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minous (Fig. 1) (see text S1 for considerations
related to determining this threshold) (2).
Luminous Supernovae
Recent Surveys and the Discovery of SLSNe
Modern studies based on large SN samples and
homogeneous, charge-coupled device–based luAvishay Gal-Yam
minosity measurements show that SLSNe are
very rare in nearby luminous and metal-rich host
Supernovae, the luminous explosions of stars, have been observed since antiquity. However,
galaxies (3, 4). Their detection therefore requires
various examples of superluminous supernovae (SLSNe; luminosities >7 × 1043 ergs per second)
surveys that monitor numerous galaxies of all
have only recently been documented. From the accumulated evidence, SLSNe can be classified
as radioactively powered (SLSN-R), hydrogen-rich (SLSN-II), and hydrogen-poor (SLSN-I, the most sizes in a large cosmic volume. The first generation of surveys covering large volumes was deluminous class). The SLSN-II and SLSN-I classes are more common, whereas the SLSN-R class is
better understood. The physical origins of the extreme luminosity emitted by SLSNe are a focus of signed to find numerous distant type Ia SNe for
cosmological use. These observed relatively small
current research.
fields of view to a great depth, placing most of the
effective survey volume at high
redshift (5).
upernova explosions play
-23
An alternative method for surimportant roles in many
SLSN−I
veying a large volume of sky is
aspects of astrophysics.
-22
SLSN−II
to use wide-field instruments to
They are sources of heavy eleSLSN−R
SLSN threshold
-21
cover a large sky area with relments, ionizing radiation, and
SN IIn
atively shallow imaging. With
energetic particles; they drive
SN Ia
-20
most of the survey volume at
gas outflows and shock waves
SN Ib/c
low redshift, one can conduct an
that shape star and galaxy for-19
SN IIb
efficient untargeted survey for
mation; and they leave behind
SN II−P
nearby SNe. Such surveys procompact neutron star and black
-18
vided the first well-observed exhole remnants. The study of super-17
amples of SLSNe, such as SN
novae has thus been actively
1999as (6), which turned out to
pursued for many decades.
-16
be the first example of the exThe past decade has seen the
tremely 56Ni-rich SLSN-R class
discovery of numerous superlu-15
minous supernova events (SLSNe;
(7), and SN 1999bd (8) (Fig. 2),
Fig. 1). Their study is motivated
which is probably the first well-14
by their likely association with
documented example of the SLSN-13
the deaths of the most massive
II class (9).
-100
0
100
200
300
400
500
600
stars, their potential contribuFurther important detections
Days from peak
tion to the chemical evolution of
resulted from the Texas Superthe universe and (at early times) Fig. 1. The luminosity evolution (light curve) of supernovae. Common SN explosions nova Survey (TSS) (10) (text S2).
to its reionization, and the possi- reach peak luminosities of ~1043 ergs s−1 (absolute magnitude > −19.5). Super- On 3 March 2005, TSS detected
bility that they are manifestations luminous SNe (SLSNe) reach luminosities that are greater by a factor of ~10. The SN 2005ap, a hostless transient
of physical explosion mecha- prototypical events of the three SLSN classes—SLSN-I [PTF09cnd (4)], SLSN-II [SN at 18.13 mag. Its redshift was z =
nisms that differ from those of 2006gy (12, 13, 77)], and SLSN-R [SN 2007bi (7)]—are compared with a normal 0.2832, which indicated an abtheir more common and less lu- type Ia SN (Nugent template), the type IIn SN 2005cl (56), the average type Ib/c solute magnitude at peak around
minous cousins.
light curve from (65), the type IIb SN 2011dh (78), and the prototypical type II-P SN −22.7 mag, marking it as the most
With extreme luminosities ex- 1999em (79). All data are in the observed R band (80).
luminous SN detected until then
tending over tens of days (Fig. 1)
(11). SN 2005ap is the first exand, in some cases, copious ultraviolet (UV) flux, ciple, almost all SLSNe belong to one of two ample of the class defined below as SLSN-I. On
SLSN events may become useful cosmic beacons spectroscopic classes: type IIn (hydrogen-rich 18 November 2006, TSS detected a bright tranenabling studies of distant star-forming galaxies events with narrow emission lines, which are sient located at the nuclear region of the nearby
and their gaseous environments. Unlike other usually interpreted as signs of interaction with galaxy NGC 1260 [SN 2006gy (12)]. Its meaprobes of the distant universe, such as short-lived material lost by the star before the explosion) or sured peak magnitude was ~ −22 mag (12, 13).
gamma-ray burst afterglows and luminous high- type Ic (events lacking hydrogen, helium, and Spectroscopy of SN 2006gy clearly showed hyredshift quasars, SLSNe display long durations strong silicon and sulfur lines around maximum, drogen emission lines with both narrow and
coupled with a lack of long-lasting environmental presumably associated with massive stellar ex- intermediate-width components, leading to a speceffects; moreover, they eventually disappear and plosions). However, the physical properties im- troscopic classification of SN IIn; this is the protoallow their hosts to be studied without interference. plied by the huge luminosities of SLSNe suggest type and best-studied example of the SLSN-II
Supernovae traditionally have been classified that they arise, in many cases, from progenitor class.
