  We present the first MIR spectrum of the z=2.2856 ultraluminous, infrared
galaxy FSC 10214+4724, obtained with the Infrared Spectrograph onboard the
Spitzer Space Telescope. The spectrum spans a rest wavelength range of 2.3-11.5
microns, covering a number of key diagnostic emission and absorption features.
The most prominent feature in the IRS spectrum is the silicate emission at
rest-frame 10 microns. We also detect an unresolved emission line at a rest
wavelength of 7.65 microns which we identify with [NeVI], and a slightly
resolved feature at 5.6 microns identified as a blend of [Mg VII] and [Mg V].
There are no strong PAH emission features in the FSC 10214+4724 spectrum. We
place a limit of 0.1 micron on the equivalent width of 6.2 micron PAH emission
but see no evidence of a corresponding 7.7 micron feature. Semi-empirical fits
to the spectral energy distribution suggest about 45% of the bolometric
luminosity arises from cold 50 K dust, half arises from warm (190 K) dust, and
the remainder, 5%, originates from hot (640 K) dust. The hot dust is required
to fit the blue end of the steep MIR spectrum. The combination of a red
continuum, strong silicate emission, little or no PAH emission, and no silicate
absorption, makes FSC 10214+4724 unlike most other ULIRGs or AGN observed thus
far with IRS. These apparently contradictory properties may be explained by an
AGN which is highly magnified by the lens, masking a (dominant) overlying
starburst with unusually weak PAH emission.
