Daniel KocevskiKavli Institute for Particle Astrophysics and Cosmology

Stanford University

On behalf of the Fermi collaboration

On The Lack of LAT Detected GRBs

Daniel Kocevski - Fermi Symposium, May 9th-12th 2011

Fermi GRB Detections

GBM Detected GRBs (until March 1st): 620 - Blue

GRBs in LAT FOV: 288 (46%) - Green

LAT Detected GRBs (>100 MeV): 23 (8%) - Red

LAT LLE Only Detected GRBs: 5 (2%)

Preliminary

Daniel Kocevski - Fermi Symposium, May 9th-12th 2011

Expected Detection RateTake BATSE spectra, extrapolate and compare to actual detection rate

Predicted: 9.3 GRBs/year > 100 MeV

Observed: 8.0 GRBs/year > 100 MeV

This includes GRBs with extra components

We are seeing fewer GRBs then predicted, especially at GeV energies

Possible explanations

High energy emission is suppressed

Extrapolations are uncertain

Extra components must be rare!

PRELIMINARY

Omodei’s Presentation

Daniel Kocevski - Fermi Symposium, May 9th-12th 2011

Spectral Fits

Fit NaI+BGO spectrum from 8 keV to 40 MeV in RMFIT

Estimate the expected flux in the 100 MeV to 10 GeV range

Compare upper limits to the expected LAT flux

LATGBM

Daniel Kocevski - Fermi Symposium, May 9th-12th 2011

Spectroscopic SampleBright BGO Sample:

GRBs with 70 cts/s in BGO in LAT FOV: 92

“LAT Dark GRBs” (i.e. no LAT detection)

“Gold” Sample:

Number of bright BGO GRBs with ΔBeta < 0.5: 30

Expected LAT Flux

Extrapolate β to find expected LAT flux

We use the full covariance matrix to estimate beta error

Daniel Kocevski - Fermi Symposium, May 9th-12th 2011

Expected Flux Comparisons

15 of the 30 GRBs have expected photon flux that exceed the T90 LAT photon flux upper limit

Same for the expected photon fluence and LAT fluence upper limit

10!5 10!4 10!3

LAT Flux Upper Limits 95% (photons cm!2 s!1) ! T100

10!7

10!6

10!5

10!4

10!3

10!2

10!1

Expe

cted L

AT F

lux (p

hoton

s cm!2

s!1)

0.0001 0.0010 0.0100 0.1000LAT Fluence Upper Limits 95% (photons cm!2)

10!5

10!4

10!3

10!2

10!1

100

Expe

cted

LAT

Fluen

ce (p

hoto

ns cm

!2)

Preliminary Preliminary

Daniel Kocevski - Fermi Symposium, May 9th-12th 2011

Beta vs Ratio

High Energy Spectral Index ( )

Expecte

d L

AT

Flu

x / L

AT

Upper

Lim

it

GRBs with values of β > -2.2 typically exceed the LAT upper limits

Preliminary

Daniel Kocevski, Annapolis, Nov 1st-4th 2010

Lorentz Factor Distribution3 LAT detected bursts have Γmin > 800

For 6 LAT dark GRBs:

Δt ~ 0.01s and 1 < z < 5

If we assume Ec ~ 100 MeV

Γmax ~ 50-600

LAT bursts may represent the high end of the Γ distribution

LAT dark bursts may represent the low end of the Γ distribution

0 1 2 3 4 5Redshift

0

200

400

600

800

1000

1200

1400

! ("

max =

20 M

eV, t

var =

0.1s

)

090902B

090510

080916C

090926

LAT Detections !minLAT Non!detections !max

Preliminary

Daniel Kocevski - Fermi Symposium, May 9th-12th 2011

Expected LAT Flux

0.1 1.0 10.0 100.0GBM Flux 20!2000 KeV (photons cm!2 s!1)

