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Gamma-ray Bursts in the Fermi Era

Pawan Kumar

June 13, 2012

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•Fermi data & developments of last 3 yearsFermi data & developments of last 3 years

Outline†

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High redshift burstsHigh redshift bursts

Review of main propertiesReview of main properties

Problems with the current paradigm and Problems with the current paradigm and possible solutions.possible solutions.

Gamma-ray Bursts

were discovered were discovered (accidentally(accidentally) by ) by Vela satellites in 1967.Vela satellites in 1967.

For about 20 years theFor about 20 years thedistance to GRBs was distance to GRBs was completely uncertain.completely uncertain.

HistoryHistory

(GRBs)

Colgate (1968) anticipated GRBs -- associated with breakout of relativistic shocks from the surfaces of SNe.

Compton Gamma-ray Observatory (launched in 1991)

established that the explosions arecoming from random directions (isotropic)& have non-Euclidean space distribution.

And therefore are at very large distances

ISOTROPY

A Italian/Dutch satellite – Beppo/SAX – launched in 96

localized long-bursts to 5-arcmin (a factor ~20 improvement)

Which led to the discovery of optical afterglow, and redshift.

Thus, it was discovered that energy (isotropic) Eiso ~ 1053 erg.

Stanek et al., Chornock et al. Eracleous et al., Hjorth et al., Kawabata et al.

SN 1998bw:local, energetic,core-collapsedType Ic

GRB 030329: z=0.17(afterglow-subtracted)

Long-GRB – collapse of a massive star(Woosley and Paczynski)

X-ray flash 020903 also seems to show a spectrum like 98bw at 25 days after the burst (Soderberg astro-ph/0502553).

Wain

wri

gh

t, B

erg

er

& P

en

pra

se,

2007

Long GRBs: Exclusively in star-forming galaxies ⇒ Progenitors are massive stars

HST/WFPC2 & ACS images of long-GRB host galaxies; each panel is 5-arcsec wide

Host galaxies are typically 0.1 L* & ~ 0.1 Zsun

Short GRBs: Host GalaxiesShort GRBs: Host GalaxiesBerger et al. 2007

Large circle: Swift/XRT positionSmall circle: optical position (if available)

Gemini & Magellan images are 20” on the side

€

v⊥ = β sinθ1−β cosθ

≈ 50 Dashed line: jet model with tj =10 days & E52/n0 =20.

Explosion speed(Taylor et al., 2004: GRB 030329)

v=5c

v=3c ≈7

An

gu

lar

Siz

e (a

s)

Days After Burst

Pan

aite

scu

& K

um

ar (

2001

)These explosions are highly beamed (break in lightcurves); The energy release is determined by theoretical modeling of multi-wavelength afterglow data, and is found to be on average ~1051 erg. (some bursts have >1052 erg & others <1050 erg)

Afterglow theory: synchrotron radiation in external shock

More energy comes out in these explosions in a few seconds than the Sun will produce in its 10 billion year lifetime!

The launch of Swift satellite – 11/20/04 – was another majormilestone in the study of GRBs

INTEGRAL satellite – Oct 17, 2002 launch – has discovered many GRBs and contributed much to our knowledge of these bursts.

O’Brien et al., 2006

Factor ~ 10Factor ~ 1044 drop in flux!drop in flux!

Another major discovery of Swift was that the x-ray flux declinesrapidly at the end of γ-ray burst; this behavior was anticipatedby Kumar & Panaitescu (2000) – 5 years before the discovery.

If the rapid turn-off is due to a fast decline of accretion rateonto the newly formed black-hole, then we can “invert” theobserved x-ray lightcurve and determine progenitor star structure.

time

flu

x

steep fall off

prompt GRB emission

rapid decline

X-ray plateau

r ≈ 9 109 cm

r ~ 1.5

1011

cm

fΩ ~

2

fΩ 2 3

1010 cm

r -2.5

f k

Progenitor Star PropertiesProgenitor Star PropertiesKumar, Narayan & Johnson (2008)

(Sophisticated simulation work of Lindner, Milosavljevic et al., 2010)

Some interesting GRBs detected by Swift

Naked Eye burst at z=0.94(redshift measured by HET)

movie made by Pi of the Sky, a Polishgroup that monitors transient events

2.5 million times more luminous (optical) than the most luminous supernova ever recorded

T = 5.5s, fluence=3.1x10-7 erg cm-2 (Ep=49 keV)(similar to bursts at low z)

Cucchiara et al. 2011

GRB 090429B: z=9.4, Eiso=3.5x1052erg

How far away can we see BurstsHow far away can we see Bursts??

