decay phase, the compact source moves in the outer wind region causing the decrease in the supply of material for accretion. This may cause weakening of the mHz QPOs below the detection limit. This is also consistent with the preliminary analysis of the orbital phase-resolved energy spectra presented in this paper., Comment: 16 pages, 10 figures, 3 tables, accepted for publication in ApJ

In this article, we derive and compute the sensitivity of measurements of coupling between normal modes of oscillation in the Sun to underlying flows. The theory is based on first-Born perturbation theory, and the analysis is carried out using the formalism described by \citet{lavely92}. Albeit tedious, we detail the derivation and compute the sensitivity of specific pairs of coupled normal modes to anomalies in the interior. Indeed, these kernels are critical for the accurate inference of convective flow amplitudes and large-scale circulations in the solar interior. We resolve some inconsistencies in the derivation of \citet{lavely92} and reformulate the fluid-continuity condition. We also derive and compute sound-speed kernels, paving the way for inverting for thermal anomalies alongside flows., Comment: 24 pages, 8 Figures; MNRAS

Large Area X-ray Proportional Counter (LAXPC) is one of the major AstroSat payloads. LAXPC instrument will provide high time resolution X-ray observations in 3 to 80 keV energy band with moderate energy resolution. A cluster of three co-aligned identical LAXPC detectors is used in AstroSat to provide large collection area of more than 6000 cm2 . The large detection volume (15 cm depth) filled with xenon gas at about 2 atmosphere pressure, results in detection efficiency greater than 50%, above 30 keV. With its broad energy range and fine time resolution (10 microsecond), LAXPC instrument is well suited for timing and spectral studies of a wide variety of known and transient X-ray sources in the sky. We have done extensive calibration of all LAXPC detectors using radioactive sources as well as GEANT4 simulation of LAXPC detectors. We describe in brief some of the results obtained during the payload verification phase along with LXAPC capabilities., Comment: 8 pages, 3 figures, To appear in Current Science 2017

Large Area X-ray Propositional Counter (LAXPC) instrument on AstroSat is aimed at providing high time resolution X-ray observations in 3 to 80 keV energy band with moderate energy resolution. To achieve large collecting area, a cluster of three co-aligned identical LAXPC detectors, is used to realize an effective area in access of about 6000 cm2 at 15 keV. The large detection volume of the LAXPC detectors, filled with xenon gas at about 2 atmosphere pressure, results in detection efficiency greater than 50%, above 30 keV. In this article, we present salient features of the LAXPC detectors, their testing and characterization in the laboratory prior to launch and calibration in the orbit. Some preliminary results on timing and spectral characteristics of a few X-ray binaries and other type of sources, are briefly discussed to demonstrate that the LAXPC instrument is performing as planned in the orbit., Comment: 11 pages, 15 Figures, Accepted for publication in Journal of Astronomy and Astrophysics

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

We present the calibration and background model for the Large Area X-ray Proportional Counter (LAXPC) detectors on-board AstroSat. LAXPC instrument has three nominally identical detectors to achieve large collecting area. These detectors are independent of each other and in the event analysis mode, they record the arrival time and energy of each photon that is detected. The detectors have a time-resolution of 10 $\mu$s and a dead-time of about 42 $\mu$s. This makes LAXPC ideal for timing studies. The energy resolution and peak channel to energy mapping were obtained from calibration on ground using radioactive sources coupled with GEANT4 simulations of the detectors. The response matrix was further refined from observations of the Crab X-ray source after launch. At around 20 keV the energy resolution of detector is about 10--15\%, while the combined effective area of the 3 detectors is about 6000 cm$^2$., Comment: Accepted for publication in ApJS

Acoustic glitches are regions inside a star where the sound speed or its derivatives change abruptly. These leave a small characteristic oscillatory signature in the stellar oscillation frequencies. With the precision achieved by {\it Kepler} seismic data, it is now possible to extract these small amplitude oscillatory signatures, and infer the locations of the glitches. We perform glitch analysis for all the 66 stars in the {\it Kepler} seismic LEGACY sample to derive the locations of the base of the envelope convection zone and the helium ionization zone. The signature from helium ionization zone is found to be robust for all stars in the sample, whereas the convection zone signature is found to be weak and problematic, particularly for relatively massive stars with large errorbars on the oscillation frequencies. We demonstrate that the helium glitch signature can be used to constrain the properties of the helium ionization layers and the helium abundance., Comment: 16 pages, 12 figures, 2 tables, Accepted for publication in ApJ

