Reverse Shocks and Prompt Emission

Reverse Shocks and Prompt Emission Mark Bandstra Astro 250 050926

Where are we? • During the intermediate “coasting” phase • Internal shocks create the actual GRB emission • External forward shocks into the ISM create the afterglow emission long after the GRB • A reverse “external” shock forms when the shell hits the ISM • Emission from this shock is in optical/IR/radio and is within seconds of the GRB • The reverse shock converts the KE of the shell into internal energy, allowing it to decelerate into the Blandford-McKee solution (Brian’s talk)

Why is the reverse shock important? • Allows confirmation of internal/external shocks scenario • Allows measurement of initial Lorentz factor of shell expansion, which the GRB and later afterglow cannot • Allows us to probe the magnetic field in the shell

Reverse Shock: 1-D Cartoon Expanding shell ISM (at rest)

Reverse Shock: 1-D Cartoon Expanding shell ISM

Reverse Shock: 1-D Cartoon Expanding shell ISM Reverse shock crosses the shell

Hydrodynamics Region 4: Unshocked shell Region 3: Shocked shell Region 2: Shocked ISM Region 1: Unshocked ISM (at rest) Forward shock Reverse shock Contact discontinuity

Hydrodynamics: Simulation Region 4 Region 3 Region 2 Region 1 slows heats compacts (from Kobayashi & Sari 2000)

Hydrodynamics: Assumptions Region 4: Unshocked shell Region 3: Shocked shell Region 2: Shocked ISM Region 1: Unshocked ISM Also, CD means p2=p3 and 2=3

Hydrodynamics: Equations Region 4: Unshocked shell Region 3: Shocked shell Region 2: Shocked ISM Region 1: Unshocked ISM (The symbol is 3 in the frame of 4, and it may be ~1 or >>1 )

Hydrodynamics: Solution • Solution depends only on f=n4/n1, n1, and  • Two regimes of the solution: • 2 >> f (ultrarelativistic reverse shock) • f >> 2 (“Newtonian” reverse shock) • The shock begins in the Newtonian regime and may end up relativistic (we will look at this soon)

Crossing Time • How long does it take the shock to travel from the CD to the edge of the shell (in obs. frame)? • General formula: • For both cases, the crossing time is about the same:

Distance Scales • l: Sedov length • R: forward shock sweeps up M/ of ISM (shell decelerates) • R: reverse shock crosses shell • RN: transition from Newtonian to relativistic reverse shock

Distance Scales: Two cases • R < R < RN: Newtonian • Shock crosses shell before transition to the relativistic case can occur • But most of these become mildly relativistic by the end of propagation, with R R  RN • RN < R < R: Relativistic • Transition occurs before crossing • Apparently, we only expect significant emission from a relativistic reverse shock…

Light Curve: Energetics • First of all, what is the characteristic energy of the reverse shock, compared with the forward shock? • Relativistic reverse shock case: • Find f at R: • Then the gamma factors at R are:

Light Curve: Energetics • Forward shock is from region 2:

Light Curve: Energetics • Forward shock is from region 2: X-rays!!!

Light Curve: Energetics The reverse shock emission is from region 3:

Light Curve: Energetics The reverse shock emission is from region 3: IR !!! (can in general be as high as optical, since sensitive to B and e)

Light Curve: Scaling relations • One important scaling relationship: t-2 after the shock crosses • From the Blandford-McKee blast wave: • Spectral properties:

Light Curve Examples In all four cases, flux fades by ~ t-2 after the critical time (from Kobayashi 2000)

Light Curve: Combined Afterglows (from Zhang, et al. 2003)

Light Curve: Combined Afterglows Reverse shock component Forward shock component (from Zhang, et al. 2003)

Observations: GRB990123 (ROTSE images, from Akerlof, et al. 1999) • Observation starting 22 sec after BATSE trigger • Peaked at 9th magnitude 50 sec after trigger

Observations: GRB990123 ROTSE lightcurve with GRB inset, from Akerlof, et al. 1999 Optical flash is not simply low-frequency extension of the GRB!

Observations: GRB990123 An interpretation of the data by Sari & Piran 1999 There was also a radio detection ~ 1 day after trigger which matched the expected flux in that band

Observations: GRB990123 Good! t-2 ! An interpretation of the data by Sari & Piran 1999 There was also a radio detection ~ 1 day after trigger which matched the expected flux in that band

So Observations have been a piece of cake, right? • Prompt optical emission only seen in about four other GRBs • GRB041219a • May have seen the t-2 decrease AND the t1/2 rebrightening • But, optical light curve tracks the GRB light curve! • Strange IR feature perhaps related to central engine

GRB041219a vs. GRB990123 Seems to be a definite relationship here! Optical lightcurves superimposed on gamma-rays Not an extension of the GRB (Vestrand, et al. 2005)

GRB041219a: Other Weirdness (Blake, et al. 2005)

GRB041219a: Other Weirdness t+1/2 ? t-2 ? (Blake, et al. 2005)

GRB041219a: Other Weirdness What is this?! t+1/2 ? t-2 ? (Blake, et al. 2005)

Observations: Other Worries • People are worried about the lack of more optical flashes • So much so, that they think that there is some physical process at work to suppress these afterglows • “Although host extinction can explain the properties of some bursts, and the natural range of burst energies and distances can explain some others, … these considerations alone cannot explain the full diversity of the burst population. Instead, one or more mechanisms must act to suppress the optical flash and provide a significantly enhanced efficiency of the prompt gamma-ray emission for some bursts.” (Roming, et al. 2005)

Other Applications • Determining initial Lorentz factor  • The peak time of the light curve is sensitive to 3, and therefore we can estimate 3 • Example: For GRB990123,  270, n1  0.2 cm-3 • Measuring B and e • Spectral properties also sensitive to these parameters

Hope you enjoyed the ride

Reverse Shocks and Prompt Emission