Kalpana Duorah

Peak of spectral energy distribution plays an important role in intra-day variability of blazars?

Blazars can be divided into two sub-classes namely high energy and low energy peaked blazars. In spectral energy distribution, the first synchrotron hump of the former class peaks in UV/X-rays and in IR/optical bands for the latter class. The peak of the spectral energy distribution seems to be responsible for variability properties of these classes of blazars in X-ray and optical bands. Since, in low energy peaked blazars, the X-ray bands lies well below the synchrotron hump, one expects that the highest energy electrons available for the synchrotron emission would have slower effect of variability on X-ray intra-day timescale. In this paper, by taking the advantage of a sample of 12 low energy peaked blazars with total 50 observations from XMM

Studies of quasi-periodic oscillations in the black hole transient XTE J1817–330

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A three-dimensional automated classification scheme for the TAUVEX data pipeline

Newton since its launch, we confirm that this class is less variable in X-ray bands. We found that out of 50 observational light curves, genuine intra-day variability is present in only two of light curves i.e 4%. Similar results we obtained from our earlier optical intra-day variability studies of high energy peaked blazars where out of 144 light curves, only genuine intra-day variability was detected in 6 light curves i.e ~ 4%. Since, X-ray bands lie below the peak of the spectral energy distribution of LSPs where inverse Compton mechanism is dominating rather than synchrotron radiation at the peak of the optical band, leads to slower variability in the X-ray bands. Hence, reducing their intra-day variability in X-ray bands as compared to the variability in optical bands.

Origin of X-rays in the low state of the FSRQ 3C 273: evidence of inverse Compton emission

We have used archival Rossi X-ray Timing Explorer (RXTE) Proportional Counter Array (PCA) data to investigate the timing and spectral characteristics of the transient XTE J 1817-330. The data pertain to 160 PCA pointed observations made during the outburst period 2006 January 27 to August 2. A detailed analysis of quasi-periodic oscillations (QPOs) in this black hole X-ray binary is carried out. Power density spectra were obtained using the light curves of the source. QPOs have been detected in the 2-8 keV band in 10 of the observations. In eight of these observations, QPOs are present in the 8-14 keV and in five observations in the 15-25 keV band. XTE J1817-330 is the third black hole source from which low-frequency QPOs are clearly detected in hard X-rays. The QPO frequency lies in ≈4-9 Hz and the rms amplitude in the 1.7-13.3 per cent range, the amplitude being higher at higher energies. We have fitted the power density spectra of the observations with Lorentzian and power-law models. Energy spectra are derived for those observations in which the QPOs are detected, in order to investigate any dependence of the QPO characteristic on the spectral parameters. These spectra are well fitted with a two-component model, which includes the disc blackbody component and a power-law component. The QPO characteristics and their variations are discussed and the implications on the origin of QPOs are examined.

Abundance analysis of s-process enhanced barium stars

In order to develop a pipeline for automated classification of stars to be observed by the Tel-Aviv University Ultra-Violet Experiment (TAUVEX) ultraviolet space telescope, we employ an artificial neural network (ANN) technique for classifying stars by using synthetic spectra in the ultraviolet (UV) region from 1250 to 3220 A as the training set and International Ultraviolet Explorer (IUE) low-resolution spectra as the test set. Both the data sets have been pre-processed to mimic the observations of the TAUVEX UV imager. We have successfully classified 229 stars from the IUE low-resolution catalogue to within three to four spectral subclass using two different simulated training spectra, the TAUVEX spectra of 286 spectral types and UVBLUE (http://www.inaoep.mx/∼modelos/uvblue/uvblue.html) spectra of 277 spectral types. Further, we have also been able to obtain the colour excess [i.e. E(B - V) in magnitude units] or the interstellar reddening for those IUE spectra which have known reddening to an accuracy of better than 0.1 mag. It has been shown that even with the limitation of data from just photometric bands, ANNs have not only classified the stars, but also provided satisfactory estimates for interstellar extinction. The ANN based classification scheme has been successfully tested on the simulated TAUVEX data pipeline. It is expected that the same technique can be employed for data validation in the UV from the virtual observatories. Finally, the interstellar extinction estimated by applying the ANNs on the TAUVEX data base would provide an extensive extinction map for our Galaxy and which could in turn be modelled for the dust distribution in the Galaxy.