mainly according to their spectroscopic properties stars that are very different from those of their
During the past few years, several untargeted
[see (1) for a review]; their luminosity does not much more common and less luminous analogs. surveys have been operating in parallel (14). The
play a role in the currently used scheme. In prin- In this review, I propose an extension of the clas- large volume probed by these surveys and their
sification scheme that can be applied to super- coverage of a multitude of low-luminosity dwarf
luminous events.
galaxies have led, as expected (15), to the detecDepartment of Particle Physics and Astrophysics, Faculty
I consider SNe with reported peak magnitudes tion of numerous unusual SNe not seen before
of Physics, Weizmann Institute of Science, Rehovot 76100,
less than −21 mag in any band as being superlu- in targeted surveys of luminous hosts; indeed,
Israel. E-mail: [email protected]
Absolute magnitude (mag)
S
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or as traces of circumstellar material (CSM) interaction]; hence, the ejecta mass directly constrains
the mass of the exploding helium core, which is
likely dominated by oxygen and heavier elements.
Scaling relations based on the work of Arnett (17),
as well as comparison of the data to custom lightcurve models (7), indicate an ejecta mass of M ≈
100 M⊙. Analysis of the nebular spectra provides
an independent lower limit on the mass, M > 50
M⊙, with a composition similar to that expected
from theoretical models of massive cores exploding via the pair-instability process. A lower ejecta
mass (M = 43 M⊙) has been proposed (18). In
any case, there is no doubt that these explosions
are produced by extremely massive stars, with
the most massive exploding heavy-element cores
known to date. The scaling relations used in (7)
also indicate extreme values of ejecta kinetic energy (approaching Ek = 1053 ergs). Finally, the
integrated radiated energy of this event over its
very long lifetime is high (>1051 ergs).
SN 1999as, one of the first genuine SLSNe
detected (6), was shown to be similar to SN 2007bi
during its photospheric phase, reaching −21.4 mag
absolute at peak (7) (Fig. 3); another analysis (19)
suggested physical attributes (56Ni mass, kinetic
energy, and ejected mass) that are close to but
somewhat lower than those of SN 2007bi. Unfortunately, no late-time data have been published
for this object, so it is impossible to conduct the
same analysis carried out for SN 2007bi, but the
similarities suggest that this was likely another
member of the SLSN-R class.
Recently, the Lick Observatory Supernova
Search [LOSS (20)] detected the luminous type
Ic SN 2010hy (21, 22). After this detection, the
event was also recovered in PTF data (and designated PTF10vwg). Although final photometry
is not yet available for this event, preliminary
Katzman Automatic Imaging Telescope and PTF
data indicate a peak magnitude of −21 mag or
brighter. The event is spectroscopically similar
to other SLSNe-R (Fig. 3), which suggests that
it is also likely a member of this class.
Emerging Classes of Superluminous SNe
Objects of this subclass are exceedingly rare,
and additional examples are scarce. During the
A total of 18 SLSNe have been discussed in the
past 2 years, the PTF survey has detected anliterature (Table 1). These objects can be grouped
other likely member, PTF10nmn (23) (Fig. 3),
into three classes that share observational and
with properties similar to those of SN 2007bi,
physical attributes.
while PS1 may have discovered another similar
SLSN-R. Of all classes of SLSNe, this seems
object at a higher redshift (24). Assembling a
to be the best understood. SLSN-R events are
reasonable sample of such events may thus be a
powered by large amounts (several solar masses,
time-consuming process.
M⊙) of radioactive 56Ni produced during the exSome photospheric spectra of SLSNe-R
plosion of a very massive star. The radioactive
(7, 23, 25) (Fig. 3) show forbidden line emisdecay chain 56Ni → 56Co → 56Fe deposits ension, notably Ca II and probably also Mg II. Such
ergy via g-ray and positron emission, which is
lines are usually only observed during the nebthermalized and converted to optical radiation
ular phase of SNe, when the ejecta are optiby the expanding massive ejecta. The luminosity
cally thin, which is clearly not the case here. The
of the peak is broadly proportional to the amount
superposition of this nebular-like emission on
of radioactive 56Ni, whereas the late-time decay
an underlying photospheric spectrum may hint
(which in the most luminous cases begins immeat a complex geometry of the emitting region (modiately after the optical peak) follows the theotivating spectropolarimetric studies), but no exretical 56Co decay rate (0.0098 mag day−1). The
planations for this phenomenon have been put
luminosity of this cobalt radioactive tail can also
forth so far.
be used to estimate the initial 56Ni mass.