We report the first analysis of data from AstroSat/LAXPC observations of Cygnus X-1 in January 2016. LAXPC spectra reveals that the source was in the canonical hard state, represented by a prominent thermal Comptonization component having a photon index of 1.8 and high temperature kT of electron > 60 keV along with weak reflection and possible disk emission. The power spectrum can be characterized by two broad lorentzian functions centered at 0.4 and 3 Hz. The r.m.s of the low frequency component decreases from 15% at around 4 keV to 10% at around 50 keV, while that of the high frequency one varies less rapidly from 13.5% to 11.5% in the same energy range. The time lag between the hard (20 to 40 keV) and soft (5 to 10 keV) bands varies in a step-like manner being nearly constant at 50 Milli-seconds from 0.3 to 0.9 Hz, decreasing to 8 Milli-seconds from 2 to 5 Hz and finally dropping to 2 Milli-seconds for higher frequencies. The time lags increase with energy for both the low and high frequency components. The event mode LAXPC data allows for flux resolved spectral analysis on a time-scale of 1 second, which clearly shows that the photon index increased from 1.72 to 1.80 as the flux increased by nearly a factor of two. We discuss the results in the framework of the fluctuation propagation model., Comment: 6 pages, 8 figures

The advent of space-based missions like $Kepler$ has revolutionized the study of solar-type stars, particularly through the measurement and modeling of their resonant modes of oscillation. Here we analyze a sample of 66 $Kepler$ main-sequence stars showing solar-like oscillations as part of the $Kepler$ seismic LEGACY project. We use $Kepler$ short-cadence data, of which each star has at least 12 months, to create frequency power spectra optimized for asteroseismology. For each star we identify its modes of oscillation and extract parameters such as frequency, amplitude, and line width using a Bayesian Markov chain Monte Carlo `peak-bagging' approach. We report the extracted mode parameters for all 66 stars, as well as derived quantities such as frequency difference ratios, the large and small separations $\Delta\nu$ and $\delta\nu_{02}$; the behavior of line widths with frequency and line widths at $\nu_{\rm max}$ with $T_{\rm eff}$, for which we derive parametrizations; and behavior of mode visibilities. These average properties can be applied in future peak-bagging exercises to better constrain the parameters of the stellar oscillation spectra. The frequencies and frequency ratios can tightly constrain the fundamental parameters of these solar-type stars, and mode line widths and amplitudes can test models of mode damping and excitation., Comment: 29 pages, 27 Figures, 8 Tables, Accepted for publication in Apj

We use asteroseismic data from the Kepler satellite to determine fundamental stellar properties of the 66 main-sequence targets observed for at least one full year by the mission. We distributed tens of individual oscillation frequencies extracted from the time series of each star among seven modelling teams who applied different methods to determine radii, masses, and ages for all stars in the sample. Comparisons among the different results reveal a good level of agreement in all stellar properties, which is remarkable considering the variety of codes, input physics and analysis methods employed by the different teams. Average uncertainties are of the order of $\sim$2\% in radius, $\sim$4\% in mass, and $\sim$10\% in age, making this the best-characterised sample of main-sequence stars available to date. Our predicted initial abundances and mixing-length parameters are checked against inferences from chemical enrichment laws $\Delta Y / \Delta Z$ and predictions from 3D atmospheric simulations. We test the accuracy of the determined stellar properties by comparing them to the Sun, angular diameter measurements, Gaia parallaxes, and binary evolution, finding excellent agreement in all cases and further confirming the robustness of asteroseismically-determined physical parameters of stars when individual frequencies of oscillation are available. Baptised as the Kepler dwarfs LEGACY sample, these stars are the solar-like oscillators with the best asteroseismic properties available for at least another decade. All data used in this analysis and the resulting stellar parameters are made publicly available for the community., Comment: 23 pages, 14 figures, ApJ, accepted

