Can a double component outflow explain the X-ray and Optical Lightcurves of GRBs? Massimiliano De Pasquale 1 P. Evans 2, S. Oates 1, M. Page 1, S. Zane 1, A. Breeveld 1, P. Schady 1, S. Holland 3, M. Still 1 1 Mullard Space Science Laboratory (UCL), UK 2 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 to Poynting flux or trail of shells X-Ray Nousek et al PromptAfterglow

Chromatic breaks between Optical and X-ray Panaitescu et al CHROMATIC BREAKS NOT IN AGREEMENT WITH THE 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 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) 110 Racusin et al Of a sample of 103 GRBs, 13 GRBs have good X-ray and Optical lightcurves, none have achromatic jet breaks in both bands.

GRB

Swift GRB : 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 Flux 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 ~10 4 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 s

Our sample

We examined all Swift GRBs with chromatic breaks claimed by Panaitescu plus , 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 model put forward for GRB

GRB and GRB : 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  = 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  = 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, ± ± ± ±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, ± ± ± ± ± ±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 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 ergs, however the beaming angle  can be ~ 0.02 rad => real E ~ a few 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 ; 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X q Expected slope  X, ± ± ± ± ± ± ± ± ± ±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X q Expected slope  X, ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±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 times N W N W N OXOX W W N N W A’ OXO

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 2  with the observed slope 1.59 ± 0.03 Oates, De Pasquale, Page et al 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 GRB ? 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 GRB ? before the X-ray break: From  X,1 = 0.41 ± 0.03 and  X = 1.10 ± 0.06 => q = /- 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,3  1.93 ± 0.13

GRB X-ray lightcurve from Ferrero et al X-ray  X,2 = 0.34  X,3 = 1.89

GRB lightcurves from Ferrero et al  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  OpticalX-ray T_break optical = 23.3 ± 1.73 ks  X,2 = 0.34  X,3 = 1.89 If we adopt Scenario A’’, E N >> E W 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 the prompt emission: Late internal shocks, produced by shells emitted by the central engine at late times. BUT: a few flares occur ~10 5 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 s breaks sometimes are absent: Chromatic break GRBs X-ray lightcurve Oates et al Zhang et al