Chandra View of the Unidentified TeV Gamma-ray Source HESS J1804-216

We present high-resolution X-ray images taken with the Chandra X-ray Observatory of the field that contains the unidentified TeV gamma-ray source HESS J1804-216. A total of eleven discrete sources were detected with a posteriori significance of > 5 sigma over the entire field of view. Among them, only one, designated as CXOU J180351.4-213707, is significantly extended. The source is about 40" away from the radio pulsar PSR J1803-2137, which was the target of the Chandra observation but was not detected in X-rays. A natural question is whether the two sources are physically related. While it is conceivable that CXOU J180351.4-213707 could be associated with a previously unknown supernova remnant (SNR), in which the pulsar was born, it seems equally plausible that it might be a pulsar wind nebula (PWN) that is powered by a different pulsar whose emission is beamed away from us. In either case, we argue that CXOU J180351.4-213707 is likely the X-ray counterpart of HESS J1804-216, based on the fact that the Galactic TeV gamma-ray sources are predominantly SNRs or PWNe. The X-ray spectrum of the source can be fitted well with a power law, although the model is not well constrained due to large statistical uncertainties. The spectrum seems to be very hard, with the best-fit photon index about 1.2. Under the assumption that CXOU J180351.4-213707 is the X-ray counterpart of HESS J1804-216, we attempted to model the X-ray and TeV emission as synchrotron and inverse Compton scattered radiation from relativistic electrons. We briefly discuss the results.