Binary star systems are important for understanding stellar structure and evolution, and are especially useful when oscillations can be detected and analysed with asteroseismology. However, only four systems are known in which solar-like oscillations are detected in both components. Here, we analyse the fifth such system, HD 176465, which was observed by Kepler. We carefully analysed the system's power spectrum to measure individual mode frequencies, adapting our methods where necessary to accommodate the fact that both stars oscillate in a similar frequency range. We also modelled the two stars independently by fitting stellar models to the frequencies and complementary parameters. We are able to cleanly separate the oscillation modes in both systems. The stellar models produce compatible ages and initial compositions for the stars, as is expected from their common and contemporaneous origin. Combining the individual ages, the system is about 3.0$\pm$0.5 Gyr old. The two components of HD 176465 are young physically-similar oscillating solar analogues, the first such system to be found, and provide important constraints for stellar evolution and asteroseismology., Comment: Accepted for publication in Astronomy and Astrophysics. 16 pages, 10 figures and 8 tables

We present the first quick look analysis of data from nine {\it AstroSat}'s LAXPC observations of GRS 1915+105 during March 2016 when the source had the characteristics of being in Radio-quiet $\chi$ class. We find that a simple empirical model of a disk blackbody emission, with Comptonization and a broad Gaussian Iron line can fit the time averaged 3--80 keV spectrum with a systematic uncertainty of 1.5\% and a background flux uncertainty of 4\%. A simple deadtime-corrected Poisson noise level spectrum matches well with the observed high frequency power spectra till 50 kHz and as expected the data show no significant high frequency ($> 20$ Hz) features. Energy dependent power spectra reveal a strong low frequency (2 - 8 Hz) Quasi-periodic oscillation (LFQPO) and its harmonic along with broad band noise. The QPO frequency changes rapidly with flux (nearly 4 Hz in ~ 5 hours). With increasing QPO frequency, an excess noise component appears significantly in the high energy regime (> 8 keV). At the QPO frequencies, the time-lag as a function of energy has a non-monotonic behavior such that the lags decrease with energy till about 15 -20 keV and then increase for higher energies. These first look results benchmark the performance of LAXPC at high energies and confirms that its data can be used for more sophisticated analysis such as flux or frequency-resolved spectro-timing studies., Comment: 9 pages, 11 figures

Context: Detailed oscillation spectra comprising individual frequencies for numerous solar-type stars and red giants are or will become available. These data can lead to a precise characterisation of stars. Aims: Our goal is to test and compare different methods for obtaining stellar properties from oscillation frequencies and spectroscopic constraints, in order to evaluate their accuracy and the reliability of the error bars. Methods: In the context of the SpaceInn network, we carried out a hare-and-hounds exercise in which one group produced "observed" oscillation spectra for 10 artificial solar-type stars, and various groups characterised these stars using either forward modelling or acoustic glitch signatures. Results: Results based on the forward modelling approach were accurate to 1.5 % (radius), 3.9 % (mass), 23 % (age), 1.5 % (surface gravity), and 1.8 % (mean density). For the two 1 Msun stellar targets, the accuracy on the age is better than 10 % thereby satisfying PLATO 2.0 requirements. The average accuracies for the acoustic radii of the base of the convection zone, the He II ionisation, and the Gamma_1 peak were 17 %, 2.4 %, and 1.9 %, respectively. Glitch fitting analysis seemed to be affected by aliasing problems for some of the targets. Conclusions: Forward modelling is the most accurate approach, but needs to be complemented by model-independent results from, e.g., glitch analysis. Furthermore, global optimisation algorithms provide more robust error bars., Comment: 29 pages, 13 figures, 20 tables, accepted in A&A

