http://hdl.handle.net/2289/142 Search the Channel search http://dspace.rri.res.in:8080/jspui/simple-search http://hdl.handle.net/2289/5373 Title: Probing primordial magnetic fields using lyα clouds<br/><br/>Authors: Pandey, Kanhaiya.L.; Sethi, S. K<br/><br/>Abstract: From previous studies of the effect of primordial magnetic fields on early structure formation, we know that the presence of primordial magnetic fields during early structure formation could induce more perturbations at small scales (at present 1-10 h –1 Mpc) as compared to the usual ΛCDM theory. Matter power spectra over these scales are effectively probed by cosmological observables such as shear correlation and Lyα clouds. In this paper we discuss the implications of primordial magnetic fields on the distribution of Lyα clouds. We simulate the line-of-sight density fluctuation including the contribution coming from the primordial magnetic fields. We compute the evolution of Lyα opacity for this case and compare our theoretical estimates of Lyα opacity with the existing data to constrain the parameters of the primordial magnetic fields. We also discuss the case when the two density fields are correlated. Our analysis yields an upper bound of roughly 0.3-0.6 nG on the magnetic field strength for a range of nearly scale-invariant models, corresponding to a magnetic field power spectrum index n –3.<br/><br/>Description: Restricted Access. Sat, 29 Dec 2012 22:58:59 GMT http://hdl.handle.net/2289/5371 Title: Low-frequency observations of the moon with the Murchison widefield array.<br/><br/>Authors: McKinley, B.; Deshpande, A.; Roshi, A.; Udhay Shankar, N; Srivani, K.S.; Prabu, T; Subrahmanyan, Ravi; +47 co athours<br/><br/>Abstract: A new generation of low-frequency radio telescopes is seeking to observe the redshifted 21 cm signal from the epoch of reionization (EoR), requiring innovative methods of calibration and imaging to overcome the difficulties of wide-field low-frequency radio interferometry. Precise calibration will be required to separate the expected small EoR signal from the strong foreground emission at the frequencies of interest between 80 and 300 MHz. The Moon may be useful as a calibration source for detection of the EoR signature, as it should have a smooth and predictable thermal spectrum across the frequency band of interest. Initial observations of the Moon with the Murchison Widefield Array 32 tile prototype show that the Moon does exhibit a similar trend to that expected for a cool thermally emitting body in the observed frequency range, but that the spectrum is corrupted by reflected radio emission from Earth. In particular, there is an abrupt increase in the observed flux density of the Moon within the internationally recognized frequency modulated (FM) radio band. The observations have implications for future low-frequency surveys and EoR detection experiments that will need to take this reflected emission from the Moon into account. The results also allow us to estimate the equivalent isotropic power emitted by the Earth in the FM band and to determine how bright the Earth might appear at meter wavelengths to an observer beyond our own solar system.<br/><br/>Description: Restricted Access. An open-access version is available at arXiv.org (one of the alternative locations) Sat, 29 Dec 2012 22:58:59 GMT http://hdl.handle.net/2289/5347 Title: New measurements of orbital period change in Cygnus X-3<br/><br/>Authors: Singh, N.S.; Naik, S.; Paul, Biswajit; Agrawal, P.C.; Rao, A.R.; Singh, K.Y.<br/><br/>Abstract: The nonlinear nature of the binary ephemeris of Cygnus X-3 indicates either a change in the orbital period oran apsidal motion of the orbit. We have made extended observations of Cygnus X-3 with the Pointed Proportional Counters(PPCs) of the Indian X-ray Astronomy Experiment (IXAE) during 1999 July 3−13 and October 11−14. Using the data fromthese observations and the archival data from ROSAT, ASCA, BeppoSAX and RXTE, we have extended the database for thissource. Adding these new arrival time measurements to the published results, we make a comparison between the variouspossibilities, (a) orbital decay due to mass loss from the system, (b) mass transfer between the stars, and (c) apsidal motion ofthe orbit due to gravitational interaction between the two components. The orbital decay due to mass loss from the companionstar seems to be the most probable scenario<br/><br/>Description: Restricted Access. Thu, 29 Aug 2002 22:58:59 GMT http://hdl.handle.net/2289/5346 Title: BeppoSAX observations of the X-ray binary pulsar GX 1+4<br/><br/>Authors: Naik, S.; Callanan, P.J.; Paul, Biswajit<br/><br/>Abstract: We present here the timing and spectral properties of BeppoSAX observations of the binary X-ray pulsar GX 1+4 carried out in August 1996, March 1997, and August 2000. In the middle of the August 2000 observation, the source was in a rare low intensity state that lasted for about 30 hours. Though the source does not show pulsations in the soft X-ray band (1.0-5.5 keV) during the extended low state, pulsations are detected in 5.5-10.0 keV energy band of the MECS detector and in hard X-ray energy bands (15-150 keV) of the PDS instrument. Broad-band (1.0-150 keV) pulse averaged spectroscopy reveals that the best-fit model comprises of a Comptonized continuum along with an iron Kα emission line. A strong iron Kβ emission line is detected for the first time in GX 1+4 during the extended low state of 2000 observation. The optical depth and temperature of the Comptonizing plasma are found to be identical during the high and low intensity states whereas the hydrogen column density and the temperature of the seed photons are higher during the low state.<br/><br/>Description: Restricted Access. Wed, 28 Sep 2005 22:58:59 GMT