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Optical Light Curves of Supernovae ?
Bruno Leibundgut 1 and Nicholas B. Suntze 2
1 European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching,
Germany
2 Cerro Tololo Inter-American Observatory, Casilla 603, La Serena, Chile
Abstract. Photometry is the most easily acquired information about supernovae. The
light curves constructed from regular imaging provide signatures not only for the energy
input, the radiation escape, the local environment and the progenitor stars, but also
for the intervening dust. They are the main tool for the use of supernovae as distance
indicators through the determination of the luminosity.
The light curve of SN1987A still is the richest and longest observed example for
a core-collapse supernova. Despite the peculiar nature of this object, as explosion of
a blue supergiant, it displayed all the characteristics of type II supernovae. The light
curves of type Ib/c supernovae are more homogeneous, but still display the signatures
of explosions in massive stars, among them early interaction with their circumstellar
material.
Wrinkles in the near-uniform appearance of thermonuclear (type Ia) supernovae
have emerged during the past decade. Subtle dierences have been observed especially
at near infrared (NIR) wavelengths. Interestingly, the light curve shapes appear to
correlate with a variety of other characteristics of these supernovae.
The construction of bolometric light curves provides the most direct link to theo-
retical predictions and can yield sorely needed constraints for the models. First steps
in this direction have been already made.
1 Physics of Supernova Light Curves
The temporal evolution of the energy release by supernovae (SNe) is one of the
major sources of information about the nature of these events. The brightness
information is relatively easy to obtain and, hence, light curves have been one of
the main stays of supernova research. It is not only the light curves themselves,
but also the absolute luminosity and the color evolution that have provided
major insights into the supernova phenomenon. Through light curves it has
been possible to distinguish between progenitor models, infer some aspects of
the progenitor evolution, measure the power sources, detail the explosion models,
and probe the local environment of the supernova explosions.
The observational data have been substantially increased over the last decade
(for a status before 1990 see [51]). In particular, there are now large databases
with light curve data for type Ia supernovae (SNIa). The data on type II su-
pernovae has been extensively expanded as well, but there is still a clear lack of
light curves for peculiar objects. Over the last decade the supernova family has
? to be published in: "Supernovae and Gamma Ray Bursters", Lecture Notes in Physics
(http://link.springer.de/series/lnpp)

2 Leibundgut and Suntze
further acquired new members and subclassications (see the chapter by Turatto
in this volume).
The energetic display of a supernova can have several dierent input sources.
The most important power comes in almost all cases from the radioactive decay
of material newly synthesized in the explosion. The major contributor is 56 Ni, the
main product of burning to nuclear statistical equilibrium at the temperatures
and densities encountered in supernovae. This nucleus is unstable and decays
with a half-life of 6.1 days due to electron capture to 56 Co emitting -photons
with energies of 750 keV, 812 keV, and 158 keV. The cobalt isotope is also
unstable and decays with a half-life of 77.1 days through electron capture (81%)
and -decay (19%) to 56 Fe. In this process -photons with energies of 1.238 MeV
and 847 keV are emitted. The kinetic energy of the electrons is about 600 keV
(for more details on the radioactive decays see [3,26,78]). The -rays are down-
scattered or thermalized in the ejecta until they emerge as optical or NIR photons
[2,48,85].
The light curves depend on the size and mass of the progenitor star and
the strength of the explosion. Additional energy input, which results in modu-
lations of the emerging radiation, comes from shock cooling, recombination of
the ionized ejecta, collision of the shock with circumstellar medium (CSM) and
possible accretion onto a compact remnant. Light curves are further shaped by
the time-variable escape fraction of -rays, dust formation and absorption in the
interstellar medium. In some cases, forward scattered light can change the light
curves, e.g., through light echos and uorescence of nearby gas ionized by the
X-ray/UV shock breakout of the explosion.
With this panoply of dierent energy contributors and modulators, light
curve displays are very rich indeed. Despite the plethora of possibilities, light
curves of dierent supernova types are rather distinct, although not suôciently
so for a solid classication. They are, however, important tools to learn about
the physics of supernovae.
Light curves are discussed in [37,51,56,80,81]. Reviews concentrating more
specically on SNIa light curves can be found in [57,58,73,107], while the light
curves of core-collapse supernovae have mostly been summarized in relation to
SN1987A [4,72]. Additional well-sampled data sets are available for SN1993J,
SN1998S and SN1999em (see references below).
