Very long baseline interferometry detection of the Galactic black hole binary candidate MAXI J1836−194




The X-ray transient MAXI J1836−194 is a newly identified Galactic black hole binary candidate. As most X-ray transients, it was discovered at the beginning of an X-ray outburst. After the initial canonical X-ray hard state, the outburst evolved into a hard intermediate state and then went back to the hard state. The existing RATAN-600 radio monitoring observations revealed that it was variable on a time-scale of days and had a flat or inverted spectrum, consistent with optically thick synchrotron emission, possibly from a self-absorbed jet in the vicinity of the central compact object. We observed the transient in the hard state near the end of the X-ray outburst with the European very long baseline interferometry (VLBI) Network (EVN) at 5 GHz and the Chinese VLBI Network (CVN) at 2.3 and 8.3 GHz. The 8.3-GHz observations were carried out at a recording rate of 2048 Mbps using the newly developed Chinese VLBI data acquisition system, twice higher than the recording rate used in the other observations. We successfully detected the low-declination source with a high confidence level in both observations. The source was unresolved (≤0.5 mas), which is in agreement with an au-scale compact jet.