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2HWC J1928+177 is a Galactic TeV gamma-ray source detected by the HighAltitude Water Cherenkov (HAWC) Observatory up to ~ 56 TeV. The HAWC source,later confirmed by H.E.S.S., still remains unidentified as a dark acceleratorsince there is no apparent supernova remnant or pulsar wind nebula detected inthe lower energy bands. The radio pulsar PSR J1928+1746, coinciding with theHAWC source position, has no X-ray counterpart. Our SED modeling shows thatinverse Compton scattering in the putative pulsar wind nebula can account forthe TeV emission only if the unseen nebula is extended beyond r ~ 4 [arcmin].Alternatively, TeV gamma rays may be produced by hadronic interactions betweenrelativistic protons from an undetected supernova remnant associated with theradio pulsar and a nearby molecular cloud G52.9+0.1. NuSTAR and Chandraobservations detected a variable X-ray point source within the HAWC errorcircle, potentially associated with a bright IR source. The X-ray spectra canbe fitted with an absorbed power-law model with $N_{\rm H} =(9\pm3)\times10^{22}$ cm$^{-2}$ and $\Gamma_X = 1.6\pm0.3$ and exhibitlong-term X-ray flux variability over the last decade. If the X-ray source,possibly associated with the IR source (likely an O star), is the counterpartof the HAWC source, it may be a new TeV gamma-ray binary powered by collisionsbetween the pulsar wind and stellar wind. Follow-up X-ray observations arewarranted to search for diffuse X-ray emission and determine the nature of theHAWC source.

Multi-wavelength observations of 2HWC J1928+177: dark accelerator or new TeV gamma-ray binary?