The X-shooter Spectral Library (XSL): Data Release 3
  We present the third data release (DR3) of the X-shooter Spectral Library
(XSL). This moderate-to-high resolution, near-ultraviolet-to-near-infrared
($350-2480$ nm, R $\sim$ 10 000) spectral library is composed of 830 stellar
spectra of 683 stars. DR3 improves upon the previous data release by providing
the combined de-reddened spectra of the three X-shooter segments over the full
$350-2480$ nm wavelength range. It also includes additional 20 M-dwarf spectra
from the ESO archive. We provide detailed comparisons between this library and
Gaia EDR3, MILES, NGSL, CaT library, and (E-)IRTF. The normalised rms deviation
is better than $D=0.05$ or 5$\%$ for the majority of spectra in common between
MILES (144 spectra of 180), NGSL (112$/$116), and (E-)IRTF (55$/$77) libraries.
Comparing synthetic colours of those spectra reveals only negligible offsets
and small rms scatter, such as the median offset(rms) 0.001$\pm$0.040 mag in
the (box1-box2) colour of the UVB arm,-0.004$\pm$0.028 mag in (box3-box4) of
the VIS arm, and -0.001$\pm$0.045 mag in (box2-box3) colour between the UVB and
VIS arms, when comparing stars in common with MILES. We also find an excellent
agreement between the Gaia published (BP-RP) colours and those measured from
the XSL DR3 spectra, with a zero median offset and an rms scatter of 0.037 mag
for 449 non-variable stars. The unmatched characteristics of this library,
which combine a relatively high resolution, a large number of stars, and an
extended wavelength coverage, will help us to bridge the gap between the
optical and the near-IR studies of intermediate and old stellar populations,
and to probe low-mass stellar systems.
