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 anexample – can be characterized by its wavelength. Gamma-rays havethe shortest wavelengths of all and are very penetrating. They pass easilythrough a block of wood but not through the Earth’s atmosphere.

Consequently cosmic sources of gamma-rays can only be seen bysatellites, rockets, and high flying balloons that go above most ofthe Earth’s atmosphere.

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.

Velar – to watch Nuclear Test Ban Treaty, 1963First Vela satellite pair launched 1963

First Gamma-Ray Burst

The Vela 5 satellites functioned from July, 1969 to April, 1979and detected a total of 73 gamma-ray bursts in the energy range 150 – 750 keV (n.b, radiation is sometimes defined by itsenergy measure in electron volts. 0.3 to 30 keV approximately isx-rays. Greater than 30 keV is gamma-rays)

Typical durations are 20 seconds but there iswide variation both in time-structure and duration.

Some last only hundredths of a second. Others lastthousands 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 frequently.

With appropriate instrumentation, we see about one of these per day at the Earth. They seem never to repeat fromthe same source.

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

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

but I do know when ...

One can get information on directionality from timingif there are more than one detectors.

1 2

If “1” hears the sound later than “2” the sound is somewhereto 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.

nb. 27, 42, 105, 126

In 1993 there were 135 models

Most of them involved neutron stars in our own Galaxy(quakes, comets falling, thermonuclear runaways, etc.)

The expected distribution on the sky if Galactic neutronstars 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).

April 5, 1991 – June 4, 2000

Compton Gamma-Ray Observatory

BATSE Module

log sensitivity

log numberof sources

Observed

Expected if haven’treached any edge yet

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

X

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* **We are not at the centerof our galaxy so we should see more burststowards the center than in the opposite direction,

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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.

High Energy Transient Explorer (HETE-1)

HETE-1

conceived July, 1981, Santa Cruz

born Nov 4, 1996died Nov 5, 1996

BeppoSax (1996-2002)

(2-30 keV; 20x20 degree FOV angular resolution 5 arc min)

The scintillator anti-coincidence shields of the Phoswich detectorare able to detect gamma-rays 60-600 keV and get crude angular information

(Italian-Dutch X-ray astronomy mission)

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

BeppoSax GRB 970228 (discovered with WFC)

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

Feb 28, 1997 (8 hr after GRB using MECS) March 3, 1997 (fainter by 20)

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

William Hershel Telescope Isaac Newton Telescope

GRB 970228

Wagner, Foltz, and Hewet IAUC 6581 using the MMTget a crude spectrum of the host galaxy. Estimate z = 0.5March 10, 1997

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.

Later ....

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 inferred.

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 shortlythereafter 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 sky16.5 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 data22 s into the event. The first three ROTSE points are shown superimposedon the BATSE light curve.

m = 11.8222 s, 5 s exposure

m = 8.9547 s, 5 s exposure

m =10.0872 s, 5 s exposure

m = 13.22259 s, 75 s exposure

m = 14.0447 s, 75 s exposure

m = 14.53612 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 inposition is then about 1.5 arcmin.

Two HST images of GRB 990123. The image on the left was takenFebruary 8, 1999, the one on the right March 23, 1999. Each pictureis 3.2 arc seconds on a side. Three orbits of HST time were used forthe first picture; two for the second – hence the somewhat reducedexposure.

The spectrum of host galaxy (Kelson et al, IAUC 7096) takenusing the Keck Telescopes gives a redshift of 1.61.

Given the known brightness of the burst (in gamma-rays) thisdistance 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 billiontimes 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 andoptical 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 about5% of the mass of the sun turned to pure energy according to E = mc2

But are the energies required really that great?

Earth

Earth

Nothing seen down here

If the energy were beamed to 0.1% of the sky, then the total energy could be 1000 times less

But then there would be a lot of bursts that we do not seefor every one that we do see. About 1000 in fact.

Earth

Microquasar GPS 1915in our own Galaxy – time sequence

Artist’s conception of SS433 based on observations

Quasar 3C273 as seen by the Chandra x-ray Observatory

Quasar 3C 175 as seen in the radio

It also turns out that in order to get the spectrumand time scales for GRBs correct the beam must be “relativistic”, that is it must travel at very closeto the speed of light.

Otherwise the bursts would be much softer andlast much longer.

It is a property of matter moving close to the speedof light that it emits its radiation in a small angle along itsdirection of motion. The angle is inversely proportional to the Lorentz factor

/1

,c/v1

122

This offers a way of measuring the beaming angle. As thebeam runs into interstellar matter it slows down.

c 0.995 v10

c 0.99995 v100.,.

gE

Measurements givean opening angle ofabout 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 thatan inverse correlation existsbetween the apparent energy ofthe burst and this angle.

Correcting for the fact that theburst is beamed to a small part of the sky, they find a typical energy in gamma-rays is 5 x 1050 erg. For a reasonableconversion efficiency between explosion energy and gamma-rays (20%), the total jet energy is about2 x 1051, not so different from an ordinary supernova.

Requirements on the Central Engineand 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 Stars

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 calledthe “short hard” bursts for which we have no counterpart informationyet.

The Collapsar Model (aka the “Hypernova”)

Usually massive stars make supernovae. Their iron corecollapses to a neutron star and the energy released explodesthe rest of the star.

But what if the explosion fizzled? What if the iron corecollapsed 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 godown 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 rotationalaxis of the black hole, by a variety of possible processes, energy is deposited.

The exact mechanism forextracting this energy either from the disk or the rotationof the black hole is fascinatingphysics, but is not crucialto the outcome, so long as the energy is not contaminated bytoo 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 980425and two NFI x-ray sources. The IPNerror 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 giveas 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 for10 seconds?

Probably not. This is spread all over the surface of the earth and the heat capacity of the Earth’s atmosphereis 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.

Distance Events Megatons Results (kpc) /10 by

10 100 – 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

Biological Hazards of Gamma-Ray Bursts

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 shorterwavelength 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 NASA2005

detector follows the paths ofelectron-positron pairs createdin foils.

Will the high energy emission of GRBs be their dominant emission?

HETE-2October 6, 2000

Last night ...