GRB : a canonical fake short burst L. Caito, M.G. Bernardini, C.L. Bianco, M.G. Dainotti, R. Guida, R. Ruffini. 3 rd Stueckelberg Workshop July 8–18, Pescara

Scheme of the talk Characteristics and many peculiarities of GRB Theoretical interpretation of this source within the Fireshell scenario Results obtained and future hints of investigation

2006 June 14 at UT RA j2000 = 21 l 23 m 27 s DEC j2000 = -53°02'02" Uncertainty= 3' Most interesting issues: The presence of both peculiarities of long bursts and peculiarities of short bursts The lack of any bright Ib/c supernova GRB (Mangano et al., 2007) ‏

About GRB generalities Long burst T 90 =(102±5)s 15/350 Kev (Barthelmy et al. 2006) ‏ Low redshift Z=0.125 ( Price et al ) A short, hard and multi-peaked episode (~5s) followed by a softer, prolonged emission (~100s) (e.g. Della Valle 2006) ‏ Strong hard to soft evolution in the first 400s of data (Mangano et al. 2006) ‏ Standard XRT, Optical and UV afterglow detected until 4 ks after the trigger (Mangano et al. 2006) Light curves show some achromatic breaks: at 29.7 ± 4.4 ks in the optical and UV energy band; at 36.6 ± 2.4 ks in the X-ray energy band; at 104 ks from optical to X-Ray frequencies (Mangano et al.,2007) ‏

About GRB energetics UPPER LIMIT TO THE AVERAGE ENERGY: E≤24 kev The peak energy decreased from ~300 kev during the initial group of peaks (20% of the total fluence) to ~8 kev during the BAT-XRT overlap time ( about 80 s) (Mangano et al., 2006) ‏ E iso =2.5x10 51 erg more energetic than a short but less than a typical long... E iso,1p =3.5x10 50 erg about one seventh of the total isotropic energy released! F=(2.17±0.04)x10 -5 erg*cm -2 F 1p =3.4x10 -6 erg*cm -2 γ-Ray kev F 2p =1.9x10 -5 erg*cm -2

Duration E iso intermediate value It fulfills all the empirical relations satisfied by long bursts: E p rest - E iso correlation (Amati) ‏ E γ - E iso correlation (Ghirlanda) ‏ L p, iso - E p rest c0rrelation It's close to its host galaxy GRB : is it a long...

...or a short burst? E iso intermediate value Moderate SSFR of the host galaxy until 20 times less than R Host ~2M s y -1 (L * ) -1 long GRBs with SN !!! M vHost ~-15.5 (Fynbo et al., 2007) ‏ (Della Valle et al., 2007) ‏ Spectral lags very small or absent ( Plot for the Peak Luminosity vs Time Lags for many bursts: GRB lies clearly in the region of the short GRBs. Gehrels et al., 2007) ‏

The lack of any bright Ib/c supernova In the standard scenario long duration GRBs are thought to be produced during the collapse of massive stars. A broad-lined and luminous type Ib/c core collapse SN should accompany these GRBs. For nearby GRBs ( Z≤1) the SN emission should be visible. Until now... SN 1998bw / GRB SN 2003dh / GRB SN 2003lw / GRB SN 2006aj / GRB CONFIRM THIS ASSOCIATION, UNTIL... GRB : the first clear example of a nearby long burst without SN Ib/c emission observed! The SN-component should be about 80/100 times fainter than the archetypal SN 1998bw It would be strongly fainter than any Ic supernova not associated to GRBs ever observed M v >-13.5 (Della Valle et al., 2006)

The lack of any bright Ib/c supernova A different type of SN? Very low luminosity Low velocity of expansion ( 1.0 Km*s -1 ) ‏ Type II SN Due to the collapse of a massive star with a burning energy so small that most of the 56 Ni fall back in the star. This system could give rise to a black hole. For the lack of broad undulations in a spectral sample at λ between 4500 Ǻ and 8000 Ǻ (Della Valle et al., 2006) ‏

The lack of any bright Ib/c supernova Other Hypothesis A chance superposition of a galaxy with the found redshift along the line of sight of the burst Probability: 5.6x10 -6 (Gal Yam et al., 2006) Strong dust obscuration and extinction this possibility has been ruled out by multi-wavelength observations and spectroscopy of the host: - Detection of the early afterglow in the UV energy band (Holland et al. 2006) ‏ - No significant reddening in the optical spectra of the afterglow E(B-V)= mag N H ‹2x10 20 cm -2 ( Fynbo et al., 2007) ‏ (Della Valle et al., 2006) ‏

The lack of any bright Ib/c supernova: a possible origin Origin of the burst: the merger of a Neutron Star and a massive White Dwarf (King et al., 2007) ‏ In a binary system, a WD donor is subject to instability if it have mass: M wd ≥0.66 M acc Since M wd cannot exceeds the Chandrasekar mass: M acc ≈1.4 M s It must be: M acc ≤2.1 M s The process is more likely with a neutron star accretor!

