gamma ray bursts - astro.sunysb.edu · discovery of gamma ray bursts i first observed in 1967 by...

Gamma Ray Bursts

I Definition

I History

I Classification

I Energetics

I Progenitors

I Rates

I Threats

J.M. Lattimer Gamma Ray Burst Lecture

Discovery

NASA:


Follow-Up

NASA:


Gamma Ray Bursts

I Flashes of gamma rays associated with energetic explosions indistant galaxies.

I Believed to be most luminous electromagnetic events since the BigBang.

I Observed fluxes are hundreds of times brighter than supernovae,although seem to be highly beamed, so that total luminosity iscomparable to that of a supernova.

I Bursts last from milliseconds to tens of seconds and show greatvariety.

I Often followed by an afterglow in longer wavelengths up to radio, insome cases resembling the light curve from a supernova.

I Thought to originate in some supernovae and mergers of binarycompact objects.

I Isotropic distribution shows they are at cosmological distances.

I Observed frequency is about 1 per day; actual rate due to beamingis much greater.


Gamma Ray Burst Light Curves

NASA: BATSE

None areidentical:

duration

# of peaks

symmetry

precursors


Gamma Ray Burst Distribution

NASA: BATSE


Discovery of Gamma Ray Bursts

I First observed in 1967 by U.S. Vela 3and 4 satellites launched inconjunction with Nuclear Test BanTreaty

I Signature unlike a nuclear weapon,but observations were classified

I Continued observations of burstscontinued, and solar and terrestrialorigins ruled out

I Observations declassified in 1973

I Controversy concerning locations ofbursts: Milky Way or cosmological?settled only after launch in 1991 ofthe Compton Gamma RayObservatory containing the Burst andTransient Source Explorer (BATSE),which showed isotropic, and thereforecosmological, distribution

I For decades, searches were made toidentify counterparts in other spectralregimes without success

I Breakthrough reached in 1997 withsatellite BeppoSAX detected the burstGRB 970228

I X-ray camera detected fading X-rayemission and optical observationsfound a fading optical counterpart.Deep imaging revealed a faint hostgalaxy at this location. Dimness ofgalaxy did not allow a redshiftmeasurement at the time.

I A second GRB detected byBeppoSAX, GRB 970508, wasidentified in optical only 4 hours afterits discovery. Redshift of z = 0.835measured (D = 6 billion lt. yr.)


Two Kinds


Bimodality of Gamma Ray Bursts

NASA: BATSE


Two Kinds


Fluence of Gamma Ray Bursts

NASA: BATSE


Unified View


Distances to Gamma Ray Bursts

A source emitting energy E at distance dwould give an integrated flux (fluence) S

S =E

4πd2.

If d = 100 AU (comets), E ∼ 1027 ergd = 1 kpc (neutron star), E ∼ 1040 ergd = 1 Gpc (galaxies), E ∼ 1052 erg.All sources with S > Smin are detectedout to a maximum distance

dmax =

√E

4πSmin.

The volume with sources havingS > Smin is

V =4π

3d3max .

Distribution is isotropic, the ’edge’ iscosmological, not galactic.

If n is the source number density,the number in volume V is

N = nV =4π

3n

(E

4πSmin

)3/2

∝ S−3/2min .

←edge

dlnN/d

lnS

=−

3/2

Un

iver

seis

fin

ite

orso

urc

eev

olu

tion→


Energetics of Gamma Ray Bursters

The energy output of GRB 080319B, ifspherically radiated, is > 1054 erg.This exceeds any reasonable source duringsuch a short timescale, so the radiation islikely highly beamed.

A black hole forms at the center of the GRBsource. It is rapidly rotating and almostcertainly has a large magnetic field. Itcreates a fireball of relativistic electrons,positrons and photons which expands andcollides with stellar material and createsgamma rays which emerge from the star inbeams ahead of the blast wave.

Additional emissions, or afterglow, arecreated by collisions of the shock (and areverse shock) with intervening matter. Wecan see both the jet and the afterglow if thebeam is directed towards us.


Beaming of Gamma Ray Bursters

The degree of beaming can be estimated by observing ’jet breaks’ in theafterglow light curves, a time after which the afterglow fades rapidly asthe jet slows down. Observations suggest jet angles from 2 to 20 degrees.The jet accelerates a thin shell, which decelerates as it expands in a time

tγ =Rγ

2γ20c

=

(3E

32πγ80nmpc5

)1/3

.

Rγ is the shell radius, γ0 = (1− v2/c2)−1/2 is the relativity parameter, nis the density and E is the total energy.

The jet break time tjb can then be connected to the relativity parameter

γ0 =

(3E

32πnmpc5t3jb

)1/8

' 320

(E51

n1t3jb,10

)1/8

.

A relativistic jet has an opening or beaming angle θ0 ' γ−10 .


Dark GRBs

Some GRBs havebright X-ray butonly extremely weakoptical afterglows.

This is due to dustobscuration withinthe host galaxy.

Perley et al. 2009

IRopticalX-ray


GRBs As Probes of Chemical Evolution

GRB light is absorbed byintervening galaxies.

Two systems, z = 3.5673and z = 3.5774, probablymerging galaxies, areilluminated.

The progenitor of theGRB could have formedin star formation trig-gered by galaxy merger.

[Zn/H] = 0.29 and [S/H]= 0.67 are highestmetallicies recorded forz > 3 objects.