SN 2007bi is hosted by a dwarf galaxy [with
The first well-observed example of this group
luminosity similar to that of the Small Magellanwas SN 2007bi, detected by the PTF “dry run”
ic Cloud (SMC)], with relatively low metallicity
experiment (7). Its large 56Ni mass was measured
(Z ≈ Z⊙ /3) (25)—somewhere between those of
using both the peak luminosity (−21.35 mag absothe Large Magellanic Cloud (LMC) and the SMC.
lute) and the cobalt decay tail, followed for >500
Thus, although the progenitor
days. Estimates derived from the
star of this explosion probably
observations as well as via comparison to other well-studied events Table 1. SLSN properties by class. We list the reported redshifts, homogenized had subsolar metal content, there
(SN 1987A and SN 1998bw) con- absolute peak magnitudes (see text S1), and total radiated energies as taken from is no evidence that it had very
the literature. The published post-peak magnitude of SN 2006tf (44) (M < −20.7
low metallicity. The host galaxy
verge on a value of M 56Ni ≈ 5 M⊙.
mag) is below our fiducial cutoff.
of SN 1999as is more luminous
The large amount of radioactive
(and thus likely more metal-rich)
material powers a long-lasting
Absolute
Radiated
Supernova
Redshift
Reference than that of SN 2007bi, but still
phase of nebular emission, durpeak (mag)
energy (ergs)
fainter than typical giant galaxies
ing which the optically thin ejecsuch as the Milky Way (3), whereta are energized by the decaying SLSN-R
SN 2007bi
0.1289
–21.35
1 to 2 × 1051
(7)
as the host galaxy of PTF10nmn
radionuclides. Analysis of lateSN 1999as
0.12
–21.4
(19)
seems to be as faint as or fainter
time spectra obtained during this
than that of SN 2007bi. This class
phase (7) provides independent SLSN-II
CSS100217
0.147
–23.07
1.3 × 1052
(58)
of objects may thus typically exconfirmation of the large initial
51
SN 2008fz
0.133
–22.34
1.4 × 10
(53)
56
plode in dwarf galaxies.
Ni mass via detection of strong
51
SN 2008am
0.2338
–22.39
2 × 10
(57)
The observations of SLSNe-R
nebular emission from the large
51
SN 2008es
0.205
–22.21
1.1 × 10
(36, 38)
strongly indicate that these events
mass of resulting 56Fe, as well as
51
SN 2006gy
0.019
–22.0
2.3 to 2.5 × 10
(12, 13)
are powered by massive-star exthe integrated emission from all
SN 2003ma
0.289
–21.52
4 × 1051
(45)
plosions that synthesize several
elements, powered by the remain50
SN 2006tf
0.074
< –20.7
7 × 10
(44)
solar masses of radioactive 56Ni,
ing 56Co.
SLSN-I
but the physical nature of the exEstimation of other physical
SN 2005ap
0.2832
–22.73
1.2 × 1051
(4, 11)
plosion is a matter of some conparameters of the SN 2007bi
51
SCP 06F6
1.189
–22.53
1.7 × 10
(4)
troversy. Theoretical work suggests
event—in particular, the total
PS1-10ky
0.956
–22.53 0.9 to 1.4 × 1051
(68)
two options. The first is an exejected mass (which provides a
51
PS1-10awh
0.908
–22.53 0.9 to 1.4 × 10
(68)
treme version of the iron core
lower limit on the progenitor star
PTF09atu
0.501
–22.03
(4)
collapse model that is generally
mass), its composition, and the
PTF09cnd
0.258
–22.03
1.2 × 1051
(4)
assumed to take place in explokinetic energy it carries—is more
SN 2009jh
0.349
–22.03
(4)
sions of massive stars that mancomplicated. There are no obSN 2006oz
0.376
–21.53
(69)
ifest themselves as common type
served signatures of hydrogen
50
SN 2010gx
0.230
–21.23
6 × 10
(4, 60)
II SNe (18, 26). The second is
in this event [either in the ejecta
the SN population in dwarf galaxies has been
shown to differ from that observed in giant hosts
(16). More details about these surveys [including the Catalina Real-Time Transient Survey
(CRTS), the Palomar Transient Factory (PTF),
and the Panoramic Survey Telescope and Rapid
Response System 1 (PS1)] and their results are
provided in the supplementary materials.
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Scaled flux (erg s-1 cm-2 Å-1)
Scaled flux (10-16 erg s-1 cm-2 Å-1)
Assuming, for the sake of the
the pair-instability mechanism
current discussion, that these ex[e.g., (27–32)]. The pair instabilplosions do arise from the pair
ity occurs during the evolution
2.2
SN 1999bd
instability, a clear prediction of
of very massive stars that develSN 2008am
the relevant theoretical models
op oxygen cores above a critical
SN 2006gy
2
PTF10qaf
Hα
[e.g., (27, 28)] is that for each lumass threshold (~50 M⊙). These
minous, 56Ni-rich explosion (from
cores achieve high temperatures
Hβ
1.8
at relatively low densities; suba core around 100 M⊙) there would
stantial amounts of electronbe numerous less luminous events
1.6
positron pairs are created prior
with smaller 56Ni masses but large
to oxygen ignition; loss of presejecta masses (M > 50 M⊙; fig. S2).
sure support, rapid contraction,
These should manifest as events
1.4
and explosive oxygen ignition
with very slow light curves (long
follow, leading to a powerful exrise and decay times) and moder1.2
plosion that disrupts the star.
ate or even low peak luminosities.
