can a double component outflow explain the x-ray and optical lightcurves of grbs? massimiliano de...
TRANSCRIPT
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