SUPERNOVA 1987A IN LARGE MAGELLANIC CLOUD
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John Pazmino
NYSkies Astronomy Inc
nyskies@nyskies.orf
www.nyskies.org
1987 June 1
Introduction
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Until February of 1987 the closest and best observed supernova was
Tycho's star of 1572. In fact, altho partly as a gag, the textbook for
astronomers entering the study of supernovae was 'De stella nova' by
Tycho himself!
On the day after SN1987A burst out in the Large Magellanic Cloud I
was at work perusing a book on -- I'm not kidding! -- supernovae.
Among its hopes was that someday there could be a nearby supernova in our
own galaxy for detailed study with modern instruments.
My desk phone rang. I answered it to hear an alert from an
astronomy colleague about the eruption. I with other City astronomers
followed the news of the supernova thru magazine and journal articles.
Some of us went to southern hemisphere locations to observe the star
directly.
I collect here a few of my articles about supernova SN1987A that
illustrate the excitement and science in it. Minor editing was done
and the date of the first one is the issue date for this file.
HOW HEAVY IS A NEUTRINO?
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1987 June 1
That's one major question in astronomy. Stars supposedly emit
neutrinos along with the visible radiation. It's commonly taken that
the neutrino has no mass and it travels at the speed of light.
But what if the neutrino has a small. tho nonzero mass? Well,
with their stupendous numbers supposedly pervading all space like
starlight their individually infinitesimal masses would accumulate to
a vast amount. Enough. according to some theorists. to solve the
question of the "missing mass",
There appears to a stability in galaxy clusters. a nonKepler
velocity-vs-radius function in galaxies. and a closure of space
curvature that cannot be explained from the mass we do see. That may
be the luminous (video and radio) matter locked up in the galaxies.
Indeed. so poor is our grasp of the "missing mass" that its
speculative composition could range from, cold dark stars to massy
neutrinos -- which covers a lot of prospects!
Now neutrinos are also supposed to burst out of a supernova along
with the light emission. Massless neutrinos would travel at the speed
of light and arrive at Earth coincidentially with the light, provided
the two were emitted simultaneously. Massy neutrinos. however.
travelling less than lightspeed. would lag the light in arrival.
We have our supernova, that in the Large Magellanic Cloud. And,
lo!, the Kamiokande and Morton-Thiokol nuclear labs did record bursts
of neutrinos! And physicists are certain these came from this very
supernova! In principle, by clocking the arrival of the neutrinos and
the light we can weigh the neutrino.
AAAer Robert Moniot. physics professor at Fordham University,
furnished the derivation of a formula for the neutrino mass, given the
light travel time to the source and the lag time of the neutrinos
behind the light. The derivation. omitted here. essentially splits the
measured total energy of the arriving neutrino into its restmass and
relativistic kinetic energy.
Since in relativity mass and energy are equated thru the square of
the lightspeed, mass is commonly expressed in electronvolts, One
electronvolt is 1.7827e-26 kilogram. By comparison an electron
restmass is 511,OOOeV or 9.1095e-31Kg
Moniot's formula is:
m * c ^ 2 = E * sqr(2 * delT / T),
m*c~2, restmass, eV
E, neutrino energy. eV
delT, dispersion of arrival times, s
T, light tine to source, s
There are two immense unknowns. T and delT, E was measured fairly
well by the labs as the tripping point of their detectors. Prof Moniot
notes that 10MeV is typical for neutrino receivers nowadays.
What is the lighttime to the supernova? It's the distance to the
star in light travel units, as LY. Astronomers so far are assuming the
star is actually in the LMC, 163.000 LY away. They don't know this for
certain, tho After all, it is so misbehaving that we can not gage its
dIstance against other normal supernovae.
The delT at presstime is still loosely determined. The two nuclear
labs did clock in the neutrinos, but when did the light get here? No
one caught the supernova in the act of erupting in light and later
reports show that the star sort of swelled up slowly with no sharp
outburst. And. with this star a nonconforming one, we can not cycle
back the postnova observations to the point of flareup.
About all we can definitely say right now is that the neutrinos
arrived at Earth within hours of the light and that they are, hence,
thoroly relatiVIstic particles.
AAAer Sandra Schwarz. astronomy professor at the New School.
brought our attention to an other complication. Supernova theories
generally have the neutrinos blow out during the initial core collapse
and the light flares out a mite later. The neutrinos have a headstart
over the light of order one hour. Thus delT is the dispersion of
arrival times minus the neutrino headstart.
What this headstart may be is unknown in the LMC specimen. We can
not offhandedly apply a stock model to such an abnormal star.