Extensive theoretical work indiSLSN-II. This is probably the
1
cates that such a result is unavoidmost commonly observed class
able for massive oxygen cores;
of SLSN. Whereas some exam0.8
when the core mass in question is
ples were identified relatively early
large enough (~100 M⊙, as in(e.g., SN 1999bd; Fig. 2), these
3000 3500 4000 4500 5000 5500 6000 6500 7000 7500 8000
objects became a focus of attenferred for SN 2007bi), many solar
Rest wavelength (Å)
tion only after the discovery of
masses of radioactive nickel are
SN 2006gy (12, 13). Since then,
naturally produced. It has been
shown (18, 26) that a carbon- Fig. 2. Spectra of SLSN-II events. A spectrum of SN 1999bd obtained on 22 March several additional examples have
oxygen core with a mass of ~43 M⊙ 1999 with the 2.5-m Dupont telescope at Las Campanas (blue) is compared with been studied in some detail.
spectra of SN 2006gy (12) (magenta), SN 2008am (57) (red), and a luminous SLSNSLSNe-II show strong hydrogen
(just below the pair-instability
II from PTF (PTF10qaf, cyan). The Balmer lines show narrow and intermediate-width
features in their spectra; these
threshold) that explodes with an
components [compare with the narrow host oxygen (O II) emission lines]. A promiad hoc large explosion energy nent emission bump around 4600 Å (short black vertical lines) is also a common explosions therefore typically oc(>1052 ergs) can produce the re- feature. At a redshift of z = 0.1512, the absolute magnitude at discovery of SN cur within thick hydrogen envequired large amounts of nickel 1999bd was –21.6. Telluric bands are marked and sections of the spectrum of SN lopes. This makes investigations
(26) as well as the light curve 1999bd affected have been excised; the telluric A band strongly absorbs the red wing of their nature more complicated
because all information carried
shape of the SLSN-R prototype, of the Ha line in this spectrum.
by electromagnetic radiation from
SN 2007bi (18). Both the pairthe exploding core is reprocessed
instability model and the masby the outer envelope. For this
sive core collapse model fit the
0.4
reason, our knowledge about their
light curve shape of SN 2007bi
Ca II
SN 2007bi
energy source (or sources) is still
equally well. However, because
SN
1999as
0.35
Mg II
mostly speculative. By contrast,
the progenitors of pair-instability
PTF10nmn
the physics responsible for conexplosions have larger cores and
SN 2010hy
Fe II
0.3
SYNOW fit
verting the explosion energy into
thus larger initial stellar masses—
the observed radiation is better
which are, assuming a declining
(Ca II)
0.25
understood.
initial mass function, intrinsically
Two main physical processes
more scarce—the core collapse
have been invoked to explain the
model has been claimed to be
0.2
conversion of explosion energy
favored for SN 2007bi (33, 34).
Mg II Ca II
to emitted radiation in SLSNe-II.
The two models agree about
0.15
The first process assumes that
the nickel mass but strongly
the explosion launches a powerdiffer in their predictions about
0.1
ful shock wave expanding outthe total ejected mass. Total heavyward from the center of the star.
element masses above the 50 M⊙
0.05
This shock heats the material it
threshold would indicate a core
traverses until it eventually esthat is bound to become pair0
capes from the effective outer
unstable, and would thereby rule
3000 3500 4000 4500 5000 5500 6000 6500 7000 7500 8000
edge of the star, where this effecout the core collapse model. The
Rest wavelength (Å)
tive edge is the radius around
core collapse model of (18), which
which the material is no longer
assumes a similar amount of radioactive 56Ni and lower total Fig. 3. Photospheric spectra of SLSN-R events SN 2007bi [blue (7)], SN 1999as optically thick to radiation (35).
ejected mass (to avoid the pair [magenta (7)], PTF10nmn [black (23)], and SN 2010hy (cyan); all spectra were The energy deposited by the
instability), predicts very strong obtained close to peak. Identification of prominent spectral features as well as a shock is then slowly reemitted
by the hydrogen-rich material as
nebular emission lines that are synthetic SYNOW fit [red, from (7)] are also shown.
photons diffuse out, in analogy to
not consistent with the data for
SN 2007bi. Thus, this model is not viable for this core collapse model applies to real supernovae; if the more common and much less luminous type
prototypical SLSN-R object, supporting instead a it does, the resulting SLSNe should show large II-P SNe, where this process occurs within the
pair-instability explosion as originally claimed. It amounts of radioactive nickel but relatively small envelope of a red supergiant star. To account for
the much higher observed SLSN-II luminosities,
remains to be demonstrated whether the massive amounts of total ejecta.
