G RB afterglows from long-lived r everse s hocks.

GRB afterglowsfrom long-lived reverse shocks. Frédéric Daigne(Institut d’Astrophysique de Paris)with Robert Mochkovitch (IAP) & Franck Genet (Univ. Herfordshire) 2008Nanjing GRB Conference

1. The internal-external shock scenario • Initial event : collapsars (« long » GRBs ?) – mergers (« short » GRBs ?) • Relativistic ejection : acceleration mechanism ( ? , ?)Long/short/… GRBs : different progenitors but same physics in the relativistic outflow ? Paczynski, Rees, Meszaros, Piran, … Log(R) [meters] ?

1. The internal-external shock scenario • Internal shocks : gamma-ray prompt emission(alternative models : reconnection in a highly magnetized outflow / a purely em outflow) Log(R) [meters]

1. The internal-external shock scenario • Reverse shock : optical flash • Forward shock (=external shock) : afterglow X  V  radio Log(R) [meters]

1. The internal-external shock scenario External (forward) shock : ■ DynamicsBlandford & McKee 1976 ■ Microphysicsee, p, eB ■ Synchrotron radiationSari, Piran, Narayan 1998 ■ Effect of a stellar windChevalier & Li 2000 ■ Jet vs spherical ejecta Rhoads 1997 Contact discontinuity Density Afterglow ? Forward shock RS radius Shocked ejecta Shocked external medium Relativistic ejecta External medium

1. The internal-external shock scenario Internal shocks : ■ Dynamics Shells : Kobayashi et al. (1997) Continuous outflow : Daigne & Mochkovitch (1998, 2000) ■ Microphysicsee, p, eBand z<1 ■ Syn. vs IC radiation ?See talk by Z. Bosnjak or something else ? Lorentz factor Lorentz factor G G1>G2 g(prompt GRB) mass mass

1. The internal-external shock scenario Standard model : results Forward shockMulti wavelength late-time (> a few hours) afterglow observations are well reproduced (i.e. Beppo SAX era). Reverse shock Dynamics : short-lived shock wave(observed duration ~ D/c + decaying tail due to slow cooling electrons) High density medium : RS crosses the relativistic shell before the end of the internal shock phaseLow density medium : RS comes after ISRadiation : synchrotron Results : optical flash ?

2. Afterglows in the pre-SWIFT era ■ nice fits by the FS model… (e.g. Panaitescu & Kumar 2001)

2. Afterglows in the pre-SWIFT era ■ nice fits by the FS model… ■ jet (achromatic) breaks e.g. GRB 990510 Harrison et al. 1999 Magnitude Usually, X-ray observations are not available at break time. Time

2. Afterglows in the pre-SWIFT era mV = 12 mV = 9 mV = 10 ■ nice fits by the FS model… ■ jet (achromatic) breaks ■ one famous optical flash GRB990123 / ROTSE (Akerlof et al. 1999) : such cases are very rare BATSE data

2. Afterglows in the pre-SWIFT era First problems… ■ Uniform medium when a wind is expected (Chevalier, Li & Fransson 2004)n ~ 0.01 – 10 cm-3 (Panaitescu & Kumar 2001)e.g. GRB030329 : uniform medium with n ~ 2 cm-3 !(Berger et al. 2003) ■ Slope p of the electron distribution is often found to be p<2p ~ 1.4 – 2.8 (Panaitescu & Kumar 2001)Acceleration theory : 2 < p < 2.5 ? ■ Radio afterglow can be shallower than in the visible,in contradiction with predictions(Panaitescu & Kumar 2004)

3. Afterglows in the SWIFT era More problems… ■ X-ray plateaux Nousek et al. 2006 O’Brien et al.

3. Afterglows in the SWIFT era More problems… ■ X-ray plateaux This plateau cannot be reproduced by the simplest version of the external shock model. Most discussed possibility : late energy injection (Sari & Meszaros 2000)→ Efficiency crisis ? This scenario requires to add a large amount of energy to the FS.(Panaitescu et al. 2006) It is not really consistent with internal shocks (requires > 90 % efficiency !) → Models of the central engine ?

3. Afterglows in the SWIFT era More problems… X ■ X-ray plateaux ■ Where are the jet breaks ?(Burrows & Racusin 2007) Jet break are expectedto be achromatic Such breaks are very rare in the SWIFT era. V ■ Chromatic breaks (Panaitescu et al. 2006) Such breaks cannot be easily explained in the standard FS model. (Varying microphysics parameters ?) Panaitescu et al. 2006

3. Afterglows in the SWIFT era More problems… Forward shock : rise and decay are too steep. Internal shocks : duration seems very long. ■ X-ray plateaux ■ Where are the jet breaks ? GRB 050502b IS t90 FS ■ Chromatic breaks a~ 6 ■ X-ray flares (Burrows et al. 2005)Usually interpreted as a late activity.This interpretation is very challenging for the models of thecentral engine. (Burrows et al; Falcone et al, 2005)

3. Afterglows in the SWIFT era More problems… XRT ■ X-ray plateaux ■ Where are the jet breaks ? ■ Chromatic breaks ■ X-ray flares ■ Short bursts : the afterglow is also complex (plateau, soft bumps, …)The requirements of late energy injection/late activity are probablyeven more difficult for NS+NS mergers than for collapsars. e.g. GRB 050724(Campana et al. 2006)

4. Afterglow from long-lived reverse shocks Several propositions to solve these problems (e.g. Ghisellini et al. 07; Panaitescu 07) Long lived reverse shocks(Genet, Daigne & Mochkovitch 2007 – Uhm & Beloborodov 2007) Two important changes in the proposed RS scenario :(1) The forward shock is radiatively inefficient (at least until late times when GFS ~ 1-2)(2) The reverse shock is long lived and produces the afterglow ■ This does not require a late activity■ This requires a tail of low-G material :G varies from a few 100 to ~ 1 during the ejection phase■ Kinetic energy in the ultra-relativistic part (G > 100) and the tail are comparable. Low ee and eB inultra-relativistic shocks ?(unefficient particle acceleration)

4. Afterglow from long-lived reverse shocks Genet, Daigne & Mochkovitch 2007 : The dynamics is followed by extending the model developped for internal shocks (the outflow is discretized in a large number of shells) to include the external medium. Uhm & Beloborodov 2007 : semi-analytical treatment (« mechanical model »). A detailed comparison has been made between this two different approachs : excellent agreement for all dynamical quantities.