Study of IGM through high energy radiation from blazar

We analyze the 2.5--10 keV X-ray spectra of the luminous quasar 3C 273 and simultaneous observations in UV wavelengths from XMM--Newton between 2000 and 2015. The lowest flux level ever was observed in 2015. The continuum emission from 3C 273 is generally best described by an absorbed power-law but during extremely low states the addition of fluorescence from the K-shell iron line improves the fit. We study the spectral evolution of the source during its extended quiescent state and also examine connections between the X-ray and ultraviolet emissions, which have been seen in some, but not all, previous work. We detect a possible anti-correlation between these two bands during the low state that characterized 3C 273 for most of this period; however, this was not present during a flaring state. A harder-when-brighter trend for the X-ray spectrum was observed in these long-term observations of 3C 273 for the first time. We suggest that the X-ray emission in 3C 273 is the result of inverse Compton scattering of soft UV seed photons (emitted from the local environment of the AGN), most likely in a thermal corona. We can explain the significant temporal variation of the spectral continuum as an outcome of changing optical depth of the comptonizing medium, along the lines of the wind-shock model proposed by Courvoisier and Camenzind (1989).

Nuclear reactions on accreting neutron star surface

Detailed chemical composition studies of stars with enhanced abundances of neutron-capture elements can provide observational constraints for neutron-capture nucleosynthesis studies and clues for understanding their contribution to the Galactic chemical enrichment. We present abundance results from high-resolution spectral analyses of a sample of four chemically peculiar stars characterized by s-process enhancement. High-Resolution spectra (R ~ 42000) of these objects spanning a wavelength range from 4000 to 6800 A, are taken from the ELODIE archive. We have estimated the stellar atmospheric parameters, the effective temperature T_eff, the surface gravity log g, and metallicity [Fe/H] from local thermodynamic equilibrium analysis using model atmospheres. We report estimates of elemental abundances for several neutron-capture elements, Sr, Y, Zr, Ba, La, Ce, Pr, Nd, Sm, Eu and Dy. While HD 49641 and HD 58368 show [Ba/Fe] > 1.16 the other two objects HD 119650 and HD 191010 are found to be mild barium stars with [Ba/Fe] ~ 0.4. The derived abundances of the elements are interpreted on the basis of existing theories for understanding their origin and evolution.

Studies on the evolved carbon cores

The high energy gamma rays from blazar affects the intergalactic medium (IGM) to large distances at different redshifts. The blazar radiation has been taken as the result of synchrotron and Inverse Compton scattering of electrons. It is found that the intergalactic medium is clumpy. Our estimated values lie within the suggested limit of ΩIGM ≊ 0.03 at redshift z = 3.

Multiband variability in the blazar 3C 273 with XMM-Newton

The production of X-rays and gamma-rays in bursts is believed to be due to the rapid burning of matter accreted onto a neutron star surface from its companion, most probably a giant star. The accreted matter consists mainly of hydrogen and helium and a very small amount of heavy elements. Due to the infall of matter, the temperature at the bottom layers is raised to a value of the order of 108 K. The neutron star surface density is>107 g cm−3. As hydrogen burning is a slow process under any temperature and density conditions, we consider the helium-burning reactions as the source of gamma-rays in the neutron star surface. Under high-density conditions the ordinary laboratory reaction rates should become modified. At high-density conditions, the strong screening effect due to the polarising cloud of electrons around the ions become important and enhances the reaction rates considerably. The helium-burning reactions are calculated under such conditions. The abundances of helium-burning products such as12C, 116O, and20Ne, etc., are computed. Under high-density and temperature conditions carbon is found to be more abundant than oxygen. Neon is completely absent in almost all the relevant physical conditions in which a strong screening effect is operative. It is suggested that explosive burning of accreted helium of 10−13M⊙ will account for the observed energy of gamm-ray burst.

Automated star–galaxy segregation using spectral and integrated band data for TAUVEX/ASTROSAT satellite data pipeline

Physical conditions prevalent in a degenerate carbon plasma lead to the enhancement of the carbon-carbon thermonuclear reaction rates. Nuclear energy generation rate in the carbon core is thereby augmented. The possible dissipation of energy due to pair-annihilation neutrinos, plasma neutrinos and neutrino bremsstrahlung are considered. Neutral current contribution to these weak processes are also taken into account. It is suggested that the enhanced nuclear gene-ration rate in the evolved core might halt the core collapse for a time, thus necessitating a reassessment of the phenomenon of core collapse as a precursor of carbon detonation supernova events.