Exploring the role of jets in the radio/X-ray correlations of GX 339-4

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
² Massachusetts Institute of Technology, Center for Space Research Rm. NE80-6077, 77 Massachusetts Ave., Cambridge, MA 02139, USA
³ Université Paris VII and Service d'Astrophysique, CEA, CE-Saclay, 91191 Gif sur Yvette, France
⁴ Astronomical Institute “Anton Pannekoek” and Center for High Energy Astrophysics, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands

Corresponding author: S. Markoff, sera@space.mit.edu

Received: 17 July 2002
Accepted: 14 October 2002

Abstract

The Galactic black hole candidate X-ray binary GX 339-4 spends most of its time in the low/hard state, making it an ideal candidate for modeling the assumedly low accretion phase. The radio emission correlates very tightly with the X-rays over more than two orders of magnitude in X-ray flux density, suggesting that the jet plasma also plays a role at the higher frequencies. We compare the predictions of our jet model, with and without acceleration, to thirteen broadband simultaneous or quasi-simultaneous spectra over this changing flux history. In addition, we consider a simple standard thin disk which transitions to an optically thin accretion flow, in order to account for the assumedly thermal optical data seen in some observations. A solution without acceleration cannot describe the data without unrealistic energy requirements, nor explain the non-thermal radio spectrum seen during recent radio outbursts. But because of the low disk luminosity, and possibly the assumed disk geometry, acceleration in the jet is limited only by synchrotron cooling and can extend easily into the X-rays. We present a model which can account for all the broadband spectra included here, by changing only two parameters in the jet model: the input power and the location of the first acceleration zone. However, the model is most sensitive to changes in the jet power, the varying of which can also account for the slope of the observed radio/X-ray correlation analytically. At the highest low/hard state luminosities, the synchrotron self-Compton emission from the jet could be detectable with missions such as GLAST, providing a way to test the extent of the synchrotron contribution. We conclude that jet synchrotron is a possible way to explain the broadband features and this correlation, and discuss ways of incorporating this component into the “standard” corona picture.