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Scaled flux (10-15 erg s-1 cm-2 Å-1)
the radius of the effective edge of the star must The derived limits on the amount of initial radio- seem to be the top of a broad distribution, with
be substantially larger than the radii of even the active nickel generally argue against SLSNe-II examples of peak magnitudes smoothly extendlargest red supergiants. Deposition of shock en- resulting from energetic pair-instability explo- ing from these extreme values down to luminosergy into more compact stars is radiatively in- sions. Spin-down of nascent magnetars (rapidly ities typical of the general SN population [e.g.,
efficient because the deposited energy is quickly spinning neutron stars with strong magnetic fields) SN 2010jl (54, 55); see (56) for a review of older
drained by adiabatic expansion. The observed has been proposed as an alternative energy source events]. The light curve shapes are quite diverse,
luminosities probably also require an energetic (47, 48). This process may be relevant at least with some SLSNe-II showing a rapid rise and deexplosion shock. The shape of the light curve is for some SLSNe-II (e.g., SN 2008es). One can cline [e.g., SN 2008es (36)], some showing light
determined mostly by the density structure and also consider the collapsar scenario, in which curves with a slow rise (>50 days) to a broad peak
composition of the material into which the energy energy is extracted from material rapidly accret- [e.g., SN 2006gy (12, 13), SN 2008fz (53)], and
was deposited. Several options have been suggested ing onto a newly formed black hole—a process some with rapid rise and very slow decline [e.g.,
to explain the large effective radii (>1015 cm) re- that may be driving cosmological g-ray bursts SN 2003ma (45), SN 2008am (57), and probably
quired for this mechanism to work. These include (49). When occurring within a massive star with also SN 2006tf (44)]. The spectra (Fig. 2) also
very large (bloated) stars [e.g., (36)], energy de- a thick hydrogen envelope, this process may show diversity, with most objects showing narrow
posited into massive (unbound) optically thick deposit the energy in the expanding envelope, hydrogen Balmer lines, and SN 2008es uniquely
shells ejected by previous eruptions of the ex- where it may be thermalized and reemitted as not showing such lines. Narrow Balmer lines arise
ploding star that have expanded to the required optical photons (11, 50). Unfortunately, because from a slow wind blown by the progenitor star
radius [e.g., (37, 38)], or energy deposited into any energy injected by such processes deep before its explosion. This wind is assumed to have
been photoionized by the exploan optically thick massive stelsion and to then recombine. The
lar wind [e.g., (39–42)] extendlack of such narrow lines in the
ing out to the required radius.
spectra of SN 2008es suggests
The second mechanism inMg II
SN 2005ap
that its progenitor star was not
voked in converting large exploSi III
SN 2010gx
blowing massive winds for an exsion energies into optical emission
C II
PTF09cnd
SN 2009jh
tended period prior to its explois strong interaction between the
PTF09atu
sion; any substantial mass loss
expanding ejecta and massive
PS1-10awh
1
must have been episodic [e.g., (38)]
CSM previously lost from the
PS1-10ky
or otherwise time-variable (41).
progenitor star. This mechanism
SCP 06F6
The environments of SLSNeconverts the kinetic energy carII are also quite diverse. This is
ried by the expanding ejecta into
the only SLSN subclass that has
radiation via strong shocks, and
been detected in luminous, Milky
is commonly invoked for type IIn
0.5
Way–like galaxies [e.g., SN 2006gy
SNe (43). Because CSM enveO II
(12, 13), CSS100217:102913+404220
lopes can be extremely extended,
(58)]. Still, like other SLSNe, most
this process can in principle reSLSN-II events reside in dwarf
main active for many years, and
star-forming hosts (3). Published
is thus more useful to explain
0
SLSN-II events in giant host galvery long-lived events [e.g., SN
2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000
axies seem to have been detected
2006tf (44), SN 2003ma (45)].
Rest wavelength (Å)
very close to their host nuclei,
On the other hand, conversion
which suggests that perhaps speof kinetic energy into radiation
should manifest itself as an ob- Fig. 4. Early spectra of all published SLSN-I events: SN2005ap (11); SN 2010gx, cific conditions that are unique
served decline in expansion ve- PTF09cnd, SN 2009jh, and PTF09atu (4); PS1-10awh and PS1-10ky (68); and SCP to this environment (e.g., cirlocities; this process is therefore 06F6 [(59); combined version from (4)]. The optical O II blends and near-UV C II, Si III, cumnuclear star-forming rings)
disfavored for events showing and Mg II lines identified by (4) are marked. The spectroscopic similarity among these somehow mimic the conditions in
star-forming dwarf galaxies.
high expansion velocities that do objects is quite striking.
SLSN-I. This class of SLSNe
not decrease substantially with
time [e.g., SN 2008es (36)]. Possible mechanisms inside exploding stars is then reprocessed by the was initially the most difficult to understand,
invoked to eject large quantities of mass from the optically thick outer hydrogen layers, investiga- with the first two reported events—SN 2005ap
star prior to explosion include luminous blue var- tions of such exotic processes in SLSNe-II are (11) and SCP 06F6 (59)—sparsely observed. It
was only after the discovery of additional memiable (LBV)–like activity [e.g., (12, 44)] and pul- difficult and have remained mostly speculative.
sational pair instability [e.g., (46)].
