Low-energy calibration of XENON1T with an internal ${}^{37}$Ar source

A low-energy electronic recoil calibration of XENON1T, a dual-phase xenon time projection chamber, with an internal $^{37}$Ar source was performed. This calibration source features a 35-day half-life and provides two mono-energetic lines at 2.82 keV and 0.27 keV. The photon yield and electron yield at 2.82 keV are measured to be ($32.3\,\pm \,0.3$) photons/keV and ($$40.6\,\pm \,0.5$$) electrons/keV, respectively, in agreement with other measurements and with NEST predictions. The electron yield at 0.27 keV is also measured and it is ($68.0^{+6.3}_{-3.7}$) electrons/keV. The ${}^{37}$Ar calibration confirms that the detector is well-understood in the energy region close to the detection threshold, with the 2.82 keV line reconstructed at ($2.83\,\pm \,0.02$) keV, which further validates the model used to interpret the low-energy electronic recoil excess previously reported by XENON1T. The ability to efficiently remove argon with cryogenic distillation after the calibration proves that ${}^{37}$Ar can be considered as a regular calibration source for multi-tonne xenon detectors.