Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: REM observations of GRB 060418 and GRB 060607A: the onset of the
  afterglow and the initial fireball Lorentz factor determination   Gamma-ray burst (GRB) emission is believed to originate in highly
relativistic fireballs. Currently, only lower limits were securely set to the
initial fireball Lorentz factor Gamma_0. We aim to provide a direct measure of
Gamma_0. The early-time afterglow light curve carries information about
Gamma_0, which determines the time of the afterglow peak. We have obtained
early observations of the near-infrared afterglows of GRB 060418 and GRB
060607A with the REM robotic telescope. For both events, the afterglow peak
could be clearly singled out, allowing a firm determination of the fireball
Lorentz of Gamma_0 ~ 400, fully confirming the highly relativistic nature of
GRB fireballs. The deceleration radius was inferred to be R_dec ~ 10^17 cm.
This is much larger than the internal shocks radius (believed to power the
prompt emission), thus providing further evidence for a different origin of the
prompt and afterglow stages of the GRB.

26
The categories of this paper are: bayes-an
11
The categories of this paper are: hep-lat
4
The categories of this paper are: cond-mat
0
The categories of this paper are: hep-ph
32
The categories of this paper are: atom-ph
9
The categories of this paper are: nlin
33
The categories of this paper are: plasm-ph
25
The categories of this paper are: patt-sol
23
The categories of this paper are: solv-int
5
The categories of this paper are: gr-qc
24
The categories of this paper are: dg-ga
31
The categories of this paper are: ao-sci
34
The categories of this paper are: chem-ph
17
The categories of this paper are: q-fin
13
The categories of this paper are: quant-ph
12
The categories of this paper are: nucl-th