• Kumar

Articles written in Journal of Astrophysics and Astronomy

• Observation of hysteresis between solar activity indicators andp-mode frequency shifts for solar cycle 22

Using intermediate degreep-mode frequency data sets for solar cycle 22, we find that the frequency shifts and magnetic activity indicators show a &amp;#x201C;hysteresis&amp;#x201D; phenomenon. It is observed that the magnetic indices follow different paths for the ascending and descending phases of the solar cycle while for radiative indices, the separation between the paths are well within the error limits.

• Development of a three channel photometer for UPSO, Naini Tal

The design and performance of a portable three channel photometer installed at the Uttar Pradesh State Observatory (UPSO), Naini Tal is described. The photometer is modular and the whole unit can be disassembled as individual channels such that the system can also be used as a single channel or two channel photometer. The system also has provision to monitor a guide star. The instrument was put into operation since November 1999 on the 1m Sampurnanand telescope at UPSO, Naini Tal. Since then, it is used extensively for the ‘Survey of rapidly oscillating Ap (roAp) stars in the northern sky’ from UPSO. Observational results using this new photometer in its initial phase of operation are discussed. The advantage of having continuous sky measurement is demonstrated.

• Data acquisition, control, communication and computation system of solar X-ray spectrometer (SOXS) mission

The Solar X-ray Spectrometer (SOXS) mission onboard GSAT-2 Indian Spacecraft was launched on 08 May 2003 using GSLV-D2 rocket by Indian Space Research Organization (ISRO). SOXS aims to study solar flares, which are the most violent and energetic phenomena in the solar system, in the energy range of 4–56 keV with high spectral and temporal resolution. SOXS employs state-of-the-art semiconductor devices, viz., Si-Pin and CZT detectors to achieve sub-keV energy resolution requirements. In this paper, we present an overview of data acquisition, control, communication and computation of low energy payload of the SOXS mission.

• Analysis of enhanced velocity signals observed during solar flares

Solar flares are known to release a large amount of energy. It is believed that the flares can excite velocity oscillations in active regions. We report here the changes in velocity signals in three active regions which have produced large X-class flares. The enhanced velocity signals appeared during the rise time of the GOES soft X-ray flux. These signals are located close to the vicinity of the hard X-ray source regions as observed with RHESSI. The power maps of the active region show enhancement in the frequency regime 5–6.5 mHz, while there is feeble or no enhancement of these signals in 2–4 mHz frequency band. High energy particles with sufficient momentum seem to be the cause for these observed enhanced velocity signals.

• Damping of Slow Magnetoacoustic Waves in an Inhomogeneous Coronal Plasma

We study the propagation and dissipation of slow magnetoacoustic waves in an inhomogeneous viscous coronal loop plasma permeated by uniform magnetic field. Only viscosity and thermal conductivity are taken into account as dissipative processes in the coronal loop. The damping length of slow-mode waves exhibit varying behaviour depending upon the physical parameters of the loop in an active region AR8270 observed by TRACE. The wave energy flux associated with slow magnetoacoustic waves turns out to be of the order of 106 erg cm-2 s-1 which is high enough to replace the energy lost through optically thin coronal emission and the thermal conduction belowto the transition region. It is also found that only those slow-mode waves which have periods more than 240 s provide the required heating rate to balance the energy losses in the solar corona. Our calculated wave periods for slow-mode waves nearly match with the oscillation periods of loop observed by TRACE.

• Solar and Interplanetary Disturbances causing Moderate Geomagnetic Storms

The effect of solar and interplanetary disturbances on geomagnetospheric conditions leading to 121 moderate geomagnetic storms (MGS) have been investigated using the neutron monitor, solar geophysical and interplanetary data during the period 1978–99. Further, the duration of recovery phase has been observed to be greater than the duration of main phase in most of the cases of MGS. It has further been noted that Ap-index increases on sudden storm commencement (SSC) day than its previous day value and acquires maximum value on the day of maximum solar activity. Generally, the decrease in cosmic ray (CR) intensity and Dst begins few hours earlier than the occurrence of MGS at Earth. Furthermore, negative Bz pointing southward plays a key causal role in the occurrence of MGS and the magnitude and the duration of Bz and Bav also play a significant role in the development of MGS. The solar features H𝛼, X-ray solar flares and active prominences and disappearing filaments (APDFs) which have occurred within lower helio-latitudinal/helio-longitudinal zones produce larger number of MGS. Solar flares seem to be the major cause for producing MGS.

• Development of a Low-order Adaptive Optics System at Udaipur Solar Observatory

A low-order Adaptive Optics (AO) system is being developed at the Udaipur Solar Observatory and we present in this paper the status of the project, which includes the image stabilization system and calibration of wavefront sensor and deformable mirror. The image stabilization system comprises of a piezo driven tip-tilt mirror, a high speed camera (955 fps), a frame grabber system for sensing the overall tilt and a Linux based Intel Pentium 4 control computer with Red Hat Linux OS. The system operates under PID control. In the closed loop, an rms image motion of 0.1–0.2 arcsec was observed with the improvement factor varying from 10–20 depending on the external conditions. Error rejection bandwidth of the system at 0 dB is 80–100 Hz. In addition to that, we report the on-going efforts in the calibration of lenslet array and deformable mirror for sensing and correcting the local tilt of the wavefront.

