Rapid decay of the neutron star transient XTE J1701-462
ATel #1165; Jeroen Homan (MIT), Rudy Wijnands, Diego Altamirano (UvA), Tomaso Belloni (INAF/OAB)
on 1 Aug 2007; 14:57 UT
Credential Certification: Jeroen Homan (jeroen@space.mit.edu)
Subjects: X-ray, Neutron Star, Transient
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.