Discovery of a 270 Hz X-Ray Burst Oscillation in the X-Ray Dipper 4U 1916-053.pdf
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Discovery of a 270 Hz X-Ray Burst Oscillation in the X-Ray Dipper
4U 1916?053
Duncan K. Galloway, Deepto Chakrabarty, Michael P. Muno, and Pavlin Savov
Center for Space Research and Department of Physics, Massachusetts Institute of Technology,
Cambridge, MA 02139
duncan,deepto,muno,pavlin@
Accepted for The Astrophysical Journal Letters, 2000 Dec 20
ABSTRACT
We report the discovery of a highly coherent oscillation in a type-I X-ray burst ob-
served from 4U 1916?053 by the Rossi X-ray Timing Explorer (RXTE ). The oscillation
was most strongly detected ≈ 1 s after the burst onset at a frequency of 269.3 Hz, and
it increased in frequency over the following 4 seconds of the burst decay to a maximum
of ?272 Hz. The total measured drift of 3.58 ± 0.41 Hz (1σ) represents the largest
fractional change in frequency (1.32 ± 0.15 %) yet observed in any burst oscillation.
If the asymptotic frequency of the oscillation is interpreted in terms of a decoupled
surface burning layer, the implied neutron star spin period is around 3.7 ms. However,
the expansion of the burning layer required to explain the frequency drift during the
burst is around 80 m, substantially larger than expected theoretically (assuming rigid
rotation). The oscillation was not present in the persistent emission before the burst,
nor in the initial rise. When detected its amplitude was 6–12% (RMS) with a roughly
sinusoidal profile. The burst containing the oscillation showed no evidence for photo-
spheric radius expansion, while at least 5 of the other 9 bursts observed from the source
by RXTE during 1996 and 1998 did. No comparable oscillations were detected in the
other bursts. A pair of kilohertz quasi-periodic oscillations (QPOs) has been previously
reported from this source with a mean separation of 348 ± 12 Hz. 4U 1916?053 is the
first example of a source where the burst oscillation frequency is significantly smaller
than the frequency
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