Is GRB050509b a genuine short?

Gustavo de Barros,

Maria Grazia Bernardini,

Carlo Luciano bianco,

Roberto Guida,

Remo Ruffini.

Characteristics---------------------------------------------------------------

This GRB is important because is the first short GRB with observed afterglow

- Z = 0.2248

- duration of BAT data: 40 ms

fig. from Nature,Vol 437, 851

Short scale + afterglow--------------------------------------------------------------------------------------------------

Gehrels et al. (2005) say that the data observed by BAT are typical of a short burst.

But there are also (after 100s from BAT data) observations from XRT.

So, how classify thisGRB?

fig. from Nature,Vol 437, 851

Fireshell model------------------------------------------------------------------

In the fireshell model canonicals GRBs have two important emission phases:

1- when the plasma reaches the moment of transparency (decouple of photons) there is the emission we call 'proper-GRB' (P-GRB).

2- After this, the fireshell (formed now mainly by barions) reaches the CBM (circumburst medium) and emits energy by inelastic collisions. This emission is called 'afterglow'.

Classification of grbs----------------------------------------------------------------------------------------

Classification is done using the B parameter: B = Mc2/E

B gives the ratio between the energy emitted in the P-GRB and the energy emitted in the afterglow. If there is more energy emmited in the P-GRB the GRB is a short one, else, will be a long one. But there is also the 'fake short': The GRB has more energy emmited in the afterglow, but the light curve gives us the impression that the P-GRB is more energetic.

What about 050509b?-----------------------------------------------------------------------------------------------

We analysed GRB050509b to see it's classification. It could be:

a fake short; just a long with an observed P-GRB (almost

called 'precursor' in the literature) a genuine short.

First Analysis ------------------------------------------------------------

In the first analisys we identifyed the prompt emission (data from BAT) with the P-GRB. In this case these data would be the 'short part' of the GRB and the XRT-data the long part. The parameters are:

B=1.6 x 10-3

ρ = 1 (#/cm3) R = 1 x 10-11

E=1.48x1048erg

The first analisys show us that this GRB is a long one because almost 80% of the total energy is emmitted in the afterglow. The point of view (in literature) that this is a 'short' with afterglow, is nothing more then our canonical picture for GRBs. The first part is the P-GRB which is 'short' in time, accompained by the afterglow.

Second analysis------------------------------------------------------------------------

In the second analisys we want to see if is possible to do a fit in which the GRB will be a genuine short. To do this we identifyed the data from BAT (prompt emission) with the peak of the afterglow. The parameters are:

B = 1.1 x 10-4

0.9 ( #/cm3) ; r > 3 x 1015 cm ρ = 0.9 x 10-2 ( #/cm3) ; r < 3 x 1015 cm

R = 6 x 10-6

E = 3.2 x 1049 erg EP-GRB

= 2.1 x 1049

Second analysis-----------------------------------------------------------------------

In the second analysis all the observed data are from the emission of the afterglow. The P-GRB is too hard to be observed, we expect a peak emission for it, about 850 kev. Since the abovethreshold for BAT is 350 kev, it wasn't observed.

The second analisys says that this GRB would be a genuine short. Because it has 65% of the total energy emmitted in the P-GRB. But it is important to note that we cannot see the 'short' characteristic of it. The short timescale data observed by BAT are part of the 'long' domain of the GRB (the afterglow).

The B parameter---------------------------------------------------------------

1o analysis: black line - B = 1.6 x 10-3. Long GRB

2o analysis:

yellow line

B = 1.1 x 10-4.

Genuine short GRB

Conclusions-----------------------------------------------------

This GRB may be a genuine short one, but has also the possibility to be long.

We expect hard emission in the begining phases (850 kev in this case).

We need data also in this range to remove ambiguity (in possible newer sources), and to constraint the parameters of the model.

We hope that Glast satellite will help us to solve these problems.