The INTEGRAL/IBIS Source AX J1838.0–0655: A Soft X-Ray-to-TeV γ-Ray Broadband Emitter

We report on INTEGRAL observations of AX J1838.0-0655, one of the unidentified objects listed in the first IBIS/ISGRI survey catalog and located in the Scutum arm region. This object, detected in the 20-300 keV band at a confidence level of 15.3 σ (9 × 10-11 ergs cm-2 s-1), is the likely counterpart of the still unidentified TeV source HESS J1837-069. It has been detected in the past by various X-ray telescopes, including ASCA, implying that it is a persistent rather than a transient source; the ASCA image is compatible with the source not being resolved. The broad 1-300 keV spectrum is characterized by an absorbed [NH = (6.7 ± 1.3) × 1022 cm-2] and hard (Γ = 1.5 ± 0.2) power-law continuum. Possible counterparts (radio and infrared) present within the X-ray error box are discussed, even if no clear association can be identified. The broadband spectrum together with the TeV detection suggest that AX J1838.0-0655 may be a supernova remnant or a pulsar wind nebula, which has so far eluded detection in the radio band. This is the second unidentified HESS source that shows a substantial soft gamma-ray emission.