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

ATel #1651; D. Altamirano (UvA), D. Galloway (Monash University), J. Chenevez (DTU/NSI, Denmark), J. in 't Zand (SRON), E. Kuulkers (ESA/ESAC, Spain), N. Degenaar(UvA), E. del Monte (INAF/IASF, Roma), M. Feroci (INAF/IASF, Roma), E. Costa (INAF/IASF, Roma), Y. Evangelista (INAF/IASF, Roma), M. Falanga (CEA, Saclay), C. Markwardt (U. Md./CRESST/NASA GSFC), R. Wijnands (UvA) & M. van der Klis (UvA).
on 8 Aug 2008; 10:51 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Diego Altamirano (diego@science.uva.nl)

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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.

We thank Jean Swank and the RXTE team for scheduling the observations.