  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.
