HARD X-RAY EMISSION FROM SH 2‐104: A NuSTAR SEARCH FOR GAMMA-RAY COUNTERPARTS

We present NuSTAR hard X-ray observations of Sh 2–104, a compact HII region containing several young massive stellar clusters (YMSCs). We have detected distinct hard X-ray sources coincident with localized VERITAS TeV emission recently resolved from the giant gamma-ray complex MGRO J2019+37 in the Cygnus region. Faint, diffuse X-ray emission coincident with the eastern YMSC in Sh 2–104 is likely the result of colliding winds of component stars. Just outside the radio shell of Sh 2–104 lies 3XMM J201744.7+365045 and a nearby nebula NuSTAR J201744.3+364812, whose properties are most consistent with extragalactic objects. The combined XMM-Newton and NuSTAR spectrum of 3XMM J201744.7+365045 is well-fit to an absorbed power-law model with NH = (3.1 ± 1.0) × 10 22 cm and photon index Γ = 2.1 ± 0.1. Based on possible long-term flux variation and the lack of detected pulsations (≤ 43% modulation), this object is likely a background AGN rather than a Galactic pulsar. The spectrum of the NuSTAR nebula shows evidence of an emission line at E = 5.6 keV suggesting an optically obscured galaxy cluster at z = 0.19 ± 0.02 (d = 800 Mpc) and LX = 1.2 × 10 44 erg s. Follow-up Chandra observations of Sh 2–104 will help identify the nature of the X-ray sources and their relation to MGRO J2019+37. We also show that the putative VERITAS gamma-ray excess south of Sh 2–104 is most likely associated with the newly discovered Fermi pulsar PSR J2017+3625 and not the HII region. Subject headings: ISM: individual (MGRO J2019+37, VER J2019+368, VER J2016+371, 3FGL J2021.1+3651, 3FGL J2017.9+3627, 3FGL J2015.6+3709) — pulsars: individual (PSR J2021+3651, PSR J2017+3625) — stars: neutron — supernova remnants