Gamma-Ray Burst in the Swift/Fermi Era and BeyondView this Special Issue
Review Article | Open Access
The Observer’s Guide to the Gamma-Ray Burst Supernova Connection
We present a detailed report of the connection between long-duration gamma-ray bursts (GRBs) and their accompanying supernovae (SNe). The discussion presented here places emphasis on how observations, and the modelling of observations, have constrained what we know about GRB-SNe. We discuss their photometric and spectroscopic properties, their role as cosmological probes, including their measured luminosity–decline relationships, and how they can be used to measure the Hubble constant. We present a statistical summary of their bolometric properties and use this to determine the properties of the “average” GRB-SN. We discuss their geometry and consider the various physical processes that are thought to power the luminosity of GRB-SNe and whether differences exist between GRB-SNe and the SNe associated with ultra-long-duration GRBs. We discuss how observations of their environments further constrain the physical properties of their progenitor stars and give a brief overview of the current theoretical paradigms of their central engines. We then present an overview of the radioactively powered transients that have been photometrically associated with short-duration GRBs, and we conclude by discussing what additional research is needed to further our understanding of GRB-SNe, in particular the role of binary-formation channels and the connection of GRB-SNe with superluminous SNe.
Observations have proved the massive-star origins of long-duration GRBs (LGRBs) beyond any reasonable doubt. The temporal and spatial connection between GRB 980425 and broad-lined type Ic (IcBL) SN 1998bw offered the first clues to their nature [1, 2] (Figure 1). The close proximity of this event (; ~40 Mpc), which is still the closest GRB to date, resulted in it becoming one of the most, if not the most, scrutinized GRB-SN in history. It was shown that SN 1998bw had a very large kinetic energy (see Section 4 and Table 3) of ~2 erg, which led it to being referred to as a hypernova . However, given several peculiarities of its -ray properties, including its underluminous -ray luminosity ( erg ), it was doubted whether this event was truly representative of the general LGRB population. This uncertainty persisted for almost five years until the spectroscopic association between cosmological/high-luminosity GRB 030329 ( erg ) and SN 2003dh [4–6]. GRB 030329 had an exceptionally bright optical afterglow (AG; see Figures 2 and 3), and a careful decomposition of the photometric and spectroscopic observations was required in order to isolate the SN features from the dominant AG light  (see Section 2.1 and Figure 4). As was seen for SN 1998bw, SN 2003dh was a type IcBL SN, and its kinetic energy was in excess of erg, showing that it too was a hypernova.
The launch of the Swift satellite  dramatically changed the way we studied GRBs and the GRB-SN association, and the number of events detected by this mission has helped increase the GRB-SN sample size by a factor of three since the pre-Swift era. This includes, among many others, the well-studied events GRB 060218/SN 2006aj, GRB 100316D/SN 2010bh, GRB 111209A/SN 2011kl, GRB 120422A/SN 2012bz, GRB 130427A/SN 2013cq, and GRB 130702A/SN 2013dx. A full list of the references to these well-studied spectroscopic GRB-SN associations is found in Table 4.
This review paper represents a continuation of other review articles presented to date, including the seminal work by Woosley and Bloom (2006) . As such, we have focused the majority of the content on achievements made in the 10 years since  was published. In this review and many others [12–19], thorough historical accounts of the development of the gamma-ray burst supernova (GRB-SN) connection are presented, and we encourage the reader to consult the detailed presentation given in section one of  for further details. In Tables 2 and 3 we present the most comprehensive database yet compiled of the observational and physical properties of the GRB prompt emission and GRB-SNe, respectively, which consists of 46 GRB-SNe. It is the interpretation of these data which forms a substantial contribution to this review. We have adopted the grading scheme devised by  to assign a significance of the GRB-SN association to each event, where A is strong spectroscopic evidence, B is a clear light curve bump as well as some spectroscopic evidence resembling a GRB-SN, C is a clear bump consistent with other GRB-SNe at the spectroscopic redshift of the GRB, D is a bump, but the inferred SN properties are not fully consistent with other GRB-SNe or the bump was not well sampled or there is no spectroscopic redshift of the GRB, and E is a bump, either of low significance or inconsistent with other GRB-SNe. This is found in Table 3.
