2nd Fermi Symposium – Washington DC ‐ Tuesday, November 3rd, 2009

• GBM has an eﬀecHve area 1/36 of its famous predecessor BATSE=> GBM required bright events

BUT• Even if smaller, GBM/BGO detectors are much thicker
12 Iodine Sodium with higher z.
detectors (NaI:=> Much beWer photo‐peak eﬃciency and eﬀecHve area
8 keV to 1 MeV)
above 1 MeV :BATSE maximal energy ~10 MeV.
GBM maximal energy ~40 MeV.
=> Spectroscopy of hard bursts possible with GBM.
2 Germanate Bismuth detectors• GBM has :(BGO: 200 keV to 40 MeV) much more available on‐board memory.
 a much higher telemetry downlink budget.
 a beWer data design for Time Tag Events (TTE).
=> Data available with a Hme resoluHon down to 2 μs,
128 spectral channels from 8 keV to 40 MeV and from‐30 to 300 s.
=> Ideal for the study of short events like short GRBs,
TGFs (see Michael Briggs talk and Jerry Fishman posteron TGFs) and SGRs (see Chryssa Koveliotou and Ersin
Gogus talks)
2/10Sylvain Guiriec – Fermi Symposium 2009• About 68 short GRBs detected with GBM
since since July, 2008.
• Short GRBs correspond to ~20% of thetotal GRBs detected with GBM• T50 < 1s

Sample criteria for this analysis• Fluence > 2e‐6 erg/cm2 => bright enough for Hme‐resolved spectroscopy with GBM=> This selecHon results in 3 brightest and hardest short GRBs
detected with GBM so far:
• GRB 090227B• GRB 090228• GRB 090510
In all the following, spectral analysis performed from 8 keV to 40 MeV.
Sylvain Guiriec – Fermi Symposium 20093/10

• Various model tested :Standard model before the Fermi Era

 Power‐law with exponenHal decay(comptonized)
 Band funcHon
 Comptonized+PL
 Band+PLAddiHonal component onen present in Fermi’s GRB spectra• Fit performed with the analysis package Rmﬁt

• Choice of the best model : staHsHcal improvement of theCastor Cstat value between models according to the addiHonal
degree of freedom
4/10
Sylvain Guiriec – Fermi Symposium 2009

Band (Cstat: 699/607 dof) Comptonized + PL (Cstat: 689/606 dof)
NaICount spectrum

BGO

υFυ spectrum

The addiHonal component dominates the standard Band funcHon at both low and high Energy
5/10
Sylvain Guiriec – Fermi Symposium 2009

Name

Model

Parameters of the Band func7on
Epeak (keV)

GRB 090227B Compt

2227

Band2116€
Compt+PL

GRB 090510+0.02

1995 €-0.36+0.05€-0.36+0.05

-0.59

+0.03

+97+96

1947

Compt

€

862

Band

€

+205

€

+50

Compt+PL860
722

Band+PL€

723€Compt€Band€+47−42+45−41€-3.44+0.10€-0.77+0.02-0.75+0.02€+290−278€
Compt+PL3731 €Band+PL€3695 €

+265

−246

+0.58

€

+0.05

-1.51

−0.80

−0.04

€
€

-3.77

+0.09

-1.63

−0.15

+1.14
−∞

-1.64

+0.03
−0.02

€

-2.80

795/727795/726€

922/851€911/850

−0.02

−0.02

686/605
813/728

€

+0.64

−0.64

-4.74

689/606
813/729

€

−0.10

−0.10

+0.06

−0.06

+0.11

-0.24

−237

4383

€

+0.03

-0.59
-0.23

€

+255

4797

-1.37

€

−0.03

−49

699/607

−0.05

−0.03

−47+0.27

−0.39

−0.13

−98

+52

-3.35

−0.02−91

CastorCstat / dof
706/608+0.02-0.50

Band+PLindex−0.02−95€

β-0.52−85€

GRB 090228+90α

PL+0.20−0.28+0.08-0.51

-1.35

−0.07

+0.04

−0.04

897/849

897/848-1.38€(GBM+LAT) Band+PL-0.58-2.83 €-1.623936 €€€ component in these 3 GRBs
• Comp + PL is systemaHcally
prefered€ => Existence€of an addiHonal• Value of the index of the€ these bursts€€ addiHonal €PL similar in all• Higher Epeak values than for long GRBs (=> quesHon during Ehud talk : short vs long GRBs with GBM)
• Steep β values (which conﬁrm the comment from Guido to Nicola in the previous talk)
6/10• Guiriec
GBM
onlyresults
Sylvain– FermiSymposium2009 and GBM+LAT ﬁts are consistent for GRB 090510+284−265

€

+0.08

-0.51

−0.08

+0.75

+0.04

−∞

−0.03

-3.65

+280

+0.06

+0.14

+0.03

−260

−0.05

−0.20

−0.03

The existence of addiHonal components in these 3 GRBs is
consistent with LAT data
See Poster Valerie Connaughton P3‐171 (Wed‐Thur)

7/10
Sylvain Guiriec – Fermi Symposium 2009

GRB 090227B

GRB 090510

GRB 090228

10000• Similar to what we observed in long GRBs
but contracted in Hme and shined to higher
energy (Ford et al.).
• Epeak tracks the light curves like for the longburst.
• The hardest part is not always at thebeginning.
• The most intense peaks are not always thehardest.
8/10

Sylvain Guiriec – Fermi Symposium 2009α < ‐3/2 :e‐ synchrotron
emission valid
for fast coolingα < ‐2/3 :e‐ synchrotron
emission valid
for slow cooling• α nearly systemaHcally violates thesynchrotron line of death of ‐2/3.

• Epeak evolves over an incredible
broad range of energy

9Sylvain Guiriec – Fermi Symposium 20099/10

Time-integrated spectra•

Time-integrated spectra are best fit with Band+Power law model=> Additional component : electron SSC or hadronic emission•The additional power law dominates the standard Band spectrum at low andhigh energy=> low energy extension of the PL challenges all the models•The hardest short GRBs have Epeak values well above those of the hardest
long GRBs.
Fine time-resolved spectroscopy•

Short GRBs have similar light curves than long GRBs but contracted in timeand shifted towards higher energy, and appear to have steeper β.

•Epeak tracks the light curves and spreads over a broad energy range=> consistent with the electron synchrotron models in the internal
shocks context (AcceleraHon and cooling of the electrons leading to ahardening with the peak rise then a sonening of the burst during the pulse decay)•α in the time resolved spectroscopy violates the synchrotron limits ( Frederic
Daigne talk: possible answer with IC ?)10/10
Sylvain Guiriec – Fermi Symposium 2009

Band (Cstat: 699/607 dof) Comptonized + PL (Cstat: 689/606 dof)
NaI

Count spectrum

BGO

υFυ spectrum

The addiHonal component dominates the standard Band funcHon at both low and high Energy
12/10
Sylvain Guiriec – Fermi Symposium 2009

Band (Cstat: 813/728 dof) Comptonized + PL (Cstat: 795/727 dof)
NaI

Count spectrum

BGO

υFυ spectrum

The addiHonal component dominates the standard Band funcHon at both low and high Energy
13/10
Sylvain Guiriec – Fermi Symposium 2009

Band (Cstat: 911/850 dof) Comptonized + PL (Cstat: 897/849 dof)
NaICount spectrum

BGO

υFυ spectrum

The addiHonal component dominates the standard Band funcHon at both low and high Energy
14/10
Sylvain Guiriec – Fermi Symposium 2009