Supernovae Supernova Remnants

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 + γ

168O + 4

2He → 2010Ne + γ

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

Observations of Supernovae

Supernovae can easily be seen in distant galaxies.

Total energy output:

∆Eνe ~ 3x1053 erg (~

100 L0 tlife,0)

∆Ekin ~ 1051 erg

∆Eph ~ 1049 erg

Lpk ~ 1043 erg/s ~ 109 L0

~ Lgalaxy!

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 νe mass: mνe < 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

Electrons are accelerated at the shock front of the supernova remnant:

Ne (

γ,t)

γ

γ-(q+1)

γ-q

Synchrotron Spectra of SNR shocks (I)

∂Ne/∂t = -(∂/∂γ)(γNe) + Q(γ,t).

Q(γ,t) = Q0 γ-q

γc

Uncooled Cooled

Resulting synchrotron spectrum:

Iν

ν

ν-(q−1)/2

Opt. thin, uncooledOpt. thick

ν5/2

Synchrotron Spectra of SNR shocks (II)

ν-q/2

Opt. thin, cooled

νsy,c = νsy (γc)

Find the age of the remnant from

t = (γc/γ[γc]).

Gamma-Rays from SNRs

HESS J1640-465

HESS J1640-465

Gamma-Ray Spectra of SNRs

SNR shocks may accelerate protons

and electrons to ultrarelativistic

energies

→ Two possible models for γ-ray

emission:

1. Leptonic Model: Compton scattering of CMB photons by ultrarelativistic electrons

2. Hadronic model: π0 decay following p-p collisions of ultrarelativistic protons.

Multiwavelength Spectra of SNRs

HESS J1640-465

e- s

ynch

rotro

n

bremsstrahlung

Com

pton

π 0 decay

Gamma-Ray Bursts (GRBs) Short (sub-second to minutes)

flashes of gamma-rays

The Burst and Transient Source Experiment (BATSE) on board the Compton Gamma-Ray Observatory

(1991 – 2000)

20 keV - 1 MeVSource Localization through orientation

effects: Aeff is proportional to

cosΘ

Detector

Shield

Θ

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

GRB Spectra in the BATSE Era• Spectra often well described by "Band Spectrum" (Band

et al. 1993): Smoothly broken power-law.

E [MeV]

νFν

N(E) ~

Eα e

-E/E

0

N(E) ~ E β

• Routinely explained as synchrotron emission from shock-accelerated, ultrarelativistic electrons

• Some GRB spectra with α > -2/3 pose problem to synchrotron interpretation!

10.1 10

BeppoSAX(1996 – 2003)

Wide-field Camera and Narrow-Field

Instrument (NFI; X-ray telescope) allowed

localization of GRBs to arc-minute

accuracy

First identification of X-ray and optical

afterglows of γ-ray bursts in 1997

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

Connection of GRBs with Star Forming Regions and Supernovae

• Spatial and temporal coincidence of GRB 980425 with SN 1998bw (Type Ic)

• Reddened supernova emissions in late-time optical afterglow spectra

• GRBs in low-metallicity hosts

Host galaxy of SN 1998bw

Galama et al. (1998)

Redshifts of Gamma-Ray Bursts

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

Compactness argument (Assignment set 1, problem 2) implies that the gamma-rays must be relativistically boosted

with Γ > 100

Blast-Wave Model of GRBs

Impulsive electromagnetic energy release

Entrainment of ions – conversion of EM to

kinetic energy

Coasting phase – internal shocks (prompt GRB)

Deceleration phase – external shock

(afterglow)

time

Lore

ntz

fact

or Γ

ρext

Γ0 M0 c2 = EBW

Γ0 Γ0 Msw = M0 Relativistic Mass swept-up from the external medium

(in co-moving frame)

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

Swift (launched 2004)

Localization of the first short GRBs;

Some with significant offsets

from host galaxies, favoring binary-compact-object merger models:

Dedicated GRB misssion with on-board soft γ-ray, X-ray, and optical telescopes;

Rapid automated localization and electronic distribution of GRBs with arc-second precision

Fermi Gamma-Ray Space Telescope(Launched June 11,

2008)

Two main science instruments:• LAT (Large Area

Telescope)

• GBM (GLAST Burst Monitor)

The GLAST Burst Monitor (GBM)All-sky Monitor optimized to detect X-ray / soft γ-ray

flashes(~ 8 keV – 30 MeV)

Source localization to < 15o

32

Fermi Autonomous Response to GRB 090902B

Fermi-LAT skymap from -120 s to 2000 s– Red cross = GRB 090902B– Dark region = occulted by Earth

– Blue line = LAT FoV (±66°)– White points = LAT events– Yellow dot: Sun

33

Spectral Complexity in GRBs: The Case of GRB 090926A

Ackermann et al. 2011

• High-Energy (GeV: Fermi-LAT) gamma-rays often delayed with respect to prompt MeV emission

• Many bursts show hard power-law component in addition to thermal (photosphere) MeV emission.

GBM: 8 – 14.3 keV

GBM: 14.3 – 260 keV

GBM: 260 keV – 5 MeV

LAT: All events

LAT: E > 100 MeV

E > GeV photons

Models of GRBs (I)

Hypernova:

Leading model for long GRBs:

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 disruption; neutron star matter accretes

onto black hole

=> Accretion disk

=> Jets => GRB!

Model works probably only for short GRBs.