Quasars are ideal targets to use for astrometric calibration of large scale astronomical surveys as they have negligible proper motion and parallax. The forthcoming 4-m International Liquid Mirror Telescope (ILMT) will survey the sky that covers a width of about 27$'$. To carry out astrometric calibration of theILMT observations, we aimed to compile a list of quasars with accurate equatorial coordinates and falling in the ILMT stripe. Towards this, we cross-correlated all the quasars that are known till the present date with the sources in the Gaia-DR2 catalogue, as the Gaia-DR2 sources have position uncertainties as small as a few milli arcsec (mas). We present here the results of this cross-correlation which is a catalogue of 6738 quasars that is suitable for astrometric calibration of the ILMT fields. In this work, we present this quasar catalogue. This catalogue of quasars can also be used to study quasar variability over diverse time scales when the ILMT starts its observations. While preparing this catalogue, we also confirmed that quasars in the ILMT stripe have proper motion and parallax lesser than 20 mas yr$^{-1}$ and 10 mas, respectively.

For the propagation of a shock (blast) in a self-gravitating perfect gas in case of spherical and cylindrical symmetry, an approximate analytical solution is investigated. The shock wave is considered to be a strong one, with the ratio $(C/V_S)^2$ to be a small quantity, where $C$ is the sound speed in an undisturbed medium and $V_S$ is the shock wave velocity. The initial density in the undisturbed medium is taken to be varying according to a power law. To obtain the approximate closed-form similarity solution, the flow variables are expanded in a power series of $(C/V_S)^2$. The first- and second-order approximations are discussed with the help of power series expansion. The analytical solutions are constructed for the first-order approximation. The distribution of the flow variables for first-order approximation in the flow field region behind the shock wave is shown in graphs for both the cylindrical and spherical geometries. The effect of flow parameters, namely, ambient density variation index $\alpha$, adiabatic exponent $\gamma$ and gravitational parameter $G_0$, are studied on the flow variables and on the total energy of disturbance in the case of the first approximation to the solutions. It is shown that the total energy of the disturbance in the flow field region behind the shock wave decreases with an increase in initial density variation index or adiabatic exponent, i.e. shock strength increases with increase in the value of adiabatic exponent or initial density variation index. A comparison is also made between the solutions obtained for non-gravitating and self-gravitating gases.

In our effort to decipher the words and identify the stars as cited in the text Sarvasiddh$\bar{a}$ntar$\bar{a}$jaby Nity$\bar{\rm a}$nanda of 15th century, we have found that most of the stars near the ecliptic belt are easily identifiable. The coordinate system used is Dhruvaka and Viksepa, which differ from the currently used ecliptic longitude–latitude and Right Ascension–declination coordinates. Most of the stars studied hitherto (Pai & Shylaja 2016, 2019; Shylaja & Pai Curr Sci 2018a, 2018b, 2019) were all in the northern hemisphere. It may be recalled that the name of the zodiacal sign (Pai & Shylaja 2016) for a group includes stars with all declinations – both north and south. For example, the group of Mesa includes the stars of Andromeda. The only two southern stars that were mentioned in the group of Gemini are Sirius and Canopus. Here, we study the stars grouped under Libra and Scorpio, which includes bright stars in the southern hemisphere. Theconfusion about the Dhruvaka (east–west coordinates) is discussed.

The effect of intense solar flares on total electron content (TEC) variability during the declining phase of solar cycle-24 is studied at the low-latitude station at Varanasi, India (Geog. Lat. 25.31$^{\circ}$ N, Geog. Long. 82.97$^{\circ}$ E, Geomag. Lat. 16.54$^{\circ}$ N, Geomag. Long. 157.09$^{\circ}$ E). In the present paper, we have chosen the intense solar flares that occurred during 9–13 March 2015, 1–2 January 2016, 12–14 February 2016, 6–8 August 2016, and 6–8 September 2017 in the solar cycle-24 period for which the data is available. Our results showed significant enhancements in TEC up to the order of 15 TECU during and after the solar flare events. We have also given a brief account of solar flare effect in TEC with and without geomagnetic disturbances, local time effects (solar zenith angle effects) and changing the location of the solar active region. In a few cases, our results revealed a delay in TEC response during the flare peak time as well as recovery time.

