Spitzer IRS Spectroscopy of Intermediate Polars: Constraints on Mid-Infrared Cyclotron Emission

We present Spitzer IRS observations of 11 intermediate polars (IPs). Spectra covering the wavelength range from 5.2 to 14 μm are presented for all 11 objects, and longer wavelength spectra are presented for three objects (AE Aqr, EX Hya, and V1223 Sgr). We also present new, moderate-resolution (R~2000) near-infrared spectra for five of the program objects. We find that, in general, the mid-infrared spectra are consistent with simple power laws that extend from the optical into the mid-infrared. There is no evidence for discrete cyclotron emission features in the near- or mid-infrared spectra for any of the IPs investigated, nor for infrared excesses at λ<=12 μm. However, AE Aqr, and possibly EX Hya and V1223 Sgr, do show longer wavelength excesses. We have used a cyclotron modeling code to put limits on the amount of such emission for magnetic field strengths of 1<=B<=7 MG. If cyclotron emission is occurring in the 5.2-14.0 μm bandpass, it constitutes less than 1% of the bolometric luminosity of any of the IPs. We were able to model the long-wavelength excess of V1223 Sgr and EX Hya with cyclotron emission from a 1 MG field, but the S/N of those data is very poor, and the reality of those excesses is not established. We attempted to model the long-wavelength excess of AE Aqr with cyclotron emission, but none of our models fit nearly as well as a simple, cool (T_BB=140 K) blackbody. Given the apparent variability of this excess, however, synchrotron radiation remains a better explanation. We discuss our results in the context of the standard model for IPs.