In any case, it is clear that SLSNe-II are ex- bers of this class by the PTF survey, bridging the
Regardless of the conversion mechanism, the plosions of massive stars that retained their hy- redshift gap between the relatively nearby SN
total emitted energy in several recently observed drogen envelopes until they exploded. For some 2005ap (z = 0.2832) and the high-redshift SCP
objects (>1051 ergs) is difficult to reproduce in objects, spectroscopy indicates that these stars 06F6 (z = 1.189), that a comprehensive view of
models of regular iron core collapse explosions have lost substantial amounts of mass prior to ex- this class of objects could be formed [(4), in(where >99% of the initial explosion energy, ~3 × plosion [e.g., SN 2006gy (12), SN 2006tf (44)], cluding spectroscopic redshifts based on Mg II
1053 ergs, is carried away by neutrinos). This led which suggests that perhaps the progenitor stars absorption lines, and the first correct identification
several authors to speculate about additional en- are similar to massive LBVs, which are known of the redshift of SCP 06F6; Fig. 4]. A major
ergy sources contributing to these powerful ex- to undergo episodic eruptions involving extreme reason for the initial difficulties was that the early
plosions. Pair-instability explosions can provide mass loss [e.g., (51, 52)].
spectra of these objects are quite featureless and
large kinetic energies and synthesize large amounts
The observational characteristics of SLSNe-II the absorption lines that do appear are mostly of
of radioactive 56Ni; however, SLSNe-II studied are quite diverse. The peak luminosities of the high-excitation low-mass elements (Fig. 4); the
at late times did not follow the expected 56Co brightest events reach well above −22 mag ab- elements commonly observed in most SN classes
radioactive decay rate, in contrast to SLSNe-R. solute [e.g., SN 2008fz (53)]. However, these (neutral oxygen, magnesium, iron, and the ubiquitous
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ionized calcium) appear only much later (60)
(fig. S1).
Observationally, these events are characterized by extreme peak luminosities (often brighter
than −22 mag absolute), very blue spectra with
copious UV flux persisting for many weeks, and
(relative to other classes of SLSNe) fast-evolving
light curves with rise times below 50 days and
post-peak slopes that decline substantially faster than radioactive cobalt decay rates (4, 60).
Contrary to early reports [e.g., (11, 36, 38)], these
events do not show hydrogen in their spectra
(4, 60) (fig. S5) and thus do not belong to the
spectroscopic class of type II SNe. Technically,
these events should be classified as type Ic SNe
because they also do not show strong He features
in spectra taken around peak. However, because
the class of type Ic SNe is not positively defined
and a physical connection has not been firmly
established between the events considered here
and more common SN Ic events (see below), this
class is denoted SLSN-I.
The similarity between the late-time spectra
of SN 2010gx (SLSN-I) and the spectra of broadline type Ic SNe (4, 52) (fig. S5) prompts discussion of a possible connection between these
two classes. However, there are several physical
differences between SLSNe-I and SNe Ic.
Detailed modeling of SNe Ic [e.g., (61–63)]
indicates that their luminosity is dominated by
radioactive 56Ni decay. Indeed, these objects show
a correlation between the peak luminosity and the
synthesized 56Ni mass [e.g., (64, 65)]. The same is
true for SLSNe-R (7) but not for SLSNe-I (4), in
the sense that the nickel mass required to power
the observed luminous peaks is in conflict with
the later evolution of the light curve [e.g., (4, 60)].
The luminosity of SLSNe-I must therefore come
from a different source.
Two very luminous broad-line SNe Ic [SN
2007D (65), SN 2010ay (66)] have been observed
with peak luminosities approaching those of
SLSNe-R (−20.6 mag and −20.23 mag absolute
for SN 2007D and SN2010ay, respectively), but
with less 56Ni (~1 M⊙). Other processes may be
contributing to the large observed peak luminosity of these events [e.g., an internal engine (66)].
Perhaps they are intermediate events between the
class powered purely by radioactivity (normal
SNe Ic and SLSNe-R) and SLSNe-I for which the
contribution from 56Ni is negligible, and which
must be powered by some other process, as discussed below.
Another important physical distinction between SLSNe-I and SNe Ic is the size of the emitting region. It has been shown that the energy
radiated by SLSNe-I must have been deposited
at large initial radii, ~1015 cm (4). However, early
observations of SNe Ic indicate that the progenitor stars had an initial radius orders of magnitude
smaller [<1011 cm (61, 67)], implying that both
the explosion shock energy and radioactivity
must be contained within a small initial radius.
It thus seems that the observed spectroscopic
similarity between SLSNe-I (at late times) and
broad-line SNe Ic suggests similar ejecta composition and a large kinetic energy (evident as a
substantial amount of mass at high velocities),
but other physical properties (energy source,
physical size) are different and suggest a different
physical mechanism powering these two classes
of objects.
