Extinction and the Dimming of KIC 8462852

To test alternative hypotheses for the behavior of KIC 8462852, we obtained measurements of the star over a wide wavelength range from the UV to the mid-infrared from 2015 October through 2016 December, using Swift, Spitzer and AstroLAB IRIS. The star faded in a manner similar to the long-term fading seen in Kepler data about 1400 days previously. The dimming rate for the entire period reported is 22.1 ± 9.7 mmag yr^-1 in the Swift wavebands, with amounts of 21.0 ± 4.5 mmag in the ground-based B measurements, 14.0 ± 4.5 mmag in V, and 13.0 ± 4.5 in R, and a rate of 5.0 ± 1.2 mmag yr^-1 averaged over the two warm Spitzer bands. Although the dimming is small, it is seen at ≳3σ by three different observatories operating from the UV to the IR. The presence of long-term secular dimming means that previous spectral energy distribution models of the star based on photometric measurements taken years apart may not be accurate. We find that stellar models with {T}_{eff}=7000{--}7100 K and {A}_V∼ 0.73 best fit the Swift data from UV to optical. These models also show no excess in the near-simultaneous Spitzer photometry at 3.6 and 4.5 μm, although a longer wavelength excess from a substantial debris disk is still possible (e.g., as around Fomalhaut). The wavelength dependence of the fading favors a relatively neutral color (I.e., {R}_V≳ 5, but not flat across all the bands) compared with the extinction law for the general interstellar medium ({R}_V=3.1), suggesting that the dimming arises from circumstellar material.