The significance increase of the plasma temperature from the solar photosphere to the corona up to 1MK is still unresolved. One of the candidates for this issue is waves and instabilities in the solar plasma. It was suggested that the energy of these waves could heat the solar atmosphere to transition-region (TR) and corona. Despite many recent kind of researches about waves in the various solar dynamic structures, the reasons for the sudden rise of solar atmospheric temperature are still not enough. By using the Interface Region Imaging Spectrograph (IRIS) telescope observations, we investigate the waves in magnetic tubes such as UV bright points (BPs) at the boundary and inside the supper-granules. The Study of these dynamic structures, between the chromosphere and transition region, will impressively increase our information ofmass and energy transportation through interface region between the chromosphere and inner corona. The magnetic BPs are the faces of magnetic flux tubes that are associated with magnetic elements. The magnetic flux tubes expand upward and appear as bright grains that make up the chromospheric network. For investigating periodical behavior of these structures and various aspect of these oscillations, we analyze intensity oscillations individually in IRIS slit-jaw images at three filters included 2796 Å, 1400 Å and 1330 Å by the wavelet analysis method. The wavelet analysis of the intensity fluctuations of these points showed periods from 2 to 8 min, which are in order of the frequencies of the atmospheric pressure modes. For investigating wave propagation in these layers we also study the correlation of oscillations betweenchromosphere (2796 Å) and TR (1400 Å and 1330 Å) heights by cross wavelet method, and found that these heights correlate with periods between 2.5 and 5.5 min, with wave speeds from 30 to 200 km s$^{-1}$. Also, byextracting Si IV line, we determined the Doppler velocity of BPs in the network and inter-network as about $-$20 to $+$30 km s$^{-1}$ and $-$21 to $+$21 km s$^{-1}$, respectively. Our results suggest that the wave propagation along the magnetic flux tubes of upward-moving plasma in BPs have an important role in solar atmosphere heating.

From a report based on an independent investigation by a senior fellow of the Indian Academy of Sciences, we find that there is evidence of academic misconduct in the paper titled, ‘‘Numerical simulation of inertial alfvenwaves to study localized structures and spectral index in auroral region’’ by Bheem Singh Jatav (https://doi.org/10.1007/s12036-018-9531-8; volume 39 article ID 37) published in Journal of Astrophysics and Astronomy. Therefore, the Indian Academy of Sciences has decided to withdraw the publication, on the grounds of academic misconduct by the author. The author has been notified and has agreed to the retraction.

Star sensor (SS) is one of the most critical instrument on-board a spacecraft when it comes to attitude determination and control. Capability of providing precise attitude information makes it even more important for high resolution imaging satellite with stringent pointing requirements. At the same time, theseinstruments are highly sensitive to bright objects like Sun and Earth Albedo. In this paper, selection of star sensor mounting has been elaborately studied and presented for missions primarily performing highly agile Earth imaging. This paper addresses selection factors like Sun and Earth avoidance from bore-sight axis, orbit altitude and local time, angular separation between SS, sensitivity of SS on body axis and functional availability during imaging. Results are presented in the form of available mounting options and the way they satisfy geometrical constraints over a year.

The stellar spectral data taken by LAMOST (Large Sky Area Multi-Object Fiber Spectroscopic Telescope) include multiple types, some of which that fall between two spectral classes, namely boundary spectra. Due to the massive and high dimensional nature of spectra data, it will take a lot of time and energyto cluster these spectra by manual operation alone. To address this problem, a new clustering method based on influence space is presented in this paper. First, we introduce the concept of influence space to reduce the amount of data involved in the operation, and reduce the dimension of the data by extracting the main feature lines. Second, a novel method for initial cluster center selection is applied. Next, based on the selected initial cluster centres, other spectra are clustered by running K-means algorithm on the whole data set. The experimental results indicate that the initial cluster centres obtained by this method are of higher quality and the problem of boundary spectra clustering is also well solved.

