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Witnessing with the ATCA the interaction of a relativistic jet from GX 339-4 with the interstellar medium

ATel #2745; S. Corbel (Univ. Paris Diderot & CEA Saclay), J. Broderick (Univ. Southampton), D. Calvelo (Univ. Southampton), P. Kaaret (Univ. Iowa), C. Brocksopp (MSSL), J. Tomsick (SSL/UC Berkeley), J. Orosz (San Diego State Univ.), M. Coriat (CEA Saclay), R. Fender (Univ. Southampton) & T. Tzioumis (ATNF).
on 19 Jul 2010; 16:38 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: S. CORBEL (corbel@discovery.saclay.cea.fr)

Subjects: Radio, Infra-Red, Optical, X-ray, Request for Observations, Black Hole, Transient

We conducted radio observations (at 5.5 GHz and 9 GHz) of GX 339-4 on 2010 June 25 with the ATCA (Australia Telescope Compact Array) and the new CABB back-end. No radio emission is detected at the location of GX 339-4, which is consistent with the source being in the soft state. However, we notice a new radio source located very close to the core of the system, with radio flux density of 2.39 +/- 0.06 mJy at 5.5 GHz and 2.02 +/- 0.06 mJy at 9 GHz. The resulting spectral index is -0.34 +/- 0.08, which is typical of optically thin synchrotron emission.

The location of the new radio source on 2010 June 25 is R.A. (J2000): 17h 02m 49.25s and Dec. (J2000): -48° 47' 22.59" with a position uncertainty of the order of 0.2". Taking the location of the core from the initial hard state (ATel #2525), the separation from GX 339-4 is of the order of 1.9 arc-second. The new radio source is in the same direction as the large scale jets already detected in GX339-4 (Gallo et al. 2004, MNRAS, 347, L52).

The recent appearance of this radio source close to the black hole suggests that we are witnessing the impact of material ejected by the system -- during the recent state transition, see ATel #2577 -- with a denser phase of the interstellar medium (such as previously observed in XTE J1550-564, Corbel et al. 2002, Science, 298, 196). As this could potentially result in re-acceleration of particles up to very high energy, follow-up multi-wavelength observations are strongly encouraged in order to constrain the spectral energy distribution of this new source. Due to the current brightness of the black hole in X-rays, no X-ray observation can be currently conducted to look for extended features. However, we plan to trigger our Chandra proposal when GX 339-4 becomes fainter, if the new radio source still exists. With a lower optical extinction than XTE J1550-564 and a not too steep radio spectral index, observations in infrared and optical would be currently very important and are clearly reachable with 8-m class telescopes.

Assuming an ejection date at the recent state transition on April 20 (ATel #2577), an average proper motion of 34 mas/day is deduced. For a distance of 8 kpc, this translates into an apparent jet velocity of ~ 1.5 c, meaning that GX 339-4 clearly displays superluminal motion. Further radio observations should allow for constraints on possible deceleration of the jets.