The following sections give a brief overview of observational data sources (x2),
describe the light curves of the main supernova types (core-collapse supernovae
in x3 and thermonuclear supernovae in x4) and the physics behind the light curve
shapes. We will discuss bolometric light curves in x5 before we summarize in x6.
2 Observations
With modern area detectors, light curves of supernovae have become much
easier to assemble. While early light curves have been compiled from photo-
graphic plates [60,82,84] observations are now recorded with CCDs. The in-
creased sensitivity has allowed astronomers to successfully move to smaller-size

Optical Light Curves of Supernovae 3
telescopes and to improve the temporal sampling. In parallel, the move to more
robotic telescopes has increased the number of supernova discoveries tremen-
dously (see, e.g., the chapter by Cappellaro in this volume). There are many
observational programs for supernovae currently in progress (for a description
of some see [58]), which contribute photometry for many supernovae. Most suc-
cessful are eorts with semi-automated telescopes. The robotic telescopes of the
Lick Observatory and Tenagra Observatory Supernova Search (LOTOSS) have
discovered and followed many supernovae in the last few years [65,90]. At the
Cerro Tololo Inter-American Observatory (CTIO) the Yale-AURA-Lisbon-Ohio
(YALO) telescope has regularly provided light curves for nearby supernovae
(see, e.g., [54,112]). light curves have been further contributed by the Padova
group (see, e.g., [9,97,116,117] and the results of the Harvard-Smithsonian Cen-
ter for Astrophysics (CfA) team [50,92]). There are additional contributions on
SN1993J [8,11,27,64,74,90,120] and more recently SN1998S [32] and SN1999em
[45,63]. Many amateur groups are also collecting supernova light curves. These
data are mostly maintained on Web sites (see [58] for a small collection of sites).
Infrared observations are still rare. A complete compilation of the available
photometry for SNIa before 2000 has been provided by Meikle [73]. More data
are being added (see, e.g., [46,54]). For core-collapse supernovae complete light
curves are available only for SN1987A [108,110], SN1993J [120], SN1998S [32],
and SN1999em [45]. There are several programs starting up that will concentrate
on NIR light curves with robotic telescopes.
3 Core Collapse Supernovae
Once the shock, which results from the reversal of the core collapse, breaks out
at the surface of the progenitor star the reworks begin. The rapid evolution
of the core burning just before the collapse is hidden from the surface due to
the long time scales in the atmosphere. The brightness of the shock break out is
mostly determined by the temperature in the shock and the size of the progeni-
tor star [20,31,53]. This early peak lasts typically from a few hours to a couple of
days and has been observed only for SN1987A [4], SN1993J [90], and SN1999em
[45,63]. After a rapid, initial cooling the supernova enters a phase when its tem-
perature and luminosity remain fairly constant [29,45,63]. For supernovae with
large progenitors the resulting light curve shows a plateau, while the evolution
of supernovae from smaller stars rst exhibits a decline before the supernova
brightens again to reach the plateau. Examples for the former are SN1990E [98]
and SN1999em [45,63], while the type II SN1987A (see, e.g., Fig. 1) together
with the SNIb/c belong to the latter (see, e.g., [56]). The plateau originates
from a balance between the receding photosphere in the expanding ejecta [29].
During this phase the supernova is powered by the recombination of hydrogen
previously ionized in the supernova shock. The length of the plateau phase is de-
termined by the depth of the envelope (i.e., the envelope mass and the explosion
energy), which is re ected in the expansion velocity of the ejecta [19,89]. For

4 Leibundgut and Suntze
0 1000 2000 3000 4000
20
15
10
5
days since outburst
V
magnitude
ring emission
ejecta emission
expanding
photosphere
recombination wave
radioactive tail ( 56 Co)
dust formation
'freeze out' phase
+ radioactive tail ( 57 Co)
radioactive tail ( 44 Ti)
Fig. 1. V light curve of SN1987A. The various phases are labeled.
some objects this plateau phase is conspicuously absent [80]. Most prominent
among these are SN1979C [15] and SN1980K [7].
