2002/05/24 by Tod E. Strohmayer, Craig B. Markwardt, Craig. B. Markwardt · 12 citations
Physics and Astronomy · #Astrophysical Phenomena and Observations #Gamma-ray bursts and supernovae #Pulsars and Gravitational Waves Research #astro-ph
paper · pdf · doi:10.1086/342152
published as Astrophys.J.577:337-345,2002 · 27 pages, 9 figures, AASTeX, accepted for publication in the Astrophysical Journal
arxiv created 2002/05/24 · openalex publication_date 2002/09/20 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/07/31
We report the discovery with the Proportional Counter Array on board the Rossi X-Ray Timing Explorer of highly coherent 582 Hz pulsations during the 2001 February 22 (UT) "superburst" from 4U 1636-53. The pulsations are detected during an ≈800 s interval spanning the flux maximum of the burst. Within this interval the barycentric oscillation 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 such an 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 range of circular orbits with velocity 90 km s -1 < v ns sin i < 175 km s -1 and fractional phase 0.336 > ϕ 0 > 0.277 for the neutron star can provide an excellent description of the frequency and phase evolution. The brevity of the observed pulse train with respect to the 3.8 hr orbital period unfortunately does not allow for more precise constraints. The average pulse profile is sinusoidal, and the time-averaged pulsation amplitude, as inferred from the half amplitude of the sinusoid, is 1%, smaller than typical for burst oscillations observed in normal thermonuclear bursts. We do not detect any higher harmonics nor the putative subharmonic near 290 Hz. The 90% upper limits on signal amplitude at the subharmonic and first harmonic are 0.1% and 0.06%, respectively. The highly coherent pulsation, with a Q ≡ ν 0 /Δν > 4.5 × 10 5 , provides compelling evidence for a rapidly rotating neutron star in 4U 1636-53 and further supports the connection of burst oscillation frequencies with the spin frequencies of neutron stars. Our results provide further evidence that some millisecond pulsars are spun up via accretion in low-mass X-ray binaries. We also discuss the implications of our orbital velocity constraint for the masses of the components of 4U 1636-53.