🔭 astrophysics

GRB 180728A and SN 2018fip: the nearest high-energy cosmological gamma-ray burst with an associated supernova

This paper presents the discovery and characterization of GRB 180728A and its associated supernova SN 2018fip as the nearest high-energy cosmological gamma-ray burst with an SN, revealing a complex, asymmetric ejecta structure and a kinetic energy that does not correlate with the burst's high isotropic energy, thereby highlighting the diverse energy budget partition in GRB-SNe.

Original authors: A. Rossi, L. Izzo, K. Maeda, P. Schady, D. B. Malesani, D. A. Kann, S. Klose, L. Amati, P. D'Avanzo, A. de Ugarte Postigo, K. E. Heintz, A. Kumar, V. Lipunov, A. Martin-Carrillo, A. Melandri, A. M. Ni

Published 2026-04-01

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: A. Rossi, L. Izzo, K. Maeda, P. Schady, D. B. Malesani, D. A. Kann, S. Klose, L. Amati, P. D'Avanzo, A. de Ugarte Postigo, K. E. Heintz, A. Kumar, V. Lipunov, A. Martin-Carrillo, A. Melandri, A. M. Nicuesa Guelbenzu, S. R. Oates, S. Schulze, J. Selsing, R. L. C. Starling, G. Stratta, D. Vlasenko, P. Balanutsa, R. Brivio, V. D'Elia, B. Milvang-Jensen, E. Palazzi, D. A. Perley, A. Rau, J. Sollerman, N. R. Tanvir, T. Zafar

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 the universe as a vast, dark ocean. Occasionally, a massive star at the end of its life doesn't just fade away; it explodes with the force of a billion suns, creating a Gamma-Ray Burst (GRB). These are the brightest, most energetic events in the cosmos.

This paper is a detailed "autopsy" of one specific explosion, GRB 180728A, which happened relatively close to us (in cosmic terms) in July 2018. The astronomers didn't just watch the flash; they tracked the aftermath for months to understand the relationship between the explosion's light show and the dying star's final act: a supernova.

Here is the story of GRB 180728A and its companion, SN 2018fip, broken down into simple concepts.

1. The "Big Bang" That Wasn't So Big

Usually, when we see a GRB from far away (high redshift), it's incredibly powerful. When we see them close by (low redshift), they are usually weak "coughs" compared to the "screams" of the distant ones.

The Analogy: Think of GRBs like fireworks. Most distant fireworks are massive, ground-shaking displays. Most nearby ones are just small sparklers.
The Surprise: GRB 180728A was a nearby sparkler that roared like a massive ground-shaker. It released an enormous amount of energy (about $2.5 \times 10^{51}$ ergs), making it one of the most energetic "local" bursts ever seen. It was a rare anomaly: a high-energy explosion happening right in our cosmic backyard.

2. The Afterglow: A Fading Echo

After the initial flash, the explosion leaves behind an "afterglow"—a fading glow of light that lasts for days or weeks.

The Twist: Even though the initial burst was huge, the afterglow was surprisingly dim.
The Metaphor: Imagine a cannon firing a massive shell (the GRB). Usually, the smoke and debris (the afterglow) are huge and loud. In this case, the cannon fired a massive shell, but the smoke cleared almost instantly, leaving a very quiet, dim trail.
The "Jet Break": The light curve (a graph of brightness over time) showed a distinct change in speed about 5 hours after the explosion. The astronomers call this a "jet break." Imagine a flashlight beam hitting a wall; as the beam spreads out, it gets dimmer faster. This break told the scientists that the explosion wasn't a sphere; it was a focused beam (a jet) shooting out into space.

3. The Supernova: The Star's Final Bow

As the afterglow faded, a new light emerged: the Supernova (SN 2018fip). This is the actual star exploding.

The "Bump": In the light curve, the supernova looked like a "bump" rising up as the afterglow faded. Because the afterglow was so dim, this bump was very easy to see and study.
The Shape: The star didn't explode like a perfect sphere (like a balloon popping). The data suggests it was aspherical (lopsided).
- The Analogy: Imagine a water balloon being squeezed. If you squeeze it evenly, it pops in a circle. If you squeeze it hard from the top, it shoots a jet of water out the top while the rest of the water falls down.
- The Two Components: The astronomers found two distinct parts to the explosion:
  1. The "Bullet": A fast, narrow jet of material shooting out at over 20,000 km/s. This was dominant in the first few days.
  2. The "Cloud": A slower, heavier, more spherical cloud of debris that took over later.
- Why it matters: If you only looked at the "Cloud," you'd think the explosion was weak. But the "Bullet" was hiding extra energy in a narrow direction, which explains why the initial burst was so bright even though the total explosion wasn't the most powerful ever.

