On the infrared coincidence: what is the jet contribution to the X-ray power law in GX 339-4?

Abstract

The hard X-ray power law, prominent in the hard state in black hole X-ray binaries, is generally due to thermal Comptonization in the corona. Optically thin synchrotron emission from compact jets is commonly seen at infrared wavelengths in the hard state. The extent of this spectrum to higher energies remains uncertain. Here, a multi-wavelength study of GX 339-4 is presented. The IR to X-ray spectral index is measured and compared to the X-ray spectral index fitted separately. On some dates in which the jet dominates the IR emission, the X-ray power law and the IR to X-ray power law spectral indices are both in the range alpha = -0.7 +/- 0.2 (where F_nu ~ nu^alpha), i.e. photon index, Gamma = 1.7 +/- 0.2. This suggests they could be the same power law with the same origin, or that this is a coincidence. On other dates in the hard state, alpha_{IR-X} < alpha_{X}, ruling out a common origin. It is likely that Comptonization dominates on most dates, as expected. However, the X-ray power law never appears to be fainter than the jet power law extrapolated from IR to X-ray, implying that the jet contribution imposes a lower limit to the X-ray flux. If confirmed, this would imply the cooling break in the synchrotron spectrum probably resides at X-ray or higher energies. It is suggested that X-ray spectral fitting should include an extra power law with a break (ideally fit to IR too).

Accepted to Astronomische Nachrichten/Astronomical Notes (proceedings of XMM-Newton workshop 'Black Hole Accretion Under the X-ray Microscope', ESAC, Madrid, Spain, 14-17 June 2022)

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