2221 - SLAC - Stanford University

22nd Texas Symposium on Relativistic Astrophysics at Stanford University, Dec. 13-17, 2004
The SN1987a: 18 Years After
O. Saavedra
Dipartimento di Fisica Generale,
Universita’ di Torino and INFN Torino Italy
It is 18 years ago, since February 23, 1987 that the explosion of the SN in the L.M.C. has been observed by means of
underground detectors. In fact, the neutrino burst has been detected by several underground experiments in the world running
on that time: Mt. Blanc in Italy, Kamioka in Japan, and Baksan in Russia and IMB in USA. It was for the first time in human
live that an astrophysical phenomenon has been observed also in underground detectors. For this astrophysical event, the Mt.
Blanc experiment detected 5 pulses on-line that, however, were not at the same time as detected by the other three detectors
near 5 hrs later. After 18 years some recent models has been proposed in order to explain a double burst due probably to a
double explosion in two different times, as is suggested recently by O. Ryazhskaya and V.S. Imshennik.
Observatories. Immediately we have bring the tape
and analyzed our Mt. Blanc data in order to see
whether our pulses has to do with the SN explosion
observed.
After check our time and the time of the probable
started the explosion comparing with optical
measurements, on Saturday February 28th we decide to
announce about our event. [1]
On Monday 9th March the Japanese group gives the
announcement in a conference press that Kamioka
experiment detected 12 pulses but at ~5 hours after the
event of Mt. Blanc time. In the same way, the IMB and
the Baksan groups give their results in coincidence
with Kamioka experiment.
1. INTRODUCTION
On 2.52 hr of February 23, 1987, the Mt Blanc
experiment, dedicated to detection of neutrinos from
collapsing stars, printed on-line a burst of 5 pulses
within 7 sec duration time. The pulses has been
analyzed on line by the computer given the probability
of simulation by the background of ~10-3.
Such event has been seen on Monday 23rd at 8.30
morning by a member of our group on shift at the
experiment.
Only on Wednesday 25 February we have the news
that a SN has been observed optically in the Southern
Figure 1: Original print-out of Mt. Blanc 23-Feb. 1987 event
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22nd Texas Symposium on Relativistic Astrophysics at Stanford University, Dec. 13-17, 2004
Ryazhskaya and V.S. Imshennik [2] that we will
analyze in ahead.
Figure 1 shows the event of five pulses recorded on
line at the LSD experiment at Mt. Blanc, on Feb. 23,
1987 at 2 hr, 52 min, UT. Figure 2 is the copy of the
reply telex by S. Cristiani, from ESO Observatory at
Chile on Feb. 27, 1987, to our request about more
information on the SN observed and where a massive
star Sanduleak is mentioned for the first time as a
candidate for the SN explosion.
What pulses Mt. Blanc experiment detected? There
was two bangs as some author claim-out? This was a
real puzzle and it probably will be so for ever because
we have not the possibility to check it in future. The
frequency of SN is very low to accumulate enough
statistics.
After 18 years from such SN explosion, some new
ideas are coming-out, and one of these is due to O.
Figure 2 The Telex from S. Cristiani from ESO Observatory, Chile, Feb. 27, 1987. Note that the Sanduleak is
mentioned for the first time as the candidate star that exploded.
The energy spectrum is given by a Fermi-Dirac
distribution:
2. THE STELLAR COLLAPSING STAR
dNν
2
1
≈ε2
e −αε
ε
dE
1+ e
The SN explosion that was observed in Feb.23 1987
has provided a unique opportunity to test the theory of
neutron star formation in the Type-II supernovae
explosion.
According to the Standard Supernovae Theory the
total binding energy of the neutron star is in the range Eb
= (2.5+1.5)x1053 ergs which is several hundred times the
energy the Sun will emit in its entire main-sequence
lifetime (1010 years).
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where
ε=
Eν
KT
All the gravitational binding energy of the residue
neutron star (~3x1053 ergs) was radiated in a few seconds
in the form of ~ 1058 neutrinos with average energy
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22nd Texas Symposium on Relativistic Astrophysics at Stanford University, Dec. 13-17, 2004
~(10-15) MeV.
A type-II supernovae explosion is physically the
types is ~Eb/6, while the temperature Tcool(νe)=Tcool( )
is ~ 5 MeV and Tcool(νµ) = Tcool( ) is ~ 10 MeV. In
addition to the basic energetic arguments, there is the
basic neutronization argument. The collapsing core has
~1057 protons that are converted to neutrons via p + en + νe
To form a neutron star each νe, so emitted from the core,
carries away on the average 10 MeV, thus around
1.3x1052 ergs are emitted by neutronization νe’s this is
<10% of the binding energy. The remainder of the
neutrinos comes from pair processes such as:
νi νi, where i=e, ν, or τ, with νµ and ντ
e+ + eproduction occurring via neutral currents, and νe via both
charged and neutral currents.