Throughout this article we use a CDM cosmology constrained by  of km , , . All published data, where applicable, have been renormalized to this cosmological model. Foreground extinctions were calculated using the dust extinction maps of [21, 22]. Unless stated otherwise, errors are statistical only. Nomenclature is as follows: denotes the standard deviation of a sample, whereas the root-mean square of a sample is expressed as RMS. A symbol with an overplotted bar denotes an average value. LGRB and SGRB are long- and short-duration GRBs, respectively, while a GRB-SN is implicitly understood to be associated with an LGRB. The term refers to the time that a given GRB was detected by a GRB satellite.
2. Observational Properties
2.1. Photometric Properties
The observer-frame, optical light curves (LCs) of GRBs span more than 8–10 magnitudes at a given observer-frame postexplosion epoch (see, e.g., Figure in ). Similarly, if we inspect the observer-frame -band LCs of GRB-SNe (redshift range ) shown in Figure 3, they too span a similar range at a given epoch. Indeed, the peak SN brightness during the SN “bump” phase ranges from for GRB 130702A (the brightest GRB-SN bump observed to date) to for GRB 021211.
For a typical GRB-SN, there are three components of flux being measured: the afterglow (AG), which is associated with the GRB event, the SN, and the constant source of flux coming from the host galaxy. A great deal of information can be obtained from modelling each component, but, for the SN component to be analysed, it needs to be decomposed from the optical/NIR LCs (Figure 4). To achieve this task, the temporal behaviour of the AG, the constant source of flux from the host galaxy, and the line of sight extinction, including foreground extinction arising from different sight-lines through the Milky Way (MW) [21, 22], and extinction local to the event itself [10, 23–26], in a given filter need to be modelled and quantified. The host contribution can be considered either by removing it via the image-subtraction technique [27–29], by simple flux-subtraction [30–32], or by including it as an additional component in the fitting routine [33–36]. The AG component is modelled using either a single or a set of broken power laws (SPL/BPL; ). This phenomenological approach is rooted in theory however, as standard GRB theory states that the light powering the AG is synchrotron in origin and therefore follows a power law behaviour in both time and frequency (, where the respective decay and energy spectral indices are and ).
Once the SN LC has been obtained, traditionally it is compared to a template supernova, that is, SN 1998bw, where the relative brightness () and width (also known as a stretch factor, ) are determined. Such an approach has been used extensively over the years [10, 18, 31–34, 38–43]. Another approach to determining the SN’s properties is to fit a phenomenological model to the resultant SN LC [10, 42–44], such as the Bazin function , in order to determine the magnitude/flux at peak SN light, the time it takes to rise and fade from peak, and the width of the LC, such as the parameter (in a given filter, the amount a SN fades in magnitudes from peak light to 15 days later). All published values of these observables are presented in Table 3.
2.2. Spectroscopic Properties
Optical and NIR spectra, of varying levels of quality due to their large cosmological distances, have been obtained for more than a dozen GRB-SNe. Those of the highest quality show broad observation lines of O I, Ca II, Si II, and Fe II near maximum light. The line velocities of two specific transitions (Si II 6355 and Fe II 5169; Figure 6) indicate that near maximum light the ejecta that contain these elements move at velocities of order km (Fe II 5169) and about km (Si II 6355). The weighted mean absorption velocities at peak -band light of a sample of SNe IcBL that included GRB-SNe were found to be km (Fe II 5169) by  (see as well Table 3). SNe IcBL (including and excluding GRB-SNe) have Fe II 5169 widths that are ~9,000 km broader than SNe Ic, while GRB-SNe appear to be, on average, about ~6,000 km more rapid than SNe IcBL at peak light . Si II 6355 appears to have a tighter grouping of velocities than Fe II 5169, though SN 2010bh is a notable outlier, being roughly 15,000 to 20,000 km more rapid than the other GRB-SNe. SN 2013ez is also a notable outlier due to its low line velocity (4000–6000 km s), and inspection of its spectrum (Figure 5) reveals fewer broad features than other GRB-SNe, where it more closely resembles type Ic SNe rather than type IcBL . Nevertheless, this relative grouping of line velocities may indicate similar density structure(s) in the ejecta of these SN, which in turn could indicate some general similarities in their preexplosion progenitor configurations. For comparison,  found that the dispersion of peak SNe Ic Fe II 5169 line velocities is tighter than those measured for GRB-SNe and SNe IcBL not associated with GRBs ( km , resp.). This suggests that GRB-SNe and SNe IcBL have more diversity in their spectral velocities, and in turn their density structures, than SNe Ic. Finally,  found no differences in the spectra of GRB-SNe relative to high-luminosity GRB-SNe.
