For the first time ever, a multinational team of astronomers from France, the UK, the United States, Germany, Spain, the Netherlands and Australia, has detected emission from the base of black-hole jets.

“One of the best-studied stellar-mass black holes is the one hosted in the binary system GX 339-4. We can monitor its evolution quite closely because the source gives rise to bright outbursts every couple of years,” explained Dr Stéphane Corbel from Laboratoire AIM, France, who led the study of this system based on far-infrared (FIR) data from ESA’s Herschel Space Observatory and observations performed at X-ray, optical, near-infrared (NIR) and radio wavelengths.

“The multi-wavelength approach is essential for us to explore the vicinity of black holes, as different regions radiate at different wavelengths. Broadly speaking, the accretion disc shines most brightly in X-rays, whereas the jets emit mainly radio waves. But there is more: the base of the jets – closer to the black hole – emit light at shorter wavelengths than radio waves, up to the infrared: this is where Herschel’s contribution proved crucial.”

While GX 339-4 has been studied extensively at radio, NIR, optical and X-ray wavelengths, astronomers had rarely observed it in the vast portion of the spectrum between radio and NIR wavelengths. In fact, until now hardly any data from any stellar-mass black hole had been collected in this broad wavelength range.

The team used the Herschel to observe GX 339-4 after they detected changes to its X-ray emission signalling that the outburst phase of this source was about to cease.

“We believe that black-hole binaries give rise to outbursts when enough material has piled up in the accretion disc: then, just like a dam that bursts because it can no longer hold any more water, the material is accreted onto the black hole, giving rise to an enormous increase of the source’s emission at soft X-ray wavelengths,” Dr Corbel said.

The outburst phase is accompanied by the release of ‘ballistic jets’ – jets that are very bright at radio wavelengths, consist of multiple ejections and extend up to 10 000 Astronomical Units (AU). When the outburst is over and the source evolves to the so-called ‘hard’ state, the appearance of the jets changes: with weaker radio emission and an extent up to only about 10 AU, these are known as ‘compact jets’.

“We had been monitoring GX 339-4′s outburst across the electromagnetic spectrum for several months. When we saw that it was transitioning to a more quiescent state, we were extremely curious to see what would happen to the jets. It is the first time that we could witness the onset of compact jets and follow their evolution. By combining radio observations with Herschel’s FIR data, we could probe the jet emission down to the base, very close to the black hole.”

The results will be published in the Monthly Notices of the Royal Astronomical Society: Letters (arXiv.org version).

The Herschel data confirmed the current view, based on radio observations, which explains the emission from jets as synchrotron radiation released by highly-energetic electrons. In particular, the most energetic electrons, present at the base of the jets, radiate at FIR wavelengths, whilst the lower-energy ones, which are more abundant at larger distances from the black hole, give rise to radio emission.

The new data, however, raise questions about what causes the emission detected at NIR and optical wavelengths; this emission is also associated with the jets but does not seem to have the same origin as the radio and FIR emission. Since the optical and NIR emission follows that at radio and FIR wavelengths, one of the possible explanations is that radio and FIR photons emitted in the jets are then reflected off the disc, gaining energy in the process and thus radiating at shorter wavelengths.

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Bibliographic information: S. Corbel et al. 2013. Formation of the compact jets in the black hole GX 339-4. MNRAS Letter, in press; arXiv: 1303.2551