The advent of space-based observatories such as CoRoT and Kepler has enabled the testing of our understanding of stellar evolution on thousands of stars. Evolutionary models typically require five input parameters, the mass, initial Helium abundance, initial metallicity, mixing- length (assumed to be constant over time), and the age to which the star must be evolved. Some of these parameters are also very useful in characterizing the associated planets and in studying galactic archaeology. How to obtain these parameters from observations rapidly and accurately, specifically in the context of surveys of thousands of stars, is an outstanding ques- tion, one that has eluded straightforward resolution. For a given star, we typically measure the effective temperature and surface metallicity spectroscopically and low-degree oscillation frequencies through space observatories. Here we demonstrate that statistical learning, using artificial neural networks, is successful in determining the evolutionary parameters based on spectroscopic and seismic measurements. Our trained networks show robustness over a broad range of parameter space, and critically, are entirely computationally inexpensive and fully automated. We analyze the observations of a few stars using this method and the results com- pare well to inferences obtained using other techniques. This method is both computationally cheap and inferentially accurate, paving the way for analyzing the vast quantities of stellar observations from past, current, and future missions., Comment: Submitted. 9 pages, 7 Figures, 1 Table, MNRAS

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., Comment: 8 pages (ApJ style), 5 figures, accepted for publication in the Astrophysical Journal

We present the first detections by the NASA K2 Mission of oscillations in solar-type stars, using short-cadence data collected during K2 Campaign\,1 (C1). We understand the asteroseismic detection thresholds for C1-like levels of photometric performance, and we can detect oscillations in subgiants having dominant oscillation frequencies around $1000\,\rm \mu Hz$. Changes to the operation of the fine-guidance sensors are expected to give significant improvements in the high-frequency performance from C3 onwards. A reduction in the excess high-frequency noise by a factor of two-and-a-half in amplitude would bring main-sequence stars with dominant oscillation frequencies as high as ${\simeq 2500}\,\rm \mu Hz$ into play as potential asteroseismic targets for K2., Comment: Accepted for publication in PASP; 16 pages, 2 figures

Measurements of oscillation frequencies of the Sun and stars can provide important independent constraints on their internal structure and dynamics. Seismic models of these oscillations are used to connect structure and rotation of the star to its resonant frequencies, which are then compared with observations, the goal being that of minimizing the difference between the two. Even in the case of the Sun, for which structure models are highly tuned, observed frequencies show systematic deviations from modeled frequencies, a phenomenon referred to as the "surface term." The dominant source of this systematic effect is thought to be vigorous near-surface convection, which is not well accounted for in both stellar modeling and mode-oscillation physics. Here we bring to bear the method of homogenization, applicable in the asymptotic limit of large wavelengths (in comparison to the correlation scale of convection), to characterize the effect of small-scale surface convection on resonant-mode frequencies in the Sun. We show that the full oscillation equations, in the presence of temporally stationary 3-D flows, can be reduced to an effective "quiet-Sun" wave equation with altered sound speed, Br\"{u}nt--V\"{a}is\"{a}la frequency and Lamb frequency. We derive the modified equation and relations for the appropriate averaging of three dimensional flows and thermal quantities to obtain the properties of this effective medium. Using flows obtained from three dimensional numerical simulations of near-surface convection, we quantify their effect on solar oscillation frequencies, and find that they are shifted systematically and substantially. We argue therefore that consistent interpretations of resonant frequencies must include modifications to the wave equation that effectively capture the impact of vigorous hydrodynamic convection., Comment: 9 pages, 4 figures

Aims. Forbush decrease (FD) is a transient decrease followed by a gradual recovery in the observed galactic cosmic ray intensity. We seek to understand the relationship between the FDs and near-Earth interplanetary magnetic field (IMF) enhancements associated with solar coronal mass ejections (CMEs). Methods. We use muon data at cutoff rigidities ranging from 14 to 24 GV from the GRAPES-3 tracking muon telescope to identify FD events. We select those FD events that have a reasonably clean profile, and magnitude > 0.25%. We use IMF data from ACE/WIND spacecrafts. We look for correlations between the FD profile and that of the one hour averaged IMF. We ask if the diffusion of high energy protons into the large scale magnetic field is the cause of the lag observed between the FD and the IMF. Results. The enhancement of the IMF associated with FDs occurs mainly in the shock-sheath region, and the turbulence level in the magnetic field is also enhanced in this region. The observed FD profiles look remarkably similar to the IMF enhancement profiles. The FDs typically lag the IMF enhancement by a few hours. The lag corresponds to the time taken by high energy protons to diffuse into the magnetic field enhancement via cross-field diffusion. Conclusions. Our findings show that high rigidity FDs associated with CMEs are caused primarily by the cumulative diffusion of protons across the magnetic field enhancement in the turbulent sheath region between the shock and the CME., Comment: accepted in A&A