Once the photosphere has receded deep enough, additional heating from the
radioactive decay of 56 Ni and 56 Co extends the plateau for a brief time [123].
Afterwards the light curve is powered solely by the radioactive decay in the
remaining nebula. The -rays are captured in the ejecta and converted into
optical photons, which can escape freely. At this moment the supernova light
curve drops onto the \radioactive tail." This happens typically after about 100
days (see Fig. 1).
For a complete trapping of the -rays the luminosity of the late decline gives
an indication of the amount of 56 Ni and 56 Co decays powering the light curve
[12,99]. This can be checked with the decline rate of the bolometric light curve,
which should re ect the 56 Co decay time.

Optical Light Curves of Supernovae 5
From such measurements a rather large range of nickel masses has been
derived [99,103,116]. These phases are especially interesting as they may show
signatures of signicant fallback of the inner explosion material onto the forming
compact object, neutron star or black hole, in the explosion [6,10,122].
Especially for SNIb/c the decline rates are steeper [21,90,104,105] at these
times, which is an indication that some of the -rays escape from the ejecta
without any energy deposition.
Very few objects have been followed beyond about 200 days and the situation
has not changed very much since about 10 years ago [80,113]. The photometry of
such objects becomes very diôcult as they fade into the glare of the underlying
galaxy. The remarkable exception is, of course, SN1987A on which all very late
phase information is based. This supernova suered from dust forming within
the ejecta, which resulted in an increase of the decline rate in the optical as
light was shifted to the infrared [111]. This occurred after about 450 days and
could also be observed as a shift of emission lines towards the blue as the redder
parts of the lines were absorbed. After about 800 days the light curves started
to atten again [111] due to energy release of ionized matter [40]. This so-called
\freeze-out" stems from tenuous material which was ionized during the original
explosion but recombines on time scales longer than the expansion time. At later
times, the attening is caused by the energy input from long-lived 57 Co (half-life
of 270 days) and 44 Ti which has a half-life of about 60 years [26].
As is apparent from Fig. 1, the very late times are, in fact, dominated by
the emission from the circumstellar inner ring, which was ionized by the shock
breakout [39]. Around 1500 days after the explosion the ring emission is stronger
than that from the supernova ejecta itself.
The closeness of SN1987A has permitted us to resolve the ring emission
and also light echos from interstellar dust from the supernova ejecta [109,124].
For any other supernova these contributions can not be separated and would
in uence the light curve shape.
Some supernovae do not follow the path described above. They often have
a much slower evolution and also display narrow lines in their spectra (see,
e.g., [37,41]). These objects remain bright for a long time and must be powered
by a dierent energy source. The mostly likely explanation for these objects
is interaction of the shock with a dense circumstellar medium (CSM). In this
process, kinetic energy is converted to light and hence an additional energy source
can be tapped for the light curve. Since many of these objects are dominated by
line emission and the line strengths critically depend on density and composition
of both the supernova ejecta and the circumstellar medium (CSM), lter light
curves can vary signicantly from object to object. A few classical cases are
known so far: SN1978K [96], SN1986J [61,95], SN1988Z [114] and SN1995N [41].
Of similar nature are probably also objects which can be observed for decades
after their explosions although at a much lower luminosity. The prime examples
are SN1979C [35,119], SN1980K [34,36,61], SN1970G [33], and SN1957D [68,69].
A summary of the observational characteristics of these objects can be found in
Leibundgut [55] and the detailed physics of the SN-circumstellar medium (CSM)

6 Leibundgut and Suntze
interaction is given in the chapter by Chevalier and Fransson in this volume. It
is noteworthy that all these objects also emit radio waves (see the chapter by
Sramek and Weiler in this volume).
On no occasion has it been possible, so far, to observe the emergence of
a pulsar within a supernova. Even for SN1987A the data do not require any
input from a pulsar. Some objects which have been observed for decades, like
SN1957D, SN1970G, SN1979C, SN1980K, still do not require the energy input
corresponding to the expectations from a pulsar powered plerion (see, e.g., [55]).
It is more likely that these objects are powered by interaction of the shock
with the circumstellar medium (CSM). In all of these cases the spectrum clearly
shows that we are still observing supernova light and not an underlying stellar
association. This is further supported by changes in the light curves as observed
for SN1957D [69] and SN1980K [36].
