Detection of compact radio emission from Circinus X-1 with the first Southern hemisphere e-VLBI experiment

Abstract

Circinus X-1 has recently returned to a state of strong radio flaring. Here we report on the first VLBI observations, and detection, undertaken in the 25 years since the 1975-1985 period of strong recurrent flaring activity. We detected Circinus X-1 with the first observations conducted by a recently developed Southern hemisphere e-VLBI array, at both 1.6 and 8.4 GHz, over a three-day period. At 1.6 GHz, the compact source has a total flux density of 11 mJy and a size of 60 ± 15 mas (Gaussian model full width at half maximum). At 8.4 GHz, the compact source is less than 60 mas. The size variation with frequency is consistent with a broadened image due to scattering in the turbulent, ionized interstellar medium of our Galaxy. However, these size measurements appear inconsistent with the λ 2.2 variation expected for strong interstellar scattering and previous VLBI observations made at 2.3 GHz in the early 1980s. To explain this apparent inconsistency, we suggest that Circinus X-1 supports a weak, non-varying component of 35 mas extent (175 au at 5 kpc distance), corresponding to compact structure in the extended radio nebula. No significant variation in the flux density at 1.6 GHz is evident between two observations 24 h apart. No jet-like structures are evident on scales of tens of mas, simply a scatter broadened source, presumably coincident with the suggested neutron star in the binary system.

A return to strong radio flaring by Circinus X-1 observed with the Karoo Array Telescope test array KAT-7

“…There is also new data from HartRAO from the period 1979-1992 (∼MJD 44000 -49000) showing the 5 GHz upper limits and 5 GHz radio detections , as well as 8.6 GHz radio detections around August 2006 (∼MJD 54000). Also shown in the figure are observations by Deller et al 2007 (see also Phillips et al 2007), Calvelo et al (2010), Moin et al (2011), Calvelo et. al (2012) and Miller-Jones et al (2012.…”

“…Fomalont, Geldzahler & Bradshaw 2001a,b). Recently, Cir X-1 has been detected (Phillips et al 2007, Moin et al 2011) and resolved (Miller-Jones et al 2012) at sub-arcsec scales using GHz VLBI and also via mm observations with ATCA (Calvelo et al 2012b). These observations do not conclusively confirm or refute the ultrarelativistic interpretation.…”

Parsec-Scale Radio Emission From the Low-Luminosity Active Galactic Nucleus in the Dwarf Starburst Galaxy Henize 2-10

“…We observed He 2-10 on 2011 July 22 with the LBA in eVLBI mode (Phillips et al 2007). Continuum observations at 1.4 GHz (21 cm) were made using the four eVLBI capable antennas at this frequency -the Australia Telescope National Facility (ATNF) observatories at Parkes (64-m single dish), the Australia Telescope Compact Array (ATCA; five 22-m antennas phased), and Mopra (22m single dish), as well as the Hobart antenna (26-m single dish) operated by the University of Tasmania.…”

Long-term Radio Behaviour of the X-ray Binary Circinus X-1

“…It does seem that the radio flux density levels became lower by comparison with the ones observed at similar orbital phases before, however the limited number of observations cannot exclude the possibility of this being an observational bias. In addition, the flare observed in 2005 June had the highest flux density in the whole data set, but this might just as well be associated with a seemingly increase in the radio activity of Circinus X-1 as was observed in the last few years [18,19,20].…”