• X-ray Observations of Eight Young Open Star Clusters: I. Membership and X-ray Luminosity

We present a detailed investigation of X-ray source contents of eight young open clusters with ages between 4 to 46 Myr using archival X-ray data from XMM-NEWTON. The probable cluster memberships of the X-ray sources have been established on the basis of multi-wavelength archival data, and samples of 152 pre-main sequence (PMS) low mass (&lt; 2𝑀), 36 intermediate mass (2-10𝑀) and 16 massive (&gt; 10𝑀) stars have been generated. X-ray spectral analyses of high mass stars reveal the presence of high temperature plasma with temperature &lt; 2 keV, and mean 𝐿𝑋/𝐿bol of 10-6.9. In the case of PMS low mass stars, the plasma temperatures have been found to be in the range of 0.2 keV to 3 keV with a median value of ∼ 1.3 keV, with no significant difference in plasma temperatures during their evolution from 4 to 46 Myr. The X-ray luminosity distributions of the PMS low mass stars have been found to be similar in the young star clusters under study. This may suggest a nearly uniform X-ray activity in the PMS low mass stars of ages ∼ 4–14 Myr. These observed values of 𝐿𝑋/𝐿bol are found to have a mean value of 10-3.6 ± 0.4, which is below the X-ray saturation level. The 𝐿𝑋/𝐿bol values for the PMS low mass stars are well correlated with their bolometric luminosities, that implies its dependence on the internal structure of the low mass stars. The difference between the X-ray luminosity distributions of the intermediate mass stars and the PMS low mass stars has not been found to be statistically significant. Their 𝐿𝑋/𝐿bol values, however have been found to be significantly different from each other with a confidence level greater than 99.999% and the strength of X-ray activity in the intermediate mass stars is found to be lower compared to the low mass stars. However, the possibility of X-ray emission from the intermediate mass stars due to a low mass star in close proximity of the intermediate mass star can not be ruled out.

• X-ray Flares Observed from Six Young Stars Located in the Region of Star Clusters NGC 869 and IC 2602

We present, for the first time, an analysis of seven intense X-ray flares observed from six stars (LAV 796, LAV 1174, SHM2002 3734, 2MASS 02191082+5707324, V553 Car, V557 Car). These stars are located in the region of young open star clusters NGC 869 and IC 2602. These flares detected in the XMM-Newton data show a rapid rise (10–40 min) and a slow decay (20–90 min). The X-ray luminosities during the flares in the energy band 0.3–7.5 keV are in the range of 1029.9 to 1031.7 erg s-1. The strongest flare was observed with the ratio ∼ 13 for count rates at peak of the flare to the quiescent intensity. The maximum temperature during the flares has been found to be ∼ 100 MK. The semi-loop lengths for the flaring loops are estimated to be of the order of 1010 cm. The physical parameters of the flaring structure, the peak density, pressure and minimum magnetic field required to confine the plasma have been derived and found to be consistent with flares from pre-main sequence stars in the Orion and the Taurus-Auriga-Perseus region.

• In-orbit Performance of UVIT and First Results

The performance of the ultraviolet telescope (UVIT) on-board AstroSat is reported. The performance in orbit is also compared with estimates made from the calibrations done on the ground. The sensitivity is found to be within ∼15% of the estimates, and the spatial resolution in the NUV is found to exceed significantly the design value of 1.8′′ and it is marginally better in the FUV. Images obtained from UVIT are presented to illustrate the details revealed by the high spatial resolution. The potential of multi-band observations in the ultraviolet with high spatial resolution is illustrated by some results.

• Early In-orbit Performance of Scanning Sky Monitor Onboard AstroSat

We report the in-orbit performance of Scanning Sky Monitor (SSM) onboard AstroSat. The SSM operates in the energy range 2.5 to 10 keV and scans the sky to detect and locate transient X-ray sources. This information of any interesting phenomenon in the X-ray sky as observed by SSM is provided to the astronomical community for follow-up observations. Following the launch of AstroSat on 28th September, 2015, SSM was commissioned on October 12th, 2015. The first power ON of the instrument was with the standard X-ray source, Crab in the field-of-view. The first orbit data revealed the basic expected performance of one of the detectors of SSM, SSM1. Following this in the subsequent orbits, the other detectors were also powered ON to find them perform in good health. Quick checks of the data from the first few orbits revealed that the instrument performed with the expected angular resolution of 12’ × 2.5 and effective area in the energy range of interest. This paper discusses the instrument aspects along with few on-board results immediately after power ON.

• Planning and Scheduling of Payloads of AstroSat During Initial and Normal Phase Observations

On 28th September 2015, India launched its first astronomical space observatory AstroSat, successfully. AstroSat carried five astronomy payloads, namely, (i) Cadmium Zinc Telluride Imager (CZTI), (ii) Large Area X-ray Proportional Counter (LAXPC), (iii) Soft X-ray Telescope (SXT), (iv) Ultra Violet Imaging Telescope (UVIT) and (v) Scanning Sky Monitor (SSM) and therefore, has the capability to observe celestial objects in multi-wavelength. Four of the payloads are co-aligned along the positive roll axis of the spacecraft and the remaining one is placed along the positive yaw axis direction. All the payloads are sensitive to bright objects and specifically, require avoiding bright Sun within a safe zone of their bore axes in orbit. Further, there are other operational constraints both from spacecraft side and payloads side which are to be strictly enforced during operations. Even on-orbit spacecraft manoeuvres are constrained to about two of the axes in order to avoid bright Sun within this safe zone and a special constrained manoeuvre is exercised during manoeuvres. The planning and scheduling of the payloads during the Performance Verification (PV) phase was carried out in semi-autonomous/manual mode and a complete automation is exercised for normal phase/Guaranteed Time Observation (GuTO) operations. The process is found to be labour intensive and several operational software tools, encompassing spacecraft sub-systems, on-orbit, domain and environmental constraints, were built-in and interacted with the scheduling tool for appropriate decision-making and science scheduling. The procedural details of the complex scheduling of a multi-wavelength astronomy space observatory and their working in PV phase and in normal/GuTO phases are presented in this paper.