During the nebular phase of SN 1998bw (one of only a few GRB-SNe that has been spectroscopically observed during this phase due to its close proximity; see as well Section 5), observed lines include [O I] 5577, 6300,6364; O [II] 7322; Ca II 3934,3963, 7291,7324; Mg I] 4570; Na I 5890,5896; [Fe II] 4244, 4276, 4416, 4458, 4814, 4890, 5169, 5261, 5273, 5333, 7155, 7172, 7388, 7452; [Fe III] 5270; Co II 7541; C I] 8727 . Nebular [O I] 6300,6364 was also observed for nearby GRB-SNe 2006aj  and 100316D , though in the latter case strong lines from the underlying HII are considerably more dominant. For SN 2006aj, [Ni II 7380] was tentatively detected , which, given the short half-life of Ni, implies the existence of roughly 0.05 of Ni. Such a large amount of stable neutron-rich Ni strongly indicates the formation of a neutron star . Moreover, the absence of [Ca II] lines for SN 2006aj also supported the lower kinetic energy of this event relative to other GRB-SNe, which is likely less than that attributed to a hypernova.
3. Phenomenological Classifications of GRB-SNe
Replicating previous works [19, 41], in this review, we divided GRB-SNe into the following subclasses based primarily on their isotropic -ray luminosity :(i)GRB-SNe: GRB-SNe associated with low-luminosity GRBs ( erg ).(ii)INT-GRB-SNe: GRB-SNe associated with intermediate-luminosity GRBs ( erg ). (Not to be confused with intermediate-duration GRBs, i.e., those with durations of 2–5 s [50–52].)(iii)GRB-SNe: GRB-SNe associated with high-luminosity GRBs ( erg ).(iv)ULGRB-SNe: ultra-long-duration GRB-SNe, which are classified according to the exceptionally long duration of their -ray emission (~104 seconds [53, 54]) rather than on their -ray luminosities.
Historically, the term X-ray flash (XRF) was used throughout the literature, which has slowly been replaced with the idiom of “low-luminosity.” Strictly speaking, the definition of an XRF  arises from the detection of soft, X-ray rich events detected by the Wide Field Camera on BeppoSax in the energy range 2–25 keV. Here we make no distinction based on the detection of a given satellite and instrumentation, where the “” nomenclature refers only to the magnitude of a given GRB’s .
The luminosity, energetics, and shape of the -ray pulse of a given GRB can reveal clues to the origin of its high-energy emission and thus its emission process. Of particular importance is whether the -rays emitted by GRBs arise from the same mechanism as high-luminosity GRBs (i.e., from a jet) or whether from a relativistic shock breakout (SBO) [30, 56–60] (see as well Section 9). It was demonstrated by [61, 62] that a key observable of GRBs are their single-peaked, smooth, nonvariable -ray LCs compared to the more erratic -ray LCs of jetted-GRBs, which become softer over time. It was shown by  that an SBO is likely present in all LGRB events, but for any realistic configuration the energy in the SBO pulse is lower by many orders of magnitude compared to those observed in the GRB prompt emission ( erg, for reasonable estimates of the ejecta mass and progenitor radii). These low energies (compared with ) suggest that relativistic SBOs are not likely to be detected at redshifts exceeding . In cases where they are detectable, the SBO may be in the form of a short pulse of photons with energies >1 MeV. Inspection of the values in Table 2 shows that only a few events have photons with peak -ray energies close to this value: GRB 140606B has keV ; however suspected GRBs 060218 and 100316D only have keV and 30 keV, respectively. It should be noted that while the SBO model of  successfully explains the observed properties (namely, the energetics, temperature, and duration of the prompt emission) of GRBs 980425, 031203, 060218, and 100316D, their SBO origins are still widely debated [63, 64], with no firm consensus yet achieved.