4 Thermonuclear Supernovae
The observational situation for type Ia SNe is quite dierent. There have been
several focused searches for SNIa, which have produced large sets of well-sampled
light curves (see, e.g., [42,44,46,50,66,73,88,91,92,97,107,112]). These data sam-
ples have produced a detailed view of SNIa. A recent summary of SNIa light
curves can be found in Leibundgut [58] and Meikle [73].
The incineration of a white dwarf, which is the most favored model today
(see, e.g., [47,123]), does not predict an observable shock breaking out at the sur-
face. The rise in brightness is due to the increase in size of the ejecta and lasts for
almost three weeks [1,23,42,93] with the color and temperature rather constant.
The earliest observed supernovae are SN1990N and SN1998bu, which were ob-
served about 17 days before maximum [58,93]. The light curves are shaped by
the progressing diusion of photons out of the ejecta (see, e.g., [85]).
The maximum is reached rst at NIR wavelengths [24,73] and is followed a
few days later in the optical. While the blue light curves display a monotonic
decrease of the brightness for the rst month after maximum, the NIR bands
I, J, H, and K display a prominent second maximum after about 20 days for
most SNIa [30,73], which is often also observed as \shoulders" in the V and R
lter light curves. This second maximum is conspicuously missing for objects of
the type Ia faint subclass with SN1991bg and SN1999de as the most prominent
examples [38,62,77,115].
SNIa show a strong color evolution towards the red through the maximum
phase. Despite the diôculty that this poses for an exact measurement, the in-
trinsic color of SNIa appears to be very uniform [88] and this is often used to
determine the amount of reddening towards the supernova.
After about 40 days the light curves settle onto an exponential (in luminosity)
decline for several months. This has been interpreted as the optically thin phase
when the ejecta nebula captures fewer and fewer -rays and the optical and NIR
light curves decline faster than the 56 Co decay rate (see, e.g., [49,59,85]). After
about 150 days the light curves change slope once more (see, e.g., [58]) when

Optical Light Curves of Supernovae 7
the importance of the positron channel in the 56 Co decay sets in (see, e.g., x1,
[5,75]. The decline at these phases should tell us about the magnetic elds in the
explosion as they determine whether the positrons are captured or escape the
ejecta [22,75,94]. Currently, the best estimate of these late light curves predicts
a positron escape similar to that of the photons [76].
Light echos can start to dominate SNIa light curves several hundred days
after the explosion. There are now at least two objects with clear signatures
of light echos, SN1991T [100,106] and SN1998bu [18]. Their light curves have
attened almost completely as the peak light is scattered o nearby dust clouds
and, since we observe time and intensity integrated light, the brightness does
not change until the edge of the dust layer is reached or the scattering angle
increases to the point where the scattering eôciency decreases. High spatial
resolution imaging shows rings around these supernovae ([106] and Garnavich,
private communication) very similar to the ones observed around SN1987A.
Despite the highly complicated hydrodynamics and the radiative transport
in SNIa ejecta the bolometric light curves can provide important insights. Most
of the emission from SNIa is emitted in the optical and NIR region [23,107].
By sampling the emission from the atmospheric cuto near 3600  A to 1 m
we are capturing about 80% of the energy emitted by these objects outside the
-ray region. Although the color changes signicantly through the observable
life of a SNIa, not much light is emitted in the near-UV (see, e.g., [52,57,107])
or the infrared [23]. In fact, the V light curve is a rather good surrogate of the
bolometric light curve after maximum [24]. In particular, during the late declines
the V light curves have been used to calculate the Ni mass synthesized in the
explosions [17] and also to estimate the positron escape [76].
Fig. 2 illustrates the connection between observed bolometric light curves
(dotted line { constructed from observations of SN1992bc) and theoretical mod-
els. The expected decay lines for 56 Ni and 56 Co are indicated by the thin, gray
lines. At early times all the energy is trapped in the ejecta and as the surface
increases the brightness increases as well. Around maximum the energy released
is almost identical to the decay energy from 56 Ni and 56 Co combined. This has
been pointed out long ago by Arnett [2] and has been conrmed by Pinto and
Eastman [85]. A compilation of typical values of early models has been given in
Branch [14]. This is an important feature of SNIa and can be used to measure
the total amount of radioactive material synthesized in the explosion (see, e.g.,
[24]). For a brief time after maximum the energy output from the supernova
exceeds the prediction from a complete trapping of the radioactive energy while
\old" photons still leak out at the surface. At later phases more and more -
rays are lost and the decline is faster than the line indicating a full trapping
(dash-dotted). The dashed line indicates a light curve in which no energy from
the original decays is converted to optical light. Only when 3% of the  decay
sets in can there be some levelling of the light curve. The observed light curve
evolves between these extreme cases.

