In this work, the effect of rotation on the oscillation frequencies of some radially and nonradially oscillating Delta-Scuti stars have been explored. Rotation has been considered as a perturbation and treated up to the second order. Series of evolutionary models have been calculated for the oscillating stars in question and compared with the observational parameters. Three stars are considered: V350 Peg with no rotation, CC And with a rotational velocity Vsini=20 km/s, and BS Tuc with Vsini=130 km/s. We find that splitting in the oscillation frequencies are conspicuous especially in fast rotating stars, with a considerable contribution from the related terms due to second order effect.

In this work, the CHANDRA ACIS-S3 data of the old classical nova RR Pic (1925) is presented. The source is detected with a count rate of 0.067 (+-0.002) c/s in the 0.3-5.0 keV energy range. The orbital period of the binary system in X-ray wavelengths is detected. X-ray spectrum of RR Pic can be represented by a composite model of bremsstrahlung with a photoelectric absorption, two absorption lines centered around 1.1-1.4 keV and 5 Gaussian emission lines centered at around 0.3-1.1 keV . The best t bremsstrahlung temperature derived from the model ranges from 0.99 to 1.60 keV and the unabsorbed X-ray flux is found to be 2.51 (+0.39 -1.21) erg/cm2/s in the 0.3-5.0 keV range.The absorption lines correspond to several transitions of Fe, Ne and Na. The emission lines correspond to various transitions of S, N, O, C, Ne and Fe / and observations with better spectral resolution are needed to determine the exact transitions. The source spectrum is better explained with photoionized plasma model rather than the cooling flow model. It has also been found that the neutral Hydrogen column density differs for orbital minimum and orbital maximum spectra with values 0.25 (+0.23 -0:18)*10^22 atoms/cm2 and 0.64 (+0.13 -0.14)*10^22 atoms/cm2 respectively at 3 sigma confidence level. The difference of neutral Hydrogen column density between the maximum and minimum phase spectra indicates existence of a warm absorbing region on the disc at the location of the impact zone.

In this study spectral and photometric data of the SAX J2103.5+4545 Be/X-ray system are given. The spectral data were taken from June 2007 to September 2008 with the 1.5 m Russian-Turkish telescope, whereas the photometric data were obtained using ROTSE-IIId archive from June 2004 to November 2008. The photometric data up to April 2007 shows that the system was in quiescence in the optical region. But, in the 23rd of April 2007, the system&#039 / s luminosity underwent a large increase, which is in agreement with X-ray data. This increase was approximately 1 mag. Also, the Halpha line was displaying an emission with increased equivalent width proportional to the outburst. Afterwards, the Halpha line profile changed from a double peaked emission into a single peaked absorption, which is in agreement with the system&#039 / s structure. This means that the Be star threw away its disc and its light curve fell back to its old luminosity.

In this thesis we analyse X-ray data of accretion powered low mass and high mass X-ray binaries to understand the nature of their accretion mechanisms by searching for some clues of viscous time-scales of their accretion discs, if they have, in their low frequency power density spectra created from their long-term X-ray observations, or by doing pulse timing analysis with much shorter X-ray data to detect the effects of torque fluctuations caused by the accreting material on the pulsar. The low mass and high mass X-ray binaries we analysed have breaks in their power density spectra, which are attributed to the role of viscosity in the formation of accretion discs. Although, the time-scales corresponding to these break frequencies are smaller than the predictions of the Standard theory of accretion discs, the sources give consistent results among themselves by displaying the expected correlation between their break and orbital frequencies. The correlation curve of LMXBs implies thicker appearing accretion discs than those assumed by the theory. The dichotomy of the HMXBs on this curve points out the different origins of accretion that these sources may have, and offers a way to distinguish the stellar-wind fed systems from the Roche-lobe overflow systems. The timing and spectral analysis of Swift J1626.6-5156 reveal a correlation between the spin-up rate and the luminosity of the source implying that the pulsar is accretion-powered. This correlation together with the characteristics of the X-ray spectra enables us to estimate the magnetic field and the distance of the source. The AXP 1E 2259+586 does not display any signs of viscous time-scale in its low frequency power density spectra, and its pulse timing analysis gives a much smaller torque noise value than that expected from accretion powered pulsars. In addition, the analysis results presented in this thesis reveal magnetar-like glitches which differ than those of radio pulsars, due to the presence of the strong magnetic field of the pulsar. These results eliminate the possibility that the AXP is an accretion-powered pulsar.

In this study, vibrational properties and stability of delta-Scuti like pulsating pre-main sequence stars have been investigated. Studies were held in the mass range 2-4 Mo and limited to radial linear adiabatic pulsations. Numerical computations were performed by the oscillation program written by Kirbiyik &amp / Al-Murad (1993). The models were selected to be at the latest phases of the pre-main sequence evolution where the luminosity starts to increase. We have limited our calculations upto the end of the radiative inner regions, since at the surface of the star, our adiabatic perturbation computation does not perfectly fit to the relatively thin non-adiabatic convective envelope of the star. The results of the stability analysis showed that the PMS models undergo an instability whose time period is a function of mass. Instability Strip of pulsating PMS stars was re-drawn with comparison to M. Marconi &amp / F.Palla (1998). The effect of gravitational contraction on stability was also investigated.

