Magnetic Field Upper Limits for Jet Formation

Context. Very high magnetic fields at the surface of neutron stars or in the accretion disk of black holes inhibit the production of jets. Aims. We quantify here the magnetic field strength for the jet formation. Methods. By using, the Alfvén Radius, RA, we study what we call the basic condition, RA/R∗ = 1 or RA/RLSO = 1 (LSO, last stable orbit), in its dependency with the magnetic field strength and the mass accretion rate, and we analyse these results in a 3-D and 2-D plot in the case of neutron star and black hole accretor systems respectively. For this purpose, we did a systematic search of all available observational data of the magnetic field strength and the mass accretion rate. Results. The association of a classical X-ray pulsar (i.e. B ∼ 1012 G) with jets is excluded even if accreting at the Eddington critical rate. Z-sources may develop jets for B < ∼ 10 8.2 G whereas Atoll-sources are potential sources for jets if B < ∼ 10 7.7 G. It is not ruled out that a millisecond X-ray pulsar could develop jets, at least for those sources where B < ∼ 10 7.5 G. In this case the millisecond X-ray pulsar could switch to a microquasar phase during its maximum accretion rate. For stellar-mass black hole X-ray Binaries, the condition is that B < ∼ 1.35 × 10 8 G and B < ∼ 5 × 10 8 G at the last stable orbit for a Schwarzschild and a Kerr black hole respectively. For Active Galactic Nuclei it reaches B < ∼ 10 5.9 G for each kind of black hole. Most of these general and theoretical results are in complete agreement with observational data. Radio emission in classical X-ray pulsars is up to now ruled out (Fender et al. 1997, Fender & Hendry 2000; Migliari & Fender 2006). The magnetic field strength has been determined in a Z-source (Scorpius X-1) using magnetoacoustic oscillations in kHz QPO reaching values of 107−8 G (Titarchuk et al. 2001). In the accreting millisecond X-ray pulsar SAX J1808.4-3658 hints for a radio jet have been found (Gaensler et al. 1999).