Thermal, black body (BB) components in UV and X-ray spectra have been detected for several events, including GRB 060218 (X-ray: keV, time averaged from first 10,000 s, ); GRB 100316D (X-ray: keV, time averaged from 144–737 s, ); GRB 090618 (X-ray: keV up to first 2500 s, ); GRB 101219B (X-ray: keV, ); and GRB 120422A (UV: eV at observer-frame d, ). A sample of LGRBs with associated SNe was analysed by  who found that thermal components were present in many events, which could possibly be attributed to thermal emission arising from a cocoon that surrounds the jet  or perhaps associated with SBO emission. Reference  analysed a larger sample of LGRBs and found that, for several events, a model that included a BB contribution provided better fits than absorbed power laws. Reference  found that, in their sample of 28 LGRBs, eight had evidence of thermal emission in their X-ray spectra, indicating such emission may be somewhat prevalent. However, the large inferred BB temperatures ( ranging from 0.16 keV for 060218 to 3.2 keV for 061007, with an average of ≈1 keV) indicates that the origin of the thermal emission may not be a SBO. Moreover, the large superluminal expansions inferred for the thermal components instead hint at a connection with late photospheric emission. In comparison, some studies indicate a SBO temperature of ~1 keV , while [60, 72–74] showed that for a short while the region behind the shock is out of thermal equilibrium, and temperatures can reach as high as ~50 keV.
The radius of the fitted BB component offers additional clues. References [58, 59] derived a BB radius of 5–8 × 1012 cm for GRB 060218;  found ≈8 × 1011 cm for GRB 100316D;  found ≈7 × 1013 cm for GRB 120422A; and  derived a radius of ≈9 × 1013 cm for GRB 140606B. The radii inferred for GRBs 060218, 120422A, and 140606B are commensurate with the radii of red supergiants (200–1500 ), while that measured for GRB 100316D is similar to that of the radius of a blue supergiant (≤25 ). These radii, which are much larger than those expected for Wolf-Rayet (WR) stars (of order a few solar radius to a few tens of solar radii), were explained by these authors by the presence of a massive, dense stellar wind surrounding the progenitor star, where the thermal radiation is observed once the shock, which is driven into the wind, reaches a radius where the wind becomes optically thin. An alternative explanation for the large BB radii was presented by  (see as well ), where the breakout occurs in an extended () low-mass (0.01 ) envelope surrounding the preexplosion progenitor star. The origin of envelope is likely material stripped just prior to explosion, and such an envelope is missing for high-luminosity GRB-SNe .
For a given GRB-SN event there are both relativistic and nonrelativistic ejecta, where the former is responsible for producing the prompt emission, and the latter is associated with the SN itself. The average mass between the two components is large: the ejecta mass of a GRB-SN is of order 2–8 , while that in the jet that produces the -rays is of order , based on arguments for very low baryon loading . A GRB jet decelerates very rapidly, within a few days, because the very low-mass ejecta is rapidly swept up into the comparatively larger mass of the surrounding CSM. Conversely, SNe have much heavier ejecta and can be in free-expansion for many years or even centuries. Measuring the amount of kinetic energy associated with each ejecta component can offer additional clues to the explosion mechanisms operating in these events. Figure 7 shows the position of SNe Ibc (green), GRBs (red), GRBs (blue), and relativistic SNe IcBL (purple) in the plane [32, 78–81], where (not to be confused with the spectral PL index of synchrotron radiation) and is the bulk Lorentz factor. Squares indicate slow-moving SN ejecta, while circles represent the kinetic energy and velocity of the nonthermal radio-emitting ejecta associated with these events (e.g., the jet in GRBs). The velocities were computed for day (rest-frame), where the value denotes the division between relativistic and nonrelativistic ejecta. The solid lines show the ejecta kinetic energy profiles of a purely hydrodynamical explosion (green) [57, 82, 83]; explosions powered by a short-lived central engine (blue/purple dashed), SBO-GRBs and relativistic IcBL SNe 2009bb and 2012ap: ; and those arising from a long-lived central engine (red), that is, jetted-GRBs: .
