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

Abstract

Circinus X-1 is a bright and highly variable X-ray binary which displays strong and rapid evolution in all wavebands. Radio flaring, associated with the production of a relativistic jet, occurs periodically on a ∼17-d time-scale. A longer term envelope modulates the peak radio fluxes in flares, ranging from peaks in excess of a Jansky in the 1970s to a historic low of milliJanskys during the years 1994–2006. Here, we report first observations of this source with the MeerKAT (Karoo Array Telescope) test array, KAT-7, part of the pathfinder development for the African dish component of the Square Kilometre Array, demonstrating successful scientific operation for variable and transient sources with the test array. The KAT-7 observations at 1.9 GHz during the period 2011 December 13 to 2012 January 16 reveal in temporal detail the return to the Jansky-level events observed in the 1970s. We compare these data to contemporaneous single-dish measurements at 4.8 and 8.5 GHz with the HartRAO 26-m telescope and X-ray monitoring from MAXI. We discuss whether the overall modulation and recent dramatic brightening is likely to be due to an increase in the power of the jet due to changes in accretion rate or changing Doppler boosting associated with a varying angle to the line of sight.