Astronomy and Astrophysics – Astrophysics
Scientific paper
Apr 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002aps..aprn17094s&link_type=abstract
American Physical Society, April Meeting, Jointly Sponsored with the High Energy Astrophysics Division (HEAD) of the American As
Astronomy and Astrophysics
Astrophysics
Scientific paper
We report the discovery with the Proportional Counter Array on board the Rossi X-ray Timing Explorer of highly coherent ~ 582 Hz pulsations during a ``superburst'' from 4U 1636-53. The superburst was observed on February 22, 2001 (UT). The pulsations are detected during an ~ 800 s interval spanning the flux maximum of the burst. Within this interval the pulsation frequency increases in a monotonic fashion from ~ 581.89 to 581.93 Hz. The predicted orbital motion of the neutron star during this interval is consistent with this increase as long as optical maximum corresponds roughly with superior conjunction of V801 Arae, the optical companion to the neutron star in 4U 1636-53. We show that a circular orbit with 70 < v sin i < 110 km s-1 for the neutron star provides an excellent description of the frequency and phase evolution and is consistent with a coherent pulsation. The orbital phase, intrinsic pulsation frequency and velocity are not highly constrained due to the relative shortness of the detected pulse train with respect to the 3.8 hr orbital period. The average pulse profile is sinusoidal, with a half amplitude of the sinusoid of 1 %, significantly smaller than the typical amplitudes of burst oscillations observed in normal thermonuclear bursts. We do not detect any higher harmonics nor the putative subharmonic near 290 Hz. Such a highly coherent pulsation, with a Q ≡ ν0 / Δν > 4.5 × 10^5 could only be produced by the rapidly rotating neutron star in 4U 1636-53, and confirms that the oscillations observed during normal thermonuclear bursts, so called ``burst oscillations,'' reflect the spin frequencies of neutron stars. We discuss the implications of our orbital velocity measurement for the masses and inclination of the components of 4U 1636-53.
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