VLBI observations of the shortest orbital period black hole binary, MAXI J1659-152

Abstract: The X-ray transient MAXI J1659-152 was discovered by Swift/BAT and it was initially identified as a GRB. Soon its Galactic origin and binary nature were established. There exists a wealth of multi-wavelength monitoring data for this source, providing a great coverage of the full X-ray transition in this candidate black hole binary system. We obtained two epochs of European VLBI Network (EVN) electronic-VLBI (e-VLBI) and four epochs of Very Long Baseline Array (VLBA) data of MAXI J1659-152 which show evidence for outflow in the early phases. The overall source properties (polarization, milliarcsecond-scale radio structure, flat radio spectrum) are described well with the presence of a compact jet in the system through the transition from the hard-intermediate to the soft X-ray spectral state. The apparent dependence of source size and the radio core position on the observed flux density (luminosity dependent core shift) support this interpretation as well. We see no evidence for major discrete ejecta during the outburst. For the source proper motion we derive 2 sigma upper limits of 115 microas/day in right ascension, and 37 microas/day in declination, over a time baseline of 12 days. These correspond to velocities of 1400 km/s and 440 km/s, respectively, assuming a source distance of 7 kpc.