So Swift can detect bursts like this to z ~ 15, when the universe was 270 million years old.

Swift/BAT sensitivity is 1.2x10Swift/BAT sensitivity is 1.2x10-8-8erg cmerg cm-2-2ss-1-1

Burst z tγ(s) flux (erg cm-2 s-1) Eiso(erg)

050904 6.3 225 3x10-8 ~1054

090423 8.2 10.2 6x10-8 1.2x1053

090429B 9.4 5.5 5x10-8 3.5x1052

SVOM: a Chinese-French mission (2017?) – more sensitive than Swift for GRBs with Epeak< 20 keV ( 4--250 keV band); 2 IR telescopes (0.4 to 0.95 μm – located in Mexico & China) to look for z~8 bursts.

JANUS: funded for phase A study, but not selected for launch; x-ray (1-20 keV) and IR telescope (0.7—1.7 μm) can determine GRB redsfit to z=12.

080913 6.7 8 7x10-8 ~1053

GRBs to probe the young Universe

The most distant quasar is at z=6.4 & galaxy at z~10. The most distant quasar is at z=6.4 & galaxy at z~10.

So we should be able to see bursts at much larger z, So we should be able to see bursts at much larger z, if they are there, which will help us explore properties if they are there, which will help us explore properties of the first stars and objects that formed.of the first stars and objects that formed.

Amati relation (2002): E_p \propto E_iso^0.52\pm0.06

Swift has seen two bursts at Swift has seen two bursts at z=8.2 & 9.4 which are among z=8.2 & 9.4 which are among the most distant objects we the most distant objects we have seen.have seen.

These bursts were NOT These bursts were NOT exceptionally bright – in fact exceptionally bright – in fact their luminosity, spectra and their luminosity, spectra and lightcurves are similar to lightcurves are similar to lower redshift GRBs. lower redshift GRBs.

GRB 090423 (z=8.2)

Pre-Swift bursts

Swift burstscomoving volume (dV/dz)

Star formation rate(Porciani & Madau 01)

Gehrels, Ramirez-Ruiz & Fox (2009)

Our understanding of GRBs has improved dramatically in last ~10 years.

However, there are a number of fundamental questions thatHowever, there are a number of fundamental questions thatremain unanswered. The foremost amongst these are:remain unanswered. The foremost amongst these are:

1. Whether a BH or a NS is produced in these explosions?

2. Composition of relativistic jets in GRBs: Baryons? e± ? or B?

We can answer these questions if we could understand how γ-rays are generated in GRBs, and use that to read the signatures of different central engine models and jet composition

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magnetarblackhole

6/11/2008Fermi8 KeV to 300 GeV

One of the goals for Fermi One of the goals for Fermi is to understand γ-ray burst is to understand γ-ray burst prompt radiation mechanismprompt radiation mechanismby observing high energy by observing high energy photons from GRBs.photons from GRBs.

However, there were surprisesin store for us:

Fermi discovered that Fermi discovered that

How are γ-rays generated?

1. >102MeV photons lag <10MeV photons (2-5s)

2. >100 MeV radiation lasts for ~103s whereas emission below 10 MeV lasts for ~30s or less!

Ab

do

et a

l. 20

09

They suggested that high energy photons (>100 MeV)are produced in the External-shock via synchrotron

Within a few months of these discoveries (by Fermi) Kumar & Barniol Duran (2009) proposed a model – now widely accepted – which will be discussed in the next few slides.