• Classical Orbital Paramagnetism in Non-equilibrium Steady State

We report the results of our numerical simulation of classical-dissipative dynamics of a charged particle subjected to a non-Markovian stochastic forcing. We find that the system develops a steady-state orbital magnetic moment in the presence of a static magnetic field. Very significantly, the sign of the orbital magnetic moment turns out to be paramagnetic for our choice of parameters, varied over a wide range. This is shown specifically for the case of classical dynamics driven by a Kubo–Anderson type non-Markovian noise. Natural spatial boundary condition was imposed through (1) a soft (harmonic) confining potential, and (2) a hard potential, approximating a reflecting wall. There was no noticeable qualitative difference. What appears to be crucial to the orbital magnetic effect noticed here is the non-Markovian property of the driving noise chosen. Experimental realization of this effect on the laboratory scale, and its possible implications are briefly discussed. We would like to emphasize that the above steady-state classical orbital paramagnetic moment complements, rather than contradicts the Bohr–van Leeuwen (BvL) theorem on the absence of classical orbital diamagnetism in thermodynamic equilibrium.

• High energy power-law tail in X-ray binaries and bulk Comptonization due to an outflow from a disk

We study the high energy power-law tail emission of X-ray binaries (XRBs) by a bulk Comptonization process which is usually observed in the very high soft (VHS) state of black hole (BH) XRBs and the high soft (HS) state of the neutron star (NS) and BH XRBs. Earlier, to generate the power-law tail in bulk Comptonization framework, a free-fall converging flow into BH or NS had been considered as a bulk region. In this work, for a bulk region we consider mainly an outflow geometry from the accretion disk which is bounded by a torus surrounding the compact object. We have two choices for an outflow geometry: (i) collimated flowand (ii) conical flow of opening angle $\theta_b$ and the axis is perpendicular to the disk. We also consider an azimuthalvelocity of the torus fluids as a bulk motion where the fluids are rotating around the compact object (a torus flow). We find that the power-law tail can be generated in a torus flow having large optical depth and bulk speed (>0.75$c$), and in conical flow with $\theta_b$ > $\sim 30^{\circ}$ for a low value of Comptonizing medium temperature. Particularly, in conical flow the low opening angle is more favourable to generate the power-law tail in both theHS state and the VHS state. We notice that when the outflow is collimated, then the emergent spectrum does not have power-law component for a low Comptonizing medium temperature.

• Identifying Li-rich giants from low-resolution spectroscopic survey

In this paper we discuss our choice of a large unbiased sample used for the survey of red giant branch stars for finding Li-rich K giants, and the method used for identifying Li-rich candidates using low-resolution spectra. The sample has 2000 giants within a mass range of 0.8 to 3.0$M_{\odot}$. Sample stars were selected from the Hipparcos catalogue with colour (B–V) and luminosity (L/L$_{\odot}$) in such way that the sample covers RGB evolution from its base towards RGB tip passing through first dredge-up and luminosity bump. Low-resolution (R $\approx$ 2000, 3500, 5000) spectra were obtained for all sample stars. Using core strength ratios of lines at Li I 6707{\AA} and its adjacent line Ca I 6717{\AA} we successfully identified 15 K giants with A(Li) > 1.5 dex, which are defined as Li-rich K giants. The results demonstrate the usefulness of low-resolution spectra to measure Li abundance and identify Li-rich giants from a large sample of stars in relatively shorter time periods.

• Halo orbit of regularized circular restricted three-body problem with radiation pressure and oblateness

In this paper, computation of the halo orbit for the KS-regularized photogravitational circular restricted three-body problem is carried out. This work extends the idea of Srivastava et al. (Astrophys. Space Sci. 362: 49, 2017) which only concentrated on the (i) regularization of the 3D-governing equations of motion, and (ii) validation of the modeling for small out-of-plane amplitude ($A_z = 110000$ km) assuming the third order analytical approximation as an initial guess with and without differential correction. This motivated us to compute the halo orbits for the large out-of-plane amplitudes and to study their stability analysis for the regularized motion. The stability indices are described as a function of out-of-plane amplitude, mass reduction factor and oblateness coefficient. Three different Sun–planet systems: the Sun–Earth, Sun–Mars and the Sun–Jupiter are chosen in this study. Stable halo orbits do not exist around the $L_1$ point, however, around the $L_2$ point stable halo orbits are found for the considered systems.

• Application of Chebyshev collocation method for relocating of spacecrafts in Hill’s frame

In this study, the Chebyshev collocation method is used for solving the spacecraft relative motion of equations in Hill’s frame. Three different models of governing equations of relative motion (M1, M2, and M3) are considered and the maneuver cost required moving the spacecraft from one state to another is computed in the form of delta velocity at the first terminal point as a function of time of flight (TOF) and inter-satellite distance (ISD). A quantitative as well as qualitative difference is observed in the maneuver cost with the inclusion ofradial and/or out of plane separation in along track separation of chaser. Also, a relative comparison of path profiles is made by considering M1, M2 and M3 models. Path profiles for M3 model are found close to M2 model for short intervals for a fixed ISD, whereas path profiles for M2 and M3 do not match even for smallvalues of ISD for a fixed but long TOF. Path profiles for M1 models match to M2 model for very low values of target orbit eccentricities.

• Optical detection of a GMRT-detected candidate high-redshift radio galaxy with 3.6-m Devasthal optical telescope

We report optical observations of TGSS J1054 $+$ 5832, a candidate high-redshift ($z = 4.8 \pm 2$) steep-spectrum radio galaxy, in $r$ and $i$ bands, using the faint object spectrograph and camera mounted on 3.6-m Devasthal Optical Telescope (DOT). The source previously detected at 150 MHz from Giant Meterwave Radio Telescope (GMRT) and at 1420 MHz from Very Large Array has a known counterpart in near-infrared bands with $K$-band magnitude of AB 22. The source is detected in $i$-band with AB24.3 $\pm$ 0.2 magnitude in theDOT images presented here. The source remains undetected in the $r$-band image at a 2.5$\sigma$ depth of AB 24.4 mag over an $1.2^{\prime\prime}\times 1.2^{\prime\prime}$ aperture. An upper limit to $i−K$ color is estimated to be $\sim$2.3, suggesting youthfulness of the galaxy with active star formation. These observations highlight the importance and potential of the 3.6-mDOT for detections of faint galaxies.

