One distinctive feature of low-luminosity active galactic nuclei ( LLAGN ) is the relatively weak reflection features they may display in the X-ray spectrum , which can result from the disappearance of the torus with decreasing accretion rates .
Some material , however , must surround the active nucleus , i.e. , the accretion flow itself and , possibly , a flattened-out or thinned torus .
In this work , we study whether reflection is indeed absent or undetectable due to its intrinsically weak features together with the low statistics inherent to LLAGN .
Here we focus on NGC 3718 ( L / L _ { Edd } \sim 10 ^ { -5 } ) combining observations from XMM–Newton and the deepest to date NuSTAR ( 0.5–79 keV ) spectrum of a LLAGN , to constrain potential reflectors , and analyze how the fitted coronal parameters depend on the reflection model .
We test models representing both an accretion disc ( Relxill ) and a torus-like ( MYTorus and Borus ) neutral reflector .
From a statistical point of view , reflection is not required , but its inclusion allows to place strong constraints on the geometry and physical features of the surroundings : both neutral reflectors ( torus ) tested should be Compton thin ( N _ { H } < 10 ^ { 23.2 } cm ^ { -2 } ) and preferentially cover a large fraction of the sky .
If the reflected light instead arises from an ionized reflector , a highly ionized case is preferred .
These models produce an intrinsic power-law spectral index in the range [ 1.81–1.87 ] , where the torus models result in steeper slopes .
The cut-off energy of the power-law emission also changes with the inclusion of reflection models , resulting in constrained values for the disc reflectors and unconstrained values for torus reflectors .