VLBI 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 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.

💡 Research Summary

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The paper reports very‑long‑baseline interferometry (VLBI) observations of the newly discovered X‑ray transient MAXI J1836‑194, a strong candidate for a Galactic black‑hole binary (BHB). After its discovery at the onset of an X‑ray outburst, the source followed the canonical spectral evolution of BHBs: an initial hard state, a transition to a hard‑intermediate state, and a return to the hard state toward the end of the outburst. Continuous monitoring with the RATAN‑600 radio telescope revealed day‑scale flux variability and a flat or slightly inverted radio spectrum (spectral index α≈0 to +0.2), characteristics typical of optically thick synchrotron emission from a compact, self‑absorbed jet close to the compact object.

To directly probe the radio emitting region, the authors performed VLBI imaging during the final hard‑state phase. Two independent campaigns were carried out. The first used the European VLBI Network (EVN) at 5 GHz in September 2011, employing a standard 128 MHz bandwidth and a recording rate of 2 Gbps. The second campaign, in October 2011, used the Chinese VLBI Network (CVN) simultaneously at 2.3 GHz and 8.3 GHz. Crucially, the 8.3 GHz observations exploited the newly developed Chinese VLBI Data Acquisition System (CDAS), which recorded at 2048 Mbps—twice the rate of the other sessions—thereby improving sensitivity and uv‑coverage.

Data reduction with AIPS and DIFMAP included fringe fitting, amplitude calibration, and self‑calibration. Both observations detected the source with high significance. The measured flux densities were ≈2.1 mJy at 5 GHz and ≈1.8 mJy at 8.3 GHz. The source remained unresolved at the longest baselines, with an upper limit on its angular size of ≤0.5 mas. Assuming a distance of ~8 kpc, this corresponds to a physical scale of ≤4 AU, indicating an extremely compact emitting region. The implied brightness temperature exceeds 10⁹ K, consistent with a partially self‑absorbed jet rather than thermal emission.

Spectral analysis between the two frequencies yields a slightly positive index (α≈+0.1), confirming a flat or mildly inverted spectrum typical of the “core” of a compact jet in the hard state. The detection of such a compact, high‑brightness component during the late hard state demonstrates that the jet remained active throughout the outburst decay. The size constraint places MAXI J1836‑194 among other well‑studied BHBs (e.g., Cyg X‑1, GRS 1915+105) that exhibit AU‑scale jets, supporting the universality of jet formation mechanisms in accreting black holes.

Beyond the astrophysical results, the work showcases the technical advantage of the CDAS system. The doubled recording rate directly translated into improved sensitivity, enabling a reliable detection of a low‑declination source that is otherwise challenging for traditional VLBI arrays. This demonstrates that high‑rate recording can extend VLBI capabilities to southern sky targets and low‑flux objects, opening new avenues for rapid follow‑up of transient phenomena.

In summary, the VLBI observations provide decisive evidence that MAXI J1836‑194 hosts a compact, self‑absorbed synchrotron jet on AU scales, confirming its status as a black‑hole binary candidate. The successful use of ultra‑high‑rate recording with the Chinese network highlights a promising path forward for high‑resolution radio studies of Galactic transients, especially those located at low declinations.