There are regions in stars, such as ionization zones and the interface between radiative and convective regions, that cause a localized sharp variation in the sound speed. These are known as "acoustic glitches". Acoustic glitches leave their signatures on the oscillation frequencies of stars, and hence these signature can be used as diagnostics of these regions. In particular, the signature of these glitches can be used as diagnostics of the position of the second helium ionization zone and that of the base of the envelope convection zone. With the help of stellar models we study the properties of these acoustic glitches in main-sequence stars. We find that the acoustic glitch due to the helium ionization zone does not correspond to the dip in the adiabatic index \Gamma_1 caused by the ionization of HeII, but to the peak in \Gamma_1 between the HeI and HeII ionization zones. We find that it is easiest to study the acoustic glitch due to the helium ionization zone in stars with masses in the range 0.9--1.2 M_\odot., Comment: Accepted for publication in ApJ

Cosmic ray Forbush decreases (FDs) are usually thought to be due to Earth-directed coronal mass ejections (CMEs) from the Sun and their associated shocks. When CMEs and their shocks reach the Earth, they cause magnetic field compressions. We seek to understand the relation between these magnetic field compressions and FDs at rigidities between 12 and 42 GV using data from the GRAPES-3 instrument at Ooty. We find that the shapes of the Forbush decrease profiles show a startling similarity to that of the magnetic field compression in the near-Earth IP medium. We seek to understand the implications of this interesting result., Comment: International Symposium on Solar Terrestrial Physics 2012, IISER Pune

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

Solar-like oscillations have been observed by Kepler and CoRoT in many solar-type stars, thereby providing a way to probe the stars using asteroseismology. We provide the mode linewidths and mode heights of the oscillations of various stars as a function of frequency and of effective temperature. We used a time series of nearly two years of data for each star. The 23 stars observed belong to the simple or F-like category. The power spectra of the 23 main-sequence stars were analysed using both maximum likelihood estimators and Bayesian estimators, providing individual mode characteristics such as frequencies, linewidths, and mode heights. We study the source of systematic errors in the mode linewidths and mode heights, and we present a way to correct these errors with respect to a common reference fit. Using the correction, we could explain all sources of systematic errors, which could be reduced to less than $\pm$15% for mode linewidths and heights, and less than $\pm$5% for amplitude, when compared to the reference fit. The effect of a different estimated stellar background and a different estimated splitting will provide frequency-dependent systematic errors that might affect the comparison with theoretical mode linewidth and mode height, therefore affecting the understanding of the physical nature of these parameters. All other sources of relative systematic errors are less dependent upon frequency. We also provide the dependence of the so-called linewidth dip, in the middle of the observed frequency range, as a function of effective temperature. We show that the depth of the dip decreases with increasing effective temperature. The dependence of the dip on effective temperature may imply that the mixing length parameter $\alpha$ or the convective flux may increase with effective temperature., Comment: Accepted by A&A, 38 pages, 35 figures, 26 tables

For the very best and brightest asteroseismic solar-type targets observed by Kepler, the frequency precision is sufficient to determine the acoustic depths of the surface convective layer and the helium ionization zone. Such sharp features inside the acoustic cavity of the star, which we call acoustic glitches, create small oscillatory deviations from the uniform spacing of frequencies in a sequence of oscillation modes with the same spherical harmonic degree. We use these oscillatory signals to determine the acoustic locations of such features in 19 solar-type stars observed by the Kepler mission. Four independent groups of researchers utilized the oscillation frequencies themselves, the second differences of the frequencies and the ratio of the small and large separation to locate the base of the convection zone and the second helium ionization zone. Despite the significantly different methods of analysis, good agreement was found between the results of these four groups, barring a few cases. These results also agree reasonably well with the locations of these layers in representative models of the stars. These results firmly establish the presence of the oscillatory signals in the asteroseismic data and the viability of several techniques to determine the location of acoustic glitches inside stars., Comment: Accepted for publication in ApJ