Discovery of mHz QPOs from the LMXB 4U 1323-619

ATel #3258; T. E. Strohmayer (NASA/GSFC), E. A. Smith (Wyle IS & NASA/GSFC)
on 5 Apr 2011; 04:33 UT
Credential Certification: Tod Strohmayer (tod.strohmayer@nasa.gov)

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We report on recent RXTE observations of the bursting, high-inclination low mass X-ray binary (LMXB) 4U 1323-619. The source was observed during two successive RXTE orbits for approximately 5 ksec beginning at 10:14:01 UTC on March 28, 2011. Significant mHz quasi-periodic oscillations (QPO) at a frequency of 8.1 mHz are detected for approximately 1600 s from the beginning of the observation until the occurrence of a thermonuclear X-ray burst at 10:42:22 UTC. The mHz oscillations are not detected following the X-ray burst. The average fractional rms amplitude of the mHz QPOs is 6.4% (3 - 20 keV), and the amplitude increases to about 8% below 10 keV. Indeed, the mHz oscillations prior to the X-ray burst can be easily seen in a light curve binned at 16 s. The X-ray burst shows a few second rise and has a two component light curve. A fast component is characterized by a duration of about 20 s, and an extended, fainter component persists for about 200 s. Such behavior is likely due to a fuel mix that includes H and He, and has been noted in previous bursts observed from the source with RXTE. The persistent flux level prior to the X-ray burst is 6.1 mCrab, and increases to 8.4 mCrab 1.5 ksec after the burst. The mHz QPO phenomenology seen in 4U 1323-619 is strikingly similar to that seen in the LMXB 4U 1636-53 (Altamirano et al. 2008, ApJ, 673, L35). Indeed, the frequency of the mHz QPOs in 4U 1323-619 prior to the X-ray burst is very similar to the transition frequency between mHz QPO and bursts found in 4U 1636-53. These results strongly suggest that the observed QPOs in 4U 1323-619 are, like those in 4U 1636-53, due to marginally stable nuclear burning that occurs near the boundary of stability between unstable and stable nuclear burning.