Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Evidence for vacuum birefringence from the first optical polarimetry
  measurement of the isolated neutron star RX\, J1856.5$-$3754   The "Magnificent Seven" (M7) are a group of radio-quiet Isolated Neutron
Stars (INSs) discovered in the soft X-rays through their purely thermal surface
emission. Owing to the large inferred magnetic fields ($B\approx 10^{13}$ G),
radiation from these sources is expected to be substantially polarised,
independently on the mechanism actually responsible for the thermal emission. A
large observed polarisation degree is, however, expected only if
quantum-electrodynamics (QED) polarisation effects are present in the
magnetised vacuum around the star. The detection of a strongly linearly
polarised signal would therefore provide the first observational evidence of
QED effects in the strong-field regime. While polarisation measurements in the
soft X-rays are not feasible yet, optical polarisation measurements are within
reach also for quite faint targets, like the M7 which have optical counterparts
with magnitudes $\approx 26$--$28$. Here, we report on the measurement of
optical linear polarisation for the prototype, and brightest member, of the
class, RX\, J1856.5$-$3754 ($V\sim 25.5$), the first ever for one of the M7,
obtained with the Very Large Telescope. We measured a polarisation degree
$\mathrm{P.D.} =16.43\% \pm5.26\%$ and a polarisation position angle
$\mathrm{P.A.}=145\fdg39\pm9\fdg44$, computed east of the North Celestial
Meridian. The $\mathrm{P.D.}$ that we derive is large enough to support the
presence of vacuum birefringence, as predicted by QED.

16
The categories of this paper are: stat
19
The categories of this paper are: eess
32
The categories of this paper are: atom-ph
14
The categories of this paper are: math-ph
1
The categories of this paper are: cs
26
The categories of this paper are: bayes-an
21
The categories of this paper are: adap-org
20
The categories of this paper are: acc-phys
28
The categories of this paper are: alg-geom
31
The categories of this paper are: ao-sci
5
The categories of this paper are: gr-qc
4
The categories of this paper are: cond-mat
9
The categories of this paper are: nlin
0
The categories of this paper are: hep-ph
2
The categories of this paper are: math
13
The categories of this paper are: quant-ph