🔭 astrophysics

Distance estimate to NGC 6951 from supernova siblings Type IIP SN 2020dpw and Type Ib SN 2021sjt

This paper presents new distance estimates to the galaxy NGC 6951 by applying the expanding photosphere method to its supernova siblings, Type IIP SN 2020dpw and Type Ib SN 2021sjt, yielding consistent results that validate the technique's applicability to Type Ib supernovae.

Original authors: Réka Könyves-Tóth, Zsófia Bodola, Tamás Szalai, Moira Andrews, Iair Arcavi, Dominik Bánhidi, Imre Barna Bíró, Federica Bianco, Jamison Burke, István Csányi, Benjamin Dilday, Joseph R. Farah, Estefania

Published 2026-03-27

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Réka Könyves-Tóth, Zsófia Bodola, Tamás Szalai, Moira Andrews, Iair Arcavi, Dominik Bánhidi, Imre Barna Bíró, Federica Bianco, Jamison Burke, István Csányi, Benjamin Dilday, Joseph R. Farah, Estefania Padilla Gonzalez, Daichi Hiramatsu, D. Andrew Howell, Levente Kriskovics, Megan Newsome, Yuan Qi Ni, Curtis McCully, András Ordasi, András Pál, Craig Pellegrino, Róbert Szakáts, Giacomo Terreran, Stefano Valenti, Krisztián Vida, Kathryn Wynn

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you are trying to measure the distance to a lighthouse in the middle of a vast, foggy ocean. You can't just walk there with a tape measure. Instead, you have to guess how far away it is based on how bright it looks and how big it appears. But here's the problem: if you don't know exactly how powerful the lighthouse bulb is, or if the fog is thicker than you think, your guess could be way off.

This is exactly the challenge astronomers face when trying to measure the distance to galaxies. In this paper, a team of researchers acts like cosmic detectives to solve this puzzle for a galaxy called NGC 6951.

Here is the story of how they did it, explained simply.

The "Supernova Sibling" Trick

Usually, astronomers look at one exploding star (a supernova) to guess how far away its home galaxy is. But that's like trying to guess the size of a city by looking at just one streetlamp. It's risky because you might be wrong about how bright that specific lamp is.

This team got lucky. They found a galaxy that had five different stars explode in it over the last few decades. Think of these as "siblings." Since they all live in the same house (the galaxy), they must all be at the exact same distance from Earth.

The researchers focused on two of these siblings:

SN 2020dpw: A Type IIP supernova (a massive star that ran out of fuel and collapsed).
SN 2021sjt: A Type Ib supernova (a star that had its outer layers stripped away before exploding).

By comparing these two, they could cross-check their math. If both methods point to the same distance, they know they are on the right track.

The "Expanding Balloon" Method

To measure the distance, the team used a clever technique called the Expanding Photosphere Method (EPM).

Imagine a supernova as a giant, glowing balloon being blown up in the dark.

The Speed: Astronomers can measure how fast the "balloon" is expanding by looking at the color of the light (the faster it moves, the more the color shifts).
The Size: They also measure how bright the balloon is. If they know how fast it's expanding and how bright it is, they can calculate how big the balloon actually is.
The Distance: Once they know the real size of the balloon and how big it looks in the sky (its apparent size), they can use simple geometry to figure out how far away it must be.

It's like if you see a car driving away. If you know the car is moving at 60 mph and you know how big a car actually is, you can figure out how far away it is just by looking at how small it appears in your rearview mirror.

The Hurdles: Fog and Glare

The researchers faced two main problems, which they had to correct for:

The "Fog" (Dust): Space isn't empty; it's full of dust that dims the light, making stars look farther away than they are. The team had to calculate exactly how much "fog" was between Earth and the galaxy and subtract it from their measurements.
The "Glare" (Dilution): The light from a supernova isn't a perfect, clean beam; it gets scattered by electrons, making it look "diluted" or less bright than a perfect light bulb. The team had to use complex math (like a special filter) to correct for this glare. They even had to invent a new version of this math for the Type Ib star, because it behaves differently than the Type IIP star.

