Gemini South Tracks Optical Microflares Around Black Hole in Binary System

April 15, 2003

Binary stars evolve in interesting ways. The more massive star in a pair evolves faster and may explode as a supernova if its original mass is greater than about 7 times the mass of our Sun. The result is that either a neutron star or a black hole is found orbiting the lower-mass companion star - as is the case with A0620-00. This system consists of an X-ray novae star, powered by the sporadic accretion material from its companion, a K5 main-sequence star. The relative faintness of the secondary star has allowed the measurement of the radial velocity of both the accretion disk around the black hole and the companion star. The black hole mass is still not well known; the most recent estimate is 11.0+/-1.9Msun (solar mass). 2668-2678, November 2001). A more robust measure is that the mass has to be greater than 2.72+/-0.06Msun.

Soft X-ray transients (SXT), also known as X-ray novae and black hole X-ray transients, are low-mass X-ray binaries characterized by long periods of quiescence punctuated by dramatic X-ray and optical outbursts. While the quiescent periods that allow us to study the accretion disk properties are not so time-dependant, they are more challenging to observe. While it is difficult to explore the quiescent state in the X-ray domain due to low photons counts, it is possible to make these observations within the optical range using a large aperture telescope that allows high time resolution. Robert I. Hynes (formerly with the University of Southampton; currently with the University of Texas at Austin) and his collaborators have embarked on a study of aperiodic optical variability of SXTs, using the Acquisition Camera on the Gemini South telescope, in order to explore the source of the microflares on short timescales.

(geminiann03008a). The changing aspect of the companion star, due to its varying distortion, causes this modulation as it orbits the black hole. Two other SXT sources, X-ray novae Muscae 1991 and XN Vel 1993 were also monitored (geminiann03008b).

Optical flares of A0620-00 are seen on much shorter timescales than ever reported before, with rise times as low as 30 seconds or less. In addition, the power-density spectrum of A0620-00 appears to exhibit band-limited noise closely resembling the X-ray power-density spectrum of black hole candidates in their low state. The flares are clearly associated with the accretion flow rather than with an active companion; this is shown by a correlation between the variability amplitude and the fractional disk contribution to the spectrum. Whether microflares originate in the outer disk, or are driven by events in the inner region, has not yet been resolved. Analysis of the power-density spectrum of A0620-00 shows a low frequency break at ~10-3 Hertz. Assuming that the break frequency scales linearly with the size of an inner evaporated region, then this region has size ~10⁴ R_{Schwarzschild} in A0620-00 during its quiescence period.