X-ray/Optical flares in Gamma-Ray Bursts Daming Wei ( Purple Mountain Observatory, China)

Piro et al suggested that the flares were the signatures of the onset of the forward shock. Piro et al suggested that the flares were the signatures of the onset of the forward shock. GRB prompt emission Afterglow Bright X-ray flares Z=0.36 E iso =2.8×10 52 erg It is a normal long GRB.

Implication of X-ray flare light curve (Fenimore et al. 1996; Kumar & Panaitescu 2000; Nakar & Piran 2003; Fan & Wei 2005; Zhang et al. 2006; Wu et al. 2006) their duration to their time of occurrence External shock internal shock External shock internal shock

Observations show that for many XRFs they have δt /t > 2+β. Therefore w Observations show that for many XRFs they have δt /t > 2+β. Therefore we proposed that the flare should be attributed to the re- activity of the central engine, i.e., they were the extension of the prompt emission but in a lower energy band (see also Zhang et al. 2006), namely, the “late internal shock”.

X-ray flares: more cases X-ray flares following long, short, and high redshift GRBs X-ray flares following long, short, and high redshift GRBs (Burrows et al. 2005; Barthelmy et al. 2005; Watson et al. 2006) (Burrows et al. 2005; Barthelmy et al. 2005; Watson et al. 2006) GRB GRB

evidences for X-ray flares arising from internal shocks (From Chincarini et al. 2007) (From Chincarini et al. 2007) Internal shock

GRB Optical flare: uncorrelated with gamma-ray emission (Akerlof, et al., 1999; Sari & Piran 1999; Meszaros & Rees 1999; Fan et al. 2002) RS emission FS emission

GRB RS FS (Wei 2003; Zhang, et al. 2003; Kumar & Panaitescu 2003)

Optical flare correlated with gamma –ray emission (Vestrand et al., 2005) The prompt optical emission is the low energy tail of the gamma-ray emission. (Vestrand et al. 2005; Wei 2007) Why not SSC? Y ～ 10 5 ! Unreasonable large !

Optical flare correlated with X-ray flare (The most distant GRB at z=6.29) Optical flare correlated with X-ray flare (The most distant GRB at z=6.29) X-ray flare Optical flare The optical flare was temporally coincident with the X-ray flare. In the “late internal shock model”, The optical flare was produced by the synchrotron radiation, while the X-ray flare was produced by the SSC process. (Wei et al. 2006) (Boër et al. 2006)

(Racusin, et al., 2008; Kumar & Panaitescu 2008; Fan & Piran 2008) GRB080319B Syn+SSC

Optical uncorrelated with gamma-ray: arising from different internal shocks? Mészáros & Rees (1999) argued that the internal shock model can well explain the temporal behavior of the optical flare. We need to calculate the emission features of the internal shock.

Emission of internal shock Racusin, et al., 2008; Kumar & Panaitescu 2008 For GRB990123, Γ ～ 800, δt ～ 2s, which is quite similar to that of GRB080319B (Racusin, et al., 2008; Kumar & Panaitescu 2008), implying to large emission site and small synchrotron-self-absorption frequency. (Wei 2007)

Optical uncorrelated with gamma-ray: arising from different internal shocks? (Wei 2007)

Neutron-rich internal shock The central engine of GRBs, usually believed to be a newly formed black hole with an accreting torus, is very compact and hot, with a temperature no less than several MeV. Such a high temperature exceeds the threshold value for nuclear dissociation, therefore GRB outflows are very likely to carry free neutrons unless they are dominated by Poynting flux (). The central engine of GRBs, usually believed to be a newly formed black hole with an accreting torus, is very compact and hot, with a temperature no less than several MeV. Such a high temperature exceeds the threshold value for nuclear dissociation, therefore GRB outflows are very likely to carry free neutrons unless they are dominated by Poynting flux ( Derishev et al. 99; Pruet et al. 03; Beloborodov 03 ). If it is the case, then the dynamics and the emission features may be very different from the normal fireball model (). If it is the case, then the dynamics and the emission features may be very different from the normal fireball model ( Derishev et al. 99; Bahcall & Meszaros 00 ).

