Context. Rapid variations in optical flux are seen in many quasars and all blazars. The amount of variability in different classes of active galactic nuclei has been studied extensively but many questions remain unanswered. Aims. We present the results of a long-term programme to investigate the intra-night optical variability (INOV) of powerful flat spectrum radio core-dominated quasars (CDQs), with a focus on probing the relationship of INOV to the degree of optical polarization. Methods. We observed a sample of 16 bright CDQs showing strong broad optical emission lines and consisting of both high and low optical polarization quasars (HPCDQs and LPCDQs). In this first systematic study of its kind, we employed the 104-cm Sampurnanand telescope, the 201-cm Himalayan Chandra telescope and the 200-cm IUCAA-Girawali Observatory telescope, to carry out R-band monitoring on a total of 47 nights. Using the CCD as an N-star photometer to densely monitor each quasar for a minimum duration of about 4 h per night, INOV exceeding ∼1-2 per cent could be reliably detected. Combining these INOV data with those taken from the literature, after ensuring conformity with the basic selection criteria we adopted for the 16 CDQs monitored by us, we were able to increase the sample size to 21 CDQs (12 LPCDQs and 9 HPCDQs) monitored on a total of 73 nights. Results. As the existence of a prominent flat-spectrum radio core signifies that strong relativistic beaming is present in all these CDQs, the definitions of the two sets differ primarily in fractional optical polarization, with the LPCDQs showing a very low median P op 0.4 per cent. Our study yields an INOV duty cycle (DC) of ∼28 per cent for the LPCDQs and ∼68 percent for HPCDQs. If only strong INOV with fractional amplitude above 3 per cent is considered, the corresponding DCs are ∼7 per cent and ∼40 per cent, respectively. Conclusions. From this strong contrast between the two classes of luminous, relativistically beamed quasars, it is apparent that relativistic beaming is normally not a sufficient condition for strong INOV and a high optical polarization is the other necessary condition. Moreover, the correlation is found to persist for many years after the polarization measurements were made. Some possible implications of this result are pointed out, particularly in the context of the recently detected rapid γ-ray variability of blazars.

Context. Significant progress has been made in recent years to understand the formation and evolution of our Galaxy, but we still lack a complete understanding of the Galaxy and its structure. Aims. Using an almost complete sample of Galactic open star clusters within 1.8 kpc, we aim to understand the general properties of the open cluster system in the Galaxy and probe the Galactic structure. Methods. We first extracted 1241 open clusters within 1.8 kpc of the Sun from the Milky Way Star Clusters (MWSC) catalog. Considering it an almost complete sample of clusters within this distance, we performed a comprehensive statistical analysis of various cluster parameters such as spatial position, age, size, mass, and extinction. Results. We find an average cluster scale height of z h = 60 ± 2 pc for clusters younger than 700 Myr, which increases to 64 ± 2 pc when we include all the clusters. The z h is found to be strongly dependent on R GC and age, and on an average, z h is more than twice as large as in the outer region than in the inner region of the solar circle, except for the youngest population of clusters. The solar offset is found to be 6.2 ± 1.1 pc above the formal Galactic plane. We derive a local mass density of ρ 0 = 0.090 ± 0.005 M ⊙ /pc 3 and estimate a negligibly small amount of dark matter in the solar neighborhood. The reddening in the direction of clusters suggests a strong correlation with their vertical distance from the Galactic plane with a respective slope of dE(B − V)/dz = 0.40±0.04 and 0.42±0.05 mag/kpc below and above the GP. We observe a linear mass-radius and mass-age relations in the open clusters and derive the slopes of dR/d(logM) = 2.08 ± 0.10 and d(logM)/d(logT ) = − 0.36 ± 0.05, respectively. Conclusions. The dependence of the spatial distribution of clusters on their age points to a complex interplay between cluster formation and survivability within the Galaxy. The geometrical characteristics of a significant number of clusters enabled us to understand large-scale spatial properties of the cluster systems within the Galaxy. The structural and physical parameters of clusters allowed us to check mutual correlations between the individual parameters.

Aims. We search for photometric variability in chemically peculiar A type stars in the northern hemisphere. Methods. High-speed photometric observations of Ap and Am star candidates have been carried out from ARIES (Manora Peak, Nainital) using a three-channel fast photometer attached to the ARIES 104-cm Sampurnanand telescope.Results. This paper presents three new variables: HD 113878, HD 118660 and HD 207561. During the time span of the survey (1999 December to 2004 January) pulsations of the δ Sct type were also found for the two evolved Am stars HD 102480 and HD 98851, as reported in Joshi et al. (2002Joshi et al. ( , 2003. Additionally, we present 140 null results of the survey for this time span. Conclusions. The star HD 113878 pulsates with a period of 2.31 h, which is typical of δ Sct stars. HD 118660 exhibits multi-periodic variability with a prominent period of nearly 1 h. These periods need to be investigated and make HD 118660 a particularly interesting target for further observations. For HD 207561, a star classified as Am, a probable pulsation with a period of 6 min was found in the light curves obtained on two consecutive nights. Both HD 102480 and HD 98851 exhibit unusual alternating high and low amplitude maxima, with a period ratio of 2:1. The analysis of the null results confirms the photometric quality of the Nainital site.

