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ATEL # 1165; Jeroen Homan (MIT), Rudy Wijnands, Diego Altamirano (UvA), Tomaso Belloni (INAF/OAB)
on 1 Aug 2007; 14:57 UT
Password Certification: Jeroen Homan (firstname.lastname@example.org)
Subjects: X-ray, Neutron Stars, Transients
Following our last report on the outburst of the transient neutron star low-mass X-ray binary XTE J1701-462 (ATel #1144), we have continued observing the source with RXTE on a daily basis. During the last week we found that the decay rate of the source increased significantly compared to before. While the light curve from the period between early May and July 21 could be fitted with an exponential decay time scale of ~90 days, in the period after July 21 the exponential decay time has shortened to ~4.5 days. The 2.5--50 keV flux on August 1 was ~3.2e-10 erg/s/cm^2, more than a factor 10 lower than the flux we reported for July 17.
In addition to the type I X-ray burst observed on July 17 (ATel #1144), two more bursts were seen last week (no indications for burst oscillations). In the same period we also observed very narrow (Q~200) kHz quasi-periodic oscillations (QPO) around 800 Hz and in the last few days we detected an increase in the strength of the broadband variability (to ~15% rms [0.01-100 Hz]). In the color-color diagram the source has started to trace out what appears to be a so-called 'island state', which, combined with the burst rate and properties of the kHz QPO and broadband variability, suggests that XTE J1701-462 has fully transitioned from a Z source to an Atoll source. This is the first time such a transition has unambiguously been observed in a neutron-star low-mass X-ray binary.
The rapid decay of the source to quiescence will be followed further with RXTE, Swift, Chandra and XMM-Newton. Observations at other wavelengths are strongly encouraged.