Since the neutronization occurs in the initial collapse,
whereas the pair ν’s comes from thermally radiating
core, the timescale for the initial νe burst will be much
less (<10-2) than the diffusion time (~seconds) that
governs the emission of the bulk of the flux. The
duration of the gravitational collapse is then, for typical
numerical models ~10 s.
implosion of an evolved massive star (M>8M◎), which
has, became an “onion-skin structure” with several
burning shells surrounding a degenerate iron core. It
cannot gain further energy by fusion so that it becomes
unstable when it has reached the Chandrasekhar mass of
1-2M◎ that can be supported by electron degeneracy
pressure. The ensuing collapse is intercepted when the
equation of state stiffens at around nuclear density
(3x1014 g cm-3) corresponding to a core size of a few tens
of kilometers. At temperatures of tens of MeV this
compact objects is opaque to neutrinos. The gravitational
binding energy of the newborn neutron star is about
3x1053 erg is thus radiated over several seconds from the
neutrino sphere.
For massive stars the central temperature in the late
stages of evolution is sufficiently high (~1010 K) to
permit the reaction: ZNA + e- z-1NA + νe the same as:
p+e- n+ νe
This neutronization process, which spreads rapidly
through the stellar core, triggers a collapse of the star
into a neutron star or possibly a black hole.
It is expected to be radiation of various neutrinos νe, ,
νµ, , etc associated with the subsequent cooling of
collapsed star.
The energy released in cooling by each of six neutrino
3. THE MT. BLANC NEUTRINO DETECTOR
The experimental groups from Mt. Blanc in Italy [3],
Kamiokande in Japan [4], IMB [5] in USA and Baksan
in Russia [6] have reported the detection of neutrinos
from the Supernova in the Large Magellanic Cloud.
Figure 3: The Mt. Blanc Liquid Scintillator Detector.
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22nd Texas Symposium on Relativistic Astrophysics at Stanford University, Dec. 13-17, 2004
The Kamioka and IMB detected anti-neutrinos reaction
in water while Mt. Blanc and Baksan in liquid
scintillation detector.
In principle all the neutrino reactions of any flavor
are possible to detect both in scintillation and
Cherenkov counters, but, due to the location of
SN1987a at very long distance (~50 kpc), the neutrino
intensity is very low and because the cross section of
neutrino reactions are very small, the only possible
reaction was the:
+ p n + e+
In fact, the neutrino signal from SN1987a was
observed through such reaction. The number of events,
their energies and the distribution in several seconds
corresponds to the well theoretical expectations.
However, the signal does show a number of
energies
“anomalies.” For example the average
inferred from IMB and Kamiokande observations are
quite different. The large time gap of 7.3 s between the
first 8 and the last 3 Kamiokande events looks
worrisome. The distribution of the final-state positrons
+p
n + e+ capture reactions should be
from the
isotropic, but is found to be significantly peaked away
from the direction of the SN. In any case, in absence of
other explanations, these features have been blamed on
statistical fluctuations in the sparse data.
It is assumed that all 4 experiments detected neutrino
pulses from SN1987a.
However, the Mt. Blanc detector registered 5 pulses 4h
44 min before the other three detectors.
The Mt. Blanc neutrino telescope is made of liquid
scintillation detector (LSD) it has bee running since
January 1985 in the Mt. Blanc tunnel at a vertical depth
of 5200 hgcm-2. The experimental characteristics of the
apparatus are described elsewhere [8]. Briefly the
detector consists of 90 tons of liquid scintillation
(C10H22) contained in 72 stainless-steel tanks
(1.0x1.0x1.5 m3) placed on three layers. Three FEU
Russian PMs watches each counter.
Since its conception the LSD experiment has been
dedicated to the detection of antineutrinos burst from
gravitational collapse of stars in our Galaxy, by the
antineutrino capture on the free protons (energy
threshold Eth = 1.8 MeV):
+ p n + e+
followed by n + p
The Mt. Blanc detector was shielded by ~200 tns of Fe
in order to avoid and eliminate the natural background
radioactivity due to the rock of the walls.
