Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: HST/FOC observations confirm the presence of a spectral feature in the
  optical spectrum of Geminga   New optical and near-UV HST observations of Geminga are presented. When
compared with previous ground-based and HST imaging, the data confirm and
better define the presence of an emission feature centered at 6,000 Angstrom
and superimposed on the thermal continuum best fitting the extreme-UV/soft
X-ray data. This feature may be interpreted in terms of cyclotron emission
originated from a mixture of H/He ions in the neutron star's atmosphere. In the
case of pure Hydrogen, the feature wavelength would imply a magnetic field of
order 3-5 10d11 G, consistent with the value deduced from the dynamical
parameters of the pulsar. If due to a cyclotron emission, the observation of
this feature would represent the first case of an in situ measurement of the
surface magnetic field of an isolated neutron star.

20
The categories of this paper are: acc-phys
3
The categories of this paper are: physics
9
The categories of this paper are: nlin
23
The categories of this paper are: solv-int
12
The categories of this paper are: nucl-th
34
The categories of this paper are: chem-ph
31
The categories of this paper are: ao-sci
10
The categories of this paper are: q-bio
19
The categories of this paper are: eess
27
The categories of this paper are: comp-gas
30
The categories of this paper are: q-alg
21
The categories of this paper are: adap-org
13
The categories of this paper are: quant-ph
17
The categories of this paper are: q-fin
26
The categories of this paper are: bayes-an
37
The categories of this paper are: supr-con