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Supernovae and Gamma-Ray Bursts
Summary of Post-Main-Sequence Evolution of Stars
M > 8 Msun
M < 4 Msun
Subsequent ignition of nuclear
reactions involving heavier
elements
Fusion stops at formation of C,O core.
Fusion proceeds; formation
of Fe core.
Supernova
Fusion of Heavier Elements
Final stages of fusion happen extremely rapidly: Si burning lasts only for ~ 2 days.
126C + 4
2He → 168O +
168O + 4
2He → 2010Ne +
168O + 16
8O → 2814Si + 4
2He
Onset of Si burning at T ~ 3x109 K
→ formation of S, Ar, …;
→ formation of 5426Fe and 56
26Fe
→ iron core
The Life “Clock” of a Massive Star (> 8 Msun)
Let’s compress a massive star’s life into one day…
12 12
3
45
67
8
9
1011
12 12
3
45
67
8
9
1011
Life on the Main Sequence
+ Expansion to Red Giant: 22 h, 24 min.
H burning
H → He
H → He
He → C, O
He burning:
(Horizontal Branch) 1 h, 35 min, 53 s
H → HeHe → C, O
C → Ne, Na, Mg, O
Ne → O, Mg
H → He He → C, O
C → Ne, Na, Mg, O12 1
2
3
45
67
8
9
1011
C burning:
6.99 s
Ne burning:
6 ms 23:59:59.996
H → HeHe → C, O
C → Ne, Na, Mg, O
Ne → O, Mg
O burning:
3.97 ms 23:59:59.99997
O → Si, S, P
H → HeHe → C, O
C → Ne, Na, Mg, O
Ne → O, Mg
Si burning:
0.03 ms
The final 0.03 msec!!
O → Si, S, P
Si → Fe, Co, Ni
Observations of Supernovae
Supernovae can easily be seen in distant galaxies.
Total energy output:
Ee ~ 3x1053 erg (~
100 L0 tlife,0)
Ekin ~ 1051 erg
Eph ~ 1049 erg
Lpk ~ 1043 erg/s ~ 109 L0
~ Lgalaxy!
SN 2006X in M 100
Observed with the MDM 1.3 m telescope
Type I and II SupernovaeCore collapse of a massive star:
Type II Supernova
Collapse of an accreting White Dwarf exceeding the Chandrasekhar mass limit
→ Type Ia Supernova.
Type I: No hydrogen lines in the spectrum
Type II: Hydrogen lines in the spectrum
Type Ib: He-rich
Type Ic: He-poor
Type II P
Type II L
Light curve shapes
dominated by delayed energy
input due to radioactive
decay of 5628Ni
The Famous Supernova of 1987:SN 1987A
Before At maximumUnusual type II
Supernova in the Large Magellanic
Cloud in Feb. 1987
Progenitor: Blue supergiant (denser than normal SN II
progenitor)
20 M0;
lost ~ 1.4 – 1.6 M0 prior to SN
Evolved from red to blue ~ 40,000 yr
prior to SN
The Remnant of SN 1987ARing due to SN ejecta
catching up with pre-SN stellar wind; also
observable in X-rays.
vej ~ 0.1 c
Neutrinos from SN1987 have been observed by
Kamiokande (Japan)
Escape before shock becomes opaque to
neutrinos → before peak of light curve
provided firm upper limit on e mass: me < 16 eV
Remnant of SN1978A in X-rays
Color contours: Chandra
X-ray image
White contours:
HST optical image
Supernova Remnants
The Cygnus Loop
The Veil Nebula
The Crab Nebula:
Remnant of a supernova observed
in a.d. 1054
Cassiopeia AOptical
X-rays
Synchrotron Emission and Cosmic-Ray Acceleration
The shocks of supernova remnants accelerate
protons and electrons to extremely high,
relativistic energies.
→“Cosmic Rays”
In magnetic fields, these relativistic electrons emit
Synchrotron Radiation.
Power-law distribution of relativistic electrons:
I
Ne() ~ -pj ~ -
p
-p
Opt. thin
~ -
pOpt. thick
5
Synchrotron Radiation
Electrons are accelerated at the shock front of the supernova remnant:
Ne (,
t)
Ne = Ne(t)
-q+1
-q
Synchrotron Spectra of SNR shocks (I)
∂Ne/∂t = -(∂/∂)(Ne) + Q(,t).
Q(,t) = Q0 -q
c
Uncooled Cooled
Resulting synchrotron spectrum:
I
-q
Opt. thin, uncooledOpt. thick
5
Synchrotron Spectra of SNR shocks (II)
-q
Opt. thin, cooled
sy,c = sy (c)
Find the age of the remnant from
t = (c/[c]).
Gamma-Ray Bursts(GRBs)
Short (sub-second to minutes) flashes of gamma-rays
GRB Light Curves
Long GRBs (duration > 2 s) Short GRBs (duration < 1 s)
Possibly two different types of GRBs: Long and short bursts
General Properties
• Random distribution in the sky• Approx. 1 GRB per day observed• No repeating GRB sources
Afterglows of GRBs
Most GRBs have gradually decaying afterglows in X-rays, some also in optical and radio.
X-ray afterglow of GRB 970228
(GRBs are named by their date: Feb. 28, 1997)
On the day of the GRB 3 days after the GRB
Optical afterglow of GRB 990510 (May 10, 1999)
Optical afterglows of GRBs are extremely difficult to localize:
Very faint (~ 18 – 20 mag.); decaying within a few days.
1 day after GRB 2 days after GRB
Optical Afterglows of GRBs
Optical afterglow of GRB 990123, observed with Hubble Space
Telescope (HST/STIS)
Long GRBs are often found in the spiral arms (star forming regions!) of very faint host
galaxies
Host Galaxy
Optical Afterglow
Energy Output of GRBs
Observed brightness combined with large
distance implies huge energy output of GRBs, if they are
emitting isotropically:
E ~ 1054 erg
L ~ 1051 erg/s
Energy equivalent to the entire mass of the sun (E = mc2), converted into gamma-rays in just a few seconds!
… another one, observed by us with the MDM 1.3 m
telescope on Kitt Peak!
BeamingEvidence that GRBs are not
emitting isotropically (i.e. with the same intensity in all
directions), but they are beamed:
E.g., achromatic breaks in afterglow light curves.
GRB 990510
Models of GRBs (I)
Hypernova:
There’s no consensus about what causes GRBs. Several models have been suggested, e.g.:
Supernova explosion of a very massive (> 25 Msun) star
Iron core collapse forming a black hole;
Material from the outer shells accreting onto
the black hole
Accretion disk =>
Jets => GRB!
Models of GRBs (II)Black-hole – neutron-star merger:
Black hole and neutron star (or 2 neutron stars) orbiting each
other in a binary system
Neutron star will be destroyed by tidal effects;
neutron star matter accretes onto black hole
=> Accretion disk
=> Jets => GRB!
Model works probably only for short GRBs.