IIAP Repository Community: Research Publications Based on data from Indian Astronomical Observatory

Intra-night optical variability of core dominated radio quasars: the role of optical polarization

Sun, 29 Jul 2012 22:58:59 GMT

Title: Intra-night optical variability of core dominated radio quasars: the role of optical polarization

Authors: Goyal, A; Gopal-Krishna; Wiita, P. J; Anupama, G. C; Sahu, D. K; Sagar, R; Joshi, S

Abstract: 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/subop/ backsimeq 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 gamma-ray variability of blazars.

FR Cnc revisited: photometry, polarimetry and spectroscopy

Mon, 27 Feb 2012 22:58:59 GMT

Title: FR Cnc revisited: photometry, polarimetry and spectroscopy

Authors: Golovin, A; Galvez-Ortiz, M. C; Hernan-Obispo, M; Andreev, M; Barnes, J. R; Montes, D; Pavlenko, E; Pandey, J. C; Martinez-Arnaiz, R; Medhi, B. J; Parihar, P. S; Henden, A; Sergeev, A; Zaitsev, S. V; Karpov, N

Abstract: This paper is part of a multiwavelength study aimed at using complementary photometric,polarimetric and spectroscopic data to achieve an understanding of the activity process inlate-type stars. Here, we present the study of FR Cnc, a young, active and spotted star.We performed analysis of All Sky Automated Survey 3 (ASAS-3) data for the years 2002–08and amended the value of the rotational period to be 0.826518 d. The amplitude of photometricvariations decreased abruptly in the year 2005, while the mean brightness remained the same,which was interpreted as a quick redistribution of spots. BVRC and IC broad-band photometriccalibration was performed for 166 stars in FR Cnc vicinity.The photometry at Terskol Observatory shows two brightening episodes, one of whichoccurred at the same phase as the flare of 2006 November 23. Polarimetric BVR observationsindicate the probable presence of a supplementary source of polarization. We monitored FRCnc spectroscopically during the years 2004–08.We concluded that the radial velocity changescannot be explained by the binary nature of FR Cnc. We determined the spectral type of FRCnc as K7V. Calculated galactic space-velocity components (U,V ,W) indicate that FR Cncbelongs to the young disc population and might also belong to the IC 2391 moving group.Based on Li I λ6707.8 measurement, we estimated the age of FR Cnc to be between 10 and120 Myr. Doppler tomography was applied to create a starspot image of FR Cnc.We optimizedthe goodness of fit to the deconvolved profiles for axial inclination, equivalent width and v sin i,finding v sin i = 46.2 kms−1 and i = 55◦. We also generated a synthetic V -band light curvebased on Doppler imaging that makes simultaneous use of spectroscopic and photometric data.This synthetic light curve displays the same morphology and amplitude as the observed one.The starspot distribution of FR Cnc is also of interest since it is one of the latest spectraltypes to have been imaged. No polar spot was detected on FR Cnc.

Description: Restricted Access

Discovery of Main-belt Comet P/2006 VW139 by Pan-STARRS1

Mon, 19 Mar 2012 22:58:59 GMT

Title: Discovery of Main-belt Comet P/2006 VW139 by Pan-STARRS1

Authors: Hsieh, H. H; Yang, Bin; Haghighipour, N; Kaluna, H. M; Fitzsimmons, A; Denneau, L; Novakovic, B; Jedicke, R; Wainscoat, R. J; Armstrong, J. D; Duddy, S. R; Lowry, S. C; Trujillo, C. A; Micheli, M; Keane, J. V; Urban, L; Riesen, T; Meech, K. J; Abe, S; Cheng, Yu-Chi; Chen, Wen-Ping; Granvik, M; Grav, T; Ip, Wing-Huen; Kinoshita, D; Kleyna, J; Lacerda, P; Lister, T; Milani, A; Tholen, D. J; Vereš, P; Lisse, C. M; Kelley, M. S; Fernández, Y. R; Bhatt, B. C; Sahu, D. K; Kaiser, N; Chambers, K. C; Hodapp, K. W; Magnier, E. A; Price, P. A; Tonry, J. L

Abstract: The main-belt asteroid (300163) 2006 VW139 (later designated P/2006 VW139) was discovered to exhibit comet-like activity by the Pan-STARRS1 (PS1) survey telescope using automated point-spread-function analyses performed by PS1's Moving Object Processing System. Deep follow-up observations show both a short (~10'') antisolar dust tail and a longer (~60'') dust trail aligned with the object's orbit plane, similar to the morphology observed for another main-belt comet (MBC), P/2010 R2 (La Sagra), and other well-established comets, implying the action of a long-lived, sublimation-driven emission event. Photometry showing the brightness of the near-nucleus coma remaining constant over ~30 days provides further evidence for this object's cometary nature, suggesting it is in fact an MBC, and not a disrupted asteroid. A spectroscopic search for CN emission was unsuccessful, though we find an upper limit CN production rate of Q CN < 1.3 × 1024 mol s–1, from which we infer a water production rate of $Q_{\rm H_2O}<10^{26}$ mol s–1. We also find an approximately linear optical spectral slope of 7.2%/1000 Å, similar to other cometary dust comae. Numerical simulations indicate that P/2006 VW139 is dynamically stable for >100 Myr, while a search for a potential asteroid family around the object reveals a cluster of 24 asteroids within a cutoff distance of 68 m s–1. At 70 m s–1, this cluster merges with the Themis family, suggesting that it could be similar to the Beagle family to which another MBC, 133P/Elst-Pizarro, belongs.

Description: Open Access

Studies of a possible new Herbig Ae/Be star in the open cluster NGC 7380

Sun, 29 Jan 2012 22:58:59 GMT

Title: Studies of a possible new Herbig Ae/Be star in the open cluster NGC 7380

Authors: Mathew, B; Banerjee, D. P. K; Ashok, N. M; Subramaniam, A; Bhavya, B; Joshi, V

Abstract: We present a study of the star 2MASS J22472238+5801214 with the aim of identifying its true nature which has hitherto been uncertain. This object, which is a member of the young cluster NGC 7380, has been variously proposed to be a Be star, a D-type symbiotic and a Herbig Ae/Be star in separate studies. Here we present optical spectroscopy, near-IR photometry and narrow band Hα imaging of the nebulosity in its environment. Analysis of all these results, including the spectral energy distribution constructed from available data, strongly indicate the source to be a Herbig Ae/Be star. The star is found to be accompanied by a nebulosity with an interesting structure. A bow-shock shaped structure, similar to a cometary nebula, is seen very close to the star with its apex oriented towards the photoionizing source of this region (i.e. the star DH Cep). An interesting spectroscopic finding, from the forbidden [SII] 6716, 6731 Å and [OI] 6300 Å lines, is the detection of a blue-shifted high velocity outflow (200±50 km s−1) from the star.