Rapid evolution of variability properties in XTE J1817-330

ATel #752; Jeroen Homan (MIT), Jon M. Miller (Michigan), Rudy Wijnands (Amsterdam)
on 28 Feb 2006; 14:33 UT
Credential Certification: Jeroen Homan (jeroen@space.mit.edu)

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We observed the black hole candidate X-ray transient XTE J1817-330 five times with RXTE on February 24, between 06:03 UTC and 17:04 UTC. The unabsorbed 4-25 keV fluxes were close to 6.2e-10 ergs/cm^2/s in all five observations, only slightly higher than in our observation of February 13 (ATel #743). However, the 10-25 keV fluxes were more than 2-2.5 times higher in our latest observations, confirming the recent spectral hardening reported in ATel #738. The 4-10 keV fluxes were ~10-20% lower.

During the ~13 hours of observations we observed a remarkably rapid evolution of the variability properties of the source. The power spectra of the first two observations (06:03-08:00 UTC) showed band-limited noise with a QPO around 8.5 Hz. The rms variability in the 0.01-50 Hz range was ~11.5% (6-60 keV). The third observation (13:20-13:43 UTC) showed 1/f noise with a weak QPO around 6.4 Hz. The strength of the overall variability was only 2.6% rms. In the last two observations (UTC 14:55-17:04) the weak QPO was replaced by a more coherent and stronger QPO around 5.0 Hz, resulting in an increase in the overall variability to ~8% rms. Based on properties such as strength, Q-value, and frequency, we identify the QPOs in observations 1/2, 3, 4/5 as type C, type A, and type B, respectively (see Remillard et al. 2002, ApJ 564, 962 for a definition of these three QPO types in black hole X-ray binaries).

Plots of the power spectra can be found at http://tahti.mit.edu/1817/atel

The changes in the variability properties coincided with subtle changes in the spectral power-law index. For observations 1/2, 3, and 4/5 we measured indices of, 2.32+/-0.02, 2.41+/-0.03, and 2.44+/-0.02, using a similar spectral model as in ATel #743. The 10-25 keV flux varied by 15% between the observations, being the highest in the first two observations, and the lowest in the third. These observations show that significant changes in the variability properties do not necessarily require large spectra changes (at least not in the RXTE/PCA energy band).

Finally, we note that the sudden changes we observed in the power spectral properties (i.e. noise changing from band-limited to 1/f and QPOs from type C to type A/B) have previously been associated with relativistic jet/ejection events in the radio (see Fender et al. MNRAS, 355, 1105).