Decoupling of protons and neutrons (Bahcall & Meszaros 2000) In the beginning the n and p are coupled together. When the scattering optical depth is smaller than ~1, the n, p will decouple and the neutrons can not be accelerated any more. The decoupling may occur in the coasting phase or in the accelerating phase which depends on whether the dimensionless entropy η is below or above the critical value

Neutron-rich internal shocks (Fan & Wei 04, ApJL)  -rays Regular internal shocks at ~10 13 cm: powering gamma-ray emission The beta-decay radius : The beta-decay products of the early neutron shells Secondary internal shocks at ~10 16 cm: powering UV/optical emission Proton shell

The secondary internal shocks are more likely to produce UV/optical flare rather than X-ray (gamma-ray) emissions for the following reasons: They are generated at a radius much larger than that of the regular internal shocks (R), so that the magnetic fields are much weaker than those in the regular internal shocks. They are generated at a radius much larger than that of the regular internal shocks (R int ～ cm ), so that the magnetic fields are much weaker than those in the regular internal shocks. The Lorentz contrast between the merged proton shell and the neutron shell is smaller than that of the regular internal shocks. So the accelerated electron energy and the emission frequency are smaller. The Lorentz contrast between the merged proton shell and the neutron shell is smaller than that of the regular internal shocks. So the accelerated electron energy and the emission frequency are smaller.

(Fan, Zhang & Wei 2008) The naked-eye optical flash of GRB B: Tracing the decaying neutrons of the outflow? (Fan, Zhang & Wei 2008)

Comparing with SSC model In the SSC model, one predicts very bright prompt GeV emission. In the SSC model, one predicts very bright prompt GeV emission. In neutron-rich model, the prompt GeV emission is weaker. In neutron-rich model, the prompt GeV emission is weaker. Being at a larger emission radius, the optical variability is smoothed by the geometric effect. This is consistent with the fact that the optical light curve is smoother than the gamma-ray light curve. The time delay between the optical and gamma-ray peaks is ~1s.

Extinction of early optical emission GRBs lie in star-forming region, large amounts of dust may exist, so early optical emission may subject to severe extinction. GRBs lie in star-forming region, large amounts of dust may exist, so early optical emission may subject to severe extinction. The prompt emission and the afterglow emission may destroy the dust grains, so the dust extinction may decrease with time. So detailed calculation on dust destruction and extinction is important when considering early optical emission. The prompt emission and the afterglow emission may destroy the dust grains, so the dust extinction may decrease with time. So detailed calculation on dust destruction and extinction is important when considering early optical emission. (Perna & Lazzati 2002; Jin & Wei 2008) (Perna & Lazzati 2002; Jin & Wei 2008)

GRB Rykoff et al Jin & Wei 2008

Absence of optical flare For many GRBs we have not detected the optical flares, there are several reasons, such as For many GRBs we have not detected the optical flares, there are several reasons, such as High self-absorption frequency. In the neutron-rich internal shock model, the self-absorption frequency is usually much smaller than that in the standard internal shock. High self-absorption frequency a. In the neutron-rich internal shock model, the self-absorption frequency is usually much smaller than that in the standard internal shock. Dust extinction. We found at least in some GRBs the dust extinction is large. (Chen, Li & Wei 2006; Li, Li & Wei 2008) Dust extinction. We found at least in some GRBs the dust extinction is large. (Chen, Li & Wei 2006; Li, Li & Wei 2008) The reverse shock region might be highly magnetized. The reverse shock region might be highly magnetized. (Fan et al. 2004; Zhang & kobayashi 2005) (Fan et al. 2004; Zhang & kobayashi 2005)

Conclusion  Internal shock not only produce the prompt gamma-ray emission, but also can produce X-ray flares and optical flares.  Prompt optical emission may include both internal shock and external shock components.  The absence of optical flares may be due to dust extinction, large self-absorption frequency, etc..  Inverse Compton scattering of internal shock can produce high energy (MeV – GeV) emission.

(Vestrand, et al., 2006)

Inner Engine Internal-External shock External Shock Afterglow  -rays Internal Shocks X/optical IC→  -rays may be neutron shell

time flux Peak in optical, SSC→gamma-ray Produced by different shells Peak in gamma-ray, Low energy extension-optical External shock gamma-rays optical

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