What to do? Considering that in the closing years of the 20th
century still the primer for supernova specialists is "De Stella Nova"
by Tycho Brahe', we have to make guesses, hopefully good ones.
Let the star be in the Cloud, so T is 163,000 years or about
5e12s. E is 10MeV, the typical trigger setting for receivers.
deIT remains the grand unknown, Never the less, from the
discussion above, delT can surely not be more than one full day and
could plausibly be identically zero,
Plugging these values into the restmass formula, we have:
deIT, cust | 0 | 1 sec | 1 min | 1 hr | 1 day
-----------+-----+---------+-------+-------+--------
deIT, s | 0 | 1 | 60 | 3600 | 86400
m*c"2, eV | 0 | 6.3 | 49 | 378 | 1852
neut/elect | 0 | 81111 | 10429 | 1352 | 276
----------------------------------------------------
From studies of large galaxies. for example, the neutrino restmass
probably can not excede 50eV, The basis for this is the orbital
velocity-vs-radius curve for the galaxy. If the mass were confined to
the visible envelope of the galaxy, particles (stars!) far from the
center would have velocities approximating to Kepler's. They do not.
The velocity is more or less constant at large radii, as if there
were mass distributed in shells far beyond the visible zone. This is
one indicator that the universe is laden with "missing mass",
If this mass be in clouds of neutrinos, the plausible numbers and
density to reproduce the velocity curve points to a neutrino restmass
of 50eV at most,
The LMC experience does not violate this restmass limit, but it
certainly does not prove or confirm it, We have no firm conclusion.
All we can say for now is that neutrino mass allowed by the ambiguities
in T and delT embrace that mass called for by the galactic (and other)
missing mass scenarios.
DE NOVA NOVISSIMA
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1987 December 1
The supernova continues to astound astronomers. amateur and
career alike. with its antics in the LMC. Its latest doings were
summarized by Dr Robert Kirshner of CfA on 24 October 1987 at the
[AAVSO] convention. Dr Kirshner's commentary here is supplemented by
discussions I had with him and other astronomers after that talk.
The prenoval and transnaval phases of the star are firmly
chronicled now. Photos of the LMC were unearthed to document the
absence of the supernova before 09:30UT on 23 February 1987 and
there's a visual account of the star on the rise. being about 6th
magnitude, at 10:38UT that same day. From these (and other) datapoints
the star turned from utter obscurity to king of the LMC in something
like one hour! The increase of light. from prenoval 12th magnitude to
4th. was only about 1600-fold. much less than expected for a "regular"
supernova. Never the less. if the star were in our Milky Way. say 500
parsecs away. it would shine at magnitude -6, easily casting shadows
on the Earth at night!
Now the nailing down of the light outburst in time makes the
neutrino-vs-light experiment tenable. The several neutrino monitors
caught a neutrino burst at 7:35UT on the 23rd, two hours before the
visual eruption. Kirshner explained that this lead of the neutrino
arrival over the light is substantially due to the expulsion of
neutrinos from the star several hours prior to the flashout of light.
Hence, recalling my article here in June 1987, the neutrino's mass is
quite. if not identically. zero.
By the way. Kirshner noted that the Kamiokande lab caught 11
neutrinos in 12 seconds. Given the sheer inertness of neutrinos
against matter and the efficacy of the monitors. this is equivalent to
about 100 trillion neutrinos per square meter! To visualize 1his
number, image all the stars of 1000 Milky Ways whizzing thru the top
of a cardtable in 12 seconds.
Separately, since particulate irradiation can cause biological
damage. I heard that the NYC Health Dept informally guesstimates that
the supernova's neutrino blast could induce 1000 new cancer cases in
the present human population.
The overall lightcurve is well determined; see Figure 1 [ASCII
substitute here]. adapted from Kirshner's viewgraphs. The star blew up
rather instantly. subsided briefly. then soared in brightness for
three months. It culminated on May 22 at 2.9 magnitude. From this peak
the star faded rapidly until June 24. Since then the decline has been
strictly exponential at 1 full magnitude over 125 days. That is, it is
losing half of its remaining light every 94 days or its decay halflife
is 94 days.
2.0 +--+----+----+----+----+----+----+----+ +-------------------+
| | | Sanduleak -69o202 |
2.5 + + | ----------------- |
a | +-2.9, 22 May | | Spectrum B3-I |
p 3.0 + / \ + | Temp 15,000K |
p | / \ | | Mass (MS) ~20 Sun |
3.5 + / \ + | Lum ~100.000 Sun |
m | / \ | +-------------------+
a 4.0 + / +-4.0, 24 Jun + figure 2
g | +- 4.2, 24 Feb \ |
m 4.5 + |\/ |\ +
| | HL=94d---| \ |
5.0 + | +----- \ +
| | 5.2, 27 Oct-\|
5.5 + | +
| | |
6.0 +--+----+----+----+----+----+----+----+
0 33 67 100 133 167 200 233
days since eruption
figure 1
The identity of the prenatal supernova seems positive. It is
Sanduleak -69o202. a 12th magnitude speck among equal and greater
specks in a congested quarter of the LMC. The vital statistics of this
star are in Figure 2 [ASCII substitute here]. it is (was!) a
blue supergiant crossing leftward at the upper end of the main
sequence. It arrived there from a previous redgiant state and sits on
the track of a star of main sequence mass 20 Suns.
