So you Think you can Explain GRB?

David Eichler

My collaborators:

Amir Levinson

Jonathan Granot

Dafne Guetta

Hadar Manis

Samir Mandal

Outstanding Questions about GRB

Outstanding Questions about GRB

(to be shot out rapid fire, and punctuated with “Huh?..Huh?...)

Why are GRB not squelched by baryon contamination?

1) Strange matter sources (Paczynski and coworkers, Usov)

2) Axion decay (Kluzniak)

3) Event horizons, GR needed (Levinson and Eichler)

4) Centrifugal barrier, GR unnecessary, (several authors)

5) Patience, wait ~100 s for baryons to settle down, (as for magnetars) then use magnetocentrifugal ejection (Thompson….?)

Why does spectrum typically peak at several hundred KeV?

Selection effects? Coincidence?

Photospheric emission? pair controlled? * (DE 1994, Thompson 1995, Levinson and Eichler 2000, Ryde,2004, Meszaros and Rees 2000, 2005, Pe’er, Meszaros and Rees 2008)

*e.g.Cavalo and Rees 1978

What carries the energy from the collapsed object to the emitting radius – EM flux or baryonic material? Does the photon pair fireball derive from or provide the kinetic energy of the baryons?

Do the photons and baryons in GRB always coincide in direction? If not, then photons with hardly any baryons is possible.

?

Did the matter that the gamma ray photons last interact with come from the collapsed object or from the host environment?

If from host environment, what is the nature of the interaction? How do the baryons acquire energy from the fireball

Does Lorentz factor saturate before or after photosphere? Rsat >, < Rph?

If prompt gamma ray energy comes from kinetic energy in matter, then probably Rsat < Rph

If Rsat > Rph, it means that the matter is still accelerating at the photosphere, hence kinetic energy of baryonic matter probably comes from EM /pair pressure.

Are there any observational clues?

Why do Fermi LAT /EGRET detections show soft to hard evolution?

Harder emission made further downstream?

Why do subpulses show hard to soft evolution?

Why are spectral lags inversely correlated with luminosity L for long bursts?

Why the spectral lag -L correlation for long bursts, not short ones?

Why is the Amati “relation” one sided?

Why does it not apply to afterglow fluences, i.e. why does it apply to photon flux but not baryonic kinetic energy?

Why does duration of flat phase of afterglow correlate with spectrum?

• Why are short GRB so far off the Amati relation?

• If short GRB are due to short central engine activity, why do many of them have long X-ray tails? Why not all of them?

Why is GRB 060218 both the longest duration burst (longer than this talk) and one of the closest?

What are the odds it is just a coincidence?

“Slow” sheath of Baryons

Ultrarelativistic fireball, nearly baryon free, except for neutron leakage

e.g. Levinson and Eichler 1993

Basic picture of central engine

Rossweg and Ramirez-Ruiz

How sharp is the distinction between the baryon poor and baryon rich parts of a GRB event?

Event horizon is either/or, and might provide sharper transverse gradients than a Newtonian centrifugal barrier in the presence of strong viscosity. Needs careful theoretical study.

Could there be opportunity for observational investigation? Can we probe the transition from baryon rich to baryon poor?

Can we probe the transition from GRB to NQGRB depending on viewing angle?

One possible observational consequence:

Neutrons can decouple from protons at some radius (Derishev 1999). If parameters vary sharply at some magnetic surface, then pickup ex- neutrons receive enormous energies in lab frame (Eichler and Levinson 1999)– get harder neutrino spectrum than from shock acceleration. Most of the GRB energy could come out as UHE neutrinos.

Collisional Avalanche

n

nnn

Neutron free streaming boundary

Nn about

A/1049A12

Neutron mist

Collisional Avalanche

n

nnn

Collisional Avalanche

n

nnn

1015 eV neutrinos

Collisional Avalanche- solves problem conversion efficiency problem, gives very hard spectra

n

nnn

1015 eV neutrinos

Could be the main output. Spectrum much harder than for shock acceleration

What do NQGRBs look like?

• Dirty fireballs?

• Kinematically softened gamma rays ?

• Was the dominant emitting matter beamed directly at the observer?

Rossweg and Ramirez-Ruiz

What do NQGRBs look like?

• Dirty fireballs?

• Kinematically softened gamma rays ?

• Was the dominant emitting matter beamed directly at the observer?

• The Amati relation may provide clues.

The Amati relation interpreted as a viewing angle effect. Soft photons always spray out beyond 1/ cone.