The Results: A New Ruler for the Galaxy

After doing all the math, correcting for the fog, and adjusting for the glare, the team got their answer:

SN 2020dpw suggested the galaxy is about 25.8 million light-years away.
SN 21sjt suggested it is about 24.6 million light-years away.

These two numbers are very close to each other (within the margin of error), which is a huge success! It means their "balloon" method works for both types of exploding stars.

Why Does This Matter?

Think of the universe as a giant map. To draw the map correctly, we need to know the distances between the "cities" (galaxies). If our distance measurements are wrong, our whole map of the universe is distorted.

This paper is important because:

It confirms the distance to NGC 6951, making our cosmic map more accurate.
It proves the method works on Type Ib supernovae, which are trickier to measure. This gives astronomers a new tool to measure distances to other galaxies that have these specific types of explosions.

In short, by watching two "siblings" explode in the same neighborhood, these astronomers were able to measure the distance to that neighborhood with much greater confidence than before, helping us understand the true scale of our universe.

1. Problem Statement

Accurate extragalactic distance measurements are fundamental to cosmology, yet they often suffer from significant systematic uncertainties. While "supernova siblings" (multiple supernovae exploding in the same host galaxy) offer a unique opportunity to cross-calibrate distance measurement techniques and reduce these uncertainties, most previous studies have focused on Type Ia supernovae.
This paper addresses two specific gaps:

Lack of Type II/Ib Sibling Studies: There is a need to validate distance measurement methods (specifically the Expanding Photosphere Method, EPM) on non-Type Ia siblings, particularly Type IIP and Type Ib supernovae.
Distance to NGC 6951: The host galaxy NGC 6951 has hosted five supernovae over the past few decades (SN 1999el, SN 2000E, SN 2015G, SN 2020dpw, and SN 2021sjt), but previous distance estimates vary significantly (ranging from ~23 to 33 Mpc) and rely on different methods (e.g., Phillips relation for Type Ia, EPM for Type II). The authors aim to provide a refined, consistent distance estimate for NGC 6951 using its two most recent and well-observed siblings: SN 2020dpw (Type IIP) and SN 2021sjt (Type Ib).

2. Methodology

The authors employed the Expanding Photosphere Method (EPM), a geometric technique that relates the observed angular radius of a supernova to its physical radius derived from photospheric velocity.

Data Acquisition:
- Photometry: BVgri (and BVgriz for SN 2020dpw) light curves were obtained from the Konkoly Observatory (RC80), Baja Observatory (BRC80), and the Las Cumbres Observatory (LCO) Global Telescope Network.
- Spectroscopy: Spectra were retrieved from the WiseRep database (SN 2020dpw) and obtained via the LCO/FLOYDS spectrograph (SN 2021sjt).
Angular Radius ( $\theta$ ) Estimation:
- Photometric data were corrected for interstellar reddening ( $E(B-V)_{tot} = 0.38$ mag).
- Spectral Energy Distributions (SEDs) were constructed and fitted with blackbody curves to determine photospheric temperatures ( $T_{ph}$ ).
- Pseudo-bolometric fluxes ( $F_{bol}$ ) were calculated by integrating fluxes, with extrapolations for unobserved UV and IR bands.
- Dilution Factors ( $\xi$ ): To correct for the dilution of blackbody radiation due to electron scattering, specific correction factors were applied:
  - For SN 2020dpw (Type IIP): $\xi$ coefficients from Dessart & Hillier (2005).
  - For SN 2021sjt (Type Ib): $\xi$ coefficients from Dessart et al. (2015).
- $\theta$ was derived using the relation $\theta = \sqrt{F_{bol} / (\xi^2 \sigma T^4)}$ .
Photospheric Velocity ( $v_{ph}$ ) Estimation:
- Spectra were modeled using the SYN++ code to identify ions and determine photospheric velocities.
- SN 2020dpw: Only one high-quality spectrum (21 days post-explosion) was available. The velocity was tied to Fe II lines. An empirical formula (Takáts & Vinkó 2012) was fitted to extrapolate velocities to other epochs.
- SN 2021sjt: Five spectra (4–19 days post-explosion) were modeled. A new, modified empirical velocity curve was developed specifically for Type Ib SNe, as their evolution is faster than Type IIP.
Distance Calculation:
- The distance ( $D$ ) and explosion time ( $t_0$ ) were derived by linearly fitting the relation $t = D \cdot (\theta/v_{ph}) + t_0$ .
- The explosion time ( $t_0$ ) was fixed to a value between the last non-detection and first detection for each object to reduce free parameters and improve precision.
Error Analysis:
- Systematic errors were rigorously estimated by varying three key parameters: the dilution factor ( $\xi$ ), the explosion date ( $t_0$ ), and the interstellar reddening ( $E(B-V)$ ).