Dr Kirshner explained that from the density and velocity of the
star's expanding shell about 3 Suns of mass were exploded out. From
the quality and quantity of radiant energy about 6 Suns are left in
the core. now a nascent neutron star. Thus. the Sanduleak star was of
9 Suns mass -- where are the missing 11 Suns??.
Stellar modeling shows that a redgiant sheds mass. This mass loss
was formerly thought quite modest, in some books only a few tenths
solar mass. Laterly the thought leans toward a prodigious mass loss. a
sizable part of the star. But. could Sanduleak -69o202 have shed 55%
of its mass?
It sure looks that way since the amount of loss in the models is
imprecise and the Sanduleak star had no assessed mass for its prenoval
state. This supernova is a rare case of directly gaging a star's mass
after it leaves the redgiant phase.
When the star supernovated it produced neutrinos, kinetic motion,
and light and other radiation. Altho the stupendous spectacle of the
supernova is its light output, the energy evected by this light is
actually the least part of all the energy expelled. The LMC supernova
threw off order 53 ergs in neutrinos, 51 in mass motion, and "only" 49
in radiant energy including light. The output is practicly all
neutrinos! The immense torrent of light is is but 1% of 1% of the
whole energy!! Even the shell -- outrushing at 30 to 40 thousand Km/s
at first -- carries only 1% of the supernova's energy!!!
Nowadays the star is fading with a halflife of 94 days. The
probable cause of this decline is radiation accompanying the decay of
elements created in the explosion. Astronomers believe that the
explosion created about 1/10 sun-mass of nickel-56. This decays into
cobalt-56 with halflife of 6.2 days by electron capture. A nickel
proton absorbs a free electron and turns into a neutron plus
radiation. The cobalt in turn decays into iron-56, a stable isotope.
also by electron capture but with halflife 77.3 days. This cobalt
decay releases strong radiation which is absorbed by the dense shell
around the star. We do not see it directly; we see the fluorescently
reemitted radiation as the declining portion of the lightcurve.
The spread of halflifes, 77.3 days for the cobalt decay and 94 for
the light, is disturbing. However, among reactions believed to occur
in supernovae, the cobalt decay is a better fit, tho seemingly a poor
one.
All eyes are watching for the shell to dilate or dissipate,
allowing the cobalt radiation to escape to the Earth. When this
happens its spectrum can be compared with the laboratory spectrum of
cobalt decay, with lines at 1238KeV, 847KeV, 732KeV. In wavelengths
these are 1.001e-3nm, 1.464e-3nm, and 1.694e-3nm.
So, folks, don't leave yet. The best is still to come!
SUPERNOVA BULLETIN BOARD
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1988 February 1
A special bulletin board for diffusing among astronomers news
about the LMC supernova began full operations in January 1988. The
board, Astronomy Information Service or ASTIS, is honchoed by the
Space Telescope Science Institute and is accessible via the comms nets
of NASA such as ARPA and SPAN. Being that it is an internal operation
of NASA, ASTIS is not reachable thru the public telephone grid. Since
most universities and NASA contractors are tied into the NASA nets,
nonNASA astronomers can benefit from ASTIS.
The need for the BBS arose because there are already some 200
astronomers actively studying the supernova, mainly from southern
hemisphere stations far from northern homebases. Conventional mail and
telegraph quickly proved to be altogether clumsy, slow, and erratic.
ASTIS has electronic mail, message bases, litterature indexing,
text search, astronomer rosters, specificly for the LMC supernova. The
tAU Central Bureau for Astronomical Telegrams posts on ASTIS the IAU
Circulars related to the exploded star.
STSI will run ASTIS for two years initially, on the belief that
thereafter the influence of supernova information will cease to be
time-critical. But, of course, if the star continues to act up, ASTIS
will be kept online indefinitely. Already the acceptance and success
of the BBS are pointing toward possible parallel boards for other
fast-breaking phaenonena.
Altho full service on ASTIS was attained in January, the BBS saw
"first signal" in September 1987. From then thru presstime about 100
customers signed up for the board and 950 logons were handled by it.