Near perimeter observers see Amati relation

1/ cone

Patchy jet also works if patches have sharp edges

Amati relation according to Butler et al 2007

Detection threshold

Pencil beam

Extended source, near perimeter viewing

Head on

Harder

brighter

Amati relation according to Butler et al 2007

Amati relation according to Butler et al 2007

Detection threshold

Extended source, near perimeter viewing

Scattering of slow material

Amati relation according to Butler et al 2007

Detection threshold

Constant photon number. e..g. dirty fireballs

Pencil beam

WHERE ARE THE NQGRBs??

ray X-ray

???

Jet

Observer

So if you believe the viewing angle interpretation of the Amati relation, then you have to believe that the directly oriented emission is subdominant, i.e. sharp edge

Amati relation according to Butler et al 2007

Detection threshold

Constant photon number

Pencil beam

Photospheric emission does not deny internal shocks

Non-thermal emission in GRB beyond dispute since SMM.

Is the emission at 300 KeV from a pair- controlled photosphere?

The baryons could come from the host environment after the fireball is going strong.

But wait: If there are enough baryons to account for non-thermal gamma rays via shock acceleration beyond the photosphere, (even if injected beyond photosphere), why do they not obscure the photospheric emission?

But wait: If there are enough baryons to account for non-thermal gamma rays via shock acceleration beyond the photosphere, (even if injected beyond photosphere), why do they not obscure the photospheric emission?

I’m glad you asked.

But wait: If there are enough baryons to account for non-thermal gamma rays via shock acceleration beyond the photosphere, (even if injected beyond photosphere), why do they not obscure the photospheric emission?

Partial coverage? (if all else fails).

But wait: If there are enough baryons to account for non-thermal gamma rays via shock acceleration beyond the photosphere, (even if injected beyond photosphere), why do they not obscure the photospheric emission?

Maybe they do sometimes, so what?

But wait: If there are enough baryons to account for non-thermal gamma rays via shock acceleration beyond the photosphere, (even if injected beyond photosphere), why do they not obscure the photospheric emission?

Back-end photospheres: slow baryon acceleration,hard to soft evolution, spectral lags, outliers to the Amati relation, subluminous events, short hard bursts with long X-ray tails, may all fit into a single picture that addresses this question

Lorentz transformation

Back-facing photosphere

Observer

Sharply rising FRED’s

Optically thick cloud?

Backscattered radiation in frame of cloud.

Shadow in frame of cloud

FRED’s

Observer

Τ(t1)

1

Optically thick cloud accelerated by photon pressure of Poynting flux

Backscattered radiation relativistically beamed in observer frame

shadow

FRED’s

Observer

Τ(t1)

1

Optically thick cloud accelerated by photon pressure of Poynting flux

Backscattered radiation relativistically beamed in observer frame

shadow

FRED’s

Observer

Τ(t1)

1

Optically thick cloud accelerated by photon pressure of Poynting flux

Backscattered radiation relativistically beamed in observer frame

shadow

Optically thin scattering cloud

Optically thin scattering cloud

theory

Optically thick cloud

Blocked by high optical depth

Switches on just when = 1/.

0 50 100 150 200 250 300 350 4000

100

200

300

400

500

600

700

800

900

to

No

=1 and =1/200

o=30

o=100

o=200

Spectral lag, hard to soft evolution

E -0,4 – 0.5

7.0 ])[(log10 keVE

)(log10 width

])[(log10 keVE

)(log10 width40657.0 Ewidth

7.0

Subpulse evolution seems to show acceleration of photosphere. Good quantitative agreement on many fronts.

Hard to soft evolution Ep flat in first phase, then softens as t-2/3.

Width proportional to Eph -0.4 or so

Inverse Lag-L correlation

Short duration events lie far off Amati line,

GRB 060218:

Longest GRB: t ~ 2000 s

One of the closest

Classic hard to soft evolution, but in slow motion

High energy channels consistent with Amati relation

Huge low energy Eiso~ 1050 erg, [Ep~1 KeV], overluminous relative to Amati relation, much more luminous than breakout shocks from host envelope

E-0.3

luminosity

lag

Mandal and Eichler 09

15-150 keV

5-10 keV

2-5 keV

0.3-2 keV

Too luminous to be envelope breakout shock

Is this it?? Is this the dirty fireball we have been waiting for?

Mandal and Eichler 09

15-150 keV

5-10 keV

2-5 keV

0.3-2 keV

Amati relation according to Butler et al 2007

Detection threshold

Constant photon number. e..g. dirty fireballs

Pencil beam

Conclusions

Closest thing we have seen to a baryon- contaminated fireball is associated with what seems to be a GRB seen way off axis, i.e. just where we would expect to see dirty fireballs.

It is much different from a GRB. So far we haven’t seen anything that bridges the gap

Prediction that LIGO signals should coincide with GRB (Eichler, Livio, Piran,

Schramm ‘89) failed to note that GRB are highly beamed. Understanding large angle manifestations of nearby GRB important for multi-messenger detections.