4. The Host Galaxy: A Tiny, Blue Factory

Where did this happen?

The Setting: The explosion occurred in a small, irregular, blue galaxy.
The Metaphor: Think of the universe as a city. Some galaxies are massive skyscrapers (like our Milky Way) full of old stars. This host galaxy was more like a small, bustling construction site in the suburbs. It was young, blue (meaning it had lots of hot, new stars), and not very massive. This is the typical neighborhood where these kinds of massive star explosions happen.

5. The Big Conclusion: Energy Mismatch

The most fascinating finding is the energy mismatch.

The Puzzle: The initial flash (GRB) was incredibly energetic. But the actual star explosion (Supernova) was actually on the fainter, lower-energy side of the spectrum for these types of events.
The Lesson: This proves that the power of the "flash" (the GRB) and the power of the "explosion" (the Supernova) are not directly linked. You can have a massive flash from a relatively modest explosion if the energy is focused into a tight beam.
The Efficiency: The scientists calculated that only about 2% of the total energy went into the relativistic jet (the GRB flash). The other 98% went into the supernova debris. This confirms that the supernova is the engine, and the GRB is just a small, focused exhaust pipe.

Summary

GRB 180728A is a cosmic detective story. It showed us a nearby explosion that was deceptively powerful. By carefully peeling back the layers of light, the astronomers discovered that:

The explosion was focused in a narrow beam (like a laser).
The star itself wasn't as massive or energetic as the flash suggested.
The universe is full of diversity; even "standard" explosions can have unique, lopsided shapes that change how we see them.

It's a reminder that in the universe, what you see isn't always what you get—sometimes a small, focused beam can look like a giant explosion, and a massive star can die with a surprisingly quiet bang.

1. Problem and Context

Gamma-ray bursts (GRBs) are the most energetic explosions in the universe, broadly categorized into long-duration GRBs (LGRBs) associated with the collapse of massive stars (collapsars) and short-duration GRBs associated with compact object mergers. While LGRBs are known to be linked to broad-lined Type Ic supernovae (SNe Ic-BL), the relationship between the energy of the GRB jet and the properties of the associated supernova remains complex and not fully understood.

Most LGRBs at low redshift ( $z < 0.2$ ) are low-energy events ( $E_{\gamma,iso} \sim 10^{48}–10^{50}$ erg). High-energy events at such low redshifts are rare; prior to this study, only GRB 030329 ( $z=0.167$ ) and the exceptionally bright GRB 221009A ( $z=0.151$ ) were known. Furthermore, while hundreds of GRBs have been detected, only $\sim50$ have confirmed associated supernovae, and detailed spectroscopic follow-up is limited to low-redshift events.

GRB 180728A presents a unique case: it is a high-energy LGRB ( $E_{\gamma,iso} \approx 2.5 \times 10^{51}$ erg) at a very low redshift ( $z=0.1171$ ). Unlike GRB 221009A, where the afterglow outshone the supernova, GRB 180728A's afterglow was faint enough to allow the associated supernova, SN 2018fip, to be detected and studied in detail from its rise. The paper aims to characterize the prompt emission, afterglow, host galaxy, and supernova of this event to better understand the energy budget partition and explosion geometry of GRB-SNe.

2. Methodology

The study employed a comprehensive multi-wavelength approach combining photometry and spectroscopy:

Observations:
- Prompt Emission: Analyzed data from Swift/BAT, Fermi/GBM, and Konus-Wind to characterize the burst duration and spectral properties.
- Afterglow Photometry: Utilized data from MASTER (South African Astronomical Observatory), REM (La Silla), GROND (MPG 2.2m), and Swift/UVOT and XRT. Observations spanned from 22 seconds to $\sim80$ days post-burst.
- Spectroscopy: Conducted extensive UV-to-NIR spectroscopy using the X-shooter instrument on the VLT (ESO) over 11 epochs (from 0.24 to 78 days).
Data Reduction & Analysis:
- Image Subtraction: A critical step involved using deep reference images (taken $>220$ days post-burst) to subtract a nearby bright star contaminating the field, allowing for precise photometry of the faint afterglow and SN.
- Light Curve Modeling: The optical light curve was modeled as a smoothly broken power-law (representing the afterglow) plus a supernova component scaled from the template SN 1998bw (using luminosity factor $k$ and stretch factor $s$ ).
- Spectral Energy Distribution (SED): Joint fitting of optical and X-ray data to determine spectral slopes, breaks, and intrinsic extinction.
- Supernova Modeling:
  - Bolometric: Used the Arnett model to derive ejecta mass, kinetic energy, and nickel mass.
  - Spectral Synthesis: Used the TARDIS code to model the spectral evolution. The authors tested spherical symmetry and found it insufficient, leading to a two-component aspherical model (high-velocity outer layer and low-velocity inner core).
- Host Galaxy Analysis: Analyzed the host galaxy morphology and spectral properties (metallicity, star formation rate) using late-time imaging and spectroscopy.

