First detection of a type-I X-ray burst from the transient Z source XTE J1701-462

ATel #1144; Jeroen Homan (MIT), Tomaso Belloni (INAF/OAB), Rudy Wijnands, Michiel van der Klis (UvA), Jean Swank (NASA/GSFC), Evan Smith, Divya Pereira (NASA/GSFC/RSIS), Craig Markwardt (NASA/GSFC/UMD)
on 18 Jul 2007; 20:13 UT
Credential Certification: Jeroen Homan (jeroen@space.mit.edu)

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We report the first detection of a short type-I X-ray burst from the transient Z source XTE J1701-462 (Homan et al. 2007, ApJ, 656, 420). The detection was made with the RXTE/PCA in an observation that started on July 17 2007 12:18 UTC. A preliminary analysis showed that the burst reached a maximum bolometric black body flux of ~2.2e-8 ergs/cm^2/s (kT~2.1 keV, assuming an Nh of 2e22 atoms/cm^2) and had an exponential decay time of ~4.5 sec. No burst oscillations were found and we saw no indications for photospheric radius expansion, suggesting that the peak luminosity of the burst is below the Eddington luminosity. Based on the burst peak flux, and using the empirically determined Eddington limit for short bursts (Kuulkers et al. 2003, A&A, 399, 663), we put an upper limit on the distance to XTE J1701-462 of ~12.0 kpc. This is roughly consistent with the initial estimate of 14.7+/-3 kpc that was based on a comparison of color-color diagrams with Sco X-1 (Homan et al. 2007).

XTE J1701-462 is still being observed with the RXTE/PCA on a daily basis. After the initial peak of the outburst in January/February 2006 (~3.4e-8 erg/cm^2/s [2.5-50 keV]) the source was at a fairly stable level of ~0.3 Crab, but since early April 2007 the source flux has been steadily going down to ~4e-9 erg/cm^2/s on July 17. The decay has been strongest at low energies (<5 keV). Around the time of the start of the decay, the source also stopped showing the ~20-25 day flux cycles that were reported earlier (Homan et al. 2007). In the color-color diagram the source is now gradually evolving from Z source-like tracks to Atoll-like tracks. As for the luminous Atoll sources such as GX 9+9, only the spectrally soft "banana' branch is currently being traced out. The average power spectrum from the last two weeks is also similar to that of the luminous Atoll sources, in terms of strength (~4% rms, 0.1-100 Hz) and variability components (broad Lorentzian near 20 Hz, on top of red noise).

RXTE/PCA will continue to observe XTE J1701-462 on a daily basis.