We revisit here a possibility of generation of gravitational wave (GW) bursts due to a very quick change in the quadrupole moment (QM) of a deformed spheroidal pulsar as a result of crustquake. Since it was originally proposed as a possible explanation for sudden spin-up (glitch) of pulsars, the occurrence ofcrustquake and its various consequences have been studied and discussed quite often in the literature. Encouraged by recent development in gravitational wave (GW) astronomy, we re-investigate the role of crustquake in the emission of GWs. Assuming exponential decay of excitation caused by crustquake, wehave performed a Fourier analysis to estimate the GW strain amplitude $h(t)$, characteristic signal amplitude $h_c(f)$ and signal-to-noise ratio (SNR) of the burst for the Crab pulsar. For exotic quark stars, multifold enhancement of these quantities are expected, which might make quark star a potential source of gravitational waves. The absence of such bursts may put several constraints on pulsars and such hypothetical stars.

We present a CCD photometric survey for the search of variable stars in four open clusters namely Berkeley 69, King 5, King 7 and Berkeley 20. The time series observations were carried out for 1 and/or 2 nights for each of the clusters in the year 1998, which have led to identify nineteen variable stars in theseclusters. Out of these 19 variable stars, five stars show $\delta$ Scuti-like variability and two stars show W UMa type variability. In other stars, we could not find the periods, and hence the type of variability due to the lack of sufficient data. The periods of $\delta$ Scuti type stars are found to be in the range of 0.13–0.21 days, whereas the two stars in the cluster Berkeley 20, which showed W UMa type variability have orbital periods of 0.396 and 0.418 days, respectively. Using the Gaia data, the basic parameters of the clusters Berkeley 69, King 7 and King 5 are also revised. The age and reddening are estimated to be $0.79\pm 0.09$ Gyr and $0.68 \pm 0.03$ mag for Berkeley 69, $0.79\pm 0.09$ Gyr and $1.22 \pm 0.03$ mag for the cluster King 7 and $1.59\pm 0.19$ Gyr and $0.63\pm 0.02$ mag for the cluster King 5, respectively. The signature of mass segregation is found in the clusters King 7 and King 5.

The work is the study of Bianchi type-I models with holographic dark energy in the framework of $f(R)$ theory of gravitation. The exact solutions of the field equations are deduced by considering the exponential and power-law volumetric expansion. Various physical and kinematical properties of the models are discussed. In addition to make our interpretation more clear for that we have taken the state finder diagnostic pair $\{r; s\}$ and cosmic jerk parameter $j(t)$ to characterize different phases of the universe. The holographic scalar fields of dark energy are reconstructed and quintessence potential for both the modelsdescribing accelerated expansion of the universe is also obtained.

We studied dust acoustic solitary waves (DASW) in a five component cometary plasma by deriving the Kadomstev–Petiashvili (KP) equation. The five components consist of two components of electrons described by kappa distributions with different temperatures and spectral indices, a lighter (hydrogen)and a heavier (oxygen) ion component, both ion components are described by Maxwellian distributions. Dust particles, with varying charge, constitute the fifth component. The system supports rarefactive DASWs whose amplitudes are larger when the charges on the dust particles vary. The amplitudes alsoincrease with increasing $z_{d0}n_{d0}$ (product of equilibrium charge number and density of dust) and increasing ion densities. It, however, decreases with increasing spectral indices of the electrons.

