Constraining the Active Galactic Nucleus and Starburst Properties of the
  IR-luminous Quasar Host Galaxy APM 08279+5255 at Redshift 4 with SOFIA
  We present far-IR photometry and infrared spectrum of the z=3.9114
quasar/starburst composite system APM 08279+5255 obtained using the
Stratospheric Observatory for Infrared Astronomy (SOFIA)/HAWC+ and the Spitzer
Space Telescope Infrared Spectrograph (IRS). We decompose the IR-to-radio
spectral energy distribution (SED), sampled in 51 bands, using (i) a model
comprised of two-temperature modified blackbodies (MBB) and radio power-laws
and (ii) a semi-analytic model, which also accounts for emission from a clumpy
torus. The latter is more realistic but requires a well-sampled SED, as
possible here. In the former model, we find temperatures of T_warm = 296^17_15
K and T_cold = 110^3_3 K for the warm and cold dust components, respectively.
This model suggests that the cold dust component dominates the FIR energy
budget (66%) but contributes only 17% to the total IR luminosity. Based on the
torus models, we infer an inclination angle of i=15^8_8 degree and the presence
of silicate emission, in accordance with the Type-1 active galactic nucleus
nature of APM 08279+5255. Accounting for the torus' contribution to the FIR
luminosity, we find a lensing-corrected star formation rate of
SFR=3075x(4/mu_L) Msun yr^-1. We find that the central quasar contributes 30%
to the FIR luminosity but dominates the total IR luminosity (93%). The 30%
correction is in contrast to the 90% reported in previous work. In addition,
the IR luminosity inferred from the torus model is a factor of two higher.
These differences highlight the importance of adopting physically motivated
models to properly account for IR emission in high-z quasars, which is now
possible with SOFIA/HAWC+.