8 Leibundgut and Suntze
­20 0 20 40 60 80 100
t (days after maximum)
41.0
41.5
42.0
42.5
43.0
43.5
44.0
log
L
(ergs/s)
Fig. 2. Bolometric light curve of a SNIa. The dotted line is the observed bolometric
light curve. The thin lines represent the 56 Ni and 56 Co decay lines, while the dashed
line would be the expected curve if all -rays from the radioactive decay escape the
ejecta and only the positrons are converted into optical emission. The dash-dotted line
indicates the expectation of full trapping of all decay energy [23].
Correlations The SNIa light curve shape has been recognized as correlating
with the peak luminosity [87]. This has become the linchpin for distance de-
terminations using SNIa (see, e.g., the chapter by Perlmutter and Schmidt in
this volume). The normalization of the peak luminosity allows the determination
of cosmological parameters. However, the correlation is not as clear-cut as one
would wish. There are three implementations [83,88,91], which are currently not
consistent with each other [28,58].
There are other parameters which correlate with the peak luminosity of SNIa.
They are the rise time to maximum [93], color near maximum light [88], line
strengths of the primary Ca and Si absorption lines [79], the velocities as mea-
sured in Fe lines at late phases [70], the host galaxy morphology [101], and host

Optical Light Curves of Supernovae 9
galaxy colors [16,43]. There may be indications that the secondary peak in the
I light curves and the shoulder in the bolometric light curves correlate with the
absolute luminosity [24,44,91].
With SN2000cx we now also have a clear example of an object which does not
follow these simple rules. For most objects the rise and decline rates correlate
fairly well [24,93], but SN2000cx violates this rule [66]. While it had a rapid
rise, its light curve did decline more slowly than many other SNIa. It is, at the
moment, not clear what fraction of SNIa show a similar behavior as the rise
phase is often not observed and the correlations have been based on the decline
rate. Nevertheless, the fact that for some SNIa the rise and decline rates do
correlate [24,93] hints at intrinsic dierences within this class.
The physical reason for these correlations are not yet clear. Possibilities are
dierences in the amount 56 Ni synthesized and the distribution of the radioactive
material, and hence the heating of the ejecta [49,71,86].
5 Bolometric Light Curves
The connection of the observations to the explosion and radiation physics has to
come through bolometric light curves as they represent the total energy output
of a supernova. This integrated quantity is easily constructed from the available
multi-lter photometry and is also rather easily extracted from model calcula-
tions. Detailed calculations of the emerging spectrum have proven diôcult for
all supernova types. While the physics of the radiation escape for core-collapse
supernovae during the plateau phase, with their large envelopes and a rather
well-dened photosphere (see, e.g., the chapter by Branch et al. in this volume),
is fairly simple to calculate, possible chemical abundance mixing and deviations
from spherical geometry can spoil direct comparison with the observations. For
thermonuclear supernovae the spectrum formation is due to multiple scattering
of the -rays in a non-thermal environment and hence the photons rather \leak"
out at the surface than originate from a thermalized photosphere. Hence, the cal-
culation of a lter light curve requires a detailed spectrum formation calculation
which, in the case of SNIa, is currently impossible at the needed detail.
The temporal evolution of the integrated light gives a rather straight forward
comparison between theory and observation. Important explosion parameters
can be calculated from these integrated quantities. Foremost, the amount of
energy available from the nucleosynthesis can be determined from bolometric
light curves. The nickel masses derived for all supernovae are an essential input
of the explosion models. So far, this observational input has been missing. It is
foreseeable that in the near future this will change and observational constraints
on the models will become available.