Gehrels, Piro & Leonard: Scientific American, Dec 2002

Abdo et al. 2009

(GRB 080916C)(GRB 080916C)

Long lived lightcurve for >102MeV (Abdo et al. 2009)

ffνν α ν α ν -1.2

-1.2 t t -1.2-1.2

α = 1.5β – 0.5 (FS)

α = 1.5β – 0.5 (FS)

>10>1022MeV data MeV data expected ES flux in the X-ray and optical band expected ES flux in the X-ray and optical band (GRB 080916C) (GRB 080916C)

We can then compare it with the available X-ray and optical data.We can then compare it with the available X-ray and optical data.

Abdo et al. 2009, Greiner et al. 2009, Evans et al. 2009

Long lived lightcurve for >102MeV (Abdo et al. 2009)

Ku

mar

& B

arn

iol D

uran

(20

09)

Or we can go in the reverse direction…

Assuming that the late (>1day) X-ray and optical flux are from ES, calculate the expected flux at 100 MeV at early times

And that compares well with the available Fermi data.And that compares well with the available Fermi data.

X-ray

Optical

> 100MeV

50 - 300keV

Abdo et al. 2009, Greiner et al. 2009, Evans et al. 2009

Ku

mar

& B

arn

iol D

ura

n (

2009

)

Using only >100MeV Fermi data

Par

amet

er s

earc

h at

t =

50

sec.

How are Magnetic fields Generated in Shocks?

30 30 μμGG

5 μG

Using late time x-ray, optical & radio data

Par

amet

er s

earc

h at

t =

0.5

day

.

30 30 μμGG

5 μG

Similar results found for two other Fermi/LAT bursts: 080916c & 090510.

Recent work has provided a surprising answer: εεBB is consistent with shock is consistent with shock

compressed magnetic field of CSM of ~ 10 μG compressed magnetic field of CSM of ~ 10 μG (Kumar & Barniol Duran 2009)

[Thompson et al. (2009) find weak magnetic field amplification in SNe shocks]

(A long standing open question)

GR

B 090902B

GR

B 090902B

Melandri et al. (2008), Antonelli et al. (2006), Panaitescu & Vestrand (2011), Schulze et al. (2011), Stratta et al. (2009), Covino et al. (2010), Perley et al. (2008), Perley et al. (2009), Uehara et al. (2010), Guidorzi et al. (2011), Perley et al. (2011), Greiner et al. (2009),Yuan et al. (2010), Melandri et al. (2010)

This result suggests a weak magnetic dynamo in relativistic shocks

San

tan

a et

al.

201

2

central engine

relativistic outflow

Jet energy dissipation

and γ-ray generation

External shock

radiation

central engine jet -rays

A good fraction of >102MeV photons appear to be generated in external shock; (photo-pion & other hadronic processes might also contribute for ~30s or so)

How about photons in the intermediate energy band, i.e. ~10keV – 0.1 GeV?

Emission in this band lasts for <102s, however it carries a good fraction of the total energy release in GRBs.

And it offers the best link to the GRB central engine.

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Synchrotron, IC or SSC in internal shocks, RS or FS, or hadronic collision or photo-pion process...

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Katz 1994; Derishev et al. 1999; Bahcall & Meszaros 2000 Dermer & Atoyan 2004; Razzaque & Meszaros 2006 Fan & Piran 2008; Gupta & Zhang 2008; Granot et al. 08;Daigne, Bošnjak & Dubus 2011 …

Meszaros & Rees 1994; Pilla & Loeb 1996; Dermer et al. 2000 Wang et al. 2001 & 06; Zhang & Meszaros 2001; Sari & Esin 01’Granot & Guetta 2003; Piran et al. 2004; Fan et al. 2005 & 08 Beloborodov 2005; Fan & Piran 2006; Galli & Guetta 2008Pe’er et al. 06; Granot et al. 08; Bošnjak, Daigne & Dubus 09

(~10(~1022 man-years of work has gone into it) man-years of work has gone into it)

Radiation mechanism

GRB: current paradigm (internal shock model)Gehrels et al. (2002); Scientific American

The next few slides will show that this paradigm is NOT working…The next few slides will show that this paradigm is NOT working…We don’t know what replaces though…We don’t know what replaces though…

It can be shown that shock-based mechanism for sub-MeV radiation from GRBs has severe problems

Also, the natural expectation is to have two breaks in the spectrum – at νc & νi – which is never seen.