3. Key Results

A. The Gamma-Ray Burst (GRB 180728A)

Energy: The burst has an isotropic equivalent energy of $E_{\gamma,iso} = (2.5 \pm 0.5) \times 10^{51}$ erg and a rest-frame spectral peak energy $E_{p,i} = 123 \pm 28$ keV.
Classification: It lies on the $E_{p,i} - E_{\gamma,iso}$ correlation typical of classical collapsar events, making it one of the most energetic GRBs at $z < 0.2$ .
Afterglow: The afterglow is surprisingly faint (under-luminous) compared to other high-energy GRBs. It exhibits a shallow early decay phase that steepens around 0.2 days (5 hours), interpreted as a jet break.
Jet Geometry: Assuming a uniform jet in a constant density medium, the jet opening angle is calculated to be $\theta_{jet} \approx 4.8^\circ$ . The true collimated energy is $E_{tot} \approx 0.4 \times 10^{50}$ erg.

B. The Supernova (SN 2018fip)

Detection: The SN component dominated the optical emission after $\sim3$ days. It is a broad-lined Type Ic SN.
Light Curve: The SN is slightly fainter ( $\sim80-90\%$ ) and evolves slightly faster than the prototype SN 1998bw.
Spectral Evolution & Asymmetry:
- Early spectra (days 6–9) show strong suppression below 4000 Å and a featureless continuum.
- Late spectra (days 18–23) show typical SN Ic features.
- Two-Component Model: To explain the spectral evolution, the authors propose an aspherical, two-component ejecta structure:
  1. High-Velocity Component: A narrow, collimated ( $\theta < 30^\circ$ ), Fe-rich outer layer moving at $>20,000$ km/s. This dominates the early spectra.
  2. Low-Velocity Component: A slower ( $\sim15,000$ km/s), more massive, quasi-spherical inner component that dominates at later epochs.
Energetics:
- Using the Arnett model (spherical assumption): $M_{ej} \approx 2.4 M_\odot$ , $E_{k,SN} \approx 3.2 \times 10^{51}$ erg, $M_{Ni} \approx 0.19 M_\odot$ .
- Including the high-velocity component (spherical approximation): Total $E_{k,SN} \approx 6.5 \times 10^{51}$ erg.
- Crucially, the kinetic energy is below the typical range for GRB-SNe ( $10^{52}–10^{53}$ erg).

C. The Host Galaxy

Properties: The host is a low-mass ( $\sim 10^{7.8} M_\odot$ ), blue, irregular, star-forming dwarf galaxy.
Environment: It has low metallicity ( $12 + \log(O/H) \approx 8.57$ ) and a low star formation rate ( $\sim0.28 M_\odot$ /yr), typical of LGRB hosts at low redshift.
Offset: The GRB occurred with negligible offset from the brightest region of the host, consistent with massive star formation.

D. Energy Partition and Efficiency

Efficiency: The GRB efficiency ( $\xi = E_{jet} / (E_{jet} + E_{SN})$ ) is calculated to be extremely low, approximately 2%.
Correlation: There is no correlation between the high energy of the GRB and the relatively low energy of the SN. This supports the view that the SN explosion is the primary energy reservoir, and the GRB jet is a byproduct of the central engine, not the driver of the SN energetics.

4. Significance and Conclusions

Diversity of GRB-SNe: GRB 180728A demonstrates that high-energy GRBs can be associated with intrinsically faint supernovae and afterglows. This challenges simple scaling laws where higher GRB energy implies a more energetic SN.
Asphericity is Key: The study provides strong evidence that asymmetry is a fundamental property of GRB-SNe. The two-component ejecta model (high-velocity jet-like cocoon + slow spherical envelope) is necessary to explain the spectral evolution, particularly the early blue suppression and the lack of high-velocity features at late times.
Energy Budget: The findings reinforce that the kinetic energy of the supernova is generally independent of the GRB's isotropic energy. The low efficiency ( $\sim2\%$ ) suggests that most of the engine's energy goes into the SN ejecta, with only a tiny fraction powering the relativistic jet.
Future Outlook: The paper highlights the need for next-generation telescopes (ELTs, JWST) to perform similar high-quality, multi-epoch spectroscopy on GRB-SNe at higher redshifts ( $z > 0.2$ ) to fully map the diversity of these explosions and their asymmetries.

In summary, GRB 180728A/SN 2018fip serves as a critical local laboratory for understanding the complex, asymmetric nature of collapsar explosions, revealing that high-energy GRBs do not necessarily require high-energy supernovae and that the geometry of the explosion plays a decisive role in the observed properties.