The bolometric light curve of SN1987A has been instrumental in decoding
the various phases of the energy release (see Fig. 1 and [72,111]). The luminos-
ity of the plateau phase indicates the size of the progenitor and the explosion
energy, while the length of the plateau phase is mostly dominated by the mass
of the exploded star [89]. For the core-collapse supernovae the luminosity on the

10 Leibundgut and Suntze
radioactive tail, after the light curve leaves the plateau due to the recombination,
together with the time since explosion gives the 56 Ni mass directly. This method
has been employed for several supernovae and it could be shown that there are
rather large dierences among these objects. This is particularly interesting as
it might provide insight into fall-back onto the forming neutron star.
It should be possible to combine some of these measurements into a picture of
the explosion. One can investigate how the progenitor mass correlates with the
explosion energy and the amount of fall-back onto the forming compact remnant.
In extreme cases, it might be possible to show that a black hole formed in the
explosion [6]. It is essential to show that the event has all the signatures of a
massive stellar explosion to avoid confusion between dierent explosion types.
Comparing the explosion parameters with direct information of the progenitor
star, which are becoming available more often now (see, e.g., [102]), will be an
important link between the post-event deductions and the models.
The bolometric light curves of thermonuclear supernovae can be used to
extract physical parameters of the explosions (see, e.g., [118]). According to Ar-
nett's rule [2,85], the peak luminosity of SNIa re ects the amount of radioactive
56 Ni produced in the explosion. This fact has been used by Contardo et al. [24]
(see also [23]) to derive the 56 Ni masses for several supernovae. They claim that
there is a signicant range of 56 Ni produced in the explosions (up to a factor of
ten when extreme objects are included). A similar result was derived by Cap-
pellaro et al. [17] based on the late light curves and the assumption that the V
light curve can serve as a surrogate for the bolometric luminosity. Independent
conrmation of this result comes from the observations of infrared spectra of
SNIa [13].
The late decline can be further used to determine the escape fraction of the
-rays. A steeper decline indicates a faster decrease of the column density. Three
possible explanations for this eect could be: 1) dierent explosion energies, 2)
a varying distribution of nickel in the explosion, or 3) dierences in ejecta mass.
All possibilities indicate fundamental variations in the explosions. Interestingly,
the expansion velocity of the iron in the ejecta correlates with peak luminosity
of SNIa [70], however not in the way expected from simple models. The brighter
supernovae have larger expansion velocities indicating, for a xed ejecta mass, a
higher explosion energy. These are also the objects which produce more 56 Ni in
the explosions. In this case, the distribution of the iron-peak elements must be
dierent to explain the slower decline at late times [23]. An alternative explana-
tion could be that the ejecta mass for SNIa is not the same in all events, quite
a radical suggestion in view of the currently favored models of Chandrasekhar-
mass white dwarf progenitors (see, e.g., [67]). Bolometric light curves of more
objects will be needed to conrm such a result.
6 Summary
SN light curves are essential for our understanding of supernova physics. The
acquisition of light curves has become relatively easy and the use of robotic

Optical Light Curves of Supernovae 11
telescopes and pipeline reductions will further advance. The success of recent
supernova searches is increasing the available data set dramatically and we will
soon be able to investigate detailed statistics on light curve parameters.
The variety of light curve shapes of core-collapse supernovae provides evi-
dence for the many physical eects which can in uence the outcome of these
events. The rapid, and sometimes violent, evolution of their massive progenitor
stars and their death within their cradle provides for vastly dierent environ-
ments and displays. We can still learn a great deal from detailed observations.
The physics background has developed considerably over the past decade and
the nearby example of SN1987A, oering extremely detailed observations, has
led to major new insights into this phenomenon.
The short lifetime of core-collapse supernovae can be used to directly measure
the star formation rates as a function of look back time (see, e.g., [25] and the
chapter by Cappellaro in this volume). However, their light and color curves are
required for a secure identication.
The more uniform appearance of thermonuclear type Ia explosions has been
challenged by the observations in redder lter bands and additional examples.
Although these objects still are rather uniform, their light curves have allowed us
to show that signicant variations in these explosions must exist. The correlation
of the light curve shape with the luminosity has provided a convenient way to
normalize these objects and make them the best cosmological distance indicator
available at the moment (see the chapter by Perlmutter and Schmidt in this
volume). Light curvesare at the heart of the cosmological applications of SNIa.
It remains a major task to establish the physical understanding of these events
in the coming years.
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