Computer Science
Scientific paper
Oct 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997phdt.........5f&link_type=abstract
Thesis (PHD). COLUMBIA UNIVERSITY , Source DAI-B 58/09, p. 4869, Mar 1998, 123 pages.
Computer Science
4
Scientific paper
I have discovered fast x-ray oscillations near 1000 Hz in the flux of the low mass x-ray binary 4U0614+091 in observations with NASA's Rossi X-Ray Timing Explorer (RXTE). These signals are likely produced within a few stellar radii of the central neutron star that powers the x-ray binary. Two peaks in the power density spectra, or quasi-periodic oscillations (QPOs) are observed simultaneously and vary in frequency; the higher frequency QPO appearing between 500 and 1145 Hz with the lower frequency QPO between 480 and 800 Hz. The difference in frequency between these two signals is constant at 323 ± 4 Hz. The existence of two QPOs with a constant frequency separation is predicted by the 'beat-frequency' model. The higher frequency QPO is associated with the Keplerian orbital motion of gas in the inner accretion disk, while the lower frequency signal arises from interference between this modulation and the spinning neutron star. The variation of QPO frequency corresponds to a variation in the radius of the inner disk; as the inner disk radius shrinks at higher mass accretion rates, the QPO frequency increases. I observe a robust correlation between the frequency of the QPO and the flux of a blackbody component of the x-ray spectrum. The correlation can be explained by this model where the higher frequency QPO is associated with the Keplerian motion of the inner disk. From the QPO frequencies one can measure the spin period of the neutron star in the low mass x-ray binary, one of the observational goals of the last two decades. The beat-frequency model predicts that the neutron star spin frequency is equal to the frequency difference between the two QPOs. In 4U 0614+091, the frequency separation implies a neutron star spin period of 3.1 msec. The QPO measurements also provide constraints on the masses and radii of the neutron stars. The highest frequency QPOs correspond to the smallest inner disk radii, approaching the marginally stable orbit predicted by general relativity. The oscillations must be produced outside this critical radius, which implies upper limits to the mass and radius of the neutron star. The upper limits in 4U 0614+091 are 2.1Msolar and 19 km, respectively. Oscillations at the highest frequencies serve as probes of this region of strong gravitational fields.
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