4. A ROTATING COLLASAR AND
POSSIBLE INTERPRETATION OF LSD
DATA
In a recent paper V.S. Imshennik and O.G. Ryazhskaya
[2] and by D.K. Nadyozhin and V.S. Inshennik [9] it is
shown that a new and more notable fact could be
occurred in the SN1987a explosion. The idea is not
only due to two-stage gravitational collapse, as it was
claimed since long time ago, particularly in the case of
resumption of the collapse in a neutron star with its
transformation into a black hole, but by the fact that in
the first collapse only electron neutrino type are
emitted and also because the second collapse occurs
~5 hr latter.
It is due because they consider the high probability of
collapsars falling into the region of dynamical
instability specified by the standard criterion
β = Erot/ Egrav > 0.27, where Erot and Egrav are the total
rotational and total gravitational energies respectively.
Note that during collapse with the conservation of total
angular momentum and local specific angular
momentum, the energy Erot greatly increases compared
to Egrav. This instability grows with characteristic
hydrodynamic time and typically leads to the breakup
of the collapsar into pieces, in the simplest case into a
binary of neutron star.
The very important and interesting parameter that they
take into account is the tgrav, which is the time,
calculated for a given orbital angular momentum Jorb
and Mt (in terms of reduced binary mass M1M2/Mt
corresponding to the typical conditions of a Fe-O-C
stellar core on the threshold of its collapse. The results
of these calculations are that tgrav must be 4.7 h= 16920
s and this is the time exactly between the first and
second collapse.
It is interesting to note that the rotational energy in
the initial conditions is actually negligible compared to
the gravitational, i.e. β<<1, and the stellar structure is
virtually spherically symmetric.
In conclusion of this new scenario of double collapse is
that the corresponding energy at the maximum of the
spectrum is Eν~50 MeV while the authors estimate the
total number of νe required the almost complete
neutronization of the material of the rotating collapsar
independent of the mean energy of the νe spectrum:
Nν = 1.0x1057.
The total energy E of these neutrinos with <E> = 50
MeV is: Eν = Nν<Eν> = 8.0x1052 erg. In other words,
there must be a double pulses: the first one due only to
the νe and the second to the normal and standard
d+ γ
This interaction gives two signals in the time
coincidence: the prompt positrons pulse with energy
Eel = Eν – 0.8 MeV followed by a gamma pulse of
energy Eγ = 2.2 MeV with an average delay of ~190
µs. This double pulses detection gives a good signature
for the reaction of neutrino capture.
A careful and systematic study of the low-energy
radioactivity background spectrum was performed.
Fig 3 shows the Mt. Blanc detector.
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22nd Texas Symposium on Relativistic Astrophysics at Stanford University, Dec. 13-17, 2004
collapsing star but delayed by about ~5 hrs compared
to the first one.
Now, with this new model and new concept of
rotating collapsar we can again reinterpreted the
experimental results obtained with neutrino detectors
during the explosion time of SN1987a on February 23,
1987.
Let us consider the various detectors operating
during the SN explosion, it must be take into account
that:
1.- two neutrino burst separated by a time tgrav ~ 5 h.
2.- The first neutrino burst are due to νe with a total
energy Eν = 8.9x1052 erg; the neutrino energy spectrum
is hard and asymmetric with mean energy in the range
25-50 MeV; the duration of the neutrino radiation is
~3-6s.
3.- The second neutrino burst corresponds to the
standard collapse theory.
They show that taking into account the detection
threshold the number of event recorded by LSD is 5, 2
for Kamioka and 1 for Baksan detector.
They shown that if only almost electron neutrinos with
mean energy of ~30-40 MeV were emitted in the first
collapse, than the experimental data correspond to the
scenario for the rotational mechanism of the SN
explosion.
6. TIME COINCIDENCES AMONG
SEVERAL DETECTORS DURING SN 1987.
If we consider the double bang of the 1987a SN
explosion as a possible explanation of the double
pulses detected on Feb. 23, it is worthwhile to
remember that there is still another anomaly effect,
which is presented during the SN explosion time. This
effect is the strange sequences of pulses in coincidence
among the detectors (three underground neutrino
detectors and two gravitational antennas).
Such coincidences pulses was found firstly among
LSD and gravitational antennas and subsequently
among LSD and Kamioka and Baksan.
The results of the analysis made by independent
groups has been presented and discussed at the 14th
Texas Symposium (Denver, 1988) by G. Pizzella [11],
by A.E. Chudakov [12] and by O. Saavedra [13].
The analysis that these groups presented at the
Symposium shows a strange activity of coincidences
(accidentals?) in a period about two hrs around the Mt.
