Shashikiran Ganesh

QUASI-PERIODIC OSCILLATIONS OF ∼15 MINUTES IN THE OPTICAL LIGHT CURVE OF THE BL LAC S5 0716+714

Over the course of 3 hr on 2008 December 27, we obtained optical (R band) observations of the blazar S5 0716+714 at a very fast cadence of 10 s. Using several different techniques, we find fluctuations with an approximately 15 minute quasi-period to be present in the first portion of these data at a >3σ confidence level. This is the fastest quasi-periodic oscillation that has been claimed to be observed in any blazar at any wavelength. While these data are insufficient to strongly constrain models for such fluctuations, the presence of such a short timescale when the source is not in a very low state seems to favor the action of turbulence behind a shock in the blazars relativistic jet.

Mass-Losing Semiregular Variable Stars in Baade's Windows

By cross-correlating the results of two recent large-scale surveys, the general properties of a well-defined sample of semiregular variable stars have been determined. ISOGAL mid-infrared photometry (7 and 15 ?m) and MACHO V and R light curves are assembled for approximately 300 stars in the Baades windows of low extinction toward the Galactic bulge. These stars are mainly giants of late M spectral type, evolving along the asymptotic giant branch (AGB). They are found to possess a wide and continuous distribution of pulsation periods and to obey an approximate log P-Mbol relation or set of such relations. Approximate mass-loss rates in the range of ~1 ? 10-8 to 5 ? 10-7 M? yr-1 are derived from ISOGAL mid-infrared photometry and models of stellar spectra adjusted for the presence of optically thin circumstellar silicate dust. Mass-loss rates depend on luminosity and pulsation period. Some stars lose mass as rapidly as short-period Mira variables but do not show Mira-like amplitudes. A period of 70 days or longer is a necessary but not sufficient condition for mass loss to occur. For AGB stars in the mass-loss ranges that we observe, the functional dependence of mass-loss rate on temperature and luminosity can be expressed as T?L?, where ? = -8.80 and ? = +1.74, in agreement with recent theoretical predictions. If we include our mass-loss rates with a sample of extreme mass-losing AGB stars in the Large Magellanic Cloud and ignore T as a variable, we get the general result for AGB stars that L2.7, valid for AGB stars with 10-8 < < 10-4 M? yr-1.

Infrared dark clouds from the ISOGAL survey ? Constraints on the interstellar extinction curve

The ISO galactic survey provides images of the inner disk in two broad lters (around 7 and 15m) over some 15 square degrees, away from the brightest star forming regions. A multiresolution analysis of the images leads to a catalogue of infrared dark clouds, most of which are condensed cores of large molecular clouds, several kpc away from the Sun, seen in absorption in front of the diuse galactic emission. The longitude distributions of the background emission and of the dark clouds correlate with known tracers of young population components. We analyse the morphology of the dark clouds and the intensity fluctuations within the cloud boundaries at the two wavelengths. The 7 to 15 m contrast ratio is 0:75 0:15 for the clouds located away from the Galactic Centre (jlj > 1 )a nd 1:05 0:15 for the clouds closest to the Galactic Centre (jlj < 1;jbj < 0:2). Using a simple absorption model, we derive a 7 to 15 m opacity ratio equal to 0:7 0:1 for the clouds located away from the Galactic Centre and estimate the opacity, , of a few objects at 15 m in the range 1 to 4. Several explanations for the variation of the contrast ratio, including absorption along the line of sight and local variations of the extinction curve are discussed.