It is seen that GRBs and high-luminosity GRBs span a wide range of engine energetics, as indicated by the range of PL indices seen in Figure 7. The two relativistic SNe IcBL considered in this review (SNe 2009bb and 2012ap), which are also thought to be engine-driven SNe [79, 81, 89], occur at the lower-end of central engine energetics. Modelling of GRB 060218  showed that ~1048 erg of energy was coupled to the mildly relativistic ejecta (). Reference  showed the presence of a very weak central engine for GRB 100316D, where ~1049 erg of energy was coupled to mildly relativistic (), quasi-spherical ejecta. It was shown by  that ≥1049 erg was associated with the relativistic (), radio-emitting ejecta of SN 2009bb. These authors also showed that, unlike GRB jets, the ejecta was in free-expansion, which implied it was baryon loaded. For SN 2012ap,  estimated there was ~1.6 × 1049 erg of energy associated with the mildly relativistic () radio-emitting ejecta. The weak X-ray emission of SN 2012ap  implied no late-time activity of its central engine, which led these authors to suggest that relativistic SNe IcBL represent the weakest engine-driven explosions, where the jet is unable to successfully break out of the progenitor. GRBs then represent events where the jet does not or just barely escapes into space. Note that  calculated an estimate to the dividing line between SBO-GRBs and jet-GRBs, finding that for -ray luminosities above erg a jet-GRB may be possible.
Next, the distribution of (the time over which a burst emits from 5% of its total measured counts to 95%) as measured by the various GRB satellites can be used to infer additional physical properties of the GRB jet duration and progenitor radii. A basic assertion of the collapsar model is that the duration of the GRB prompt phase (where is used as a proxy) is the difference of the time that the central engine operates minus the time it takes for the jet to break out of the star: . A direct consequence of this premise is that there should be a plateau in the distribution of for GRBs produced by collapsars when . Moreover, the value of found at the upper limit of the plateaus seen for three satellites (BATSE, Swift, and Fermi) was approximately the same ( s), which is interpreted as the typical breakout time of the jet. This short breakout time suggests that the progenitor star at the time of explosion is quite compact (~5 ). Reference  then used these distributions to calculate the probability that a given GRB arises from a collapsar or not based on its and hardness ratio. Note however that might not always be the best indicator of the engine on-time. For example,  showed that while GRB 120422A had s, the actual duration of the jet was actually 86 s, as constrained by modelling of the curvature effect. Though see  who state that curvature radiation is not from a central engine that is still on but from electrons that were off-axis and hence had a lower Lorentz factor and which are received over a time interval that is long compared to the duration of the burst.
Finally, Figure 8 shows the properties of the prompt emission for the various GRB-SN subclasses in the plane, that is, the Amati relation . Data from [85–87] are shown in grey along with their best fit to a single power law (index of ) and the 2 uncertainty in their fit. Several events do not appear to follow the Amati relation, including GRBs 980425 and 031203, INT-GRB 150818A, and high-luminosity GRB 140606B. Both ULGRBs are consistent with the Amati relation, so are GRBs 030329 and 130427A, while GRB 120422A and GRB 100316D are marginally consistent. It was once supposed that the placement of a GRB in the plane could be a discriminant of GRB’s origins, where it is seen that SGRBs also do not follow the Amati relation. However, over the years many authors have closely scrutinized the Amati relation, with opinions swinging back and forth as to whether it reflects a physical origin or is simply due to selection effects [96–103]. To date, no consensus has yet been reached.