Shows star formation andmetallicities heightenedby interaction of galaxies.

Savaglio et al. 2011


Most Distant GRBs

ESO

GRB 090423

z = 8.26

t = 630 Myr

z = 10

t = 480 Myr


Long Gamma Ray Burst Progenitor

I GRB 980425 was followedwithin a day by SN 1998bw(type Ib) at the same location,providing the first clues aboutprogenitors.

I BATSE ended in 2000 and wasfollowed by HETE-2 from200-2007

I Swift launched in 2004 and stilloperating; this also containsX-ray and optical telescopes forrapid deployment to search forcounterparts.

I Fermi Gamma-ray Large AreaTelescope (GLAST) launchedin 2008 and now detectsseveral hundred bursts per year

SN 1998bw


Long GRB Is a Hypernova

ESO


GRBs and Supernovae

Della Valle et al. 2003

Type Ib/c


Collapsar Model

Meszaros 2002


GRBs and Progenitors

Almost every long GRB has been associated with a galaxy with rapid starformation, and some long GRBs are linked to supernovae.The evidence favors that the parent SN population of GRBs arehypernovae, or Type Ib/c SNe from massive progenitors characterized byhigh luminosity, high expansion veclocites and no H/He in spectra.The brightest SNe are associated with relatively faint GRBs.

Short GRBs account for about 30% of total, and not until 2005 weretheir origins clarified. Several short GRB afterglows have been assoicatedwith large elliptical galaxies or centers of large clusters, both regions oflittle or no star formation. They are more offset from galactic centers.

Short GRBs have no supernova link, and must be physically distinct fromlong GRBs. The most prevalent suggestion is that short GRBs are formedin mergers of neutron stars or black holes and neutron stars. Afterglowsof minutes to hours in X-rays are consistent with fragments oftidally-disrupted neutron star material (r-process radiation).

A fraction of low-luminosity short GRBs may be giant flares from softgamma ray repeaters (magnetized neutron stars) in nearby galaxies.


GRB Models

The Encylopedia of Science


Double Neutron Star Mergers

Podsiadlowski

Initial mass transfer

First supernova and kick

Be/X-ray binary

Common-envelope evolution

Helium star–neutron star binary

Secondary mass transfer

Second supernova and kick

Double neutron star binary


Mergers–Maximum Mass

Belczynski et al. 2007


Mergers–Neutron Star Radii

Bauswein, Stergioulas, Janka (2015)

Mtot =

3M�

2.7M�

2.4M�


Rates of Gamma Ray Bursters

More than 1 per day is observed, but due to beaming, the actual rate isperhaps 1 per minute in the universe.

Within our galaxy, the rate is estimated to lie between 1 per 100,000 and1,000,000 years. Less than 10% of these would be beamed in ourdirection.

It has been suggested that the Ordovician-Silurian extinction, 450 Myrsago, was caused by a GRB. Evidence is the extinction of tribolites whichwere upper ocean dwellers. Other species living on land or in shallow seaswere also particularly hard hit. Species that were deep-water dwellerswere relatively unharmed.

The rate per volume of long GRBs is estimated to be between 100 and1000 per Gpc3 per year, which is 1 to 10% of the rate of Type Ib/csupernovae. This difference is probably due to beaming, i.e., ∼ θ−1.


Gamma Ray Bursters and the Fermi Paradox

Fermi’s Paradox:

The universe is over 1010 years old and the galaxy is about D = 100, 000lt. yrs. across with about 1 light year between stars on average. Anadvanced civilization will take about 10,000 years to advance from star tostar, traveling with the Earth’s orbital velocity, or about 100 years ifadvanced technologies allow v = 0.01c .

There will be a delay between successive colonizations, and colonizationwould tend to be somewhat random, but the time needed to colonize thegalaxy could be as short as 10D/v ∼ 108 yrs.

Fermi’s Paradox is the mismatch between the Galaxy’s age and thecolonization timescale.

There is no evidence aliens have ever been nearby. Perhaps gamma raybursts, causing repeated sterilizations, have prevented intelligence fromdeveloping and colonization occurring until now.


A gamma ray burst occurs in our Galaxy every 106 years now but the ratewas higher in the past. About 1% might be beamed in our direction.Assume each such burst causes an extinction in the beam path. 1010

years ago, a spot in the Galaxy could expect an extinction every 106

years; now it would be perhaps 1− 2× 108 years between extinctions.

Life in seas might be protected better than life on land; but life on landbecame prominent only 3− 4× 108 years ago. The relevant time is thetimescale for the rise of intelligent life, not from single-celled life, butfrom multicellular life in the oceans. It could take 1− 3× 108 years todevelop sufficient complexity for intelligence.

The important point is that the two relevant timescales are aboutthe same, 1− 2× 108 yrs. This should set up an equilibrium.

Once the gamma-ray burst rate decreases below the intelligent evolutionrate (both are inversely equal to their respective timescales), intelligentcivilizations will begin to develop. It is interesting that the colonizationtimescale is also the same, so this answers Fermi’s question: ”Theyhaven’t had time to get here yet!”

J. Annis, J. Br. Int. Soc. 52, 19 (1999)


GRB Real-Time Monitor

grb.sedona.edu


gamma ray bursts - astro.sunysb.edu · discovery of gamma ray bursts i first observed in 1967 by...

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