LOFAR 150-MHz observations of SS 433 and W 50
  We present LOFAR high-band data over the frequency range 115-189 MHz for the
X-ray binary SS 433, obtained in an observing campaign from 2013 February -
2014 May. Our results include a deep, wide-field map, allowing a detailed view
of the surrounding supernova remnant W 50 at low radio frequencies, as well as
a light curve for SS 433 determined from shorter monitoring runs. The complex
morphology of W 50 is in excellent agreement with previously published
higher-frequency maps; we find additional evidence for a spectral turnover in
the eastern wing, potentially due to foreground free-free absorption.
Furthermore, SS 433 is tentatively variable at 150 MHz, with both a debiased
modulation index of 11 per cent and a $\chi^2$ probability of a flat light
curve of $8.2 \times 10^{-3}$. By comparing the LOFAR flux densities with
contemporaneous observations carried out at 4800 MHz with the RATAN-600
telescope, we suggest that an observed $\sim$0.5-1 Jy rise in the 150-MHz flux
density may correspond to sustained flaring activity over a period of
approximately six months at 4800 MHz. However, the increase is too large to be
explained with a standard synchrotron bubble model. We also detect a wealth of
structure along the nearby Galactic plane, including the most complete
detection to date of the radio shell of the candidate supernova remnant G
38.7-1.4. This further demonstrates the potential of supernova remnant studies
with the current generation of low-frequency radio telescopes.
