Can a double component outflow explain the X-ray and OpticalLightcurves of GRBs?

Massimiliano De Pasquale1

P. Evans2, S. Oates1, M. Page1 , S. Zane1 , A. Breeveld1, P. Schady1, S. Holland3 , M. Still1

1 Mullard Space Science Laboratory (UCL), UK2 University of Leicester, UK 3 NASA GSFC, USA

Not predicted before the Swift era

Present both in the optical and the X-ray

No change in X-ray spectrum at the end of plateau: hydro dynamical or geometrical cause only!

Plateau is already forward shock emission

Plateau: A mysterious new feature in GRB lightcurves

Likely cause: Energy injection,

with L~ t-q into the ejecta, due toPoynting flux or trail of shells

X-Ray

Nousek et al. 2006

Prompt Afterglow

Chromatic breaks between Optical and X-ray

Panaitescu et al. 2006

CHROMATIC BREAKS NOT IN AGREEMENT WITHTHE PREVIOUS SCENARIO

Strong evidence of X-ray uncoupled from the Opt

X-ray Optical

In a few (but it can be as high as in 50%) GRBs:

No break in the optical!

Further problem: where are the late jet breaks in the X-ray?

• Pre-Swift: breaks in the Optical were commonly seen, a few days after the trigger

• Thought to be jet breaks, and expected in the X-ray at the same time

GRB990510, Pian et al. 2001

Swift years:

- Of 230 well sampled GRB X-ray lightcurves, ≤ 50% show evidence or strong indication of a jet break

X-ray

Optical

Liang et al 2008

Time (d)1 10

Racusin et al 2009

- Of a sample of 103 GRBs, 13 GRBs have good X-ray and Optical lightcurves, none have achromatic jet breaks in both bands.

GRB 050802

Swift GRB 050802: a single component outflow is ruled out! (Oates, De Pasquale, Page et al 2007)

Implications:

After correction for extinction, the U and B band would lie above the extrapolated X-ray spectrum at late times.

We have therefore to assume a multi-component outflow to explain the observed properties.

Enegy, keV

SED

Ratio

= 0.63

= 1.59

X-Ray

Optical

= 0.86

= 0.89

Our model: double outflow + continuous energy injection

Flu

x

time

Jet

Spher.

N(t)=N

Opt

X

Spher.

Model ingredients:

- An outflow made up of 2 components:

- A very narrowly collimated and faster component, responsible for the X-ray emission, with a jet break ~104 s after the trigger;

- A broader and slower component, responsible for the Optical emission, which does not usually show a jet break within follow up time.

- A continuous energy injection to both components, which lasts 105-106 s

Our sample

We examined all Swift GRBs with chromatic breaks claimed by Panaitescu plus 060605, which has well sampled lightcurves.

Method:

- Built up the lightcurves and the SEDs of GRB afterglows to first test the standard scenario

- Tried to interpret the result within the same modelput forward for GRB050802.

GRB 050319 and GRB 060605: an unique component is ruled out

- Optical and X-ray lie on the same spectral segment => after break =X

- Or spectral break and decay = 3/2 + 0.5 = 1.28±0.08

X-ray and optical fluxes are NOT originating from the same component.

- Spectral break between Optical and X-ray, with = 3/2 + 0.5 = 1.40 ± 0.10

O = 0.62

X, 2 = 0.48

X, 3 = 1.41

X,3 = 1.93

X,2 = 0.41

O = 0.83

Lightcurves of all GRBs with chromatic break can be reproduced by a continuous energy injection and a jet break (De Pasquale et al 2009)

GRB Expected slope X,3

050319 1.31±0.10 1.41±0.09

060605 1.48±0.20 1.93±0.11

GRBs with achromatic breaks may also be explained by the Jet + Energy Injection scenario, assuming a single component outflow.The ‘normal decay’ would be a jet expansion phase.