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e± cool rapidly

1. 1. Thermal radiation + ICThermal radiation + ICThompson (1994 & 06); Liang et al. 1997;Thompson (1994 & 06); Liang et al. 1997;Ghisellini & Celloti 1999; Meszaros & Rees (2001); Ghisellini & Celloti 1999; Meszaros & Rees (2001); Beloborodov (2009)Beloborodov (2009)Daigne & Mochkovitch (2002); Pe’er et al. (2006)…Daigne & Mochkovitch (2002); Pe’er et al. (2006)…

(for prompt (for prompt -rays)-rays)

Problem: we don’t see a thermal component in GRB prompt emission – Ryde (2004, 05) claims to find evidence for thermal spectrum, but Ghirlanda et al. 2007 do not.

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There are three possible solutions

2. Continuous acceleration of electrons

If electrons can be continuously accelerated (as they lose energyIf electrons can be continuously accelerated (as they lose energyto radiation) then some of the problems mentioned earlier to radiation) then some of the problems mentioned earlier disappear (disappear (Kumar & McMahon, 2008Kumar & McMahon, 2008). ).

However, is it NOT possible to accelerate electronscontinuously in shocks.

So this solution requires a magnetic jet and reconnection.

3. Relativistic turbulence (probably requires magnetic jet)Lyutikov & Blandford 03’; Narayan & Kumar 09’; Lazar, Nakar & Piran 09’

Line of Sight

1/

Rs t

Relativistic Turbulence ModelRelativistic Turbulence Model

Variability time = Rs(1+z)

————(2c2) t

2

Synchrotron emission in quiescent part of shell less variable optical

IC scattering of synchrotron off of blobs γ-rays (more variable)IC scattering of synchrotron off of blobs γ-rays (more variable)

Lyutikov & Blandford 03’; Narayan & Kumar 09’; Lazar, Nakar & Piran 09’

Consistent solutions for Consistent solutions for -rays are-rays arefound in this case & Rfound in this case & Rss~R~Rdd as as suggested by observations.suggested by observations.

One possibility:

Black-hole vs. Magnetar & jet composition

One of the best ways to determine jet composition is by looking for One of the best ways to determine jet composition is by looking for optical/IR radiation from RS–heated jetoptical/IR radiation from RS–heated jet (and ~TeV ν (and ~TeV νee ν νμμ).).

Improved sensitivity (~10) in optical/IR is needed on a timescale of Improved sensitivity (~10) in optical/IR is needed on a timescale of less than 10less than 1022s from GRB trigger; recent ICECUBE result for s from GRB trigger; recent ICECUBE result for neutrinos is interesting, but not yet too constraining of jet neutrinos is interesting, but not yet too constraining of jet composition. composition.

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Swift found that the x-ray flux at the end of GRBs declines very rapidly –– t-3 or faster.

The expected decline of dipole luminosity for a magnetar is t-2

Some GRBs have E > 1052 erg – more than expected of a magnetar.

Recent work of Metzger et al. (2011) offers interesting suggestions regarding magnetars, but I see some problems...

Although pulsar breaking index n (dΩ/dt α Ωn) is found to be between 1.4 and 2.9 for 6 pulsars which implies a faster decay.

Summary

We have learned many things about GRBs in the last 10 years:

Produced in core collapse (long-GRB) & binary mergers (short-GRB)

Highly relativistic jet (Γ ≥ 102), beamed (θj ~ 50), Ej~1051 erg

They do occur at high redshifts (current record Z=9.4)

High energy photons (>100 MeV) are produced in external shock

Generation of magnetic fields in relativistic shocks is clarified

But we don’t yet have answers to several basic questions:Are blackholes produced in these explosions (or a NS)?

What is the GRB-jet made of?

How are gamma-rays of ~MeV energy produced?

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