Variability of radio sources due to black hole activity
4 December 2015
The universe is a violent and dynamic environment – dramatic changes in the brightness of an object on a variety of timescales are often an indicator that extreme astrophysical processes are taking place, or that the intervening medium is severely affecting the propagation of the electromagnetic waves. In either case, measuring temporal variations enables us to investigate the changes in density, temperature, pressure, velocity and gravitational/magnetic fields experienced in these environments which are often an excellent probe of "extreme physics". A variety of exciting astronomical objects detectable at radio wavelengths fall into the variable classification such as: Active Galactic Nuclei (AGN), Black Hole X-ray Binaries, flare stars, Gamma-Ray Bursts afterglows, pulsars and Radio Supernovae, to name a few.
The Chandra Deep Field South (CDFS) is a region of the sky that has been observed extensively and to very deep depth with a plethora of telescopes across the electromagnetic spectrum. It covers ~0.3 square degrees of the sky, and the majority of the objects within this region have been classified and studied in great detail. In a recent paper, CAASTRO Affiliate Dr Martin Bell and his colleagues revisited this particular region of sky – two years after their first intense survey at radio wavelengths (Huynh et al. 2012) – to study how the objects had changed and to search for new objects that had "appeared" or become transient in the intervening time. The team again used the Australia Telescope Compact Array at a frequency of 5.5 GHz. Choosing the CDFS, instead of another region of the sky that was not so well studied, ensured that identification of objects of interest was easy and did not require additional telescope observations at different frequencies.
Within the CDFS, the researchers identified four highly variable radio sources out of a total of 124 objects. Using the multi-wavelength data available, they identified these objects as AGN and concluded that the variability was driven by accretion onto a supermassive Black Hole. The radio emission traces the after-effect of the accretion process via collimated relativistic "jets" of material. These jets respond to increases and decreases in the amount of material plummeting into the Black Hole and are hence time variable. The remarkable thing about these variable AGN is that they all had inverted radio spectra. Such spectra are a hall mark of potentially young objects that have "knots" within their compact radio jets.