Analysis of spectropolarimetric observations of two circular sunspots located close to disk centre in H$\alpha$ (6562.8 Å) and Fe i (6569.22 Å) is presented in this paper. The corresponding active region numbers areNOAA 10940 and NOAA 10941 referred to as AR1 and AR2, respectively. The vector magnetic field at the photosphere is derived through inversion of Stokes profiles of Fe I under Milne–Eddington atmospheric model. The chromospheric vector magnetic field is derived from H$\alpha$ Stokes profiles under weak-field approximation. Azimuthally averaged magnetic field as a function of radial distance from the centre of sunspot at the photosphere and chromosphere are studied. At the photosphere, the radial variation shows a well known behaviour that the total field and the line-of-sight (LOS) component monotonically decrease from centre to the edge of the sunspot and the transverse component initially increases, reaches a maximum close to half the sunspot radius and thendecreases. LOS and the transverse components become equal close to half the sunspot radius consistent with the earlier findings. At the chromosphere, all the components of the magnetic field decrease with the sunspot radius. However, the LOS component decreases monotonically whereas the transverse component decreases monotonically up to about 0.6 times the sunspot radius after which it reaches a constant value. Azimuthally averaged magnetic field gradient from photosphere to chromosphere is also presented here.

We study the distributions of $\gamma$-ray properties of a sample of $\gamma$-ray loud blazars taken from the third catalogue of blazars detected by Fermi-Large Area Telescope (Fermi-LAT).We compute the $\gamma$-ray dominance ($D_{\rm g}$) of the sample which includes 415 flat spectrum radio quasars (FSRQs) and 535 BL Lacertae objects (BL Lacs). We find that BL Lacs and FSRQs are highly dominated by $\gamma$-ray emission, which is consistent with diffuse high-energy neutrino flux associated with $\gamma$-ray loud blazars. The $\gamma$-ray dominance fairly scales with $\gamma$-ray luminosity ($r \sim +0.5$) in both BL Lacs and FSRQs, but shows little or no correlation ($r \leq 0.2$) with radio luminosity in either sample. BL Lacs and FSRQs occupy separate and parallel regions on the $D_{\rm g}$-luminosity plane. There is a fairly significant correlation ($r \sim 0.5$) between $\gamma$-ray dominance and frequency at synchrotron peak ($\nu_{\rm pk}$) in BL Lacs, which disappears in FSRQs. On the other hand, there is a tight correlation ($r \geq +0.8$) between $\gamma$-ray and radio luminosity with a smooth transition from BL Lacs at low luminosities to FSRQs at high luminosities. Nevertheless, the presence of few BL Lac-like FSRQs is noted. These results suggest that while there may be intrinsic differences between BL Lacs and FSRQs, some form of a unified scheme can also be relevant.

Ongoing research on pulsar timing array (PTA) to detect gravitational radiation is reviewed. Here, we discuss the use of millisecond pulsars as a gravitational wave detector, the sources of gravitational radiation detectable by PTAs and the current status of PTA experiments pointing out the future possibilities.

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.

Archival Vilnius CCD photometric observations are presented for the heavily reddened star cluster Melotte 105, resulting in colour-magnitude diagrams and spectral class estimates. There is considerable lack of agreement between studies for reddening, age and distance for this cluster explaining why the archival data are being made available by this paper. The derived reddening $E(B − V) = 0.34 \pm 0.04$ mag and the distance $V − M_V = 12.9 \pm 0.3$ mag are directly from the Vilnius photometry. The Gaia Data Release 2(DR2) and Vilnius photometric data of the cluster were used to estimate the structural parameters of the cluster, probability of stellar membership in the cluster, the distance modulus and the cluster age. Lack of $Y$ bandobservations prevented determination of metal abundance. The values of the colour excess and distance module are determined by two different methods (i.e., Q and Zero Age Main Sequence or ZAMS methods). A distance modulus of $12.85 \pm 0.07$ mag was derived by ZAMS fitting, in good agreement with the above estimate. ZAMS fitting indicates a reddening of $0.403\pm 0.02$ mag, within two sigma of the estimate above. The cluster’s metallicity and age are estimated to be 0.24 dex and $240 \pm 25$ Myr, respectively. The derived mass function is in good agreement with the Salpeter slope. The cluster space velocity components (U, V,W) were determined as ($−3.90 \pm 3.34$, $−13.76 \pm 5.69$, $+3.45 \pm 0.41$) km/s. Perigalactic and apogalactic distances were obtained as $R_ p = 6.85$ and $R_a = 7.44$ kpc respectively. The maximum vertical distance from the Galactic plane was calculated as $Z_{\rm max} = 84$ pc and the eccentricity of the orbit was determined as $e = 0.042$.