10!8

10!6

10!4

10!2

100

Expe

cted

LAT

Flux

0.1

!10

GeV

(pho

tons

cm!2

s!1)

Bright BGO SampleKaneko et al. 2006

Simulated BATSE Sample

LAT Detected Bursts

Simulate a population of GRBs using BATSE Epk, α, and β distributions

Roughly 65-75% of a simulated BATSE sample have expected flux values that exceed the median 30s LAT sensitivity

High energy extrapolations must be misleading in order to explain the number of LAT “dark” bursts

Preliminary

Daniel Kocevski - Fermi Symposium, May 9th-12th 2011

Joint GBM+LAT Spectral Fits

Very different beta value if we include LAT limits in the spectral fits.

For bright BGO sample, median β = -2.2 -> -2.5

Which fit is statistically preferred?

β = -1.8Cstat = 408.55

DOF = 370

β = -2.2Cstat = 416.52

DOF = 380

GBM Only Fit GBM + LAT Fit

GBM OnlyBand Fit

GBM+LATBand Fit

We cannot statistically compare these two scenarios using ∆C-Stat because we are using different data sets for the two fits

Model Comparisons?

beta = -2.10 beta = -2.54

GBM+LATBand + Step Fit

GBM+LAT FitBand + Cutoff

We have to compare the ∆C-stat values for the Band only, Band+Step Function, and Band+Cutoff fits to the same GBM+LAT data

beta ~-2.10beta = -2.10

Nested Model Comparisons

∆C-Stat

Band vs. Band + Step Function, change of 1 degree of freedom

Only 6 of 30 (20%) GRBs result in ΔC-Stat > 10

We can reject the null hypothesis (the Band model) only for these bursts

!5 0 5 10 15 20 C!Stat

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Band fit to GBM+LAT data results in softer beta values compared to fits to GBM data alone

Possibly consistent with suggestions by Hascoet (this conference)

!3.0 !2.8 !2.6 !2.4 !2.2 !2.0 !1.8 !1.6High Energy Spectral Index ( ! )

0

2

4

6

8

10Band: GBMBand: GBM+LAT

Preliminary

ΔC-Stat Correlations

Correlation between over-prediction of LAT flux and ΔC-Stat. Likewise, anti-correlation between σβ and ΔC-Stat

Statistical errors on β do not reflect the true, systematic, uncertainty in the parameter estimation

0.001 0.010 0.100 1.000 10.000 100.000 1000.000! C!Stat

0.001

0.010

0.100

1.000

10.000

100.000

1000.000

Expe

cted L

AT F

lux / L

AT U

pper

Limi

t

3" Fit Improvement

0.001 0.010 0.100 1.000 10.000 100.000 1000.000! C!Stat

0.01

0.10

1.00

High

Ene

rgy S

pectr

al Ind

ex U

ncer

tainty

(")

3# Fit Improvement

Preliminary

Preliminary

Daniel Kocevski - Fermi Symposium, May 9th-12th 2011

ConclusionsGBM to LAT extrapolations can be misleading!

Statistical uncertainties may not fully reflect the systematic uncertainties and cross-correlations among the spectral parameters

ΔC-stat for a nested model comparison is the proper method of distinguishing between fits of increasingly complexity

24 (80%) GRBs in our spectroscopic sample are consistent with having a steeper beta value

6 of 30 (20%) prefer a spectral break

Two of these bursts show this break in the LLE selection

Our previous estimates of the β distribution may be biased

Use of future LLE data may help distinguish between cutoffs and softer β

Simulation TestsGRB 101113483

GBM Only: β = -1.8

GBM+LAT: β = -2.2

GBM+LAT+Step: β = -1.8

ΔC-stat ~ 5

Simulated GRB: β = -1.8

GBM Only: β ~ -1.8

GBM+LAT: β ~ -2.2

GBM+LAT+Step: β = -1.8

ΔC-stat ~ 25Nested model comparison can distinguish the difference between the two scenarios, even though the different beta values are statistically excluded