Low-luminosity GRBsand

Relativistic shock breakouts

Ehud Nakar Tel Aviv University

• Omer Bromberg• Re’em Sari• Tsvi Piran

GRBs in the Era of Rapid Follow-upLiverpool, 2012

Low-luminosity GRBs• ~10-4 lower luminosity, <1048 erg/s

• Much more frequent

• Smooth light curve

• E << total energy

• Not highly collimated

• Mildly relativistic ejecta with

energy ~ E

• Delayed X-ray emission, with

energy ~ E

• Not “successful” jets

Long

Short

Low luminosity

Wanderman & Piran 2011

• Low-luminosity GRB is NOT a regular GRB with low luminosity• Connection to long GRBs is mainly via the common association to broad-line Ic SNe

Outline

• Propagation of a relativistic jet in a stellar envelope

• Why low-luminosity GRBs are not generated by “successful” jets (as long GRBs)

• Theory of relativistic shock breakout (>0.5)

• Comparison of relativistic shock breakout predictions to low-luminosity GRB observations

• A note on short GRB classification

Jet propagation in a stellar envelope

NumericalMacfadyan, Woosley, Zhang ,Morsony, Lazzati, Mizuta, Aloy, Nagakura, Tominaga, Nagataki, Ioka ...

AnalyticBegelman, Matzner, Meszaros, Waxman, Lazzati, Bromberg, ...

JetMedium

JetMedium

Head

JetMedium

Cocoon

Head

JetMedium

Cocoon

Head

Collimation Shock

Morsony et al., 07

• The head is slower than the jet material, and dissipates the jet energy.

• In order to propagate the head needs to be pushed by the jet material.

• The engine must work continuously until the jet breaks out.

To observe a long GRB: the jet must break out

• The head is slower than the jet material, and dissipates the jet energy.

• In order to propagate the head needs to be pushed by the jet material.

• The engine must work continuously until the jet breaks out – or it will fail.

• Breakout time:

To observe a GRB: the jet must break out

Failed jet

313232

0

31

51 15510/10sec15

M

M

R

R

serg

Lt isob

ʘ ʘ

Bromberg, EN, Piran & Sari 11

Are low-luminosity GRBs produced by “successful” jets?

(Bromberg, EN & Piran 2011)

tγ = te - tb

ttbb ttγγ

ttee

After the jet breaks out energy flows (relatively) freely to large distances where the prompt GRB emission is emitted.

ttbb tt

tteeLess lik

ely

Less likely

The engine is unaware that the jet breaks out

0.01 0.1 1 10T90/tb

# of

bur

sts

Low-luminosity

Long GRBs

Low-luminosity GRBs are most likely (2) not produced by jets that successfully punches through their progenitor envelope

Bromberg, EN & Piran 2012

If not a successful jet then what is the -ray source of low-luminosity GRBs?

Even “failed” jets drive shocks that breakout of the stellar surface!

“failed” jets are much more frequent than successful ones

What are the observed signatures of the resulting shock breakouts?

Relativistic shock breakout(EN & Sari 2012)

Shock breakout

Shock accelerates while its energy decreases

Shock width = distance to edge

Radiation dominated shock: =c/v

Shock breakout

radiation-dominatedshock

Relativistic shock breakout

Main physical properties:

• Constant post shock rest frame temperature ~100-200 keV

• Temperature dependent (pair) opacity

• Significant post breakout acceleration 31 initialfinal

104

105

10-2

10-1

100

101

102

V (km/s)

T (

ke

V)

TBB

pairs

Katz et. al., 10Budnik et. al., 10

Three observables depend on two physical parameters: R and

Relativistic breakout relationfor quasi-spherical breakout without wind

s22

sunbo

bobo R

Rt

keV 50 boboT

7.22/1

46 keV 50erg 10s 20

bobobo TEt

The breakout emission - A flash of -raysColgate was correct after all (for wrong reasons)

erg 102

2/3144

sun

bobobo R

RE

s22

sunbo

bobo R

Rt

keV 50 boboT

The breakout emission - A flash of -raysColgate was correct after all (for wrong reasons)

erg 102

2/3144

sun

bobobo R

RE

Important note The photosphere is not in thermal equilibrium

The blackbody radius (Rbb2 = L/4T4) is meaningless

Emission following the shock breakout

EN & Sari 12

-rays

X-rays

Ep shifts from -rays to X-rays (Ex > E)~

Are low-luminosity GRBs produced by relativistic shock breakouts?

Colgate 1968, Kulkarni et al., 1998, Tan et al., 2001, Campana et al., 2006, Waxman et al., 2007, Wang et al., 2007, Katz et al., 2010

Predictions of relativistic shock breakouts from “failed” jets and a comparison to low luminosity GRBs:

• Smooth light curve

• E << total energy

• Relativistic ejecta with energy ~ E

• Delayed X-ray emission, with energy ~ E(e.g., X-ray echo of GRB 031203)

Relativistic breakout relation

7.22/1

46 keV 50erg 10 s 20

bobo

bo

TEt

?

Low luminosity GRBs

GRB Ebo

(erg)Tbo

(keV)tbo

(s)Relation

tbo (s)Rbo

(cm)bo

980425 1048 150 30 10 61012 3

031203 5104

9

>200 30 <35 21013 >4

060218 5104

9

40 2100 1500 51013 1

100316D 5104

9

40 1300 1500 51013 1

Relativistic breakout relation

7.22/1

46 keV 50erg 10s 20

bobobo TEt

Shock breakout from long GRBs

s~ mtbo

MeV boT

erg 5

10~2

48

sun

bobo R

RE

A short, hard and faint pulse at the beginning of the burst

-ray flares from relativistic shock breakouts are expected in a range of other explosions. For example,

White dwarf explosions (Type Ia and .Ia SNe and AIC):

erg 1010~ 4240 boE

ms 301~ bot

MeV ~boT

Extremely energetic and compact supernovae (e.g., SN 2002ap):

erg 1010~ 4644 boE

s 303~ bot

keV 100~boT

BATSE T90 (50 - 300 keV) Swift T90 (15 - 350 keV)

The threshold duration for Swift sample must be shorter than for BATSE sample !!!

We show that it is 0.6-0.7 s (Bromberg, EN, Piran & Sari 12)

A note on short GRB classification

Summary• Relativistic breakouts produce -ray flares with characteristic properties:

• Ebo – Tbo – tbo relation (if quasi-spherical without wind)• smooth• a small fraction of total explosion energy• to X-ray evolution• generate a relativistic outflow with E~Ebo

• Low-luminosity GRBs, which are fundamentally different than long GRBs, show all these characteristics

• Failed jets is the most natural mechanism (explains also the high low luminosity GRB rate)

• Swift GRBs with 1s < T < 2s are most likely (>50%) collapsars

Thanks

t

25

12

6

3/115

3/211

3/2

1050

3/1

s 15

MRLt ob

L 51

Zhang et al., 04

Comparison with simulations

Bromberg, EN, Piran Sari 11

Which explosions are expected to have relativistic breakouts?

EN & Sari 11

95.0

*

2.1

sun

7.1

53

exp

5M5erg 10 14

sun

ejectalosionbo R

RME