SLIDE 1 Collaborators: Dafne Guetta (INAF – Rome Observatory; ORT Braude – Carmiel, Israel), Elena Pian (INAF – IASF, Bologna; SNS – Pisa), Lorenzo Amati (INAF – IASF, Bologna), Simonetta Puccetti (ASI – Frascati), Simone Dichiara (Univ. Ferrara)
Gamma-ray bursts as laboratories for quantum effects of gravity
Gianluca Castignani (SISSA)
Based on Castignani et al. (2014), A&A, 565, 60
SLIDE 2 Gamma-ray bursts: general properties
Current scenario: GRBs are powerful explosions,
possibly asymmetric (jets), originated by stellar collapse or binary mergers Peak isotropic luminosities 1050-53 erg s-1 at ~100 keV 10 – 1000 keV energy output 1051-54 erg A few tens are detected at energies >~20 MeV (Fermi LAT, AGILE grid) [Marisaldi 2009, Ackermann et al. 2013] Cosmological distances (~10 at z>4, Coward et al. 2013) Short time-scale observed variability (δt>~1ms)
SLIDE 3 Short vs Long GRBs
stellar collapse
(broadly accepted scenario)
binary mergers(?)
Time distribution of GRBs detected by the BATSE instrument
- n the Compton Gamma-ray Observatory.
[Credits: NASA Goddard Space Flight Center]
SLIDE 4 Gamma-ray Burst Monitor (GMB, Meegan et al. 2009); 8 keV – 40
MeV Large Area Telescope (LAT,Atwood et al. 2009);20 MeV–300 GeV The onset of the emission of the brightest
GRBs at >100 MeV is systematically delayed with respect to the start of the GBM signal by <~a few sec (or a fraction of sec, for
short GRBs) (Abdo et al. 2009a, b, c; Giuliani et al. 2010; Del Monte et al. 2011; Ackermann et al. 2011; Piron et al. 2012)
Fermi Gamma-ray space Telescope and GRBs
SLIDE 5 Two possible explanations for the delay
Astrophysical processes occur
Different emitting regions associated with GBM and LAT emissions Prompt emission
(e.g. Ghirlanda et al. 2010)
Non classical relation dispersion E ≠ pc
(Amelino-Camelia et al. 1998; Nemiroff et al. 2011)
Δt ~= ΔE / (MQGc2)*(D/c) ΔE = Ehigh - Elow
Time delays are used to set lower limits on MQG and test QG
[Amelino-Camelia+13, Guetta+13, Vasileiou+13; see also Stecker's and Granot's talks]
SLIDE 6
Goals
Determine whether the time lags really exist and possibly quantify their
uncertainties on the basis of the entire light curves
small LAT number count statistics + poor understanding of emission models (especially at LAT energies)
SLIDE 7 The Sample
1st Fermi LAT GRB catalog (35 GRBs up to 08/2011)
100MeV–10GeV fluence >0.6 10-5 erg cm-2 Test Statistic TS >460
→ GRB 080916C, 090510, 090902B, 090926A, and 110731A
SLIDE 8
The DCF (Edelson & Krolik 1988)
Originally introduced to cross correlate (unevenly sampled) light curves of AGNs But also used for GRBs (Pian+2000, Ackermann+13) Adopted here to cross correlate discrete LCs with a possible different sampling
The DCF binning partially overcomes the limitations deriving from small LAT number counts with respect to standard CCF
SLIDE 9
Simulations
N=10,000 GBM and LAT light curves randomly generated Poisson Statistics Simulated light curves pairwise compared by means of the DCF Peterson et al. (1998) approach
...the DCF maxima/minima might still be spurious and due to statistical fluctuations + more robust and accurate estimates of the DCF peak locations
SLIDE 10
GRB 080916C
Castignani+14
100 MeV – 300 GeV light curve 8-1000 KeV NaI light curve
SLIDE 11
GRB 080916C
Castignani+14
SLIDE 12
GRB 080916C
Castignani+14
SLIDE 13
GRB 090510
Castignani+14
SLIDE 14
GRB 090926A
Castignani+14
SLIDE 15
GRB 090926A
Castignani+14
>=4σ significance
SLIDE 16
Results
Single DCFs: time lags are formally consistent with zero (as e.g. Del Monte+2011)
When simulations are considered time lags different from zero are always detected!!
The presence of secondary maxima suggests the complexity of the LCs and the fact that the delays may be ascribed to intrinsic GRB physics
SLIDE 17
Summary and future work
First estimates of the gamma- vs. hard- X-rays time lags in the five brighest LAT GRBs on the basis of the full Lcs Non-zero time lags are detected in all GRBs!!! Time lags may be due to intrinsic GRB physics Larger samples are nevertheless required to understand the origin of the time delays.
