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.