Imaging search for the unseen companion to ε Ind A - improving the detection limits with 4 μm observations

epsilon Ind A is one of the nearest Sun-like stars, located only 3.6 pc away. It is known to host a binary brown dwarf companion, epsilon Ind Ba/Bb, at a large projected separation of 6.7 arcmin, but radial velocity measurements imply that an additional, yet unseen, component is present in the system, much closer to epsilon Ind A. Previously, direct imaging has excluded the presence of any stellar or high-mass brown dwarf companion at small separations, indicating that the unseen companion may be a low-mass brown dwarf or high-mass planet. We present the results of a deep high-contrast imaging search for the companion, using active angular differential imaging at 4 mu m, a particularly powerful technique for planet searches around nearby and relatively old stars. We also develop an additional point spread function reference subtraction scheme based on locally optimized combination of images to further enhance the detection limits. No companion is seen in the images, although we are sensitive to significantly lower masses than previously achieved. Combining the imaging data with the known radial velocity trend, we constrain the properties of the companion to within approximately 5-20 M-jap, 10-20 au and i > 20 degrees, unless it is an exotic stellar remnant. The results also imply that the system is probably older than the frequently assumed age of similar to 1 Gyr.