GRB spectral evolution: from complex profile to basic structure 1 P.Minaev А.Pozanenko S.Grebenev S.Molkov Space Research Institute (IKI)

Spectral evolution of GRBs Spectrum analysis Light curve analysis (spectral lag)

“Traditional” method of spectral lag analysis Calculation of spectral lag between light curves in two different energy bands using cross-correlation analysis In most cases 4 channels of BATSE are used (25-50, 50-100, 100-300, 300-1000 keV) One can derive 3 values of spectral lag in the best case One can only get a view of general behavior of spectral evolution CCF 3-1 From Hakkila et al, 2007

“New” method of spectral lag analysis Calculation of spectral lag between light curves in two different energy bands using cross-correlation analysis. Analysis of light curves in narrow energy channels. A number and width of energy channels depend only on GRB intensity. In bright events a number of channels can be more than 20. Using new method one can investigate spectral evolution in more details.

IBIS/ISGRI and SPI experiments IBIS/ISGRI experiment Array of 128х128 CdTe elements Energy range - 15-200 keV Geometrical area of detectors – 2600 cm2 Full field of view - 30о SPI spectrometer Array of 19 Ge detectors Energy range - 20-8000 keV Geometrical area of detectors 508 cm2 Spectral resolution – 2.5 keV @ 1.3 MeV

Data analysis GRBs from catalog of SPI transients were investigated (Minaev et al., submitted). 28 most intense bursts were selected. 6 of them have multi-pulse structure with well separated pulses (events). 43 events were investigated in total.

Algorithm of analysis Building a energy-time diagram Pulse-resolving procedure in wide energy band Forming of light curves in narrow energy channels with a high temporal resolution. Cross-correlation analysis of light curves: calculation of spectral lag between the first (the softest) and other channels. Fitting a dependence “spectral lag – energy”

Results (1) For 85% events In other cases: GRB031203 GRB040323 GRB050525 For 85% events In other cases: The slope of the curve – “spectral lag index” is a new parameter characterizing spectral evolution

Results (2)

Results (3) Duration of the pulse, s

Main results New method of spectral evolution investigation was proposed. Using this method one can investigate spectral evolution in more detail. 28 GRBs were investigated in data of SPI and IBIS/ISGRI experiments. It was found that for simple structure bursts and for well separated pulses of bursts the dependence of spectral lag on energy band is described by logarithmic function lag ~ A log(E). Spectral lag index А is a new parameter characterizing spectral evolution of GRBs. For simple structure bursts negative value of spectral lag index and break in the lag – energy relation were not found. Correlation between spectral lag index and duration of the pulses was found. Short GRB obeys the correlation.

To the dichotomy of short GRBs

Extended emission of single events ~ Discovered in light curves of single events: - Konus – 10% (Mazets et al, 2002) - BATSE, Swift, HETE-2 – 1-3% (Norris, Bonnell, 2006) - SPI-ACS/INTEGRAL – 4% (Minaev et al., 2010) - BATSE – 7% (Bostanci et al., 2012) - Swift – 25% (Norris et al., 2010) Features of short GRBs with EE: - total duration ~ 100 sec - strong variability - zero spectral lag of both IPC and EE - spectrum of EE is softer - no association with SN GRB921022b ~ GRB931222 ~ GRB961225 BATSE Norris, Bonnell 2006

Extended emission of single events GRB 060221 Minaev et al 2010 (SPI-ACS) GRB 031214 Minaev et al 2010 (SPI-ACS) σ σ T, seconds T, seconds BATSE Norris, Bonnell 2006

Extended emission of single events GRB 060614 Gehrels et al 2006 (BAT/Swift) BATSE Norris, Bonnell 2006

Dichotomy problem Norris, et al. 2010. 53 short Swift GRBs were investigated EE was found in light curves of 25% short GRBs Light curve simulations showed that Swift could detect EE in a half of cases observations simulations

of averaged light curves Extended emission of averaged light curves Experiment Energy range (keV) N Duration of emission BATSE (Connaughton, 2001) 50-300 100 Up to 100 sec BATSE (Lazzati et al., 2001) 25-110 76 Up to 30 sec BeppoSAX (Montanari et al.,2005) 40-700 93 Konus (Frederiks et al., 2004) 10-750 SPI-ACS (Minaev et al, 2010) > 80 204 Up to 35 sec Norris et al. (Swift)

of averaged light curves Extended emission of averaged light curves EE was found in SPI-ACS experiment in energy range > 80 keV. 204 confirmed short GRBs from paper Savchenko et al., 2012 were analyzed. 106 unconfirmed short GRBs were also analyzed. Minaev et al (SPI-ACS) Confirmed GRBs Minaev et al (SPI-ACS) Unconfirmed GRBs 4 σ 3.5 σ

Theoretical models of Extended Emission Metzger et al., 2008: Magnetar is formed after NS-NS or NS-WD merging. It’s relativistic outflow generates Extended Emission. Confirms dichotomy. Lazzati et al., 2010: Short GRBs with EE are the long GRBs observing at large angles to jet axis. Barkov and Pozanenko, 2011: Two-jet model. IPC – wide-angle neutrino heating jet. EE - Blandford-Znajek jet. Does not confirm dichotomy.

Conclusions Extended emission was found in light curves of individual short GRBs Extended emission was found in averaged light curve of all stacked short GRBs. Extended emission significantly complicates phenomenological classification of GRBs. Several theoretical models describing EE were developed Is the Extended Emission a common property of all short GRBs?

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