Blanc event.
Although we did not expected to have such
coincidences among the various experiments the
independent analysis of Chudakov shows clearly a
positive result of the fantastic nature of the
phenomenon in question. Chudakov [12] made an
independent analysis that unambiguously confirmed
the analysis made by our LSD group.
Whether the pulses from several detectors are real
coincidence or not, probably we will never know
because we cannot experience it even for the next SN.
5. A POSSIBLE EXPLANATION OF LSD
EFFECT
The neutrinos with energies 30-40 MeV and duration
of the burst of ~3-6 s can be recorded by the detector
nuclei by the reactions:
νe + (A,Z) e- +(A,Z+1)
νe + (A,Z) e- + (A,Z+1)*,
νe + (A,Z) ν'e + (A,Z)*
The detectors operating on February 23, 1987
contained either oxygen, mainly 16O (Kamioka and
IMB) or carbon, mainly 12C and iron 56Fe (LSD and
Baksan). It was seeing that the cross section σνe for
iron at Eν, 40 Mev exceed the σνe for oxygen by more
than a factor 20 (σνe (56Fe) >20(σνe (16O)) Therefore,
at this energies, the number of (νe A) interactions in
LSD (200 ts of Fe) is larger than that in the Kamioka
(1900t of 16O).
The partial cross sections calculated for the reactions
56
νe + 56Fe
Co* + e- for Eν = 40 MeV is calculated.
The threshold for such a reaction is 8.16 MeV. An
electron can be produced with an energy in the interval
31.8-24.8 MeV accompanied by a cascade and γ-ray
photons with a total energy from 3.54 to 10.54 MeV.
The calculation the authors made, indicate that the
interactions of electron neutrinos with Eν = 40 MeV in
a 2-3 cm-thick iron layer located between two
scintillation layers many more γ-rays photons than
electrons fall into the scintillation detector. The mean
energy of these particles are ~(7-9) MeV.
The authors calculated the number of events recorded
in the experiments operating during the SN explosion
and in particular at the time of Mt. Blanc experiment
.The results estimated are consistent with the
experimental data.
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7. CONCLUSION
It appear now that a new model can explain the
pulses detected by LSD at Mt Blanc, without
contradiction with the other detectors during the SN
explosion. After 18 years we are still learning some
more details of the SN explosion mechanism.
The most important thing now is the difference
between the LSD and the other detectors: at LSD we
have used ~200 t of Fe enough for detection of the first
burst of electron neutrinos. According to this new
mechanism, the first phase is peculiar in that a rotating
collapsar is formed in it with the emission of very hard
electron neutrino spectrum and almost complete
absence of electron anti-neutrinos and other type of
neutrinos. This not only allows us to confirm the above
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22nd Texas Symposium on Relativistic Astrophysics at Stanford University, Dec. 13-17, 2004
properties but also justify why other neutrino detectors
have not recorded the first neutrino signal.
The second neutrino pulse detected by Kamioka,
IMB and Baksan and not in the LSD detector (although
LSD detected only one pulse) is due to the normal SN
standard explosion. Why LSD was not detected such
pulses is merely because the intensity of neutrinos is
very low and LSD is not large enough to detect it.
As far as the strange pulses coincidences it could be
so easy to eject it as pure coincidence or due to
fluctuations, just two hrs around the Mt. Blanc
detector, because we have not any other way to test it,
however it is worth while to remember that such effect
is present in our data.
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
References
[1]
[2]
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[13]
Castagnoli et al. I.A.U.
Circular No. 4323 (1987)
V.S. Imshennik and O.G. Ryazhskaya, Astron.
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Lett. 30,14 (2004)
M. Aglietta et al. Europhys. Lett. 3, 1321 (1987)
K.Hirata et al. Phys. Rev. Lett. 58, 1490 (1987)
M. Aglietta et al. Europhys. Lett. 3, 1321 (1987)
R. Bionta et al. Phys. Rev. Lett. 58, 1494 (1987)
E.N. Alexeev et al. JETP lett. 45, 589 (1987)
G.G. Reffelt astro-ph/0105250 and references
therein
G. Badino et al. Nuovo Cimento 7, 573 (1984)
D.K. Nadyozhin and V.S. Imshenik
astro-ph/0501002.
E. Amaldi et al. Proc. 14th Texas Symposium
Ed. E.J. Fenives, Ann. NY Acad. Science 571
561 (1989).
A. E. Chudakov 14th Texas Symposium, ibid
577 (1989)
M.Aglietta et al. 14th Texas Symposium, ibid
584 (1989)