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Gamma-Ray Bursts The Brightest Explosions Since the Big Bang. H. A. Bethe Lecture March 6, 2002 . Electromagnetic radiation – of which ordinary visible light is an example – can be characterized by its wavelength. Gamma-rays have

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Gamma-Ray Bursts

The Brightest Explosions Since the Big Bang

H. A. Bethe Lecture March 6, 2002

Electromagnetic radiation – of which ordinary visible light is an

example – can be characterized by its wavelength. Gamma-rays have

the shortest wavelengths of all and are very penetrating. They pass easily

through a block of wood but not through the Earth’s atmosphere.

Consequently cosmic sources of gamma-rays can only be seen by

satellites, rockets, and high flying balloons that go above most of

the Earth’s atmosphere.

Nuclear Test Ban Treaty, 1963

First Vela satellite pair launched 1963

Velar – to watch

The Vela 5 satellites were placed in orbit by the Advanced

Research Projects of the DoD and the AEC. Launched on

May 23, 1969 into high earth orbit (118,000 km), this pair

of satellites and their predecessors, Vela 4, discovered the

first gamma-ray bursts. The discovery was announced

by Klebesadel, Strong, and Olson (ApJ, 182, 85) in 1973.

First Gamma-Ray Burst

The Vela 5 satellites functioned from July, 1969 to April, 1979

and detected a total of 73 gamma-ray bursts in the energy

range 150 – 750 keV (n.b, radiation is sometimes defined by its

energy measure in electron volts. 0.3 to 30 keV approximately is

x-rays. Greater than 30 keV is gamma-rays)

Typical durations are

20 seconds but there is

wide variation both in time-

structure and duration.

Some last only hundredths

of a second. Others last

thousands of seconds.

A Cosmic Gamma-Ray Burst, GRB for short, is a brief,

bright flash of gamma-rays lasting typically about 20

seconds that comes from an unpredictable location in

the sky.

Some, in gamma-rays, are as bright as the planet Venus.

Most are as bright as the visible stars. It is only because of

the Earth’s atmosphere and the fact that our eyes are not

sensitive to gamma-rays that keeps us from seeing them


With appropriate instrumentation, we see about one of

these per day at the Earth. They seem never to repeat from

the same source.

A problem in 1973 was – and still is – that gamma-ray detectors(CsI and NaI) do not give much information on directect.

Yes, there are gamma-raysbut I don’t know where they

came from.....

but I do know when ...

One can get information on directionality from timing

if there are more than one detectors.




If “1” hears the sound later than “2” the sound is somewhere

to the right.

Interstellar warfare
  • Primordial black hole evaporation
  • Flares on nearby stars
  • Distant supernovae
  • Neutron star quakes
  • Comets falling on neutron stars
  • Comet anti-comet annihilation
  • Thermonuclear explosions on neutron stars
  • Name your own ....

uncertainty in distance –

a factor of one billion.

In 1993 there were 135 models

nb. 27, 42, 105, 126

Most of them involved neutron stars in our own Galaxy

(quakes, comets falling, thermonuclear runaways, etc.)

The expected distribution on the sky if Galactic neutron

stars were responsible should center around the Galactic equator.

Plane of the MilkyWay Galaxy

So basically we wandered in the

wilderness for twenty years ...

(1973 – 1993).

Compton Gamma-Ray Observatory

April 5, 1991 – June 4, 2000

BATSE Module


Expected if haven’t

reached any edge yet

log number

of sources

log sensitivity

This posed a problem for the models that had GRBs in our

own Galaxy ..





























We are not at the center

of our galaxy so we

should see more bursts

towards the center than

in the opposite direction,










Isotropy could mean three things:
  • Very nearby bursts centered on the Earth – e.g.,
  • the Oort cloud of comets
  • A very extended spherical halo around the Galaxy – much bigger than the distance from here to the center of the Galaxy
  • Bursts very, very far away – billions of light years

What we really needed was source identifications.


conceived July, 1981, Santa Cruz

born Nov 4, 1996

died Nov 5, 1996

BeppoSax (1996-2002)

(Italian-Dutch X-ray astronomy mission)

MEC S and LECS (medium and lowenergy x-ray sensors, 1 arc min positions)

(2-30 keV; 20x20 degree FOV

angular resolution 5 arc min)

The scintillator anti-coincidence shields of the Phoswich detector

are able to detect gamma-rays 60-600 keV and get crude angular


BeppoSax GRB 970228 (discovered with WFC)

Feb 28, 1997 (8 hr after GRB using MECS)

March 3, 1997 (fainter by 20)

Each square is about 6 arc min or 1/5 the moon’s diameter

GRB 970228

William Hershel Telescope

Isaac Newton Telescope

Groot, Galama, von Paradijs, et al IAUC 6584, March 12, 1997

Wagner, Foltz, and Hewet IAUC 6581 using the MMT

get a crude spectrum of the host galaxy. Estimate z = 0.5

March 10, 1997

Later ....

