The recent evolution of radio emission from GX 339-4

ATel #107; Rob Fender (Amsterdam), Stephane Corbel (Paris VII & CEA Saclay), Tasso Tzioumis (ATNF), Steven Tingay (ATNF), Catherine Brocksopp (Liverpool John Moores), Elena Gallo (Amsterdam)
on 19 Sep 2002; 11:38 UT
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Credential Certification: Rob Fender (rpf@astro.uva.nl)

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We report on the evolution of the radio emission from the black hole candidate GX 339-4, as observed with the Australia Telescope Compact Array (ATCA). These observations are associated with the re-appearance of GX 339-4 as a bright X-ray source following approximately three years in an "Off" state, and subsequent transition to the current extremely bright X-ray state.

Three sets of observations between MJD 52368-52382 revealed a radio source with an approximately flat spectrum between 1 - 9 GHz, at a level between 5 - 14 mJy, characteristic of the low/hard X-ray state.

The source subsequently brightened (and softened) dramatically in X-rays around MJD 52408, associated with which we have observed a rapid (hours) radio flare, during which GX 339-4 reached a peak flux density of approximately 50 mJy, the highest radio flux ever measured from this source.

Subsequent ATCA observations revealed a decaying, optically thin spectrum until MJD 52483, when the radio flux was observed to have again increased to more than 15 mJy at 5 GHz, again with an optically thin spectrum. Since then this radio emission has faded, with a flux density of ~3 mJy recorded on MJD 52516, and has shown evidence of spectral steepening.