Spectral evolution and kHz QPOs in the transient Z source XTE J1701-462

ATel #748; Jeroen Homan (MIT), Tomaso Belloni (INAF/OAB), Michiel van der Klis, Piergiorgio Casella (U. Amsterdam), Mariano Mendez (SRON), Walter Lewin (MIT), Rob Fender (U. Southampton), Elena Gallo (UCSB), and Neil Gehrels (NASA/GSFC)
on 24 Feb 2006; 20:59 UT
Credential Certification: Jeroen Homan (jeroen@space.mit.edu)

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Since our last report on the transient Z source XTE J1701-462 (ATel #725), RXTE has continued observing the source on an almost daily basis. Spectral evolution similar to that reported by Homan et al. (ATel #725) is being observed, albeit at progressively lower luminosities, suggesting that the source has begun its decay. By following the count rate of a fixed location along the 'Z-track' (we chose the middle of the normal branch) we estimate that the luminosity has decreased by a factor of two since maximum. In addition to the lower luminosities the overall shapes in the color-color and color-intensity diagrams seem to be evolving and moving to harder colors. In particular, the upturn in the horizontal branch and the dips observed in the flaring branch are no longer observed. This suggests that the source might be evolving from a 'Cyg X-2 like' Z source to a 'Sco X-1 like' Z source (see Kuulkers et al. 1994, A&A, 289, 795 for a discussion).

Lately, the source has spent a relatively large fraction of its time on the normal and flaring branches. During one brief excursion onto the horizontal branch we saw strong indications for twin kHz QPOs at 651 Hz and 908 Hz. Although these detections are only a little more significant than 3 sigma (single trial), it should be noted that their frequencies fall right on top of well established frequency-frequency relations for similar pairs of kHz QPO in other neutron star LMXBs.

Finally, the broad bump in the power spectrum around 6 Hz that we tentatively identified as the normal-branch QPO (ATel #725) has evolved into a narrower feature, just like the normal-branch QPO seen in other Z sources.

More RXTE observations have been planned.