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Understanding GRBs at LAT Energies

Understanding GRBs at LAT Energies

Robert D. Preece

Dept. of Physics

UAH

Robert D. Preece

Dept. of Physics

UAH

Mar. 3, 2006 Data Challenge II 2

Example Spectrum: GRB990123Example Spectrum: GRB990123

10-4

10-3

10-2

10-1

100

101

102

103

BATSE SD0 BATSE SD1 BATSE LAD0 BATSE SD4 OSSE COMPTEL Telescope COMPTEL Burst Mode EGRET TASC

GRB 990123

Low-Energy Indexα = -0.6 ± 0.07

- High Energy Indexβ = -3.11 ± 0.07

νFν Peak EnergyEp = 720 ± 10 keV

10-8

10-7

10-6

0.01 0.1 1 10 100

Photon Energy (MeV)

10-4

10-3

10-2

10-1

100

101

102

103

BATSE SD0 BATSE SD1 BATSE LAD0 BATSE SD4 OSSE COMPTEL Telescope COMPTEL Burst Mode EGRET TASC

GRB 990123

Low-Energy Indexα = -0.6 ± 0.07

- High Energy Indexβ = -3.11 ± 0.07

νFν Peak EnergyEp = 720 ± 10 keV

10-8

10-7

10-6

0.01 0.1 1 10 100

Photon Energy (MeV)

Mar. 3, 2006 Data Challenge II 3

OT Synchrotron ‘Line of Death’

Cooling ‘Line of Death’

Kaneko et al. 2006

~8900 spectral fits from 350 bright BATSE GRBs

Spectral Observations by BATSE: αSpectral Observations by BATSE: α

‘Band’ Function:

Synchrotron emission constrains alpha < –2/3

Significant fraction of spectra fail

If cooling is taken into account, there is a second limit

‘Band’ Function:

Synchrotron emission constrains alpha < –2/3

Significant fraction of spectra fail

If cooling is taken into account, there is a second limit

Mar. 3, 2006 Data Challenge II 4

~ 6 Decades of full energy coverage

Precise determinationof high-energy power

law index

Good photon countingstatistics at highest

energies

LAT will be very good at localization; all it needs is one high-energy photon!

Expected Spectral Performance of GLAST

Expected Spectral Performance of GLAST

GBM NaI

GBM BGO

LAT

Mar. 3, 2006 Data Challenge II 5

GRB 990123 Simulation: LAT + GBMGRB 990123 Simulation: LAT + GBM40

30

20

10

BATSE24–120 keV

GRB 990123

-4

-3

-2

100806040200

( )Time Since BATSE Trigger s

-1

-0.5 ( )Assumed BATSE LAT Fit + GBM LAT Joint Fit

1

0.5

Mar. 3, 2006 Data Challenge II 6

GLAST GRB Science: EPeakGLAST GRB Science: EPeak

• Narrow distribution: GLAST will determine upper limit: esp. for COMP model

• Some fits unbounded: (beta > –2) Epeak in LAT range

• Red-shift? Cosmological + intrinsic

• GLAST will verify Ghirlanda relation (Swift has limited bandpass)

Kaneko et al. 2006

GMB + LAT Coverage

BATSE

Mar. 3, 2006 Data Challenge II 7

GLAST GRB Science: βGLAST GRB Science: β• β > –2 can not continue forever: infinite energy!

• No high-energy spectral cut-off has been observed

• GLAST will be able to observe 10 keV to ~300 GeV: long baseline

• Low deadtime allows good photon statistics (c.f. Hurley ‘94)

• No High-Energy (NHE) bursts exist (no emission > 300 keV)

Kaneko et al. 2006

Mar. 3, 2006 Data Challenge II 8

Spectral Observations by BATSE: βSpectral Observations by BATSE: β 1st order Fermi: Electrons are

accelerated by successively reflecting off of 2 converging fluids; magnetic field conveys them across the boundary

PIC simulations of relativistic shocks unanimously predict a constant electron power-law index ~ -2.4, or equivalent photon spectral index ~ -2.2

BATSE observations of high-energy photon power-law indices clearly contradicts this

However, if there were no acceleration, cooling would take place much faster than observed

1st order Fermi: Electrons are accelerated by successively reflecting off of 2 converging fluids; magnetic field conveys them across the boundary

PIC simulations of relativistic shocks unanimously predict a constant electron power-law index ~ -2.4, or equivalent photon spectral index ~ -2.2

BATSE observations of high-energy photon power-law indices clearly contradicts this

However, if there were no acceleration, cooling would take place much faster than observed

1st order Fermi

Power Law Decay

Mar. 3, 2006 Data Challenge II 9

EGRET Observation of 940217EGRET Observation of 940217 Persistent hard emission lasted nearly 92 minutes after the

BATSE emission ended. A single 18 GeV photon is observed at ~T+80 min: hardest

confirmed event from any GRB. We have no idea what the spectrum was, nor how it

evolved with time (given EGRET’s deadtime)!

Persistent hard emission lasted nearly 92 minutes after the BATSE emission ended.

A single 18 GeV photon is observed at ~T+80 min: hardest confirmed event from any GRB.

We have no idea what the spectrum was, nor how it evolved with time (given EGRET’s deadtime)!

Hurley et al. 1994, Nature

Mar. 3, 2006 Data Challenge II 10

GRB 941017: Gonzalez et al. (2003)GRB 941017: Gonzalez et al. (2003)

BATSE

Continuum only

EGRET-TASC:Continuum+PL

Hard Gamma-ray excess

Mar. 3, 2006 Data Challenge II 11

GLAST and NHE BurstsGLAST and NHE Bursts

GRB970111: no-high-energy GRB Initial, very hard, (alpha ~ +1)

portion smoothly transitions to classical GRB

First 6 s spectra consistent with BB

BB kT falling with increasing flux: fading fireball

May be best example of initial fireball becoming optically thin

LAT can determine HE emission with good statistics

LAT upper limits on normal bursts will still provide good science

GRB970111: no-high-energy GRB Initial, very hard, (alpha ~ +1)

portion smoothly transitions to classical GRB

First 6 s spectra consistent with BB

BB kT falling with increasing flux: fading fireball

May be best example of initial fireball becoming optically thin

LAT can determine HE emission with good statistics

LAT upper limits on normal bursts will still provide good science

GRB970111

Mar. 3, 2006 Data Challenge II 12

GLAST and Quantum GravityGLAST and Quantum Gravity

If certain QG theories are correct, very high energy (VHE) photons will be delayed: If Spacetime is corrugated, photon travels ‘farther’ Lower energy limit depends somewhat upon theory Observation is quite tricky:

VHE photon count rate must be actually observable Must assume a particular relation between energy and time

within a GRB: A relation has already been observed: spectral lag - Norris, et al. Lag is somewhat correlated with luminousity

Chance coincidence: bright, very hard GRB with very sharp leading edge pulse - increases with mission lifetime

If certain QG theories are correct, very high energy (VHE) photons will be delayed: If Spacetime is corrugated, photon travels ‘farther’ Lower energy limit depends somewhat upon theory Observation is quite tricky:

VHE photon count rate must be actually observable Must assume a particular relation between energy and time

within a GRB: A relation has already been observed: spectral lag - Norris, et al. Lag is somewhat correlated with luminousity

Chance coincidence: bright, very hard GRB with very sharp leading edge pulse - increases with mission lifetime