  The potentially hazardous asteroid (PHA) (99942) Apophis is one of the most
remarkable near-Earth asteroids (NEA) in terms of impact hazard. A good
determination of its surface thermal inertia is very important in order to
evaluate the Yarkovsky effect on its orbital evolution. We present thermal
infrared observations obtained on January 29, 2013, with CanariCam mid-infrared
camera/spectrograph attached to the Gran Telescopio CANARIAS (GTC, Roque de los
Muchachos Observatory, La Palma, Spain) using the Si2-8.7, Si6-12.5, and
Q1-17.65 filters with the aim of deriving Apophis' diameter ($D$), geometric
albedo ($p_V$), and thermal inertia ($\Gamma$). We performed a detailed
thermophysical model analysis of the GTC data combined with previously
published thermal data obtained using Herschel Space Observatory PACS
instrument at 70, 100, and 160 $\mu$m.The thermophysical model fit of the data
favors low surface roughness solutions (within a range of roughness slope
angles $rms$ between 0.1 and 0.5), and constrains the effective diameter,
visible geometric albedo, and thermal inertia of Apophis to be $D_{eff} =$~380
-- 393 m, $p_V = $~0.24--0.33 (assuming absolute magnitude $H = 19.09 \pm
0.19$) and $\Gamma =$~50 -- 500 Jm$^{-2}$ s$^{-0.5}$ K$^{-1}$, respectively.
