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

TL;DR

This study reports the first successful observations of strong radio flaring in Circinus X-1 using the KAT-7 array, demonstrating the array's capability to monitor variable astrophysical sources and revealing a return to high flux levels similar to the 1970s.

Contribution

It demonstrates the scientific utility of the KAT-7 array for studying variable and transient sources, specifically capturing the re-emergence of strong radio flares in Circinus X-1.

Findings

KAT-7 detected Jansky-level radio flares in Circinus X-1.

The flaring activity resembles the levels observed in the 1970s.

Possible causes include increased jet power or changes in Doppler boosting.

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-day timescale. A longer-term envelope modulates the peak radio fluxes in flares, ranging from peaks in excess of a Jansky in the 1970s to an historic low of milliJanskys during the years 1994 to 2007. Here we report first observations of this source with the MeerKAT test array, KAT-7, part of the pathfinder development for the African dish component of the Square Kilometre Array (SKA), demonstrating successful scientific operation for variable and transient sources with the test array. The KAT-7 observations at 1.9 GHz during the period 13 December 2011 to 16 January 2012 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.