The Galactic Centre black hole in close-up

The research into the nature and properties of the black hole at the centre of the Milky Way galaxy is one of the highlights of astronomical discovery of the last two decades. Using the biggest telescopes on the planet and state of the art observing technology, we’ve been able to track the young massive stars that are whizzing around the black hole in a dense cluster, and shown with a high level of certainty that the galaxy’s central object really is a supermassive black hole, referred to as Sagittarius A*. Using these stellar orbits, we’ve also determined its mass – 4 million solar masses.

Now you see it, now you don't! The square arcsecond surrounding the galactic centre black hole, seen in the near-infrared. On the left, no source is visible, later on (right) a flare brought it into view. The star marked S2 is the closest known star to Sgr A*. Click to embiggen. (ESO)

With the next generation of infrared instrumentation, we’re planning to take the next step in the study of Sgr A*. For this, we’ll use interferometry – the combination of light beams from a number of telescopes – to zoom into the black hole closer than ever before. In a paper posted to the Arxiv late last year, Vincent et al discuss the potential of a new interferometric instrument, Gravity, for testing black hole physics near Sgr A*.

Having a supermassive black hole in our galactic backyard presents the unique opportunity to check out some of the exotic predictions of general relativity in the strong gravity regime. The extreme mass of a black hole causes a strong curvature of the surrounding spacetime, and this is the sort of environment where general relativistic effects should become pronounced, and potentially observable.

One such prediction is the precession of the peribothron – the orbiting stars’ closest approach to the black hole.  For S2, the star with the best-known orbit around the black hole today, this effect is too small to witness in a reasonable amount of time. S2 moves fast, but still takes 15 years to complete a single orbit. To measure this tiny orbit precession, we have to find and track stars that are around ten times closer to the edge of the black hole’s event horizon than the S stars we know today.  More than 100 stars are already known to exist within one arcsecond from Sgr A*, and this number will keep growing with our continuing efforts to monitor the region.

Gravity is a 2nd generation instrument for the VLT Interferometer (VLTI), that will link up all 4 of the 8.2-m Unit Telecopes. The resolution of the resulting image is vastly improved over what a single dish can deliver; in interferometric mode, the resolution is determined by the telescope separation, rather than simply the mirror diameter. The price to pay for this boost in image detail is a tiny field of view, and because of the long and complex optical path from the telescope through the instrument to the detector, only the brightest of sources can be observed.

The current interferometric instruments at VLTI, AMBER and MIDI, have delivered some neat results, but could not yet achieve VLTI’s ultimate goal: the linking up of all four large telescopes. This milestone was recently achieved with a new visiting instrument called PIONIER. For Gravity and its fellow 2nd generation instrument, MATISSE, this will be part of routine operations. Operating in the near-infrared K band, Gravity’s design is optimised to carry out the ultra-precise orbit tracking measurements in the galactic centre.

In their paper, Vincent et al work through the formalism of Gravity’s galactic centre observations and attempt to simulate the precision with which the position of a star in this crowded and heavily obscured region can be measured (the astrometric precision) with the instrument, as a function of the number of stars found in this tiny region and their brightnesses. The innermost stable circular orbit (ISCO) around the black hole is just 30 micro-arcseconds in extent, so the required precision for tracking anything on or close to such an orbit is extremely high.

Their calculations show much promise for Gravity to succeed in these measurements. But depending on the brightness of the targets found, the required precision may only be achievable from a whole night’s observation, to be repeated once a month during the 6-month period when the galactic centre is visible from Paranal, for several years. Observing with just one Unit Telescope for a night costs 50,000 euro per night; taking up all four at once for an entire night, for half a dozen nights a year makes this a very expensive science project. The brightness of the kind of sources we’re likely to find will push the telescopes and the instrument to its limits, and I think it’s fair to say that this work lies in the “high risk, high gain” category. The gain, however, is substantial.

The PIONIER instrument, the first to combine the light from all four VLT unit telescopes (ESO/B. Lazareff)

That’s not the only cool science Gravity will be capable of doing. Additionally in the galactic centre, Gravity may be able to identify the source of the peculiar flaring seen in the brightness of the source associated with the black hole. The black hole is essentially invisible in the infrared for most of the time (see the image above), but a few times day it suddenly brightens. The flares are particularly marked at near-infrared and X-ray wavelengths, and last around an hour. At their typical brightnesses, the flares come well within the magnitude range observable with Gravity.

Several complex theories have been put forward to explain this odd behaviour: interaction of matter with the black hole’s magnetic field, expansion of a hot plasma blob, jets or accretion disk instabilities. Vincent et al show that with its excellent astrometric precision, Gravity should be able to track any motion in the centre of brightness of the source of the flare, which will help pin down the underlying mechanism.

Research into the black hole at the centre of the galaxy has in recent years exploited the newest observational technologies, such as adaptive optics, with great success. With Gravity and MATISSE, the field is actively driving the development of innovative technology in observational astronomical. I’m excited to be involved in both the science and engineering for this instrument, and I’m sure I’ll be writing more about some of the cool aspects of the instrument and the science it will enable. In the US, similar instrumentation is being developed for the Keck telescopes’ interferometric mode. When these new facilities come online in a few years’ time, it will be an interesting time for black hole research.

References

F. H. Vincent, T. Paumard, G. Perrin, L. Mugnier, F. Eisenhauer, & S. Gillessen (2011). Performance of astrometric detection of a hotspot orbiting on the
innermost stable circular orbit of the galactic centre black hole MNRAS arXiv: 1011.5439v1