4. Physical Properties: Observational Constraints
The bolometric LCs of a sample of 12 GRB-SNe, which includes GRB-SNe and ULGRB-SN 2011kl, are shown in Figure 9. The Bazin function was fit to the GRB-SN bolometric LCs in order to determine their peak luminosity (), the time of peak luminosity (), and the amount the bolometric LC fades from peak to 15 days later (). (NB. that SNe 2001ke, 2008hw, and 2009nz were excluded from the fitting and the subsequent calculated averages, as their bolometric LCs contained too few points to be fit with the Bazin function, which has four free parameters. As such, their luminosities and peak times were approximated by eye and are not included in the average GRB-SN properties presented here.) These values are presented in Table 3.
The average peak luminosity of the GRB-SN sample, excluding SN 2011kl, is erg , with a standard deviation of erg . The peak luminosities of SNe 2003dh and 2013dx are ≈1 × 1043 erg , meaning that they are perhaps better representatives of a typical GRB-SN than the archetype SN 1998bw ( erg ). The peak luminosity of SN 2011kl is erg , which makes it more than 5 more luminous than the average GRB-SN. This is not, however, as bright as superluminous supernovae (SLSNe), whose luminosities exceed >7 × 1043 erg . This makes SN 2011kl an intermediate SN event between GRB-SNe and SLSNe and perhaps warrants a classification of a “superluminous GRB-SNe” (SLGRB-SN); however, in this chapter we will stick with the nomenclature ULGRB-SN. When SN 2011kl is included in the sample, erg , with erg . Even using this average value, SN 2011kl is still 2.5 more luminous than the average GRB-SN.
The average peak time, when SN 2011kl is and is not included in the sample, is d ( d) and d ( d), respectively. Similarly, mag ( mag) and 0.8 mag ( mag), respectively. As such, the inclusion/exclusion of SN 2011kl has little effect on these derived values. The fact that SN 2011kl peaks at a similar time as the average GRB-SN, but does so at a much larger luminosity, strongly suggests that ULGRB-SNe do not belong to the same class of standardizable candles as GRB-SNe. This can be readily explained in that SN 2011kl is powered by emission from a magnetar central engine [36, 105–107], whereas GRB-SNe, including GRB-SNe, are powered by radioactive heating . Whether ULGRB-SNe represent the same set of standardizable candles as type I SLSNe [108, 109], which are also thought to be powered by a magnetar central engine, their own subset, or perhaps none at all, requires additional well-monitored events.
Over the years, and since the discovery of SN 1998bw, the bolometric properties (kinetic energy, , ejecta mass, , and nickel mass, ) of the best-observed GRB-SNe have been determined by sophisticated numerical simulations (hydrodynamical models coupled with radiative transfer, RT, codes) [3, 7, 36, 47, 48, 107, 110–119] and analytical modelling [10, 18, 31, 32, 40, 41, 43, 106, 120–122]. A summary of the derived bolometric properties for individual GRB-SNe is presented in Table 3, while a summary of the average bolometric properties, broken down by GRB-SN subtype and compared against other subtypes of SNe Ibc, is shown in Table 1. It should be noted that the values presented have been derived over different wavelength ranges: some are observer-frame , while others include UV, -band, and NIR contributions. Further discussion on the effects of including additional filters when constructing a bolometric LC of a given SN can be found in [30, 41, 49, 120, 123], who show that including NIR flux leads to brighter bolometric LCs that decay slower at later times and including UV flux leads to an increase in luminosity at earlier times (during the first couple of weeks, rest-frame) when the UV contribution is nonnegligible.
| (Section 3): GRBs: GRB-SNe associated with low-luminosity GRBs ( erg ); INT-GRBs: GRB-SNe associated with intermediate-luminosity GRBs ( erg ); GRBs: GRB-SNe associated with high-luminosity GRBs ( erg ); ULGRBs: GRB-SNe associated with ultra-long-duration GRBs (see Section 3).|
: erg .
Note: average bolometric properties of SNe Ib and Ic are from .
-ray properties calculated by  for a redshift range of .
GRB: GRB-SN associated with a low-luminosity GRB ( erg ); INT: GRB-SN associated with an intermediate-luminosity GRB ( erg ); GRB: GRB-SN associated with a high-luminosity GRB ( erg ); ULGRB: GRB-SN associated with an ultra-long-duration GRB (see Section 3).