Chandra observation of the relativistic binary J1906+0746

PSR J1906+0746 is a young radio pulsar (τ = 112 kyr, P = 144 ms) in a tight binary (P _orb = 3.98 hr) with a compact high-mass companion (M _comp ≃ 1.36 M _ȯ), at the distance of about 5 kpc. We observed this unique relativistic binary with the Chandra Advanced CCD Imaging Spectrometer detector for 31.6 ks. Surprisingly, not a single photon was detected within the 3″ radius from the J1906+0746 radio position. For a plausible range of hydrogen column densities, n _H = (0.5-1) × 10²² cm^-2, the nondetection corresponds to the 90% upper limit of (3-5) × 10³⁰ erg s^-1 on the unabsorbed 0.5-8 keV luminosity for the power-law model with Γ = 1.0-2.0, and 10 ³² erg s^-1 on the bolometric luminosity of the thermal emission from the neutrons star surface. The inferred limits are the lowest known for pulsars with spin-down properties similar to those of PSR J1906+0746. We have also tentatively detected a puzzling extended structure which looks like a tilted ring with a radius of 16 centered on the pulsar. The measured 0.5-8 keV flux of the feature, ≈3.1 × 10^-14 erg cm^-2 s^-1, implies an unabsorbed luminosity of 1.2 × 10³² erg s^-1 (4.5 × 10^-4 of the pulsar's ) for n _H = 0.7 × 10²² cm^-2. If the ring is not a peculiar noise artifact, the pulsar wind nebula around an unusually underluminous pulsar would be the most plausible interpretation.