It has been shown (4) that the observed luminosity of these objects requires the deposition
of a large amount of internal energy, taking
place at large radii (1015 cm, about 10 times the
size of the largest red supergiants), into material
expanding at high velocities (104 km s−1). The
data can rule out the traditional energy conversion mechanisms discussed above (radioactivity, photon diffusion, interaction with massive
hydrogen-rich CSM). Viable options include interaction with expanding shells of hydrogen-free
material (40), perhaps ejected by the pulsational
pair instability [e.g., (46)], or reemission of energy
injected by an internal engine, such as magnetar
spin-down (47, 48) or a “collapsar”-like accreting
black hole [e.g., (11, 60, 68, 69)]. Even though
these events are not enshrouded by massive, opaque
hydrogen shells, the physical nature of the energy
source remains speculative; the energy conversion
mechanism is also not clearly understood.
The host galaxies of these events are again
typically dwarf galaxies, although at higher redshifts, luminosity upper limits on undetected hosts
are less constraining (3, 4). The most natural
explanation for these objects not occurring in
more luminous galaxies is that a lower metallicity
is required to form the progenitor stars of these
events, but other explanations are also possible
(e.g., different star formation modes or a topheavy initial mass function in dwarf galaxies).
The extreme intrinsic luminosity and plentiful UV
flux of these sources make them ideal probes of
dwarf galaxies at high redshifts.
Rates of SLSNe
The only measurement of the rate of SLSNe is a
rough estimate based on TSS statistics (4), which,
normalizing the rate of SLSNe-I at z ≈ 0.3 relative
to that of SNe Ia, yields ~10−8 Mpc−3 year−1. This
rate is substantially lower than the rates of corecollapse SNe (~10−4 Mpc−3 year−1) and is also well
below those of rare subclasses such as broad-line
SNe Ic (hypernovae; ~10−5 Mpc−3 year−1) or long
gamma-ray bursts [>10−7 Mpc−3 year−1 (70, 71)].
The reported discovery statistics suggest that the
rate of SLSNe-II is comparable or larger than that
of SLSNe-I, whereas SLSNe-R are rarer by a
factor of ~5, correcting for their slightly lower
peak luminosities. SLSNe-R are the rarest type of
explosions studied so far, and quite possibly they
arise from stars that are at the very top of the
initial mass function.
Summary
During the past dozen years, numerous superluminous SN events have been discovered and
studied. The accumulated data suggest that these
can be grouped into three distinct subclasses
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VOL 337
according to their observational and physical
attributes. Radioactively powered SLSNe-R
seem to be the best understood (and rarest) class;
hydrogen-rich SLSNe-II and the most luminous
hydrogen-poor SLSNe-I are more common, but
the physical origins of the extreme luminosity
they emit is not clear at this time. With several
ongoing surveys efficiently detecting additional
examples, the amount of information about these
objects is likely to increase substantially in the
next few years.
References and Notes
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2. Even the brightest SNe associated with cosmological
gamma-ray bursts (e.g., the nickel-rich SN 1998bw) fall
well below our SLSN threshold.
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4. R. M. Quimby et al., Nature 474, 487 (2011).
5. Possible early detections of SLSNe by these surveys
and their modern counterparts are discussed in text S2.
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7. A. Gal-Yam et al., Nature 462, 624 (2009).
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9. The study of the peculiar SN 1997cy (72) was perhaps
the first modern study of a supernova that was
substantially more luminous than the norm. The absolute
peak magnitude of this event (–20.1 mag), as well as
that of similar objects detected since [e.g., SN 2002ic
(73)], falls below our fiducial limit defined above, and
thus we do not discuss them here.
10. R. M. Quimby, thesis, University of Texas (2006).
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34. The models considered by (33) require stars with
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pair-unstable cores at the moderate metallicity indicated for
SN 2007bi (25). However, alternative models (74) predict
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This effective edge does not necessarily coincide with the
physical edge of the star, which we define as the radius
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There is no conflict between this process (converting
kinetic energy to radiation) and the previous one
(converting shock energy stored as internal heat energy
into radiation) and both can contribute in any given
object, although there is debate about which one is
dominant for particular cases.
N. Smith et al., Astrophys. J. 686, 467 (2008).
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A similar relation has been established in the case of a
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Mon. Not. R. Astron. Soc. 408, 87 (2010).
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C. W. Stubbs, Astrophys. J. 533, 320 (2000).
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Gamma-Ray Bursts
Neil Gehrels1* and Péter Mészáros2
Gamma-ray bursts (GRBs) are bright flashes of gamma rays coming from the cosmos. They
occur roughly once per day, typically last for tens of seconds, and are the most luminous events
in the universe. More than three decades after their discovery, and after pioneering advances from
space and ground experiments, they still remain mysterious. The launch of the Swift and Fermi
satellites in 2004 and 2008 brought in a trove of qualitatively new data. In this Review, we survey
the interplay between these recent observations and the theoretical models of the prompt GRB
emission and the subsequent afterglow.
amma-ray bursts (GRBs) are the most
extreme explosive events in the universe.
The initial (prompt) phase lasts typically
less than 100 s and has an energy content of
~1051 ergs, giving a luminosity that is a million
times larger than the peak electromagnetic luminosity of the bright emission from an explodingstar supernova. The GRB name is a good one
because their spectra peak in the gamma-ray
band between ~100 keVand ~1 MeV. The source
of the energy powering the bursts is thought to be
the gravitational collapse of matter to form a
black hole or other compact object.
