Discovery of 7-13 Hz quasi-periodic oscillations in the new X-ray transient XTE J1806-246
ATel #17; Rudy Wijnands & Michiel van der Klis (Astronomical Institute "Anton Pannekoek", University of Amsterdam)
on 8 May 1998; 01:06 UT
Credential Certification: Rudy Wijnands (rudy@astro.uva.nl)
Subjects: X-ray, Binary, Transient
We have discovered quasi-periodic oscillations (QPO) with frequencies
of 7-13 Hz (see the power density
spectrum) in the recently discovered X-ray transient XTE J1806-246
(IAUC 6891 ) in data obtained on May 3 1998 21:03-22:31 UT with the
proportional counter array (PCA) onboard the Rossi X-ray Timing
Explorer. The total PCA count rate varied between approximately 7000
and 8000 counts/sec. The rms amplitude of the QPO in the 2-60 keV
energy band varied between 2 and 6.5 % (rms) and the FWHM of the QPO
peak varied between 2 and 9.5 Hz. Their is no clear correlation
between X-ray count rate and the QPO parameters. In the X-ray
color-color diagram a two-branched structure was traced out. The QPO
parameters are well correlated with the position of the source on
these branches. During earlier observations on 1998 April 27, 28 and
29 the count rates were between 5000 and 7000 counts/sec. The 7-13 Hz
QPO was not detected during these observations. In the X-ray
color-color diagram those observations are slightly displaced from the
data obtained on May 3.
The frequency of the QPOs and the behavior of the QPO parameters with
respect to the position of the source on the two branches is similar
to the behavior of the 5-7 Hz QPO usually found in the brightest
low-mass X-ray binaries, the Z sources, which accrete matter from a
companion star at close to the Eddington mass accretion rate. It is
possible that XTE J1806-246 is similar to these systems. If so, then
it would be the first discovery of a Z source in over more then twenty
years. It would also be the first transient Z source known.
