We present results from NuSTAR and XMM-Newton observations of the new black hole X-ray binary MAXI J1820+070 at low accretion rates ( below 1 % of the Eddington luminosity ) .
We detect a narrow Fe K \alpha emission line , in contrast to the broad and asymmetric Fe K \alpha line profiles commonly present in black hole binaries at high accretion rates .
The narrow line , with weak relativistic broadening , indicates that the Fe K \alpha line is produced at a large disk radius .
Fitting with disk reflection models assuming standard disk emissivity finds a large disk truncation radius ( a few tens to a few hundreds of gravitational radii , depending on the disk inclination ) .
In addition , we detect a quasi-periodic oscillation ( QPO ) varying in frequency between 11.6 \pm 0.2 mHz and 2.8 \pm 0.1 mHz .
The very low QPO frequencies suggest a large size for the optically-thin Comptonization region according to the Lense-Thirring precession model , supporting that the accretion disk recedes from the ISCO and is replaced by advection-dominated accretion flow at low accretion rates .
We also discuss the possibility of an alternative accretion geometry that the narrow Fe K \alpha line is produced by a lamppost corona with a large height illuminating the disk .