• Modified Chaplygin gas with bulk viscous cosmology in FRW (2$+$1)-dimensional spacetime

In this paper we study the bulk viscous cosmology by considering modified Chaplygin gas in the framework of ($2 + 1$)-dimensional spacetime. For this we consider various form of bulk viscosity coefficient $\zeta$ and then obtain the physical parameters energy density $\rho$, Hubble and deceleration parameters, $H$ and $\rho$, respectively. Finally we discuss the stability of the model by using the speed of sound.

• Characteristics of SEPs during solar cycles 21–24

The study of the solar energetic particle events (SEPs) and their association with solar flares and other activities are very crucial to understand the space weather. Keeping this in view, here we present the study of SEPs (intensity $\geq$10 pfu) during the solar cycles 21–24 (1976–2017) in >10 MeV energy channels associated with solar flares. For our analysis, we have used the data from different instruments onboard SOHO satellite. We have examined the flare size, source location, coronal mass ejections (CMEs) characteristics of associated SEPs. About 31% and 69% of the SEPs were originated from the eastern and western solar hemisphere, respectively. The average CME speed and width were 1238 km s$^{−1}$ and 253$^{\circ}$, respectively. About58% SEPs were associated with halo CMEs and 42% of SEPs associated with CMEs width varying from 10$^{\circ}$ to 250$^{\circ}$, respectively.

• Prediction of meteoroid stream structure based on meteoroid fragmentation

Every day, large number of meteoroids enter the Earth’s atmosphere and deposit their mass either in atomic form or in ionic form depending on whether it has undergone ablation or fragmentation. The heavier meteoroids undergo fragmentation while the lighter ones are more prone to ablate. In this paper, wewould like to speculate meteoroid stream structure of Leonid meteor shower based on fragmenting meteoroids. A 23 revolutions old meteoroid trail left behind by the comet 55P/Tempel-Tuttle in the year 1213 AD, which instigated Leonid meteor shower in the year 2010 is considered for our study. We have calculatedmass of the meteoroids, echo durations and percentage of fragmentation. From the observed echo durations of meteoroids, estimated masses and from the percentage of fragmentation, we visualize the stream structure to be like the lighter particles wrapping up the heavier ones. The results we draw from these three different studies are matching with each other. To our knowledge, we are the first to speculate on the meteor stream structure based on fragmentation and making it a new tool in meteor stream evolution. Based on echo durations, it has been observed that 72% of the activity during the shower is contributed by lighter particles of the stream. It is found that about 20% of the meteoroids have undergone fragmentation indicating the minimal role of heavier particles (>10$^{-6}$ g) during Leonid Meteor Shower (LMS). The masses of the meteors are estimated to be in the range of 10$^{-10}$–10$^{-5}$ g.

• Detection of PAH and nbL features in planetary nebulae NGC 7027 and BD $+$30$^{\circ}$ 3639 with TIRCAM2 instrument on 3.6 m DOT

High resolution infrared imaging observations of the young planetary nebulae (PNe) NGC 7027 and BD $+$30$^{\circ}$ 3639, taken with the newly installed TIFR near infrared camera-II (TIRCAM2) on 3.6 m Devasthal optical telescope (DOT), ARIES, Nainital, are being reported. The images are acquired in J, H, K,polycyclic aromatic hydrocarbon (PAH) and narrow-band L (nbL) filters. The observations show emission from warm dust and PAHs in the circumstellar shells. The imaging of the two objects are among the first observations in PAH and nbL bands using TIRCAM2 on DOT. The NGC 7027 images in all bands showsimilar elliptical morphology with $\sim$6$^{\prime\prime}$.7 and $\sim$4$^{\prime\prime}$.5 semi-major and semi-minor axes. Considering size up to 10% of peak value the nebula extends upto 800 from the central star revealing a multipolar evolution. The relatively cooler BD $+$30$^{\circ}$ 3639 shows a rectangular-ring shaped nebula. In J and H bands it shows an angular diameter of $\sim$8$^{\prime\prime}$, while a smaller $\sim$6$^{\prime\prime}$.9 size is observed in K, PAH and nbL bands. The 3.28 $\mu$m emission indicates presence of PAHs at about 6000 and 5000 AU from the central stars in NGC 7027 and BD $+$30$^{\circ}$ 3639 respectively. Analysis suggests domination of neutral PAHs in BD $+$30$^{\circ}$ 3639, while in NGC 7027 there is higher ionization and more processed PAH population.

• Observations with the 3.6-meter Devasthal optical telescope

The 3.6-meter Indo–Belgian Devasthal optical telescope (DOT) has been used for optical and nearinfrared (NIR) observations of celestial objects. The telescope has detected stars of $B=24.5 \pm 0.2$, $R = 24.6 \pm 0.12$ and $g= 25.2 \pm 0.2$ mag in exposure times of 1200, 4320 and 3600 s respectively. In one hour of exposure time, a distant galaxy of $24.3 \pm 0.2$ mag and point sources of $\sim$25 mag have been detected in the SDSS $i$ band. The NIR observations show that stars up to $J = 20\pm 0.1$, $H = 18.8 \pm 0.1$ and $K = 18.2 \pm 0.1$ mag can be detected in effective exposure times of 500, 550 and 1000 s respectively. The $nbL$ band sources brighter than $\sim$9.2 mag and strong ($\geq$0.4 Jy) $PAH$ emitting sources like Sh 2-61 can also be observed with the 3.6-meter DOT. A binary star with angular separation of 0:$''$4 has been resolved by the telescope. Sky images with sub-arcsec angular resolutions are observed with the telescope at wavelengths ranging from optical to NIR for a good fraction of observing time. The on-site performance of the telescope is found to be at par with the performance of other, similar telescopes located elsewhere in the world. Owing to the advantage of its geographicallocation, the 3.6-meter DOT can provide optical and NIR observations for a number of frontline galactic and extra-galactic astrophysical research problems, including optical follow-up of GMRT and AstroSat sources and optical transient objects.