Jet + Energy injection: predicted vs observed decay slopes

GRB Expected slope X,3

050401 1.74±0.09 1.44±0.07

050607 1.57±0.41 1.33±0.14

050713 1.44±0.11 1.21±0.03

Theoretical side: does a double component model work?

Framework: standard expressions for m , c and peak flux p = 2.4, energy injection q = 0.5 (average)

Constant density medium

Conditions on flux F required:

X-ray: F NARROW > 2 F WIDE from 300s to 8000s

Optical: F WIDE > 2 F NARROW from 300s to 8000s

Hierarchy of characteristic frequencies (6 scenarios investigated)

Conditions on physical parameters: kinetic energy E, fraction of energy given to electron and magnetic field e and B, density n

SEDs of Plausible Scenarios 4 Scenarios satisfy

the whole set of inequalities!

Caveat: Fine tuning of parameters

is needed; Energies,

B, e, n, cannot change

more than 1.5 - 2 times

W

O

X

Wide

Narrow

N

- As for X-ray data only, bursts with chromatic breaks show the usual ‘canonical’ lightcurve: an energy injected decay followed by the ‘normal’ decay.

- Optical data rule out this scenario and require a two component outflow. We propose: 1) the break in the X-ray lightcurve is a jet break 2) the ‘normal decay’ is actually a jet expansion with energy injection.

- No need to worry anymore for the lack of jet break in the X-ray lightcurves!

- This scenario may be generally applied to all GRBs; it would change our interpretation of the canonical lightcurve and have extremely important consequences on the physics of GRBs.

Conclusions

Even a few X-ray lightcurves alone require the Jet + Energy Injection model

• 11 of 40 GRB X-ray lightcurves with jet breaks REQUIRE THIS SCENARIO to explain the model fits.

• Other 53 cases, deemed ‘unlikely jets’, could actually either be Spherical + EI followed by Spherical OR Jet + EI.

• Monte Carlo simulations show that errors should not cause more than 7.4 +/- 1 cases.

Racusin et al 2009 have examined 230 Swift GRB X-ray lightcurve. They find:

The jet break time could have a much wider range than thought before Swift, being masked by Energy Injection.

Issues on Energy budget and Efficiency

• The isotropic kinetic energy of the wide component can approach 1056 ergs, however the beaming angle can be ~ 0.02 rad => real E ~ a few 1052 erg

• Late and slow shells are not able to power the prompt emission – efficiency problem?

NO. If all the kinetic energy is taken into account, ~ a few 10-3; even if it were 10 times higher, it would not be a problem

What happens to lightcurves at late times? -I

• In 2 of the scenarios mentioned, the X-ray flux from the narrow component is comparable to that of the wide component only after 2-3 days after the trigger

What happens to lightcurves at late times? - II

• Question 1 - "We should see steep decay, = -2 only when the energy injection ends.This should be true both in the X-ray and in the Optical. Then why don't we see jet breaks in both bands at the same time?”

Answer:• If the energy injection ends when the optical has not yet undergone a jet break: the X-ray will show a

slope = -2 and the optical will become steeper; but the steepening in the optical is less evident than in the case of a jet break.

• If the energy injection ends after the optical has undergone a jet break, both X-ray and optical will show a slope -2.

• In this model, optical and X-ray may have different decay slopes even after the end of the energy injection.

After plateau

Lightcurves of all GRBs with chromatic break can be reproduced by a continuous energy injection and a jet break (De Pasquale et al 2009)

GRB X,2 X q Expected slope

X,3

050319 0.48±0.03 1.02±0.02 0.46±0.06 1.31±0.10 1.41±0.09

060605 0.41±0.03 1.10±0.06 0.20±0.06 1.48±0.20 1.93±0.11

GRBs with achromatic breaks may also be explained by the Jet + Energy injection scenario, assuming a single component outflow.The ‘normal decay’ would be a jet expansion phase.

