Unmasking LHAASO J2108 + 5157: near-infrared insights into a mysterious TeV source

ABSTRACT LHAASO J2108 + 5157 is one of the few ultra-high energy $\gamma$-ray sources in the LHAASO catalogue without secure counterpart at longer wavelengths. Several Galactic scenarios have been proposed, including an evolved supernova remnant and a pulsar wind nebula. Yet, no shocked gas, shell-like structure, or compact pulsar candidate has been identified. Follow-up observations with VERITAS and the LST-1 prototype have not firmly clarified its nature. A recent microquasar candidate from GMRT radio data remains uncertain. Here, we present the first dedicated near-infrared study of the field, combining deep $JHK_s$ imaging with narrow band observations targeting the H$_2$ v = 1–0 S(1) line. Our observations were initially planned to encompass the full source region, but now only partially cover the latest updated position and size of LHAASO J2108 + 5157. We find no evidence of shocked emission, extended nebular structures, or an accreting compact object signature in the covered field. The GMRT radio source, despite its jet-like morphology, exhibits near-infrared properties incompatible with both a Galactic microquasar and a nearby radio galaxy, discouraging an association with the gamma-ray emission. Our analysis reveals no convincing counterpart within the positional uncertainty, leaving LHAASO J2108 + 5157 as an enigmatic ultra-high energy emitter that requires deeper observations.