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
Credential Certification: Katja Pottschmidt (katja@milkyway.gsfc.nasa.gov)
Subjects: X-ray, Binary, Transient, Pulsar
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