Megahertz emission of massive early-type stars in the Cygnus region

Abstract

Massive, early-type stars have been detected as radio sources for many decades. Their thermal winds radiate free–free continuum and in binary systems hosting a colliding-wind region, non-thermal emission has also been detected. To date, the most abundant data have been collected from frequencies higher than 1 GHz. We present here the results obtained from observations at 325 and 610 MHz, carried out with the Giant Metrewave Radio Telescope, of all known Wolf-Rayet and O-type stars encompassed in area of $\sim$ 15 sq degrees centred on the Cygnus region. We report on the detection of 11 massive stars, including both Wolf-Rayet and O-type systems. The measured flux densities at decimeter wavelengths allowed us to study the radio spectrum of the binary systems and to propose a consistent interpretation in terms of physical processes affecting the wide-band radio emission from these objects. WR 140 was detected at 610 MHz, but not at 325 MHz, very likely because of the strong impact of free–free absorption (FFA). We also report—for the first time—on the detection of a colliding-wind binary system down to 150 MHz, pertaining to the system of WR 146, making use of complementary information extracted from the Tata Institute of Fundamental Research GMRT Sky Survey. Its spectral energy distribution clearly shows the turnover at a frequency of about 600 MHz, that we interpret to be due to FFA. Finally, we report on the identification of two additional particle-accelerating colliding-wind binaries, namely Cyg OB2 12 and ALS 15108 AB.

Strong low-frequency radio flaring from Cygnus X-3 observed with LOFAR

“…Indeed, we inspected the 147.5-MHz TGSS image products and calculated a 3σ upper limit for the quiescent flux density of about 30 mJy beam −1 . Furthermore, the 235-and 610-MHz results from Pandey et al (2007) suggest that the average quiescent baseline at 143.5 MHz is well below this TGSS upper limit (see Section 1), as do recently published 325-and 610-MHz GMRT flux densities (Benaglia et al 2020a(Benaglia et al ,b, 2021.…”

Radio Counterparts of Gamma-Ray Sources in the Cygnus Region

“…These are listed in Table 5 and offer results collected with instruments working from radio to X-ray bands. Regarding the search of radio counterparts on catalogs of massive, early-type stars, involving the same observations and images used here, see Benaglia et al (2020a).…”

“…This work is part of a series of studies of the same celestial area in the Cygnus region, mainly the catalog introduced in Benaglia et al (2020b). The latter lists radio sources at 610 MHz and 325 MHz bands, for flux densities greater or equal to 7σ, where σ is the local rms; see also Ishwara-Chandra et al (2019); Benaglia et al (2020a); Isequilla et al (2020). Here we have studied sources above 3σ.…”

Cygnus survey with the Giant Metrewave Radio Telescope at 325 and 610 MHz: the catalog

“…We also searched the 610 MHz image and point sources with well-determined optical positions, that is, Wolf-Rayet and O-type stars. For the cases with radio emission at or near the position of these stars (13 in total), the differences in coordinates were in the range 0.1-3.1 , and the standard deviation was 1.34 (see Benaglia et al 2020, for a study of the massive early-type stars detected in the current databases). The synthesized beam is 10 × 10 , and the average rms is 0.5 mJy beam −1 .…”

“…The GMRT observations that gave rise to our catalog allowed us simultaneously to carry out research on individual populations of astronomical objects. Specifically, different types of objects that can produce nonthermal emission were or are being studied in separated investigations: AGNs and two-lobed sources, counterparts to HE sources, massive early-type stars (Benaglia et al 2020), protoplanetary disk-like sources (Isequilla et al 2019) and young stellar objects (Isequilla et al 2020). Finally, the survey images will be presented elsewhere.…”