47 research outputs found
Origin of X-rays in the low state of the FSRQ 3C 273: Evidence of inverse Compton emission
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).Comment: 17 pages, 8 figures, 3 tables; Accepted for publication in MNRA
Spectral energy distributions and age estimates of 40 massive young stellar objects
In this paper, we present the spectral energy distributions (SEDs) of 40 massive young stellar objects (YSOs), detected from the NIR imaging survey carried out by Varricatt et al. 2010 and estimated their ages and masses. The SEDs of YSOs in 40 massive star forming regions have been reconstructed using 2MASS, MSX, IRAS, IRAC & MIPS, SCUBA, WISE, SPIRE and IRAM data, partly available from previous works, using the on-line SED Fitting tool (SED Fitter) developed by Robitaille et al. 2006, 2007. Apart from IRAS catalogue fluxes, the fluxes in the Mid-IR and sub-mm/mm were derived directly from the images. With the help of the analysis of SEDs, we have extracted important physical and structural parameters for each of the massive young stellar objects, along with the associated circumstellar disk and envelope. The cumulative distribution of the stellar ages and masses of the massive YSOs lead to a scenario for the formation history of massive stars in their respective star forming regions
Studies of Quasi Periodic Oscillations in the Black Hole Transient XTE J 1817-330
We have used archival RXTE PCA data to investigate timing and spectral
characteristics of the transient XTE J1817-330. The data pertains 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 8 of these observations, QPOs are present in the 8-14
keV and in 5 observations in the 15-25 keV band. XTE J1817-330 is the third
black hole source from which the low frequency QPOs are clearly detected in
hard X-rays. The QPO frequency lies in ~ 4-9 Hz and the rms amplitude in
1.7-13.3% range, the amplitude being higher at higher energies. We have fitted
the PDS of the observations with Lorentzian and power law models. Energy
spectra are derived for those observations in which the QPOs are detected to
investigate any dependence of the QPO characteristic on the spectral
parameters. These spectra are well fitted with a two component model that
includes the disk black body component and a power law component. The QPO
characteristics and their variations are discussed and its implication on the
origin of the QPOs are examinedComment: 16 page, 9 figures and 2 tables. Accepted in MNRAS Journa
A 3D Automated Classification Scheme for the TAUVEX data pipeline
In order to develop a pipeline for automated classification of stars to be
observed by the TAUVEX ultraviolet space Telescope, we employ an artificial
neural network (ANN) technique for classifying stars by using synthetic spectra
in the UV region from 1250\AA to 3220\AA 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
ultraviolet imager. We have successfully classified 229 stars from the IUE low
resolution catalog to within 3-4 spectral sub-class using two different
simulated training spectra, the TAUVEX spectra of 286 spectral types and UVBLUE
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 magnitudes. 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 ultraviolet 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 modeled for the dust distribution in the galaxy.Comment: 8 pages, 12 figures, Accepted for publication in MNRAS; High
resolution figures available from the authors on reques
Physical Acceptability of Isolated, Static, Spherically Symmetric, Perfect Fluid Solutions of Einstein's Equations
We ask the following question: Of the exact solutions to Einstein's equations
extant in the literature, how many could represent the field associated with an
isolated static spherically symmetric perfect fluid source? The candidate
solutions were subjected to the following elementary tests: i) isotropy of the
pressure, ii) regularity at the origin, iii) positive definiteness of the
energy density and pressure at the origin, iv) vanishing of the pressure at
some finite radius, v) monotonic decrease of the energy density and pressure
with increasing radius, and vi) subluminal sound speed. A total of 127
candidate solutions were found. Only 16 of these passed all the tests. Of these
16, only 9 have a sound speed which monotonically decreases with radius. The
analysis was facilitated by use of the computer algebra system GRTensorII.Comment: 25 pages. To appear in Computer Physics Communications Thematic Issue
on "Computer Algebra in Physics Research