[Paper Review] Discovery of the peculiar supernova 1998bw in the error box of GRB980425

T. J. Galama, P. M. Vreeswijk+43 more|CERN Bulletin|Jun 12, 1998

Gamma-ray bursts and supernovaePhysics and Astronomy8 references972 citations

TL;DR

This paper reports the discovery of SN 1998bw, a highly luminous and peculiar Type Ic supernova, in the error box of the weak gamma-ray burst GRB 980425. The supernova's extreme radio properties—indicating relativistic outflow—suggest a physical link to the burst, implying that GRBs may arise from diverse mechanisms despite similar gamma-ray characteristics, challenging the assumption of a single progenitor model for all GRBs.

ABSTRACT

The discovery of X-ray, optical and radio afterglows of gamma-ray bursts (GRBs) and the measurements of the distances to some of them have established that these events come from Gpc distances and are the most powerful photon emitters known in the Universe, with peak luminosities up to 10^52 erg/s. We here report the discovery of an optical transient, in the BeppoSAX Wide Field Camera error box of GRB980425, which occurred within about a day of the gamma-ray burst. Its optical light curve, spectrum and location in a spiral arm of the galaxy ESO 184-G82, at a redshift z = 0.0085, show that the transient is a very luminous type Ic supernova, SN1998bw. The peculiar nature of SN1998bw is emphasized by its extraordinary radio properties which require that the radio emitter expand at relativistical speed. Since SN1998bw is very different from all previously observed afterglows of GRBs, our discovery raises the possibility that very different mechanisms may give rise to GRBs, which differ little in their gamma-ray properties.

Motivation & Objective

• To investigate the nature of the optical transient discovered in the BeppoSAX error box of GRB 980425.
• To determine whether the transient is physically associated with GRB 980425 by analyzing its optical, radio, and X-ray properties.
• To assess whether SN 1998bw represents a rare type of supernova with relativistic outflow, potentially explaining the origin of low-luminosity GRBs.
• To evaluate the statistical likelihood of a chance coincidence between the supernova and the gamma-ray burst.
• To explore the implications for GRB progenitor models, particularly the hypernova hypothesis, given the extreme energy and kinematic properties observed.

Proposed method

• Optical monitoring using the 30 and 50-inch telescopes at Mt. Stromlo Observatory and the 40-inch telescope at Siding Spring Observatory to detect and track the transient's light curve in UBVRI bands.
• Spectral and photometric analysis of the transient using the 3.5m New Technology Telescope (NTT) and other ESO telescopes to determine redshift, absolute magnitudes, and spectral features.
• Radio observations to measure brightness temperature and infer relativistic outflow, using the super-Compton brightness temperature as a diagnostic for relativistic motion.
• X-ray follow-up with BeppoSAX narrow-field instruments to identify and characterize X-ray sources in the error box and assess their temporal variability.
• Statistical modeling of supernova rates and coincidence probabilities to evaluate the likelihood of a random association between the transient and GRB 980425.
• Comparison of the burst's gamma-ray properties (duration, fluence, spectrum) with those of other GRBs to assess its typicality or peculiarity.

Experimental results

Research questions

• RQ1 Is the optical transient SN 1998bw physically associated with GRB 980425, or is the proximity a chance coincidence?
• RQ2 What are the physical properties of SN 1998bw, and how do they differ from standard Type Ic supernovae?
• RQ3 What causes the extreme radio brightness temperature of SN 1998bw, and what does it imply about the presence of relativistic outflow?
• RQ4 Can the gamma-ray properties of GRB 980425 be reconciled with the supernova's energetics if they are physically linked?
• RQ5 What do the combined optical, radio, and X-ray properties suggest about the progenitor mechanism of GRB 980425 and the nature of its associated supernova?

Key findings

• SN 1998bw is a very luminous Type Ic supernova at a redshift of z = 0.0085, located in the spiral arm of the galaxy ESO 184-G82.
• The supernova exhibits a plateau in its R-band light curve followed by a rise to maximum at a rate of ~0.25 mag day⁻¹, suggesting shock breakout at the progenitor's surface.
• The radio emission from SN 1998bw has a brightness temperature exceeding the Compton limit, indicating a relativistic outflow with Lorentz factor Γ > 10.
• The gamma-ray peak luminosity of GRB 980425 is 5.5 ± 0.7 × 10⁴⁶ erg s⁻¹, and its total energy budget is 8.5 ± 1.0 × 10⁴⁷ erg, significantly lower than typical GRBs.
• The probability of a chance coincidence between SN 1998bw and GRB 980425 is estimated at 1.1 × 10⁻⁴, supporting a physical association.
• The data suggest that GRB 980425 and SN 1998bw may be linked via a hypernova-like explosion, but the low energy output implies a different mechanism than for typical, highly energetic GRBs.