Observational and Physical Classification of Supernovae + Black holes and their mergers

Notes on: Observational and Physical Classification of Supernovae by Avishay Gal-Yam and Black-hole binaries, gravitational waves, and numerical relativity.

Day 19 - October, 30, 2025

Agenda:

• Reminders - HW3 - Due: Nov. 4, before class (2m)
• Lecture (25m)
• ICA 17 + Report out - 3/4 Groups - Due: Not for Credit (45m)

Recap - The Delayed-Neutrino Driven Mechanism

Neutrino heating - neutrinos able to diffuse out of the core heat the region where the shock has already passed (the post shock region) - from about 30 km to 100-300 km.

• Region becomes convective.
• Provides a way to convert thermal energy deposited by neutrinos into kinetic energy potentially leading to revival of stalled shock overcoming ram pressure to lead to explosion. Example 3D CCSN explosion - O’Connor et al. 2018.

Type Ia Supernovae - how to ignite a white dwarf?

Single degenerate scenario (SD)

• The mass transfer rate of H/He that can facilitate steady burning and growth of the mass of the WD is very narrow. Example Video.

  • too fast: a H-rich envelope would form on the WD and be seen observationally - its not.
  • too slow: the matter burns in an unstable manner and leads to a nova outburst, that limits mass growth.

Type Ia Supernovae - how to ignite a white dwarf?

Double degenerate scenario (DD)

Merger of two WDs, merger product exceeds the and ignites the WD. Example 3D WD Merger Simulation.

• These systems would be formed in close binary systems underoing common envelope evolution followed by orbital decay due to gravitational radiation.

Main challenges is the nature of the burning of the merger product: is it enough to unbind the the WD or does it lead to core-collapse?

Spectral Classification Recap

• Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the colors interspersed with spectral lines.
• Each line indicates a particular chemical element or molecule, with the line strength indicating the abundance of that element.

Spectral Classification of Supernovae

Classification of supernovae, based on their spectra and lightcurve shapes. The main supernova types are shown as black squares. Figure from Turatto (2003, LNP 598, 21).

Below we will look at examples for each of the sub-classes in those black squares.

Regular Type Ia supernovae - white dwarf explosions - Early Time Spectra

• The spectrum does not show any lines of hydrogen or helium.
• dominated by absorption lines from intermediate-mass elements, especially SiII absorption around 6100 , large Si/O depth ratio.
• estimated expansion velocities of 11,000 km s, typical peak magnitudes of = −19.1 mag. Credit: Mazzali et al. (2014)

Regular Type Ia supernovae - white dwarf explosions - Late Time (Nebular) Spectra

• Late-time (nebular) spectra of the normal Type Ia SN 2011fe (from Mazzali et al. 2015) are dominated by strong lines of Fe-group element.
• SN Ia-CSM PTF11kx (Silverman et al. 2013b) are dominated by strong hydrogen Balmer lines, likely from the shocked CSM. Credit: Mazzali et al. (2015).

Regular Type Ib supernovae - hydrogen-poor massive star explosions - Early Time Spectra

• Spectra of events considered to be regular Type Ib SNe (top two) compared with a spectrum of a regular Type Ic SN (bottom). Credit: Mazzali et al. (2015).

• Q - What is the difference between the top two spectra (Ib) and the bottom (Ic)?

Regular Type Ib supernovae - hydrogen-poor massive star explosions - Early Time Spectra

• The top spectra shows weak hydrogen and strong helium features.
• The bottom shows no weak hydrogen and no strong helium.
• estimated expansion velocities of 9,000 to 10,000 km s, typical peak magnitudes of = −17.9 mag.

Regular Type Ic supernovae - H-poor, He-poor, massive star explosions - Early Time Spectra

Spectra of events considered to be regular Type Ic SNe.

Credit: Mazzali et al. (2015)

• Si II absorption is detected, its strength with respect to other elements (notably OI) is weaker than SNe Ia. SiII/OI depth ratio () = Ib or Ic. See here.

Regular Type II supernovae - hydrogen-rich massive star explosions - Photospheric Phase Spectra

Regular Type II SNe in the photospheric phase, the spectrum can be broadly understood in terms of a (pseudo-)continuum, usually with discrete spectral features.

• notable H-emission features
• estimated expansion velocities of 9,600 km s, typical peak magnitudes of = −17.4 mag.

Stellar Mass Black Holes

For this discussion we will be focusing on black holes formed via stellar origin. Not primordial black holes potentially created in the early universe via other means.

Single-star BH mass spectrum. Credit: Ebraheem Farag et al 2022 ApJ 937 112.

Stellar Mass Black Holes

We identify three physically motivated regions of the distribution:

• Stars with initial mass of set the first boundary via the maximum allowed mass of a neutron star, EoS dependent.

  • This results in a Core-Collapse Supernova and Black Hole.

• Stars with initial mass of can allow for electron–positron pairs from photons with the net result being and the core becoming dynamically unstable prior to O-ignition.

  • This leads to a Pulsational Pair Instability Supernova (PPISN) and eventually a Black Hole.

Stellar Mass Black Holes

We identify three physically motivated regions of the distribution:

• Stars with initial mass of set the next boundary, which also defines the peak of the stellar black hole mass spectrum/lower limit of the black hole mass gap.

  • The pulse initiated by the pair-instability completely unbinds the star and no remnant is left behind.

• Stars with initial mass of undergo endothermic photo-disintegration, efficient at absorbing energy and halting unbinding.

  • This leads (again) to Core-Collapse SNe and a Black Hole.

Stellar Mass Black Holes

• Maximum NS mass
• Pair production.
• Pulse strong enough.

Final BH as a function of the core mass. Credit: R. Farmer et al 2019.

Q: What about that BH? observed by LIGO? See here.

BHs in the Mass Gap? Dynamical Formation?

Probability distribution of the primary and secondary black holes for LIGO sources. Credit: Ruiz-Rocha et al 2025.

• Solutions to this result include the possibility of hierarchical mergers, containing the remnants of previous black-hole mergers.

• Promising locations for efficient production of hierarchical mergers include nuclear star clusters and accretion disks surrounding active galactic nuclei.

Pulsational Pair-Instability Supernovae (PPISN)

Steps in a PPISN include:

• The core contracts rapidly due to the pair instability and softening of the EoS.
• Explosive O- or Si-burning occurs.
• The core expands and cools until contracting again.
  • Occurs on a hydrodynamic or KH timescale depending on the mass of the Helium core.
  • The duration from first pulse to iron core-collapse can span hours to 10,000 years.

Pulsational Pair-Instability Supernovae (PPISN)

• Pulse duration in years (blue crosses)
• total kinetic energy in all of the ejected shells in units of erg (solid green circles)
• at , we see the pulse energy approaches a few erg in a single pulse!

Credit: S. E. Woosley 2017 ApJ 836 244.

Observed Stellar Mass Black Holes

Announced gravitational-wave detections and black holes and neutron stars previously constrained through electromagnetic observations up to O3 with p_astro > 0.5.

Credit: LIGO-Virgo / Aaron Geller / Northwestern University.

In-Class Assignment 17

In class: Work on ICA here with groups per usual. Discuss conceptual questions together and prepare answers to share at the end of class.

• Choose someone that will report out the groups responses ahead of time!

After Class: Due: Not for Credit

Note: The goal of ICAs are to use plots produced in the notebook for discussion and interpretation of results presented in lecture in groups and as a class.