A simulation of the radiation field inside a habitable module (a diameter of 6 m and length of 12 m) of a spacecraft generated by isotropic Galactic Cosmic Radiation (GCR) in deep interplanetary space is carried out for minimum and maximum solar activity using the FLUKA code. Protons, alpha-particles,deuterons, ${}^3$He, and nuclei with $Z$ > 2 are considered as primary GCR irradiating the spacecraft isotropically. The following particles are included in FLUKA radiation transport through the module shell (15 g/cm$^2$ of Al): protons, neutrons, $\gamma$-rays, electrons, $\pi^{\pm}$-mesons, $\mu^{\pm}$-mesons d, t, and nuclei from He to Ni. The inner particle spectra are needed to assess the astronaut’s radiation risk in a long-term interplanetary mission.

In view of the negative results from various dark matter detection experiments, we propose alternate models by postulating a minimal field strength (minimal curvature) and also a minimal acceleration. These postulates lead to modified Newtonian dynamics, modified Newtonian gravity, and modified relativistic gravity. Through these postulates, we show that the observed flat rotation curves of galaxies can be accounted for. This can also set constraints on the size of large scale structures such as galaxies and clusters, which are consistent with observations. This minimum curvature in general relativity, and consequently the modification of the Hilbert action leads naturally to a term equivalent to cosmological constant. This model thus neatly ties up, with these two postulates: the observed flat rotation curves and the accelerated expansion of the universe.

Solar flare activity is characterized by different classification systems, both in optical and X-ray ranges. The most generally accepted classifications of solar flares describe important parameters of flares such as the maximum of brightness of the flare in the optical range – $H_{\alpha}$ flare class (change from F to B), area of the flare in $H_{\alpha}$ (change from S for areas less than 2 square degrees to 4 for areas more than 24.7 square degrees) and the maximum amplitude of the soft X-ray (SXR)-flux in the band 0.1–0.8 nm ($F^{\rm max}_{0.1−0.8}$) – X-ray flares of classes from C to X. A new classification parameter of solar flares is proposed here – the X-ray index of flare XI, based on GOES measurements of solar radiation in the SXR-range. The XI-index has a clear physical interpretation associated with the total flare energy in the SXR-range. XI is easily calculated for each flare with the use of available GOES data. The XI-index can be used along with other geoeffective parameters of Solaractivity to assess both flares and coronal mass ejections that are connected with them.

MHD shock waves and discontinuities are often observed in the solar wind. We studied a strong shock wave event on 7 June 2014, during a solar maximum. The properties of forward shock are investigated and all physical parameters of interplanetary shock are analyzed. The upstream parameters, such as Alfvenvelocity and sound speed are calculated, and the angle ($\theta$) between the upstream magnetic field direction and the shock normal is estimated. To determine the propagation speed of the shock, a full numerical solution ofshock adiabatic equation is carried out. The thermal pressure for electrons and ions are calculated and also the pressure ratio and the entropy change across the shock are estimated. The results show that the shock strengthis about 3.88 with $\theta \approx 72^{\circ}$ (a quasi-perpendicular case), and shock propagation velocity is about 678 km/s. The increase in the specific entropy is also evaluated.

The paper deals with the dynamics of particles which are on the surface of the cometary nucleus. The key point is to research the behavior of the particles as a result of local sublimation. Generally, we can describe three mechanisms related to the particles behavior. Relatively small particles (of the order ofmicrometers) are ejected into the atmosphere of the comet due to sublimation. Slightly larger particles (of the order of centimeters) can migrate across the comet’s surface towards the equator, while much larger rubbles ofcometary matter remain at rest. The angular width of the particles is the main factor influencing the migration time towards the equator of the comet. The measure of angular width and particles’ migration time is its sizeand coefficient of friction. The numerical calculations presented in the paper refer to a hypothetical comet X/P and the comet 67P/Churyumov–Gerasimenko.