GRBs were discovered in the late 1960s by
the Vela satellites monitoring the Nuclear Test
Ban Treaty between the United States and the
Soviet Union (1). It was slow progress for 20
G
1
Astrophysics Science Division, NASA Goddard Space Flight
Center, Greenbelt, MD 20771, USA. 2Department of Astronomy
and Astrophysics, Pennsylvania State University, University
Park, PA 16802, USA.
*To whom correspondence should be addressed. E-mail:
[email protected]
932
years learning the origin of these brilliant flashes.
Gamma-ray instruments of the time had poor positioning capability, so only wide-field, insensitive
telescopes could follow up the bursts to look for
counterparts at other wavelengths. Nothing associated with the GRBs was seen in searches that
took place from hours to days after their occurrence. In the 1990s, the Burst and Transient Source
Experiment (BATSE) onboard the Compton Gamma Ray Observatory (CGRO) obtained the positions of ~3000 GRBs and showed that they were
uniformly distributed on the sky (2), indicating
either an extragalactic or galactic-halo origin.
BATSE also found that GRBs separate into two
duration classes, short GRBs (SGRBs) and long
GRBs (LGRBs), with a dividing line at ~2 s (3).
The detection of x-ray afterglows and the more
accurate localizations delivered by the BeppoSAX
mission (4, 5) enabled optical redshifts of GRBs
to be measured and their extragalactic origin to
be confirmed. The long bursts were found to be
associated with faraway galaxies at typical redshifts z = 1 to 2, implying energy releases in
excess of 1050 ergs. The afterglow time decay
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SCIENCE
80. See text S3 for additional details.
81. O. Yaron, A. Gal-Yam, http://arxiv.org/abs/1204.1891 (2012).
Acknowledgments: I thank O. Yaron; Caltech Core-Collapse
Project members E. Enriquez, A. Soderberg, S. B. Cenko,
D. Leonard, D. Fox, D. Moon, and D. Sand; M. Phillips and
P. Nugent; E. Chatzopoulos; L. Chomiuk and R. Quimby for
use of data presented here; E. Nakar, P. Mazzali, D. Xu,
R. Waldman, A. Pastorello, I. Arcavi, A. Howell, E. O. Ofek,
A. Drake, S. Smartt, C. Wheeler, and A. Miller for useful advice;
members of the PTF collaboration, and in particular J. S. Bloom,
M. M. Kasliwal, S. R. Kulkarni, N. M. Law, and D. Poznanski, for
use of unpublished PTF material; and the anonymous reviewers
for useful and constructive suggestions and comments. Supported
by grants from the Israeli Science Foundation, the U.S.-Israel
Binational Science Foundation, the German-Israeli Foundation,
the Minerva Foundation, ARCHES (Award for Research
Cooperation and High Excellence in Science), and the Lord
Sieff of Brimpton Fund. This research has made use of the
NASA/IPAC (Infrared Processing and Analysis Center) Extragalactic
Database, which is operated by the Jet Propulsion Laboratory,
California Institute of Technology, under contract with NASA.
All spectra shown are available in digital form from the Weizmann
Interactive Supernova Data Repository (WISeREP) (81) at
www.weizmann.ac.il/astrophysics/wiserep.
Supplementary Materials
www.sciencemag.org/cgi/content/full/337/6097/927/DC1
Texts S1 to S3
Figs. S1 and S2
References (82–103)
10.1126/science.1203601
often steepened after a day, indicating a geometrical beaming of the radiation into a jet of opening angle ~5° (6).
Key open issues before the Swift and Fermi
era included (i) the origin of SGRBs, (ii) the
nature of the high-energy radiation from GRBs,
(iii) the redshift distribution of bursts and their
usage for early universe studies, and (iv) the
physics of the jetted outflows.
Swift and Fermi, launched in 2004 and 2008,
respectively, have opened a new era in GRB research. Swift is a NASA mission, Fermi is a
NASA–Department of Energy partnership, and
both have major international contributions. The
two missions have different and complementary
capabilities. Swift has a wide-field imaging camera in the hard x-ray band that detects the bursts at
a rate of ~100 per year, providing positions with
arc-minute accuracy. The spacecraft then autonomously and rapidly (100 s) reorients itself for
sensitive x-ray and ultraviolet (UV)/optical observations of the afterglow. Fermi has two wide-field
instruments. One detects bursts in the gamma-ray
band at a rate of ~300 per year, providing spectroscopy and positions with 10° accuracy. The
other observes bursts in the largely unexplored
high-energy gamma-ray band at a rate of ~10 per
year. Combined, the two missions are advancing
our understanding of all aspects of GRBs, including the origin of short bursts, the nature of
bursts coming from the explosion of early stars
in the universe, and the physics of the fireball
outflows that produce the gamma-ray emission.
Swift GRB Observations
Mission and statistics. The Swift mission (7) has
three instruments: the Burst Alert Telescope (BAT)
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