Spectrum of the host galaxy of GRB 970228 obtained at the Keck 2 Telescope.

Prominent emission lines of oxygen and neon are indicated and show that the

galaxy is located at a redshift of z = 0.695. (Bloom, Djorgovski, and Kulkarni

(2001), ApJ, 554, 678. See also GCN 289, May 3, 1999.

From the red shift a distance can be inferred – billions

of light years. Far, far outside our galaxy.

From the distance and brightness an energy can be


1.6 x 1052 erg in gamma rays alone

This is 13 times as much energy as the sun will radiate in

its ten billion year lifetime, but emitted in gamma-rays

in less than a minute. It is 2000 times as much as a

really bright supernova radiates in several months.

GRB 990123

This shows the light curve as seen by BATSE starting at 9:46:56 UT on

January 23, 1999. The burst was seen simultaneously by the WFC on BeppoSax.

The position was promptly determined to a few degrees by BATSE and shortly

thereafter to 5 arc min by BeppoSax.

The Robotic Optical Transient Search Telescope (ROTSE-1)

by Akerlof et al, located at Los Alamos, images a piece of the sky

16.5 degrees across. It is able to slew anywhere in the sky in about 10 s.

It was notified of GRB 990123 within 4 s of the onset and was taking data

22 s into the event. The first three ROTSE points are shown superimposed

on the BATSE light curve.

m = 8.95

47 s, 5 s exposure

m = 11.82

22 s, 5 s exposure

m =10.08

72 s, 5 s exposure

m = 13.22

259 s, 75 s exposure

m = 14.0

447 s, 75 s exposure

m = 14.53

612 s, 75 s exposure

And Palomar finds a light where no light was before.

This ID was made 3 hours after the burst following a 5AM “wake-up” call to Palomar from Italy

GRB 990123

BeppoSax observations of

GRB 990123 6 to 33 hours

after the burst and 34 to 64

hours after the burst.

The x-ray source is clearly

fading. Each grid box is

12 x 15 arc min. The error in

position is then about 1.5 arc


Two HST images of GRB 990123. The image on the left was taken

February 8, 1999, the one on the right March 23, 1999. Each picture

is 3.2 arc seconds on a side. Three orbits of HST time were used for

the first picture; two for the second – hence the somewhat reduced


The spectrum of host galaxy (Kelson et al, IAUC 7096) taken

using the Keck Telescopes gives a redshift of 1.61.

Given the known brightness of the burst (in gamma-rays) this

distance implies an energy of over several times 1054 erg.

About the mass of the sun turned into pure energy.

Had this burst occurred on the far side of our Galaxy,

at a distance of 60,000 light years, it would have been

as bright – in gamma-rays – as the sun. This is ten billion

times brighter than a supernova and equivalent to seeing a

one hundred million trillion trillion megaton explosion.

As of February, 2002, 129 GRBs had been

promptly localized by various satellites

(starting in 1997).

X-ray afterglows had been detected from 40 and

optical afterglows from 28.

Red shifts (distances) were determined for about 21.

Typical values are z ~1. The largest is 3.42 (GRB 971214)

corresponding to emission when the universe was

about 15% its present age.

The typical energy is 1053 erg or about

5% of the mass of the sun turned to pure

energy according to E = mc2

But are the energies required really that great?


If the energy were

beamed to 0.1% of the

sky, then the total

energy could be

1000 times less


Nothing seen down here

But then there would be a lot of bursts that we do not see

for every one that we do see. About 1000 in fact.

Quasar 3C 175 as seen in the radio

Quasar 3C273 as seen by the

Chandra x-ray Observatory

Artist’s conception of SS433 based on observations

Microquasar GPS 1915

in our own Galaxy – time sequence

It also turns out that in order to get the spectrum

and time scales for GRBs correct the beam must

be “relativistic”, that is it must travel at very close

to the speed of light.Otherwise the bursts would be much softer and

last much longer.