Jet + Energy injection: predicted vs observed decay slopes

GRB X,2 X q Expected slope

X,3

050401 0.56±0.02 0.99±0.02 0.39±0.03 1.74±0.09 1.44±0.07

050607 0.54±0.10 1.07±0.11 0.59±0.23 1.57±0.41 1.33±0.14

050713 0.58±0.03 1.17±0.03 0.38±0.06 1.44±0.11 1.21±0.03

SEDs of Plausible Scenarios 4 Scenarios satisfy

the whole set of inequalities!

Caveat: Fine tuning of parameters

is needed; Energies,

B, E, n, cannot change

more than 1.5 - 2 times

N

W

N

W

N

O X O X

W

W

N

N

W

A’

O XO

GRB050802: Is our scenario consistent with data?

before the X-ray break:

From X,2 = 0.63 ± 0.03 and X = 0.89 ± 0.04 => q = 0.51 ± 0.06 (Zhang et al. ) in the case of spherical expansion, constant density medium, and C > X

after the X-ray break:

Assuming again constant density, Energy injection with parameter q = 0.51 and C > X In the case of jet, the predicted decay slope is 1.83 ± 0.16 (Panaitescu et al.)

consistent within 2with the observed slope 1.59 ± 0.03

Oates, De Pasquale, Page et al. 2007

Therefore, the break seen in the X-ray is consistent with being a jet break; it is not very steep because of energy injection. The optical does not break because its outflow is scarcely collimated.

Jet break + energy injection for GRB050319 ?

before the X-ray break:

From X,1 =0.48 ± 0.03 and X, = 1.02 ± 0.03 => q = 0.46 ± 0.09, for constant density medium and C < X

after the X-ray break:

Assuming again constant density and Energy injection with parameter q=0.46 and C < X

In the case of jet, the predicted decay slope is 1.31 ± 0.10, consistent within 1 with the observed slope 1.41 ± 0.08

Jet break + energy injection for GRB060605 ? before the X-ray break:

From X,1 = 0.41 ± 0.03 and X = 1.10 ± 0.06 => q = 0.20 +/- 0.07, for constant density medium and C > X

after the X-ray break:

Assuming again constant density, Energy injection with parameter q = 0.20, and C > X,

In the case of a jet, the predicted decay slope is 1.50 ± 0.22, consistent within 2 with the observed slope X,31.93 ± 0.13

GRB060605 X-ray lightcurve from Ferrero et al. 2008

X-ray

X,2 = 0.34

X,3 = 1.89

GRB060605 lightcurves from Ferrero et al. 2008

O,2 = 0.88

UVOT data only do not constraint any optical break very well, but we can state: T_break > 13 ks at 3

Optical X-ray

T_break optical = 23.3 ± 1.73 ks

X,2 = 0.34

X,3 = 1.89

If we adopt Scenario A’’, EN >> EW

and W > N

Work in progress:

- Exploring other hierarchies of frequencies;

- Application to a wind circumburst environment

V – X-ray Flares: late “spikes” of internal shocks

- Flares are typically visible in the X-ray band only

- Fast rise and decay => not reconcilable with forward shock emission, produced by slow , wide outflow

- If t0 is chosen just behind the beginning the rise, then the decay obeys

Behaviour reminiscent of the “spikes” of theprompt emission:

Late internal shocks, produced by shellsemitted by the central engine at late times.

BUT: a few flares occur ~105 s after the trigger. How long the central engine can be active?

Lightcurves in X-ray band and Optical provided by Swift X-ray canonical lightcurve:

0 - prompt emission

I – fast decay phase

II – slow decay

III – “normal decay”

IV – post jet break, fast decay

V – flares

VI - ?

UVOT canonical lightcurve:

- Variety of initial behaviours: rising,early plateau, or already decaying

- Slow decay

- 103-104 s breaks sometimes are absent: Chromatic break GRBs

X-ray lightcurve

Oates et al. 2008

Zhang et al. 2008

0