• Evolution of lithium in low-mass giants: an observational perspective

The overabundance of lithium in low-mass red giants has been a topic of interest for over four decades. Low-mass stars are expected to destroy lithium gradually throughout their lifetimes. Against this expectation, about 1% of red giants in the Galaxy show anomalously large Li which, in the literature, areknown as lithium-rich giants. The advent of large-scale stellar surveys (LAMOST, GALAH, Kepler, Gaia) coupled with high-resolution spectra enabled to find important clues about Li enhancement origin in redgiants. These new studies suggest Li enhancement is mostly associated with the red clump region, post-Heflash. Here, we will describe our recent results along with current updates in the field.

• On sensitivity of the stability of equilibrium points with respect to the perturbations

The present investigation considers the effect of small perturbations given in the Coriolis and centrifugal forces on the location and stability of the equilibrium points in the Robe’s circular restricted three-body problem with non-spherical primary bodies. The felicitous equations of motion of $m_3$ are obtainedby taking into account the shapes of primaries $m_1$ and $m_2$, the full buoyancy force of the fluid which is filled inside $m_1$ of density $\rho_1$, the forces due to the gravitational attraction of the fluid and $m_2$. We assume that the massive body $m_1$ is an oblate spheroid and the $m_2$ a finite straight segment, and they move under a mutual gravitational attraction described by the Newton’s universal law of gravitation. In the present problem, $m_3$ is moving in the fluid and the rotating reference frame is used, its motion is bound to be affected by the perturbed Coriolis and centrifugal forces. In this attempt these effects along with the effects caused by the oblateness and length parameters $A$ and $l$ respectively, on the location and stability of the equilibrium pointsare observed. A pair of collinear equilibrium points $L_1$ and $L_2$ and infinite number of non-collinear equilibrium points are obtained. The stability of all the equilibrium points depends on the coefficients of their corresponding characteristic polynomials that are obtained with the help of linear variational equations.

• Novel payload planning and scheduling approaches implemented for AstroSat mission

AstroSat is an astronomical observatory from India which is in a low earth orbit of 650 km altitude with a 6-degree inclination and has been functioning successfully in orbit for the last 5 years. The simultaneous observation by the payloads allows imaging of celestial sources in various wavebands that has resulted in scientific achievements which are published in various forum. Uniqueness of its orbit, and scientific objectives calls for a different type of scheduling approach and a hands-free automated tasking of payloads, on this satellite. Several software architectures involving a Centralized data-basecalled Mission Control And Proposals (MCAP) with the Scheduler, AstroSat Scheduler TeRminus for PAyLoads (ASTRAL), AstroSat Scheduler Software for OpeRaTions (ASSORT), Command Sequence Generation (CSG), AstroSat Schedule Viewer (ASV), Astroviewer and AstroSat Long Term Planner(ASPlanner) were developed and used for celestial source observations. This paper describes briefly the scheduling approaches, their goals, utilization, lessons learnt and improvements carried out over the last 5 years.

• In-orbit performance of UVIT over the past 5 years

Over the last 5 years, UVIT has completed observations of more than 500 proposals with $\sim$800 unique pointings. In addition, regular planned monitoring observations have been made and from their analysis various key parameters related to in orbit performance of UVIT have been quantified. The sensitivities of the UV channels have remained steady indicating no effect of potential molecular contamination confirming the adequacy of all the protocols implemented for avoiding contamination. The quality of the PSF through the years confirms adequacy of thermal control measures. The early calibrations obtained during the Performance Verification (PV) phase have been further revised for more subtle effects. These include flat fields and detector distortions with greater precision. The operations of UVIT have also evolved through inorbit experience, e.g. tweaking of operational sequencing, protocol for recovery from bright object detection (BOD) shutdowns, parameters for BOD thresholds, etc. Finally, some effects of charged particle hits on electronics led to optimised strategy for regular resetting. The Near-UV channel was lost in one of suchoperations. All the above in-orbit experiences are presented here.

• UVIT study of UV bright stars in the globular cluster NGC 4147

We present far ultraviolet (FUV) observations of globular cluster NGC 4147 using three FUV filters, BaF2 (F154W), sapphire (F169M), and silica (F172M) of Ultra-Violet Imaging Telescope (UVIT) onboard the AstroSat satellite. We confirmed the cluster membership of the UVIT observed sources usingproper motions from Gaia data release 2 (GAIA DR2). We identified 37 blue horizontal branch stars (BHBs), one blue straggler star (BSS) and 15 variable stars using UV-optical color magnitude diagrams (CMDs). We find that all the FUV bright BHBs are second generation population stars. Using UV-optical CMDs, weidentify two sub-populations, BHB1 and BHB2, among the UV-bright BHBs in the cluster with stars count ratio of 24:13 for BHB1 and BHB2. The effective temperatures ($T_{\rm eff}$) of BHB1 and BHB2 were derived using color-temperature relation of BaSTI-IAC zero-age horizontal branch (ZAHB). We found that BHB1 stars are more centrally concentrated than BHB2 stars. We also derive physical parameters of the detected FUV bright BSS by fitting younger age BaSTI-IAC isochrones on optical and UV-optical CMDs.