RAPID OPTICAL VARIABILITY IN BLAZAR S5 0716+71 DURING 2010 MARCH

We report rapid optical variability for the blazar S5 0716+71 during 2010 March 8-10 and 19-20 in the CCD observations made from Mt. Abu Infrared Observatory. The light curves are constructed for a duration longer than 3 hr each night, with very high temporal resolution ({approx}45 s in the R band). During 2010 March 8, the source smoothly decayed by about 0.15 mag in 2.88 hr, apart from a fast flicker lasting about 30 minutes. S5 0716+71 brightened during March 9 and 10, showing high activity, while it was relatively faint (>14 mag in the R band) albeit variable during March 19-20. During March 9 and 10, rapid flickers in the intensity modulated the long-term intra-night ({approx}3 hr) variations. The present observations suggest that the blazar S5 0716+71 showed night-to-night and intra-night variability at various timescales with a 100% duty cycle for variation along with microvariability at significant levels. On a night-to-night basis, the source exhibits mild bluer-when-brighter nature. The interaction of shocks with local inhomogeneities in the jet appears to cause intra-night variations, while microvariations could be due to small-scale perturbations intrinsic to the jet.

ISO mid-infrared spectroscopy of Galactic Bulge AGB stars ,

Aims. To study the nature of Bulge AGB stars and in particular their circumstellar dust, we have analysed mid-infrared spectra obtained with the ISOCAM CVF spectrometer in three Bulge fields. Methods. The ISOCAM 5–16.5 µm CVF spectra were obtained as part of the ISOGAL infrared survey of the inner Galaxy. A classification of the shape of the 10 µm dust feature was made for each case. The spectra of the individual sources were modelled using a radiative transfer model. Different combinations of amorphous silicates and aluminium-oxide dust were used in the modelling. Results. Spectra were obtained for 29 sources of which 26 are likely to be Bulge AGB stars. Our modelling shows that the stars suffer mass loss rates in the range of 10 −8 −5 × 10 −7 M� /yr, which is at the low end of the mass-loss rates experienced on the thermally %

Primary Black Hole Spin in OJ 287 as Determined by the General Relativity Centenary Flare

OJ 287 is a quasi-periodic quasar with roughly 12 year optical cycles. It displays prominent outbursts that are predictable in a binary black hole model. The model predicted a major optical outburst in 2015 December. We found that the outburst did occur within the expected time range, peaking on 2015 December 5 at magnitude 12.9 in the optical R-band. Based on Swift/XRT satellite measurements and optical polarization data, we find that it included a major thermal component. Its timing provides an accurate estimate for the spin of the primary black hole,

OH/IR stars in the inner bulge detected by ISOGAL

\chi =0.313\pm 0.01

Optical intraday variability studies of 10 low energy peaked blazars

. The present outburst also confirms the established general relativistic properties of the system such as the loss of orbital energy to gravitational radiation at the 2% accuracy level, and it opens up the possibility of testing the black hole no-hair theorem with 10% accuracy during the present decade.

Interstellar extinction and long-period variables in the Galactic centre

We present a study of known OH/IR stars in the inner bulge, observed by the ISOGAL survey at =6 :8 ma nd =1 4:9 m. Bolometric corrections and luminosities are computed, based on near and mid-infrared data. The vast majority of the sources exhibit mass-loss rates in the range: 3 10 7 up to a few times 10 5 M/year. The bolometric magnitude distribution peaks at Mbol = 5:0. There is no clear evidence that the luminosity is related to the expansion velocity of the envelope for the sample in the bulge observed by ISOGAL. We nd that the bulge OH/IR stars do not follow a period-luminosity (PL) law and that they are systematically less luminous than the OH/IR extension of the PL relationship for Miras.

CLOUDS search for variability in brown dwarf atmospheres. Infrared spectroscopic time series of L/T transition brown dwarfs ⋆

We have carried out optical (R band) intraday variability (IDV) monitoring of a sample of ten bright low energy peaked blazars (LBLs). Forty photometric observations, of an average of � 4 hours each, were made between 2008 September and 2009 June using two telescopes in India. Measurements with good signal to noise ratios were typically obtained within 1–3 minutes, allowing the detection of weak, fast variations using N-star differential photometry. We employed both structure function and discrete correlation function analysis methods to estimate any dominant timescales of variability and found that in most of the cases any such timescales were longer than the duration of the observation. The calculated duty cycle of IDV in LBLs during our observing run is �52%, which is low compared to many earlier studies; however, the relatively short periods for which each source was observed can probably explain this difference. We briefly discuss possible emission mechanisms for the observed variability.