NuSTAR Hard X-ray Observation of the Gamma-ray Binary Candidate HESS J1832-093

Abstract

We present a hard X-ray observation of the TeV gamma-ray binary candidate HESS J1832−093, which is coincident with the supernova remnant G22.7−0.2, using the Nuclear Spectroscopic Telescope Array. Non-thermal X-ray emission from XMMU J183245−0921539, the X-ray source associated with HESS J1832−093, is detected up to ~30 keV and is well-described by an absorbed power-law model with a best-fit photon index Γ = 1.5 ± 0.1. A re-analysis of archival Chandra and XMM-Newtondata finds that the long-term X-ray flux increase of XMMU J183245−0921539 is 50^(+40)_(-20)% (90% C.L.), much less than previously reported. A search for a pulsar spin period or binary orbit modulation yields no significant signal to a pulse fraction limit of f_p < 19% in the range 4 ms < P < 40 ks. No red noise is detected in the FFT power spectrum to suggest active accretion from a binary system. While further evidence is required, we argue that the X-ray and gamma-ray properties of XMMU J183245−0921539 are most consistent with a non-accreting binary generating synchrotron X-rays from particle acceleration in the shock formed as a result of the pulsar and stellar wind collision. We also report on three nearby hard X-ray sources, one of which may be associated with diffuse emission from a fast-moving supernova fragment interacting with a dense molecular cloud.