• Study of Galactic structure using UVIT/AstroSat star counts

The structure of our Galaxy has been studied from ultraviolet (UV) star counts obtained with the Ultra-Violet Imaging Telescope (UVIT) on board the AstroSat satellite, in Far-UV (FUV) and Near-UV (NUV) bands. The F154W (BaF$_2$) and N263M (NUVB4) filters were used in the FUV and NUV bands,respectively. The point sources are separated from the extra-galactic sources of UVIT observations using infrared (IR) color cut method. The observed UVIT star counts match well with the simulations obtained from the Besanc¸on model of stellar population synthesis towards several Galactic directions. We alsoestimated the scale length and scale height of the thick disc and the scale height of the thin disc using the space density function and the exponential density law for the stars of intermediate galactic latitudes. The scale length of the thick disc ranges from 3.11 to 5.40 kpc whereas the scale height ranges from $530 \pm 32$ pc to $630 \pm 29$ pc. The scale height of the thin disc comes out to be in the range of $230 \pm 20$ pc to $330 \pm 11$ pc.

• Contamination control of UVIT

Ultra Violet Imaging Telescope (UVIT) is one of the 5 instruments on AstroSat satellite, which was launched on September 28, 2015. UVIT was designed to make images with a resolution of <1:8$''$, simultaneously in two ultraviolet channels: Far Ultraviolet (130–180 nm) and Near Ultraviolet (200–300 nm). Images are also made in visible region (320–550 nm) for tracking drifts in pointing. The shortest wavelengths to be observed with UVIT can be heavily absorbed by mono-molecular deposits/contamination on the optical surfaces.Keeping contamination under control in UVIT was a major challenge and it required a variety of actions: (i) strict control of the payload materials and process, (ii) mechanical configuration, (iii) baking of all the parts to release all the adsorbed molecules etc., (iv) assembly in ultra cleanrooms, (v) pre-inspection and auditing of all the areas, in which UVIT was placed, for any potential for contamination, (vi) continuous purging, with ultrapure nitrogen gas, till a few days before the launch, etc. In order to minimise any possible cross contaminationsfrom the other payloads/satellite, the doors of UVIT were opened 2 months after the launch. The high performance in the orbit and high stability of the sensitivity over 4 years in the orbit shows that the contamination was negligible. This paper presents the processes and protocols followed during the integration and testingphase to minimise the contamination in order to prevent any performance degradation.

• Innovations in the AOCS of AstroSat spacecraft

AstroSat is India’s first mission dedicated to space-based Astronomy and carries a complement of instruments sensitive over a wide spectral region covering visible, ultraviolet, soft X-ray and hard X-ray bands. The spacecraft platform provides the required stability, pointing, power, thermal control, and interface requirements. Among the critical functions of the Attitude and Orbit Control System (AOCS) were, manoeuvre the spacecraft from one target source to another in an optimal way while avoiding the sun, compensate for the rotational disturbance torque created by the scanning of the SSM payload on the other payloads, primarily the UVIT payload, provide for calibration of all the payloads, which involved manoeuvring through 2-dimensional target patterns. All the above novel control functions, along with the routine house-keeping operations, were designed, developed, and tested extensively on ground, beforedeclaring them as flight-worthy. The payload performance and the resulting science are a testimony to the excellent performance of the on-board Control System.

• Exploring sub-MeV sensitivity of AstroSat–CZTI for ON-axis bright sources

The Cadmium–Zinc–Telluride Imager (CZTI) onboard AstroSat is designed for hard X-ray imaging and spectroscopy in the energy range of 20–100 keV. The CZT detectors are of 5-mm thickness and hence have good efficiency for Compton interactions beyond 100 keV. The polarisation analysis using CZTIrelies on such Compton events and have been verified experimentally. The same Compton events can also be used to extend the spectroscopy up to 380 keV. Further, it has been observed that about 20% pixels of the CZTI detector plane have low gain, and they are excluded from the primary spectroscopy. If these pixels are included, then the spectroscopic capability of CZTI can be extended up to 500 keV and further up to 700 keV with a better gain calibration in the future. Here we explore the possibility of using the Compton events as well as the low gain pixels to extend the spectroscopic energy range of CZTI for ON-axis bright X-ray sources. We demonstrate this technique using Crab observations and explore its sensitivity.

• Sub-MeV spectroscopy with AstroSat-CZT imager for gamma ray bursts

Cadmium–Zinc–Telluride Imager (CZTI) onboard AstroSat has been a prolific Gamma-Ray Burst (GRB) monitor. While the 2-pixel Compton scattered events (100–300 keV) are used to extract sensitive spectroscopic information, the inclusion of the low-gain pixels ($\sim$20% of the detector plane) aftercareful calibration extends the energy range of Compton energy spectra to 600 keV. The new feature also allows single-pixel spectroscopy of the GRBs to the sub-MeV range which is otherwise limited to 150 keV. We also introduced a new noise rejection algorithm in the analysis (‘Compton noise’). These new additionsnot only enhances the spectroscopic sensitivity of CZTI, but the sub-MeV spectroscopy will also allow proper characterization of the GRBs not detected by Fermi. This article describes the methodology of single, Compton event and veto spectroscopy in 100–900 keV combined for the GRBs detected in the first year of operation. CZTI in last five years has detected $\sim$20 bright GRBs. The new methodologies, when applied on the spectral analysis for this large sample of GRBs, has the potential to improve the results significantly and help in better understanding the prompt emission mechanism.

• Effect of intense geomagnetic storms on low-latitude TEC during the ascending phase of the solar cycle 24

The results presented in this paper are obtained from low-latitude ionospheric total electron content (TEC) variation during the chosen geomagnetic storm events happening during the solar cycle 24.We include the four intense geomagnetic storms that occurred on 26 September 2011, 15 July 2012, 19 February 2014 and 20 December 2015, depending upon the availability of TEC data. For this, we have used the TEC data from low-latitude station Varanasi (geographic latitude 25$^{\circ}$, 16$'$N, geographic longitude 82$^{\circ}$, 59$'$E and geomagnetic latitude 16$^{\circ}$, 24$'$N) and an equatorial station Bengaluru (geographic latitude 13$^{\circ}$, 02$'$N, geographic longitude 77$^{\circ}$, 34$'$E and geomagnetic latitude 04$^{\circ}$, 68$'$N). The storm-induced TEC changes at chosen stations have been discussed in terms of local time, storm wind effect, neutral wind, composition changes and variation in the dawn–dusk component of the interplanetary electric field (IEF $E_y$).