3. Key Contributions

First EPM Application to Type Ib Siblings: The paper successfully applies the EPM to a Type Ib supernova (SN 2021sjt), demonstrating that the method is viable for stripped-envelope supernovae, not just Type IIP.
Refined Distance to NGC 6951: By analyzing two distinct supernova types in the same galaxy, the authors provide a robust, cross-validated distance estimate that reconciles previous conflicting literature values.
Velocity Curve Calibration: The authors developed a specific empirical velocity evolution formula for Type Ib supernovae (Eq. 6), adapting the standard Type IIP formalism to account for the faster evolution of Ib progenitors.
Systematic Error Quantification: The study provides a detailed breakdown of how different choices in dilution factors, explosion times, and reddening values impact the final distance, highlighting that dilution factors are the dominant source of systematic uncertainty.

4. Results

The analysis yielded the following distance estimates for the host galaxy NGC 6951:

SN 2020dpw (Type IIP):
- Distance: 25.76 ± 0.34 (random) ± 5.51 (systematic) Mpc
SN 2021sjt (Type Ib):
- Distance: 24.57 ± 1.27 (random) ± 4.64 (systematic) Mpc

Key Findings:

Consistency: The two independent distance estimates agree within their error bars and are consistent with the majority of previous literature values (specifically the 23–27 Mpc range), validating the EPM approach for these objects.
Dominant Systematic Errors:
- For SN 2020dpw, the choice of the dilution factor ( $\xi$ ) contributed the largest systematic error (~3.5 Mpc).
- For SN 2021sjt, the uncertainty in the explosion date ( $t_0$ ) was the largest contributor (~1.9 Mpc), followed by the dilution factor choice.
Velocity Sensitivity: The results for SN 2020dpw were shown to be highly sensitive to the single available velocity measurement; a $\pm 500$ km/s shift in velocity resulted in a distance variation of ~3 Mpc.

5. Significance

Validation of EPM for Non-IIP SNe: This work proves that the Expanding Photosphere Method can be successfully extended to Type Ib supernovae, expanding the toolkit for distance measurements in galaxies lacking Type Ia events.
Calibration of the Cosmic Distance Ladder: By providing a geometric distance to NGC 6951 that is independent of the Cepheid variable or Type Ia standard candle calibrations, this study helps refine the local distance scale and reduces systematic uncertainties in the Hubble constant determination.
Future Prospects: The authors suggest that future studies should focus on galaxies hosting multiple supernovae of different types to further calibrate EPM and Standard Candle Method (SCM) techniques, ultimately leading to more precise cosmological parameters.

In conclusion, the paper demonstrates that despite the challenges of systematic errors (particularly regarding dilution factors and explosion timing), the EPM applied to supernova siblings provides a powerful and consistent method for determining extragalactic distances.