Context. The Nainital-Cape Survey is a dedicated ongoing survey program to search for and study pulsational variability in chemically peculiar (CP) stars to understand their internal structure and evolution. Aims. The main aims of this survey are to find new pulsating Ap and Am stars in the northern and southern hemisphere and to perform asteroseismic studies of these new pulsators. Methods. The survey is conducted using high-speed photometry. The candidate stars were selected on the basis of having Strömgren photometric indices similar to those of known pulsating CP stars. Results. Over the last decade a total of 337 candidate pulsating CP stars were observed for the Nainital-Cape Survey, making it one of the longest ground-based surveys for pulsation in CP stars in terms of time span and sample size. The previous papers of this series presented seven new pulsating variables and 229 null results. In this paper we present the light curves, frequency spectra and various astrophysical parameters of the 108 additional CP stars observed since the last reported results. We also tabulated the basic physical parameters of the known roAp stars. As a part of establishing the detection limits in the Nainital-Cape Survey, we investigated the scintillation noise level at the two observing sites used in this survey, Sutherland and Nainital, by comparing the combined frequency spectra stars observed from each location. Our analysis shows that both the sites permit the detection of variations of the order of 0.6 milli-magnitude (mmag) in the frequency range 1-4 mHz, Sutherland is on average marginally better.

We present the results of our power spectral analysis for the BL Lac object PKS 0735+178 utilizing the Fermi-LAT survey at high-energy γ-rays, several ground-based optical telescopes, and single-dish radio telescopes operating at GHz frequencies. The novelty of our approach is that, by combining long-term and densely sampled intra-night light curves in the optical regime, we were able to construct for the first time the optical power spectrum of the blazar for a time domain extending from 23 years down to minutes. Our analysis reveals that: (i) the optical variability is consistent with a pure red noise, for which the power spectral density can well be approximated by a single power-law throughout the entire time domain probed; (ii) the slope of power spectral density at high-energy γ-rays (∼1), is significantly flatter than that found at radio and optical frequencies (∼2) within the corresponding time variability range; (iii) for the derived power spectra we did not detect any low-frequency flattening, nor do we see any evidence for cut-offs at the highest frequencies down to the noise floor levels due to measurement uncertainties. We interpret our findings in terms of a model where the blazar variability is generated by the underlying single stochastic process (at radio and optical frequencies), or a linear superposition of such processes (in the γ-ray regime). Along with the detailed PSD analysis, we also present the results of our extended (1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015) intra-night optical monitoring program and newly acquired optical photo-polarimetric data for the source.

We present results of a variability search in the intermediate‐age open cluster NGC 6866 from 29 nights over two observing seasons. We find 28 periodic variables, of which 19 are newly identified. The periods of these variables, which have V magnitudes from 11.5 to 19.3 mag, range from ∼48 min to 37 d. We detected several δ Scuti stars, some of which are of high amplitude, as well as γ Doradus and rotational variables, and eclipsing binaries. In order to study the physical properties of the cluster, we obtained UBVRI photometry of all the stars on a good photometric night. The radial distribution of the stellar surface density shows that the cluster has a radial extent of about 7 arcmin (∼3 pc) with a peak density of 5.7 ± 0.7 stars arcmin−2 at the cluster centre. The colour–colour diagram indicates a reddening of E(B−V) = 0.10 mag towards NGC 6866. A distance of ∼1.47 kpc and an age of ∼630 Myr are estimated from the colour–magnitude diagram using the theoretical isochrones of solar metallicity.

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Abstract.A new survey for pulsating, chemically peculiar A-and F-type stars in the northern hemisphere has been initiated using the 1-m telescope at Uttar Pradesh State Observatory in Naini Tal. The survey is primarily structured to reveal high-overtone pulsations in chemically peculiar A-F stars, but it is also revealing low-overtone δ Scuti-type pulsations in stars with peculiar colours. This paper describes the scope and methods of the survey. Preliminary observations of 28 candidate stars are presented and the discovery of three new pulsators, HD 12098, HD 13038 and HD 13079, is announced. Null results for the remaining 25 stars are also discussed.