SN 1998bw

SN 1998bw was a rare broad-lined Type Ic^[3] gamma ray burst supernova detected on 26 April 1998 in the ESO 184-G82 spiral galaxy, which some astronomers believe may be an example of a collapsar (hypernova).^[4] The hypernova has been linked to GRB 980425, which was detected on 25 April 1998, the first time a gamma-ray burst has been linked to a supernova.^[5] The hypernova is approximately 140 million light years away, very close for a gamma ray burst source.^[6]

SN 1998bw

Occurred on the galaxy ESO 184-G82
Event type	Supernova
Type Ic
Date	26 April 1998
Constellation	Telescopium
Right ascension	19h 35m 03.17s[1]
Declination	−52° 50′ 46.1″[1]
Galactic coordinates	344.99°, −27.72°[2]
Redshift	0.0085
Host	ESO 184-G82

The region of the galaxy where the supernova occurred hosts stars 5-8 million years old and is relatively free from dust. A nearby region hosts multiple Wolf-Rayet stars less than 3 million years old, but it is unlikely that the supernova progenitor could be a runaway from that region. The implication is that the progenitor was a star that originally had a mass of 25-40 M_☉, if it exploded as a single star at the end of its life.^[7]

Observations

Light curves in four photometric bands for SN 1998bw, plotted from data published by Clocchiatti et al. (2011)^[8]

On 25 April 1998, a gamma ray burst was detected by the BeppoSAX satellite and assigned the identifier GRB 980425. The event lasted for 30 seconds,^[9] and was about average in terms of burst flux.^[10] A check of images from the ESO New Technology Telescope showed a rapidly brightening point source within the error box of GRB 980425. It was located in the face-on spiral galaxy ESO 184-G82, in a spiral arm at an offset from the nucleus. This candidate supernova event lacked spectral lines of hydrogen, ruling out a normal core-collapse Type II supernova, and it was a missing line of silicon that indicated it is not a typical Type Ia supernova.^[11]

A few weeks after full light, the spectrum of supernova SN 1998bw showed no clear indications of helium. This suggested the event be classed as a Type Ic supernova, although it showed some peculiarities compared to other supernovae of this type. The expansion velocity measured from calcium lines was measured as 11,700 km/s, and 9,100 km/s from silicon lines.^[12] The maximum recorded expansion velocities reached 3×10⁴ km/s.^[13] Energy emissions from the supernova showed polarization, which supported a core-collapse scenario with asymmetry.^[14]

Radio measurements of the supernova showed that it was unusually luminous in this band. The data suggested a shock wave moving at a relativistic velocity, whereas most supernovae ejecta are non-relativistic.^[15] This was the first evidence found for a relativistic shock from a supernova.^[16] The supernova light curves from radio to X-ray bands also indicated a blast wave that was highly relativistic. The data was consistent with a physical association between SN 1998bw and GRB 980425,^[17][18] and supported the idea of a hypernova or collapsar event. This scenario results in the formation of a black hole from the collapse of a massive star.^[17][19] Spherically-symmetrical models failed to reproduce an event with this energy level, indicating a highly asymmetrical explosion that produced the gamma-ray burst from a relativistic jet.^[20] In this case, only a fraction of the progenitor's stellar mass was ejected, with the remainder collapsing to form a black hole.^[3]

The supernova transitioned to the nebular phase around 100 days after the explosion.^[21] Expansion velocities remained very high compared to other core-collapse supernovae at similar phases.^[13] The unusual spectrum observed during the nebular phase matched a model for a strongly aspherical explosion observed from near the direction of a relativistic jet.^[22][23] The radio emission from the supernova can best be explained by interaction between the relativistic shock and clumpy circumstellar medium previously ejected by a strong stellar wind.^[24] Observations from the Chandra X-ray Observatory in 2004 found X-ray emission that supported this scenario.^[25] It also lent support to the idea that the supernova and gamma-ray burst were the same event.^[26]

Environment

Observations with the Hubble Space Telescope indicated that the host for the supernova event is a sub-luminous galaxy with a morphological classification of SBc. This indicates ESO 184-G82 is a barred spiral galaxy with loosely-wound spiral arms. The galaxy is undergoing strong star formation and the supernova occurred in an active star forming area that includes an H II region. This environment is fairly typical for Type II supernovae. The supernova afterglow was about a magnitude brighter than expected from a radioactive decay model, suggesting a contribution from a surrounding stellar cluster.^[27]

The host galaxy appears morphologically disturbed, which suggests interaction from nearby galaxies. This could explain the amplified star forming process.^[27] Six galaxies lie within the field of ESO 184-G82 but none of them have a matching redshift of 0.0087±0.0006. Thus, it appears to be an isolated dwarf galaxy and another explanation is needed for the star formation.^[28] A 2020 study with the Atacama Large Millimeter Array discovered the galaxy has a ring of dense neutral hydrogen, which includes clumps of gas. One of these clumps was the host of SN 1998bw. The presence of a ring indicates a past collision with a companion galaxy.^[29]