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

Abstract

We present Low-Frequency Array (LOFAR) 143.5-MHz radio observations of flaring activity during 2019 May from the X-ray binary Cygnus X-3. Similar to radio observations of previous outbursts from Cygnus X-3, we find that this source was significantly variable at low frequencies, reaching a maximum flux density of about 5.8 Jy. We compare our LOFAR light curve with contemporaneous observations taken at 1.25 and 2.3 GHz with the RATAN-600 telescope, and at 15 GHz with the Arcminute Microkelvin Imager (AMI) Large Array. The initial 143.5-MHz flux density level, ∼2 Jy, is suggested to be the delayed and possibly blended emission from at least some of the flaring activity that had been detected at higher frequencies before our LOFAR observations had begun. There is also evidence of a delay of more than four days between a bright flare that initially peaked on May 6 at 2.3 and 15 GHz, and the corresponding peak (≳ 5.8 Jy) at 143.5 MHz. From the multi-frequency light curves, we estimate the minimum energy and magnetic field required to produce this flare to be roughly 1044 erg and 40 mG, respectively, corresponding to a minimum mean power of ∼1038 erg s−1. Additionally, we show that the broadband radio spectrum evolved over the course of our observing campaign; in particular, the two-point spectral index between 143.5 MHz and 1.25 GHz transitioned from being optically thick to optically thin as the flare simultaneously brightened at 143.5 MHz and faded at GHz frequencies.

The Green Bank 820 MHz Pulsar Survey. I. Survey Overview and Initial Results

“…Many of these studies have focused on the relatively nearby (;1.4 kpc; Rygl et al 2012;Orellana et al 2021) star-forming region along this line of sight, known as Cygnus X (Piddington & Minnett 1952). Beyond it lie two more Galactic arms, one of which is home to the extensively studied source Cygnus X-3, a microquasar in a binary with a W-R star that exhibits both X-ray and radio emission (Trushkin et al 2017;Broderick et al 2021;Suryanarayanan et al 2022;McCollough et al 2022). These studies illuminate a rich stellar history in the Cygnus region, making it a prime target for pulsar surveys since pulsars are formed during the supernovae of massive stars.…”

The innermost jet in the hidden ultra-luminous X-ray source Cygnus X-3

“…Frequently, it undergoes bright outbursts with peak flux densities reaching ∼20 Jy (e.g. Gregory et al 1972;Waltman et al 1994;Tudose et al 2010;Kim et al 2013;Egron et al 2017;Kim & Kim 2020;Broderick et al 2021;Spencer et al 2022). During some giant outbursts, high-resolution observations revealed episodic (mildly) relativistic or curved jets roughly in the north-south direction (e.g.…”