4. Afterglow from long-lived reverse shocks Internal shock(1 pulse) High latitude emission Dissipated power (erg/s) Observer time (s) Naked GRB Genet, Daigne & Mochkovitch 2007 : Eg,iso = 1053 erg Ejection time (s) Initial Lorentz factor G Relativistic ejectionTotal duration = 10 s

4. Afterglow from long-lived reverse shocks RS propagation n=10 cm-3 Dissipated power (erg/s) A*=0.1 Observer time (s) Genet, Daigne & Mochkovitch 2007 : Ejection time (s) Initial Lorentz factor G Low-G tail Relativistic ejectionTotal duration = 10 s

4. Afterglow from long-lived reverse shocks Genet, Daigne & Mochkovitch 2007 : This dissipated power is a robust feature of the model (pure dynamical calculation). The shape is mainly determined by dG/dM in the ejecta. If dG/dM is irregular, bumps or wiggles car be observed (without any late activity). n=10 cm-3 Dissipated power (erg/s) A*=0.1 Observer time (s)

4. Afterglow from long-lived reverse shocks Genet, Daigne & Mochkovitch 2007 : Microphysics : Internal shocks and the reverse shock are very similar :►they propagate in the relativistic ejecta ►they are mildly relativistic We adopt the same parameters:large ee and eB, small z n=10 cm-3 Dissipated power (erg/s) A*=0.1 Observer time (s) ee=eB=1/3 ; z=0.01

4. Afterglow from long-lived reverse shocks Genet, Daigne & Mochkovitch 2007 : n = 1000 cm-3 Wind Uniform medium A*=0.5 n = 10 cm-3 A*=0.1 n = 0.1 cm-3 A*=0.05 Observer time (s) Observer time (s) Early X-ray afterglow : XRT flux (0.3-10 keV)

4. Afterglow from long-lived reverse shocks Genet, Daigne & Mochkovitch 2007 : Wind A*=0.5 The wind environment is preferred, as expected in the collapsar model. A*=0.1 A*=0.05 Observer time (s) Early X-ray afterglow : XRT flux (0.3-10 keV)

4. Afterglow from long-lived reverse shocks Genet, Daigne & Mochkovitch 2007 : A*=0.1 z = 0.003 X-rays Visible Achromatic breaks(X-rays are produced by fast-cooling electrons : the X-ray lightcurve follows the dynamics;Visible is produced by slow-cooling electrons : more complicated evolution)

4. Afterglow from long-lived reverse shocks Genet, Daigne & Mochkovitch 2007 : A*=0.1 z = 0.03 X-rays Visible Achromatic breaks(X-rays are produced by fast-cooling electrons : the X-ray lightcurve follows the dynamics;Visible is produced by slow-cooling electrons : more complicated evolution)

4. Afterglow from long-lived reverse shocks radio X-rays Genet, Daigne & Mochkovitch 2007 : Shallow decay visible z = 0.003 ; 0.03 ; 0.3 Long-term behaviour : X-rays  radio (Daigne & Mochkovitch, in preparation)

4. Afterglow from long-lived reverse shocks Afterglows produced by long-lived reverse shocks can explain : ► the plateau observed in the early X-ray afterglow ► chromatic breaks ► the shallow decay observed in some radio afterglows In addition : - Better results are obtained for a wind environment, in agreement with what we know about the progenitors of long GRBs. - The duration of the activity of the central engine remains comparable with the observed duration of the prompt GRB. - The energetics is consistent with internal shocks (consistent treatment including both phases).

5. Perspectives (1) : « short » GRBs Uniform medium n = 10 cm-3 In principle, the RS scenario can also be applied to short GRBs.► Based on what we find for long GRBs in a uniform medium,soft X-ray bumps are expected (without any need for a late activity) (work in progress)

5. Perspectives (2) : « long » GRBs Work in progress : ► Effect of irregularities in dG/dM (bumps, wiggles, flares, …) ► Late time evolution : can we detect the rise of the FS component when it becomes mildly/non relativistic ? ► Jet breaks / Constraints on energetics Note : uniform medium  tbreak  (E/n)1/3 qjet8/3 wind  tbreak  (E/A)qjet4 much stronger dependance : tbreak can be rejected at late times even for highly non-spherical ejecta. ► Predicted high-energy emission (GLAST) ?

6. Conclusions (1) Afterglows produced by long-lived reverse shocks can explain : ► the plateau observed in the early X-ray afterglow, ► chromatic breaks, ► the shallow decay observed in some radio afterglows. (2) Complex afterglow shape can be obtained without any late activity of the central engine. (3)Many more tests will be necessary to confirm that the reverse shock is responsible for the afterglow of GRBs. However, considering all the difficulties encountered by the standard external shock model, we believe it already represents a very interesting alternative.

G RB afterglows from long-lived r everse s hocks.