Quasiperiodic oscillations (QPOs) at frequencies of 5-8 Hz are a common feature of low-mass X-ray binaries (LMXBs) during their normal branch (NB) spectral states. We propose that this frequency band is essentially a band of rotation frequencies in a thick accretion disk. The high luminosities in the NB state give rise to the thick disk and slow down the dynamical timescales. Density and optical-thickness perturbations in a subsonic region of the disk have the frequency spectrum of sound in a rotating medium. X-rays scattered through the subsonic region are modulated by long wavelength optical-thickness oscillations whose frequencies, determined by the rotation rate and vorticity, are observed as the NB QPOs.


Frequency Resolved Spectroscopy of XB 1323-619 and EXO 0748-676
Balman, Şölen (2009-09-11)
We present the frequency resolved energy spectra (FRS) of the low-mass X-ray binary dipper XB 1323-619 and EXO 0748-676 using archival XMM-Newton observations. The FRS of XB 1323-619 is well described by a single blackbody component with kT in a range 1.2-1.6 keV. We attribute this to an existing boundary layer, the likely source of the soft emission, supported by radiation pressure. We detect a different form of FRS for the frequency ranges 0.9-6 Hz and 8-30 Hz which is modeled best with a power law and a ...
X-ray flux and pulse frequency changes of three high mass X-ray binary pulsars: Vela X-1, GX 301-2 and OAO 1657-415
Inam, SC; Baykal, Altan (2000-01-01)
Using archival BATSE (Burst and Transient Source Experiment) 20-50 keV band X-ray flux and pulse frequency time series, we look for correlations between torque, luminosity and specific angular momentum for three high mass X-ray binary pulsars Vela X-1, GX 301-2 and OAO 1657-415. Our results show that there is no correlation between pulse frequency derivative and flux which may be an indication of the absence of stable prograde accretion disk. From the strong correlation of specific angular momentum and torq...
Numerical analysis of long wavelength infrared HgCdTe photodiodes
Kocer, H.; Arslan, Y.; Beşikci, Cengiz (2012-01-01)
We present a detailed investigation of the performance limiting factors of long and very long wavelength infrared (LWIR and VLWIR) p on n Hg1-xCdxTe detectors through numerical simulations at 77 K incorporating all considerable generation-recombination (G-R) mechanisms including trap assisted tunneling (TAT), Shockley-Read-Hall (SRH), Auger and radiative processes. The results identify the relative strengths of the dark current generation mechanisms by numerically extracting the contribution of each G-R mec...
Viscous time scale in accreting powered pulsars and anomalous X-ray pulsars
İçdem, Burçin; Baykal, Altan; Department of Physics (2011)
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 hig...
Quantitative electrostatic force measurement in AFM
JEFFERY, Steve; Oral, Ahmet; Pethica, John B. (2000-04-02)
We describe a method for measuring forces in the atomic force microscope (AFM), in which a small amplitude oscillation(similar to 1 Angstrom(p-p)) is applied to a stiff(similar to 40 N/m) cantilever below its first resonant frequency, and the force gradient is measured directly as a function of separation. We have used this instrument to measure electrostatic forces by applying an ac voltage between the tip and the sample, and observed a variation in contact potential difference with separation. We also sho...
Citation Formats
M. ALPAR, G. HASINGER, J. SHAHAM, and S. YANCOPOULOS, “6 HZ QUASI-PERIODIC OSCILLATIONS FROM LOW-MASS X-RAY BINARIES - THE SOUND OF AN ACCRETION DISK,” ASTRONOMY & ASTROPHYSICS, pp. 627–631, 1992, Accessed: 00, 2020. [Online]. Available: https://hdl.handle.net/11511/67411.