Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Discovery of Very High Energy gamma - ray emission from the extreme BL
  Lac object H2356-309 with H.E.S.S   The understanding of acceleration mechanisms in active galactic nuclei (AGN)
jets and the measurement of the extragalactic-background-light (EBL) density
are closely linked and require the detection of a large sample of
very-high-energy (VHE) emitting extragalactic objects at varying redshifts. We
report here on the discovery with the H.E.S.S. (High Energy Stereoscopic
System) atmospheric-Cherenkov telescopes of the VHE Gamma-ray emission from
H2356 - 309, an extreme BL Lac object located at a redshift of 0.165. The
observations of this object, which was previously proposed as a
southern-hemisphere VHE candidate source, were performed between June and
December 2004. The total exposure is 38.9 hours live time, after data quality
selection, which yields the detection of a signal at the level of 9.0$\sigma$
(standard deviations) .

3
The categories of this paper are: physics
17
The categories of this paper are: q-fin
34
The categories of this paper are: chem-ph
12
The categories of this paper are: nucl-th
9
The categories of this paper are: nlin
0
The categories of this paper are: hep-ph
36
The categories of this paper are: cmp-lg
27
The categories of this paper are: comp-gas
29
The categories of this paper are: funct-an
7
The categories of this paper are: hep-th
28
The categories of this paper are: alg-geom
22
The categories of this paper are: chao-dyn
20
The categories of this paper are: acc-phys
25
The categories of this paper are: patt-sol
13
The categories of this paper are: quant-ph
37
The categories of this paper are: supr-con