This thesis is composed of three published articles. Each chapter is devoted to an article. In the first part the origin of some of the single radio pulsars with relatively low magnetic fields (B &lt / 1e12 G) and with characteristic ages (tau) less than 1e7 years is questioned. We proposed that such pulsars might occur as a result of the disruption of high-mass X-ray binary systems after a second supernova explosion. In these binaries, mass accretion on to the surface of X-ray pulsars may lead to the decrease in the magnetic field from its value at birth (B similar to 1e12 &ndash / 1e13 G) down to B &lt / 1e12 G similar to the processes in low-mass X-ray binaries. In the second part we put together many observational data of SGRs and AXPs and analyzed them with the main purpose of the removal of contradiction between the real age (t) of these objects and their characteristic times of period change (tau). SGRs and AXPs are neutron stars that undergo star-quakes. Magnetic activity increases from time to time. We suggest that as a result of these processes plasma is ejected from the NS and propeller mechanism starts to work. Due to propeller effect dP/dt increases, tau decreases. Indeed, high dP/dt values are observed in SGRs and in half of the AXPs. Then, for a long time NS looses its activity, its dP/dt decreases, tau increases and rapid cooling begins. It seems that there is a possible transition between each NS stage (AXP,SGR,dim). This transient cycle may be repeated once or several times until the spin period of the neutron star becomes P &gt / 10 - 12 s. Observational data and mainly the data of AXP 1E 1048-5937 and DRQNS RX J1308.8+2127 support this idea. In the third part dependence of the X-ray luminosity (Lx) of young single pulsars, due to ejection of relativistic particles, on electric field intensity, rate of rotational energy loss (dE/dt), magnetic field, period and some other parameters of neutron stars are discussed. Influence of the magnetic field and effects of some other parameters of neutron stars on the Lx - dE/dt and the Lx - tau(characteristic time) relations are considered. Evolutionary factors also play an important role in our considerations. Only the pulsars whose X-ray luminosity in the 2 &ndash / 10 keV energy band is greater than 1033 erg/s have pulsar wind nebula around them. The pulsars from which gamma-ray radiation has been observed have low X-ray luminosity in general.

In this work, we analyze the data from 1.5 m telescope of Tubitak National Observatory (TUG) for two objects V2275 Cyg and RW UMi. After applying aperture and PSF photometry to the CCD data, we derive the light curve of the objects. Also we apply discrete Fourier transform and Scargle time series analysis to obtain power spectrum and search for periodicities in their power spectra in which both systems show interesting properties. We discover large variations in V2275 Cyg due to irradiation effects on the secondary, with an orbital period Porb = 0.316&plusmn / 0.007 day. We also detect a very short orbital period for RW UMi Porb = 1.96&plusmn / 0.073h and also a spin period Pspin = 1.29&plusmn / 0.036h in the presence of several beat frequencies. These results (if con&amp / #64257 / rmed) lead us to say that RW UMi can be an intermediate of polar system.

In this thesis, RXTE timing analysis of Galactic Microquasars GRO J1655-40 and GRS 1915+105, the Chandra and RXTE joint spectral analysis of GRS 1915+105 are presented. We have investigated quasi periodic oscillations (QPOs) in the black hole binaries GRO J1655-40 and GRS 1915+105 observed in 99 and 122 observations made by the Proportional Counter Array (PCA) on board Rossi X-Ray Timing Explorer (RXTE) in both low energy band (2-12 keV) and high energy band (13-27 keV), respectively. Four different X-ray states are seen in the combined characteristics of power spectra, light curves extracted by using All Sky Monitor (ASM) and spectra during 1996 and 2005. Timing analysis of RXTE observations of both of two black hole binaries GRO J1655-40 and GRS 1915+105 displays twin high frequency quasi periodic oscillations (QPOs) which are sometimes simultaneous in high energy band. It is also shown that the time averaged 30 ksec Chandra grating spectrum analysis and RXTE spectrum analysis of recent observation of GRS 1915+105 in the very high state are consistent with the parameters which were mentioned before. We briefly discussed our results and the models on black hole spin and mass.

In this thesis, we present the spectral and timing variabilities of anomalous X-ray pulsar 1E 2259+586 observed with European Photon Imaging PN Camera (EPIC PN) on board X-ray Multi Mirror Mission (XMM), Proportional Counter Array (PCA) on board Rossi X-ray Timing Explorer (RXTE) and Advanced CCD Imaging Spectrometer (ACIS) on board Chandra X-ray Observatory. We presented the results of spectral analysis of 2000 January 11 ACIS observation. Pulse phase spectroscopy was performed on two XMM observations before and after the outburst. Pulse profiles of two XMM observations before the outburst and three XMM observations after the outburst were studied. Results are consistent with the those presented by Patel et al. (2001) and Woods et al. (2004). We searched for the spectral variations versus spin rate during the outburst. Long-term spectral, frequency and spin-down rate variations are presented between 1996 and 2006 including 98 RXTE and 4 XMM observations. However, except outburst region (Woods et al. 2004) no significant spectral and spin rate variabilities were seen. Around the outburst region we confirmed the spectral hardening with increasing spin rate and flux.

The spectroscopic and photometric observations of the optical counterparts to the Be/X-ray binary systems GRO J2058+42 and V0332+53 (BQ Cam), taken with RTT150 (Russian-Turkish 1.5 meter Telescope), are presented in this study. The distance, color and the reddening estimates for both sources, obtained via photometric observations, are consistent with the previous results. The results of our spectroscopic observations performed between May 2006 and June 2008 for optical counterpart to GRO J2058+42 indicate that the double-peaked emission line profile turns into a single-peaked emission after the last outburst of the system. Furthermore, the spectra of the source show clear evidence for the changes in the ratio of the double peaks of H alpha emission line indicative of the precession of the high-density regions confined in the disk. Unlike the spectra of counterpart to GRO J2058+42, the spectra of BQ Cam, taken between September 2006 and December 2007, exhibit single-peaked H alpha and HeI (Lambda 7065 Angstrom) emission lines. In addition, the equivalent width values of H alpha emission lines, shifts from the the laboratory wavelengths for H alpha and HeI emission lines and the variation in optical brightness of BQ Cam show a close correlation.