2023ApJ...955...44S

The early spectra of the kilonova AT2017gfo have a remarkably smooth blackbody continuum, which reveals information on the thermal properties and radioactive heating within the ejecta. However, the widespread use of a single-temperature blackbody to fit kilonova data is theoretically invalid, because (1) the significant travel-time delays for a rapidly cooling surface result in a broad distribution of temperatures and (2) the relativistic Doppler correction varies across different surface elements. Thus, the observed spectrum should be a modified blackbody with a range of temperatures over the surface. In this paper we quantify the impact of these effects and illustrate the typical wavelength-dependent spectral corrections. We apply the multitemperature blackbody framework to the first-epoch X-shooter AT2017gfo spectrum, to deconvolve the underlying physical temperature at the photosphere from the relativistic Doppler shift. We show that the cooling and Doppler effects individually result in a variation of temperatures over the photosphere of up to 30%, but in combination these effects nearly cancel and produce the single-temperature blackbody observed. Finally, we show that fitting the UV, optical, or near-infrared separately yields blackbody temperatures consistent at the percent level, which puts stringent limits on any proposed modification of the spectral continuum.