The lack of any bright Ib/c supernova: a possible origin For its characteristics lengthscale and amount of energies involved, the unstable merger of a massive WD and a NS can produce long GRBs without accompanying supernova (King et al., 2007) ‏ GRB ! and also GRB presents similar peculiarities... (T 90 =4s, Z= NO SN observed!) ‏

Data analysis of GRB We realized a detailed analysis of the observational data in: keV energy band, corresponding to the Gamma-Ray Peak of the Afterglow. Data from the Burst Alert Telescope (BAT) on the Swift satellite keV energy band, the X-Ray Afterglow. Data from the X-Ray Telescope (XRT) on the Swift satellite We plan to analyze also the Optical emission of the source.

Theoretical interpretation Brief reminder of the model The explosion of a “canonical” GRB consists of two different processes: the emission of a flash of relativistic photons when the optically thick fireshell reaches the transparency condition. This is the Proper GRB (P-GRB) the strongly hard to soft emission of energy due to the inelastic collision of the optically thin fireshell of baryonic matter with the Circum Burst Medium (CBM). This is the Afterglow phase ( Ruffini et al., 2001b, 2007a ) ‏

In this scenario, GRB o6o614 is naturally interpreted as a canonical GRB The first hard emission is the P-GRB The long, softer Gamma-Ray tail is the Peak of the Afterglow Theoretical interpretation Brief reminder of the model (Mangano et al., 2007) ‏

Theoretical interpretation Brief reminder of the model “ Both short and long GRBs originate from the gravitational collapse to a black hole.” (Ruffini et al., 2001b) CANONICAL BURSTS LONG BURSTS “GENUINE” SHORT BURSTS B<10 -5, P-GRB is dominant

Theoretical interpretation Brief reminder of the model Two free parameters describe completely the energetics and dynamics of the phenomenon: E tot e± it's the total energy of the plasma B=M b c 2 /E tot e± it represents the contribution of the baryonic matter to the total amount of energy. It oscillates between and 10 -2

Theoretical interpretation Brief reminder of the model We assume the hypothesis of the interaction of the expanding fireshell with the CBM to give rise of the total multi wavelength emission (Afterglow). This interaction takes place with fully inelastic collisions. in the distribution of the peak luminosities have a fundamental role: The variation of density of CBM, n cbm The variation of the ratio between the effective area of emission and the total area of the shell in expansion: R = A eff /A tot

The emission from the baryonic matter shell is isotropic. in the first approximation, we assume a modeling of spherical shell for the distribution of CBM: thin shell around the fireshell... we can consider just the radial coordinate of the expansion. ( Ruffini et al., 2002) With these assumptions, our theoretical fitting curves are in good agreement with the observational data. Theoretical interpretation Brief reminder of the model

The fit of the observed luminosity keV energy band E tot e± = 2.94x10 51 erg B=2.8x10 -3 E P-GRB = 1.15x10 50 erg γ Trans =346 (the fit starts after the P-GRB) ‹ n cbm ›= 4.45x10 -4 part*cm -3 ‹ R ›=1.72x10 -8 very low density......did GRB explode in a galactic halo?

The fit of the observed luminosity The role of the very low density The total energy of the peak of the Afterglow is larger than the one of the P-GRB of about one order of magnitude E iso,1p =1.15x10 50 erg P-GRB E iso =2.83x10 51 erg Afterglow The morphology of the light curve manifests an inverse trend......an high, hard P-GRB and a much lower afterglow emission! This is due to the very low density of the CBM!!!

The fit of the observed luminosity GRB : a fake short burst GRB is an example of fake short burst : the Afterglow is dominant although a peculiar low density of the environment produces an hard initial spikelike emission and a deflated tail (See Bernardini et al., 2007 on the very similar case of GRB ) ‏ LONG BURSTS < B < Afterglow is dominant FAKE SHORT BUSTS < B < Afterglow is dominant but they manifest an hard P-GRB emission GENUINE SHORT BURSTS B < P-GRB is dominant

The fit of the observed luminosity keV energy band E tot e± = 2.94x10 51 erg B=2.8x10 -3 E P-GRB = 1.15x10 50 erg γ Trans =346 n cbm = 4.70x10 -6 part*cm -3 ‹ R ›=1.27x10 -2 Making R to variate and n cbm to be constant

Density vs radius, R vs radius

Final Remarks The peculiar source GRB finds a natural interpretation in our canonical GRB scenario: two sharply different components in the phenomenon, the P-GRB and the Afterglow GRB is a fake short burst: the high value of the peak luminosity of the P-GRB compared with the lower afterglow one (although the opposite behavior of the corresponding total energies) is a consequence of the very low density of the environment Our results are consistent with the merging of a Neutron Star and a White Dwarf in a galactic halo. We are still working on this issue, as well as on the interpretation of the optical emission observed