Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: VERITAS Observations of the BL Lac Object PG 1553+113 Between May 2010
  and May 2011   VERITAS, an array of imaging atmospheric-Cherenkov telescopes, studies
blazars in the energy range between ~100 GeV and ~30 TeV. With its excellent
sensitivity at these energies, and ever-deepening source exposures, VERITAS is
in a position to probe distant blazars for clear absorption signatures in their
very-high-energy gamma-ray spectra due to interactions with the extragalactic
background light (EBL). We discuss results from recent VERITAS observations of
PG 1553+113 (z > 0.4) which have resulted in the most significant
very-high-energy detection ever obtained for this source. The most recent
VERITAS spectral measurements are used to place an upper limit on the source
redshift of z < 0.5 at the 95% confidence level. Also discussed are the
prospects of using these observations, along with those of other hard- spectrum
blazars, to place constraints on the EBL.

7
The categories of this paper are: hep-th
36
The categories of this paper are: cmp-lg
30
The categories of this paper are: q-alg
4
The categories of this paper are: cond-mat
37
The categories of this paper are: supr-con
19
The categories of this paper are: eess
21
The categories of this paper are: adap-org
24
The categories of this paper are: dg-ga
10
The categories of this paper are: q-bio
34
The categories of this paper are: chem-ph
5
The categories of this paper are: gr-qc
13
The categories of this paper are: quant-ph
23
The categories of this paper are: solv-int
14
The categories of this paper are: math-ph
9
The categories of this paper are: nlin
31
The categories of this paper are: ao-sci