Kilohertz QPOs, spectral state transitions and the distance to the neutron star X-ray transient IGR J17473-2721

We report on RXTE monitoring observations of the transient X-ray binary IGR J17473-2721 since the beginning of its latest outburst on March 26th, 2008 (see ATEL #1445, #1459, #1460, #1461 and #1468). IGR J17473-2721 reached a maximum unabsorbed flux of ~1.30E-8 ergs cm-2 s-1 (2-10 keV, assuming nH=3.8e22cm-2, ATEL #1459) on June 22nd, and has been steadily decreasing since.

The X-ray spectral state (based on the X-ray colors in the 2-16 keV band) has switched from hard to soft and back to hard. These state changes are also evident from the Swift/BAT Hard X-ray monitoring light curves. During the soft state, 6 observations show a significant (between 3.5 and 10 sigma) kHz QPO at frequency up to ~900 Hz, that reach up to 8% rms amplitudes (2-60 keV).

The combination of energy spectral and power spectral variations are typical of neutron star atoll sources. RXTE has detected 16 type-I (thermonuclear) X-ray bursts so far during this outburst. Two bursts observed on July 17 and 19 exhibited photospheric radius expansion, reaching identical peak (bolometric) fluxes of 9.5e-8 erg/cm2/s.

The corresponding distance is likely between 4.9 kpc (using the theoretical Eddington limit for a pure He atmosphere) and 5.7 kpc (using the empirical value for the Eddington luminosity from Kuulkers et al. 2003, A&A, 399, 663). We found no evidence for burst oscillations.