It is a property of matter moving close to the speed

of light that it emits its radiation in a small angle along its

direction of motion. The angle is inversely proportional to the

Lorentz factor

This offers a way of measuring the beaming angle. As the

beam runs into interstellar matter it slows down.

Measurements give

an opening angle of

about 5 degrees.

Frail et al. Nature, (2001), for 17 GRBs

with known redshifts and afterglow

light curves.

Using the angles inferred from

this sort of analysis of the

afterglows, Frail et al find that

an inverse correlation exists

between the apparent energy of

the burst and this angle.

Correcting for the fact that the

burst is beamed to a small part of the sky, they find a typical

energy in gamma-rays is

5 x 1050 erg. For a reasonable

conversion efficiency between

explosion energy and gamma-rays (20%), the total jet energy is about

2 x 1051, not so different from

an ordinary supernova.

Requirements on the Central Engine

and its Immediate Surroundings

  • Provide adequate energy to material moving close to the speed of light (2 x 1051 erg)
  • Collimate the emergent beam to approximately 5 degrees
  • Last approximately 10 s
  • Make bursts in star forming regions
Merging neutron star -

black hole pairs

Strengths: a) Known event

b) Plenty of angular momentum

c) Rapid time scale

d) High energy

e) Well developed numerical models

Weaknesses: a) Outside star forming regions

b) Beaming and energy may be inadequate

for long bursts

But this model may still be good for a class of bursts called

the “short hard” bursts for which we have no counterpart information


Usually massive stars make supernovae. Their iron core

collapses to a neutron star and the energy released explodes

the rest of the star.

But what if the explosion fizzled? What if the iron core

collapsed to an object too massive to be a neutron star –

a black hole.

A star without rotation would then simply disappear....

But what if the star had too much rotation to all go

down the (tiny) black hole?

If supernovae are the observational signal that a

neutron star has been born, what is the event that

signals the birth of a black hole?

In the vicinity of the rotational

axis of the black hole, by a

variety of possible processes,

energy is deposited.

The exact mechanism for

extracting this energy either

from the disk or the rotation

of the black hole is fascinating

physics, but is not crucial

to the outcome, so long as the

energy is not contaminated by

too much matter.

7.6 s after core collapse; high viscosity case.

SN 1998bw/GRB 980425

NTT image (May 1, 1998) of SN 1998bw in

the barred spiral galaxy ESO 184-G82[Galama et al, A&A S, 138, 465, (1999)]

WFC error box (8') for GRB 980425

and two NFI x-ray sources. The IPN

error arc is also shown.

1) Were the two events the same thing?

2) Was GRB 980425 an "ordinary" GRB

seen off-axis?

A 1053 erg event situated 30,000 light years away

(distance from here to the Galactic center) would give

as much energy to the earth in 10 seconds as the sun –

equivalent to a 200 megaton explosion.

Does it matter having an extra sun in the sky for

10 seconds?

Probably not. This is spread all over the surface of

the earth and the heat capacity of the Earth’s atmosphere

is very high. Gamma-rays would deposit their energy

about 30 km up. Some bad nitrogen chemistry would happen.

Noticeable yes, deadly to all living things – No.

Biological Hazards of Gamma-Ray Bursts

Distance Events Megatons Results

(kpc) /10 by

  • 100 – 1000 200 Some ozone damage, EMP acid rain
  • 1 1 – 10 20,000 Ozone gone, acid rain, blindness 2nd and 3rd degree burns*
  • 0.1 0.01 – 0.1 two million Shock waves, flash incineration, tidal waves, radioactivity (14C)
  • End of life as we know it.

* Depends on uncertain efficiency for conversion of energetic electrons to optical light

Milagrito detection of GRB 970417a

Atkins et al. (2000), Astrophysical Journal Letters,533, L119

Large watertight detector near Los Alamos.

Threshold about 100 GeV (gamma-rays with 100,000 times more energy and shorter

wavelength than “ordinary” GRB energies)

Fluence of this burst in BATSE was1.5 x 10-7 erg cm-2.

100 times more energy in TeV radiation??

Gamma-Ray Large Area Space Telescope (GLAST)

Sensitive to GRBs in the

energy range 20 MeV to 300 GeV

Scheduled for launch by NASA


detector follows the paths of

electron-positron pairs created

in foils.

Will the high energy emission

of GRBs be their dominant emission?


October 6, 2000