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Light curve oscillations and flux decline in the Be/X-ray binary Swift J1626.6-5156

ATEL # 2036; M. E. DeCesar (University of Maryland), K. Pottschmidt (CRESST/GSFC/UMBC), J. Wilms (Dr. Karl Remeis-Observatory, Bamberg/ECAP, Erlangen)
on 28 Apr 2009; 16:31 UT
Password Certification: Katja Pottschmidt (katja@milkyway.gsfc.nasa.gov)

Subjects: X-ray, Binaries, Pulsars, Transients

We report on the long-term light curve periodicity and recent decline in flux of the Be/X-ray binary Swift J1626.6-5156 (Palmer et al., ATel #678). The compact object is a X-ray pulsar, with spin period ~15.37s (Markwardt & Swank, ATel #679). The source has been monitored by RXTE since late December 2005, and the light curve can be viewed through the link given below. On ~JD 2453910, or late June 2006, the source began to show quasi-periodic behavior in its X-ray flux. We have used a Lomb-Scargle periodogram with false alarm probability 0.01 (corresponding to a power threshold of 9.9) to find two dominant periodicities in the light curve. The section of the light curve from ~JD 2454000-2454350 (2006 September-2007 September) has a period of ~47 days, with a peak power density of 19.5. The next section of the light curve, from JD 2454359-2454790 (2007 September-2008 November) has a period of ~72.5 days, or ~3/2 the initial period, again with a power density of 19.5. The count rate during this time varied from ~4-20 c/s. On JD 2454791 (2008 November 11), the oscillatory behavior ceased, and the X-ray count rate dropped to ~1 count/s.

We speculate that the neutron star may orbit the Be star every ~23 days. During the early part of the oscillatory stage, the wind from the Be star takes 2 orbital periods to replenish the circumstellar disk, and in the later part it takes 3 orbital periods. We are working on an orbital solution to determine how accurate such a hypothesis may be.

We have obtained Swift data of the source before and after its fall to the current low luminosity state. These data include UVOT images in all 6 filters for photometry. We encourage spectral observations at optical wavelengths to determine the current state of the Be star's disk.

The 4-22 keV RXTE/PCA spectrum of the source during the oscillatory stage, built from the combination of observations with average count rate > 10 c/s, can be roughly described by an unabsorbed cut-off power law with spectral index 1.38 0.07 plus a Gaussian emission line at 6.48 0.02 keV. Fitting the spectrum of the low state with a simple, unabsorbed power law plus a Gaussian emission line at 6.34 0.13 keV yields a spectral index of 2.02 0.10.

RXTE-PCA long-term light curve


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