LOFAR 150-MHz observations of SS 433 and W 50

Abstract : We present Low-Frequency Array (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 to 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 χ^2 probability of a flat light curve of 8.2 × 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 ∼0.5–1 Jy rise in the 150-MHz flux density may correspond to sustained flaring activity over a period of approximately 6 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.
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J.W. Broderick, R.P. Fender, J.C. A. Miller-Jones, S.A. Trushkin, A.J. Stewart, et al.. LOFAR 150-MHz observations of SS 433 and W 50. Mon.Not.Roy.Astron.Soc., 2018, 475 (4), pp.5360-5377. ⟨10.1093/mnras/sty081⟩. ⟨hal-01730032⟩

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