ZTF SN~Ia DR2: Cosmology-independent constraints on Type Ia supernova standardisation from supernova siblings

Abstract

Understanding Type Ia supernovae (SNe~Ia) and the empirical standardisation relations that make them excellent distance indicators is vital to improving cosmological constraints. SN~Ia ``siblings", i.e. two or more SNe~Ia in the same host or parent galaxy offer a unique way to infer the standardisation relations and their diversity across the population. We analyse a sample of 25 SN~Ia pairs, observed homogeneously by the Zwicky Transient Factory (ZTF) to infer the SNe~Ia light curve width-luminosity and colour-luminosity parameters $α$ and $β$. Using the pairwise constraints from siblings, allowing for a diversity in the standardisation relations, we find $α= 0.218 \pm 0.055 $ and $β= 3.084 \pm 0.312$, respectively, with a dispersion in $α$ and $β$ of $\leq 0.195$ and $\leq 0.923$, respectively, at 95$\%$ C.L. While the median dispersion is large, the values within $\sim 1 σ$ are consistent with no dispersion. Hence, fitting for a single global standardisation relation, we find $α= 0.228 \pm 0.029 $ and $β= 3.160 \pm 0.191$. We find a very small intrinsic scatter of the siblings sample $σ_{\rm int} \leq 0.10$ at 95\% C.L. compared to $σ_{\rm int} = 0.22 \pm 0.04$ when computing the scatter using the Hubble residuals without comparing them as siblings. Splitting the sample based on host galaxy stellar mass, we find that SNe~Ia in both subsamples have consistent $α$ and $β$. The $β$ value is consistent with the value for the cosmological sample. However, we find a higher $α$ by $\sim 2.5 - 3.5 σ$. The high $α$ is driven by low $x_1$ pairs, potentially suggesting that the slow and fast declining SN~Ia have different slopes of the width-luminosity relation. We can confirm or refute this with increased statistics from near future time-domain surveys. (abridged)

Figures (7)

• Figure 1: The ZTF RGB image of an example sibling pair from our sample, ZTF20abatows and ZTF20abcawtk. The crosses mark the position of the SNe Ia in the field. These siblings were closest in the time separation ($\sim 5$ days between peak for the two SNe) between their peaks and hence, were detectable at the same time.
• Figure 2: Lightcurves in the ZTF $g,r,i$ filters along with the SALT2 fits overplotted for the SN Ia pair from the spec subsample with the largest difference in $x_1$, i.e. ZTF18abdmgab and ZTF20abqefja. As discussed in the text, the high $\Delta x_1$ (and low $\Delta c$) are important for constraining the width-luminosity relation. We can see the difference in $x_1$ in the lightcurve shapes of the two SNe Ia, as well as the time of the second maximum in the $i$-band and the $r$-band shoulder.
• Figure 3: Parameters for the sibling SNe Ia in this study. The panels show the redshift (top left), SALT2 $x_1$ (top right), $c$ (bottom left) and host galaxy stellar mass (bottom right) distributions. We do not make any selection cuts on the values of $z$, $x_1$ and $c$, unlike for the cosmological sample. The equivalent distribution for the entire DR2 sample is overplotted as dashed lines.
• Figure 4: The difference in the inferred SALT2 $m_B$ versus the difference in the inferred $x_1$ (top) and $c$ (bottom) for each siblings pair in the sample. The siblings with large differences in $x_1$ (and similar values of $c$) predominantly constraint $\alpha$ precisely whereas those with large $\Delta c$ constrain $\beta$. The color bar shows the $x_1$ for the wider SN (i.e. higher $x_1$; top) and $c$ for the redder (i.e. higher $c$; bottom) SN Ia in the pair.
• Figure 5: Constraints on $\alpha$, $\beta$ and the intrinsic dispersion ($\sigma_{\rm int}$) for the complete (black), spectroscopic (violet) and photometric (brown) samples.