• Core-collapse supernova from a possible progenitor star of 100 $M_{\odot}$

In this work, we study the synthetic explosions of a massive star. We take a 100 $M_{\odot}$ zero-age main-sequence (ZAMS) star and evolve it until the onset of core-collapse using $\mathtt{MESA}$. Then, the resulting star model is exploded using the publicly available stellar explosion code, $\mathtt{STELLA}$. The outputs of $\mathtt{STELLA}$ calculations provide the bolometric light curve and photospheric velocity evolution along with other physical properties of the underlying supernova. In this paper, the effects of having a large Hydrogen-envelope on the supernova light curve have been explored.We also explore the effects of the presence of different amounts of nickel mass and the variation of the explosion energy of the supernovae from such heavy progenitors, on thebolometric light curves and photospheric velocities.

• Deep $V$ and $I$ CCD photometry of young star cluster NGC 1893 with the 3.6m DOT

Young star clusters consisting of massive stars are the ideal sites to study the star formation processes and influence of massive stars on the subsequent star formation. NGC 1893 is a young star cluster associated with the Hii region Sh2-236. It contains about five ‘O’-type stars and several early ‘B’-type stars. It is located at a moderate distance of $\sim$3.25 kpc and has a reddening, $E(B - V)\sim 0.4$ mag. To characterize the young low-mass stellar population in the central portion of the cluster, we carried out deep $VI$ band observations of the region using the $4{\rm K} \times 4{\rm K}$ CCD IMAGER mounted on the 3.6-m Devasthal Optical Telescope. Our analysis shows that the present data are deep enough to detect stars below $V \sim 24$ mag. Wefound optical counterparts of $\sim$220 candidates, including young stars and unclassified cluster members from Caramazza et al. (2008). We estimated the membership probabilities of the Gaia sources (mostly bright starswith $G$ < 19 mag) located within the cluster radius using the Gaia EDR3. Toward the fainter end, we used the optical color-magnitude diagram (CMD) to select the cluster members from a sample of young stars. The locations of young stars on the CMD show that a majority of them are low-mass stars with age <10 Myr. The unclassified candidates and X-ray sources from Caramazza et al. (2012) are also found to be young low-mass stars. In total, we identified $\sim$425 young stars with age <10 Myr, and 110 of these are new. Most of these stars appear as kinematic members of the cluster. By examining the CMD for the stars in the cluster region, we suggest that the cluster has insignificant contamination due to field stars in the pre-main-sequence zone ofthe CMD. The slope of the mass function in the mass range $0.2 \leq M/M_{\odot} \leq 2.5$ is found to be $\Gamma=-1.43\pm 0.15$, consistent with those of other star-forming complexes. The spatial distribution of the young stars as a function of mass suggests that toward the cluster center, most of the stars are massive.

• Upcoming 4m ILMT facility and data reduction pipeline testing

The 4m international liquid mirror telescope (ILMT) installation activities have recently been completed at the Devasthal observatory (Uttarakhand, India). The ILMT will perform continuous observation of a narrow strip of the sky ($\sim$27$'$) passing over the zenith in the SDSS $g'$, $r'$ and $i'$ bands. Incombination with a highly efficient 4k$\times$4k CCD camera and an optical corrector, the images will be secured at the prime focus of the telescope using the time delayed integration technique. The ILMT will reach $\sim$22.5mag ($g'$-band) in a single scan and this limiting magnitude can be further improved by co-adding the nightly images. The uniqueness of the one-day cadence and deeper imaging with the ILMT will make it possible to discover and study various galactic and extra-galactic sources, specially variable ones. Here, we present the latest updates of the ILMT facility and discuss the preparation for the first light, which is expected during early 2022. We also briefly explain different steps involved in the ILMT data reduction pipeline.

• Revealing nature of GRB 210205A, ZTF21aaeyldq (AT2021any) and follow-up observations with the 4K$\times$4K CCD imager $+$ 3.6m DOT

Optical follow-up observations of optical afterglows of gamma-ray bursts are crucial to probe the geometry of outflows, emission mechanisms, energetics and burst environments. We performed the follow-up observations of GRB 210205A and ZTF21aaeyldq (AT2021any) using the 3.6m Devasthal opticaltelescope (DOT) around one day after the burst to deeper limits due to the longitudinal advantage of the place. This paper presents our analysis of the two objects using data from other collaborative facilities, i.e., 2.2m Calar Alto Astronomical Observatory (CAHA) and other archival data. Our analysis suggests that GRB 210205A is a potential dark burst once compared with the X-ray afterglow data. Also, comparing results with other known and well-studied dark GRBs samples indicate that the reason for the optical darkness of GRB210205A could either be intrinsic faintness or a high redshift event. Based on our analysis, we also found that ZTF21aaeyldq is the third known orphan afterglow with a measured redshift except for ZTF20aajnksq (AT2020blt) and ZTF19abvizsw (AT2019pim). The multiwavelength afterglow modeling of ZTF21aaeyldq using the afterglowpy package demands a forward shock model for an ISM-like ambient medium with a rather wider jet opening angle. We determine circumburst density of $n_0 =0.87$ cm$^{-3}$, kinetic energy $E_k=3.80 \times 10^{52}$ erg and the afterglow modeling also indicates that ZTF21aaeyldq is observed on-axis ($\theta_{\rm obs}$ < $\theta_{\rm core}$) and a gamma-ray counterpart was missed by GRBs satellites. Our results emphasize that the 3.6m DOT has a unique capability for deep follow-up observations of similar and other new transients for deeper observations as a part of time-domain astronomy in the future.

