VLBI detection of the Galactic black hole binary candidate MAXI J1836-194

TL;DR

This paper reports VLBI observations of the Galactic black hole candidate MAXI J1836-194, detecting a compact jet at AU scale, providing insights into its radio emission during the X-ray hard state.

Contribution

First VLBI detection of MAXI J1836-194's compact jet at multiple frequencies, demonstrating the presence of an AU-scale jet in this black hole candidate.

Findings

Detected unresolved source at 8.3 GHz indicating a compact jet

Achieved high-confidence detection with new Chinese VLBI system

Source size constrained to <=0.5 mas, consistent with AU-scale jet

Abstract

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 timescale 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 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 (CDAS), 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.