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

We present Spitzer Infrared Spectrograph (IRS) observations of eleven intermediate polars (IPs). Spectra covering the wavelength range from 5.2 to 14 μm are presented for all eleven 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 nearor mid-infrared spectra for any of the IPs investigated. Binning the spectra to create photometry, we find no evidence for infrared excesses at λ ≤ 12 μm. However, AE Aqr, and possibly EX Hya and V1223 Sgr, show longer wavelength excesses. The lack of discrete cyclotron features suggests that if there is any such emission in the bandpass of the IRS, it must be from optically thick cyclotron emission. 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. For those objects with high quality spectra, we find that any optically thick cyclotron emission must be less than 50% of the local continum. If cyclotron emission is occurring in the 5.2 to 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 are 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 (TBB = 140 K) blackbody. Given the apparent variability of this excess, however, synchrotron radiation remains a better explanation. We