X-ray/Optical flares in X-ray/Optical flares in Gamma-Ray BurstsGamma-Ray Bursts

Daming WeiDaming Wei ( Purple Mountain Observatory, China)( Purple Mountain Observatory, China)

Piro et al. 2005Piro et al. 2005 suggested that the flares wesuggested that the flares were the signatures of re the signatures of the onset of the forwarthe onset of the forward shockd shock. .

GRB011121

prompt prompt emissionemission

AfterglAfterglowow

Bright X-ray flaresBright X-ray flares

Z=0.36Eiso=2.8×1052erg

It is a normal long GRB.

Implication of X-ray flare light curveImplication of X-ray flare light curve(Fenimore et al. 1996; Kumar & Panaitescu 2000; Nakar & Piran 2003; (Fenimore et al. 1996; Kumar & Panaitescu 2000; Nakar & Piran 2003;

Fan & Wei 2005; Zhang et al. 2006; Wu et al. 2006)Fan & Wei 2005; Zhang et al. 2006; Wu et al. 2006)

theirtheir duration to their time of occurrenceduration to their time of occurrence

External shock internal shockExternal shock internal shock

Observations show that for many XRFs they have Observations show that for many XRFs they have δδt /t <t /t << 1, and the decay index < 1, and the decay index αα>> 2+>> 2+ββ. Therefore w. 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 casesX-ray flares: more cases

X-ray flares following X-ray flares following longlong, , shortshort, and , and high redshifthigh redshift GRBs 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 050724050724

GRB GRB 050904050904

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

Internal shockInternal shock

GRB990123

Optical flare: uncorrelated with gamma-ray emissionOptical flare: uncorrelated with gamma-ray emission

(Akerlof, et al., 1999; Sari & Piran 1999; Meszaros & Rees 1999; Fan et al. 2002)

RS emission

FS emission

GRB021211

RS FS

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

Optical flare correlated withOptical flare correlated with gamma –ray emission 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 ～ 105 ! Unreasonable large !

Optical flare correlated with X-ray flareOptical flare correlated with X-ray flare((The most distant GRB050904 at z=6.29)The most distant GRB050904 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)(Racusin, et al., 2008; Kumar & Panaitescu 2008; Fan & Piran 2008)

GRB080319B

Syn+SSC

Optical uncorrelated with gamma-ray: Optical uncorrelated with gamma-ray: arising from different internal shocks?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

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

cy. (Wei 2007)

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

Neutron-rich internal shock

The central engine of GRBs, usually believed to be a newly fThe central engine of GRBs, usually believed to be a newly formed black hole with an accreting torus, is very compact anormed black hole with an accreting torus, is very compact and hot, with a temperature no less than several MeV. Such a hd hot, with a temperature no less than several MeV. Such a high temperature exceeds the threshold value for nuclear dissoigh temperature exceeds the threshold value for nuclear dissociation, therefore GRB outflows are very likely to carry free ciation, therefore GRB outflows are very likely to carry free neutrons unless they are dominated by Poynting flux (neutrons unless they are dominated by Poynting flux (Derishe

v et al. 99; Pruet et al. 03; Beloborodov 03).).

If it is the case, then the dynamics and the emission features If it is the case, then the dynamics and the emission features may be very different from the normal fireball model (may be very different from the normal fireball model (Derishe

v 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 shockNeutron-rich internal shockss

(Fan & Wei 04, ApJL)(Fan & Wei 04, ApJL)

-rays

Regular internal shocks at ~1013 cm: powering gamma-ray emission

The beta-decay radius :

The beta-decay products of the early neutron shells

Secondary internal shocks at ~1016 cm: powering UV/optical emission

Proton shell

The secondary internal shocks are more likely to produce The secondary internal shocks are more likely to produce UV/optical flare rather than X-ray (gamma-ray) emissions UV/optical flare rather than X-ray (gamma-ray) emissions for the following reasons:for the following reasons:

They are generated at a radius much larger than that oThey are generated at a radius much larger than that of the regular internal shocks (Rf the regular internal shocks (Rint ～ 1013cm), so that ), so that the magnetic fields are much weaker than those in the the magnetic fields are much weaker than those in the regular internal shocks. regular internal shocks.

The Lorentz contrast between the merged proton shell The Lorentz contrast between the merged proton shell and the neutron shell is smaller than that of the regulaand the neutron shell is smaller than that of the regular internal shocks.r internal shocks. So the accelerated electron energy aSo the accelerated electron energy and the emission frequency are smaller.nd the emission frequency are smaller.

The naked-eye optical flash of GRB 080319B: Tracing the decaying neutrons of the outflow?

(Fan, Zhang & Wei 2008)(Fan, Zhang & Wei 2008)

Comparing with SSC modelComparing with SSC model

In the SSC model, one predicts very bright prompt GeIn the SSC model, one predicts very bright prompt GeV emission.V emission.

In neutron-rich model, the prompt GeV emission is wIn neutron-rich model, the prompt GeV emission is weaker.eaker.

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 emissionExtinction of early optical emission

GRBs lie in star-forming region, large amounts of dusGRBs lie in star-forming region, large amounts of dust may exist, so early optical emission may subject to st may exist, so early optical emission may subject to severe extinction. evere extinction.

The prompt emission and the afterglow emission may The prompt emission and the afterglow emission may destroy the dust grains, so the dust extinction may decdestroy the dust grains, so the dust extinction may decrease with time. So detailed calculation on dust destrurease with time. So detailed calculation on dust destruction and extinction is important when considering eaction and extinction is important when considering early optical emission.rly optical emission.

(Perna & Lazzati 2002; Jin & Wei 2008)(Perna & Lazzati 2002; Jin & Wei 2008)

GRB030418

Rykoff et al. 2004Jin & Wei 2008

Absence of optical flareAbsence of optical flare

For many GRBs we have not detected the optical flares,For many GRBs we have not detected the optical flares, there are several reasons, such as there are several reasons, such as

High self-absorption frequency High self-absorption frequency a. In the neutron-rich . In the neutron-rich

internal shock model, the self-absorption frequency is internal shock model, the self-absorption frequency is usually much smaller than that in the standard internusually much smaller than that in the standard internal shock.al shock.

Dust extinction. We found at least in some GRBs the dDust extinction. We found at least in some GRBs the dust extinction is large. ust extinction is large. (Chen, Li & Wei 2006; Li, Li & (Chen, Li & Wei 2006; Li, Li & Wei 2008)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)

ConclusionConclusion

Internal shock not only produce the prompt gammInternal shock not only produce the prompt gamma-ray emission, but also can produce X-ray flares a-ray emission, but also can produce X-ray flares and optical flares.and optical flares.

Prompt optical emission may include both internal Prompt optical emission may include both internal shock and external shock components.shock and external shock components.

The absence of optical flares may be due to dust exThe absence of optical flares may be due to dust extinction, large self-absorption frequency, etc..tinction, large self-absorption frequency, etc..

Inverse Compton scattering of internal shock can Inverse Compton scattering of internal shock can produce high energy (MeV – GeV) emission.produce high energy (MeV – GeV) emission.

(Vestrand, et al., 2006)

InnerEngine

Internal-External shockInternal-External shock

ExternalShock

Afterglow-rays

InternalShocks

X/optical

IC→-rays

may be neutron shell

time

flux

Peak in optical,SSC→gamma-ray

Produced bydifferent shells

Peak in gamma-ray,Low energy extension-optical

External shock

gamma-rays

optical

Thank You !Thank You !