High Frequency Quasi-Periodic Oscillations in the X-ray Flux of the Black Hole Candidate H 1743-322 = IGR/XTE J1746(4)-3213

ATel #162; J. Homan (OAB, Merate), J. M. Miller (CfA), R. Wijnands (St. Andrew's), D. Steeghs (CfA), T. Belloni (OAB, Merate), M. van der Klis (Amsterdam), and W. H. G. Lewin (MIT)
on 31 May 2003; 17:13 UT
Credential Certification: Jon M. Miller (jmmiller@cfa.harvard.edu)

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We report on a ~25 ks RXTE observation of the soft X-ray transient and black hole candidate H 1743-322 (= IGR/XTE J1746(4)-3213, see ATELs 146, 142, 141, 140, 139, 138, 137, 136, 133, 132) performed on May 28 2003 (05:02 UTC - 16:53 UTC). The source was observed at a 3-100 keV flux of ~2.5x10e-8 ergs/cm^2/s with a spectrum that could be fitted well with a disk black body (kT~1.2 keV) and a power law (index~2.6) extending to above 200 keV with no apparent cutoff. Assuming a distance of 8.5~kpc, this translates to a luminosity of 2.2x10e+38 erg/s (3-100 keV). About two and a half hours after the start of the observation the source showed a sudden spectral hardening that occurred within 100 s and which was the result of the power law flux contribution increasing from 48% to 58%. After this transition a high frequency QPO was detected (~4.5 sigma) at a frequency of 240 Hz, with a width of 30 Hz and a strength of 2% rms (6-21 keV), making this source the 7th black hole candidate to show such fast variations. An additional QPO may be present (~2.5 sigma significance) around 160 Hz, suggestive of a 2:3 frequency ratio. At lower frequencies two sharp QPOs were present at 4.8 and 9.3 Hz before the transition, of which the first seemed to evolve into a less coherent and stronger asymmetric QPO around 6.6 Hz after the transition. The total power between 0.01 and 100 Hz remained more or less constant at a level of ~10% rms (6-21 keV). No QPOs were found in a ~17 ks RXTE observation performed on May 1 2003 when the source had a flux of ~2x10e-8 erg/cm^2/s but a considerably softer spectrum.

Recent RXTE/ASM observations suggest that moderate spectral transitions like the one reported above are lately occurring more frequently. We encourage multi-wavelength observations of the source to study the optical/IR and radio response to such X-ray transitions, which are probably related to changes in the inner regions of the accretion flow.