Two c's in a pod: Cosmology independent measurement of the Type Ia
  supernova colour-luminosity relation with a sibling pair
  Using Zwicky Transient Facility (ZTF) observations, we identify a pair of
"sibling" Type Ia supernovae (SNe Ia), i.e., hosted by the same galaxy at z =
0.0541. They exploded within 200 days from each other at a separation of
$0.6^{"} $ corresponding to a projected distance of only 0.6 kpc. Performing
SALT2 light curve fits to the gri ZTF photometry, we show that for these
equally distant "standardizable candles", there is a difference of 2 magnitudes
in their rest frame B-band peaks, and the fainter SN has a significantly red
SALT2 colour $c = 0.57 \pm$ 0.04, while the stretch values $x_1$ of the two SNe
are similar, suggesting that the fainter SN is attenuated by dust in the
interstellar medium of the host galaxy. We use these measurements to infer the
SALT2 colour standardization parameter, $\beta$ = 3.5 $\pm$ 0.3, independent of
the underlying cosmology and Malmquist bias. Assuming the colour excess is
entirely due to dust, the result differs by $2\sigma$ from the average
Milky-Way total-to-selective extinction ratio, but is in good agreement with
the colour-brightness corrections empirically derived from the most recent SN
Ia Hubble-Lemaitre diagram fits. Thus we suggest that SN "siblings", which will
increasingly be discovered in the coming years, can be used to probe the
validity of the colour and lightcurve shape corrections using in SN Ia
cosmology while avoiding important systematic effects in their inference from
global multi-parameter fits to inhomogeneous data-sets, and also help constrain
the role of interstellar dust in SN Ia cosmology.