• Photometric calibrations and characterization of the 4K$\times$4K CCD imager, the first-light axial port instrument for the 3.6m DOT

In the present work, recent characterization results of the 4K$\times$4K CCD imager (a first light instrument of the 3.6m devasthal optical telescope; DOT) and photometric calibrations are discussed along with measurements of the extinction coefficients and sky brightness values at the location of the 3.6m DOTsite basedon the imaging data taken between 2016 and 2021. For the 4K$\times$4K CCD imager, all given combinations of gains (1, 2, 3, 5 and 10 e$^-$/ADU) and readout noise values for the three readout speeds (100 kHz, 500 kHz and 1 MHz) are verified using the sky flats and bias frames taken during early 2021; measured values resemble well with the theoretical ones. Using color–color and color–magnitude transformation equations, color coefficients ($\alpha$) and zero-points ($\beta$) are determined to constrain and examine their long-term consistencies and any possible evolution based on UBVRI observations of several Landolt standard fields observed during 2016–2021. Our present analysis exhibits consistency among estimated a values within the 1$\sigma$ and does not show any noticeable trend with time. We also found that the photometric errors and limiting magnitudes computed using the data taken using the CCD imager follow the simulated ones published earlier. The average extinction coefficients, their seasonal variations and zenith night-sky brightness values for the moon-less nights for all ten Bessell and SDSS filters are also estimated and found comparable to those reported for other good astronomical sites.

• Combined effect of transverse electric and magnetic fields on THz generation by beating of two amplitude-modulated laser beams in the collisional plasma

In this paper, we have provided a theoretical analysis on enhanced terahertz (THz) generation by beating two amplitude-modulated laser beams in the collisional plasma under the effect of static electric and magnetic fields. Two amplitude-modulated laser beams of slightly different frequencies ($\omega_1$, $\omega_2$) with wavenumbers ($k_1$, $k_2$) are propagating along the same direction under the effect of these fields. The coupling between the various nonlinear terms like the nonlinear velocity and electron density in the collisional plasma results in the THz generation. The electric field, magnetic field, and propagation direction of lasers are mutually perpendicular to each other. The applied static magnetic and electric fields assist to enhance the nonlinear current density and normalized THz amplitude. The normalized THz amplitude showsnotable enhancement with the increase of applied static electric and magnetic fields. This scheme can be employed to generate and detect the THz radiations for making interesting astronomical observations.

• Restricted $2 + 2$ body problem with oblateness and straight segment

The present study investigates the combined effects of the oblateness and straight segment on the positions and linear stability of the equilibrium points in the restricted $2 + 2$ body problem. The present model holds fourteen equilibrium points, out of which six are collinear with the centers of the primaries and the rest are non-collinear. It has been observed that the positions of all the equilibrium points are subsequently affected by the oblateness and length of the primary bodies. The linear stability of the equilibrium points is also presented by slightly perturbing the position of the equilibrium points. It is observed that for aconsidered set of parameters, all the fourteen equilibrium points are unstable. An application of the present model is also studied, for which the position and stability of the equilibrium points are investigated for the Earth-22 Kalliope-dual satellite system. It has been observed that for this system, all the equilibrium points are unstable except for two non-collinear equilibrium points that are found to be stable.

• GRB 210217A: a short or a long GRB?

Gamma-ray bursts are traditionally classified as short and long bursts based on their $T_{90}$ value (the time interval during which an instrument observes 5% to 95% of gamma-ray/hard X-ray fluence). However, $T_{90}$ is dependent on the detector sensitivity and the energy range in which the instrument operates. As a result, different instruments provide different values of $T_{90}$ for a burst. GRB 210217A is detected with different duration by Swift and Fermi. It is classified as a long/soft GRB by Swift-BAT with a $T_{90}$ value of 3.76 s. On the other hand, the sub-threshold detection by Fermi-GBM classified GRB 210217A as a short/hard burst with a duration of 1.024 s. We present the multi-wavelength analysis of GRB 210217A (lying in the overlapping regime of long and short GRBs) to identify its actual class using multi-wavelength data. We utilized the $T_{90}$-hardness ratio, $T_{90}-E_p$ and $T_{90}-t_{\rm mvts}$ distributions of the GRBs to find the probability of GRB 210217A being a short GRB. Further, we estimated the photometric redshift of the burst by fitting the joint XRT/UVOT SED and placed the burst in the Amati plane. We found that GRB 210217A is an ambiguous burst showing properties of both short and long class of GRBs.

• Effect of heating–cooling imbalance on slow mode with time-dependent background temperature

We study the effect of heating–cooling imbalance on slow magnetohydrodynamic waves in solar coronal loops with time-varying background temperature in the presence of thermal conduction, opticallythin radiation and heating. The MHD equations governing the plasma motion are solved numerically to examine the effects of heating–cooling imbalance on slow waves in the presence of thermal conduction and radiation. It is found that the amplitude of perturbed velocity decreases in the case of increasing background temperature, whereas the perturbed velocity amplitude increases in the case of decaying background temperature. The heating–cooling imbalance influences the damping of slow waves. Damping of waves is stronger for characteristic time $\tau=1000$ s than the damping for $\tau=3000$ s in both time-varying background temperature plasmas.

• # Journal of Astrophysics and Astronomy

Volume 43, 2022
All articles
Continuous Article Publishing mode

• # Continuous Article Publication

Posted on January 27, 2016

Since January 2016, the Journal of Astrophysics and Astronomy has moved to Continuous Article Publishing (CAP) mode. This means that each accepted article is being published immediately online with DOI and article citation ID with starting page number 1. Articles are also visible in Web of Science immediately. All these have helped shorten the publication time and have improved the visibility of the articles.

• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019