Renewed radio activity from H1743-322 observed with the ATCA

ATel #1766; S. Corbel (Univ. Paris Diderot & CEA Saclay), T. Tzioumis (ATNF), M. Coriat (Univ. Paris Diderot & CEA Saclay), C. Brocksopp (MSSL) & R. Fender (Univ. Southampton)
on 7 Oct 2008; 12:45 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: S. CORBEL (corbel@discovery.saclay.cea.fr)

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Following the reactivation of H1743-322 (also known as IGR J17464-3213) as observed by INTEGRAL (Atel #1745 and #1739), we have conducted radio observations with the Australia Telescope Compact Array on 2008 October, 5 . A radio source is detected at a position consistent with the location of H1743-322 with preliminary flux densities of 2.30 +/- 0.10 mJy at 4.8 GHz and 1.80 +/- 0.15 mJy at 8.6 GHz, giving a spectral index of -0.42 +/- 0.16. The radio spectral index during this ATCA observation seems to favour optically thin synchrotron emission from H1743-322, which is surprising for a source that was still -- until recently -- in the hard state (Atel #1745 ). However, we note that now in the RXTE/ASM weather map (http://heasarc.gsfc.nasa.gov/cgi-bin/xte_weather/xte_lc?source=H1743-322), H1743-322 is detected at a level of ~5 c/s, while it seems to have reached a plateau in hard X-rays as observed with Swift/BAT (http://heasarc.gsfc.nasa.gov/docs/swift/results/transients/weak/IGRJ17464-3213 ). This seems to suggest that that H1743-322 may be undergoing a state transition from the hard state to a softer state. The detected radio emission would likely arises from the compact jets (possibly in the "unstable phase" observed during the brightest part of the initial hard state, see Fender et al. 2004, MNRAS 355, 1105). If this interpretation is correct, a major relativistic ejection event would then be expected in the coming days. Further observations are strongly encouraged to cover this transient episode.