ABSTRACT
Magnetohydrodynamic dynamo modelling shows that the large-scale solar meridional plasma flow plays an important role in governing the dynamics of the sunspot cycle. Observations indicate ...that meridional flow velocities at each solar latitude and depth vary over time and are asymmetric across the equator. Here, using helioseismic observations we explore the temporal variation in the hemispherical asymmetry of near-surface residual (time-varying) component of the Sun’s meridional flow velocity. The meridional flow velocities obtained from Global Oscillation Network Group (GONG) and Helioseismic and Magnetic Imager (HMI) onboard Solar Dynamics Observatory (SDO) ring-diagram pipelines are used in this work. Our data set covers the declining phase of cycle 23 and cycle 24 (from July 2001 till December 2018) and the flow velocities are poleward for the observed depth range. We observe a time delayed anticorrelation between the hemispherical asymmetry in near-surface meridional flow velocities and the sunspot cycle quantified in terms of magnetic flux and sunspot number. Interestingly, asymmetry in meridional flow velocity precedes the asymmetry in sunspot cycle by 3.1–3.5 yr. We propose that meridional flow asymmetry is a precursor of asymmetry in hemispherical cycle strength. The symmetric component of meridional flow is observed to be positively correlated with the corresponding symmetric components of the magnetic cycle, also with a time delay. Our analysis sets important constraints on theories for the origin of meridional plasma flow asymmetries and its temporal variations and is relevant for understanding the role of plasma flux transport processes in determining hemispheric asymmetry in the sunspot cycle.
Abstract
The Galactic microquasar GRS 1915+105 shows rich variability that is categorized into different classes. In this paper, we report the temporal and spectral analysis of GRS 1915+105 to study ...the properties of the accretion flow when the light curve shows
θ
class variability. For this purpose, we use the Large Area X-ray Proportional Counter data from the Target of Opportunity observations of India’s first multiwavelength astronomy satellite AstroSat. The
θ
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 the first two orbits (02345 and 02346) on 2016 March 4. In both 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 energy flux is also enhanced. From the evolution of the spectra, we determine the evolution of the accretion flow parameters in these two observations. Fitting the spectra with the transonic flow solution-based two-component advective flow (TCAF) model in the 4–25 keV energy band shows that the Keplerian disk accretion rate increases with the increase in 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.
Accurate inferences of solar meridional flow are crucial for understanding solar dynamo processes. 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 can 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. 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 Solar Dynamics Observatory/Helioseismic and Magnetic Imager 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 onto 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 a return flow occurring at depths below 0.78 R .
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 min between the 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 at ∼5 Hz. However, we did not find any evidence of kilo-hertz quasi-periodic oscillations and/or X-ray burst oscillations, perhaps due to the hard spectral state of the source during this observation.
ABSTRACT
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 ∼125 ks 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 δf ∼ 12 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). Broad-band spectra in the range 0.7–23 keV, obtained with Soft X-ray Telescope and Large Area X-ray Proportional Counter, could be fitted well with combination of a thermal Comptonization and a multicolour disc component model. During AstroSat observation, the source was in the soft-intermediate state (SIMS) as observed with 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 (Γ ∼ 2.1–2.4 and Tmax ∼ 0.6–0.8 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 ∼25: 1. We discuss the implications of our findings and comment on the physical origin of these exotic variabilities.
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 signatures can be used as diagnostics of these regions. In particular, the signatures of these glitches can be used as diagnostics for 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 sub(1) caused by the ionization of He II, but to the peak in Gamma sub(1) between the He I and He II ionization zones. We find that it is easiest to study the acoustic glitch that is due to the helium ionization zone in stars with masses in the range 0.9-1.2 M sub(middot in circle).
We present the discovery of HD 221416 b, the first transiting planet identified by the Transiting Exoplanet Survey Satellite (TESS) for which asteroseismology of the host star is possible. HD 221416 ...b (HIP 116158, TOI-197) is a bright (V = 8.2 mag), spectroscopically classified subgiant that oscillates with an average frequency of about 430 Hz 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 (R = 2.943 0.064 R ), mass (M = 1.212 0.074 M ), 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" (Rp = 9.17 0.33 R⊕) with an orbital period of ∼14.3 days, irradiance of F = 343 24 F⊕, and moderate mass (Mp = 60.5 5.7 M⊕) and density ( p = 0.431 0.062 g cm−3). The properties of HD 221416 b show that the host-star metallicity-planet mass correlation found in sub-Saturns (4-8 R⊕) 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%, HD 221416 b is one of the best characterized Saturn-size 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.
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 yr 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 of 0.231 to 0.251 and that of 16 Cyg B lies in the range of 0.218 to 0.266.
Context. Solar-like oscillations have been observed by Kepler and CoRoT in several solar-type stars, thereby providing a way to probe the stars using asteroseismology Aims. We provide the mode ...frequencies of the oscillations of various stars required to perform a comparison with those obtained from stellar modelling. Methods. We used a time series of nine months of data for each star. The 61 stars observed were categorised in three groups: simple, F-like, and mixed-mode. The simple group includes stars for which the identification of the mode degree is obvious. The F-like group includes stars for which the identification of the degree is ambiguous. The mixed-mode group includes evolved stars for which the modes do not follow the asymptotic relation of low-degree frequencies. Following this categorisation, the power spectra of the 61 main-sequence and subgiant stars were analysed using both maximum likelihood estimators and Bayesian estimators, providing individual mode characteristics such as frequencies, linewidths, and mode heights. We developed and describe a methodology for extracting a single set of mode frequencies from multiple sets derived by different methods and individual scientists. We report on how one can assess the quality of the fitted parameters using the likelihood ratio test and the posterior probabilities. Results. We provide the mode frequencies of 61 stars (with their 1-σ error bars), as well as their associated échelle diagrams.
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 was 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 KIC 7341231, 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 few decades.
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