Sgr A and it’s importance to Gamma-Ray astronomy – by Albert Lim (2000)

Sgr A or Sagittarius A is a complex located at the centre of the Milky Way galaxy. The exact centre of the galaxy is designated Sagittarius A* ; the asterisk being used to distinguish the central point source which is a compact radio source from the entire Sagittarius A complex. The Sagittarius A complex contains one of the brightest infrared source in the sky called IRS16 which is known even before IRAS (Infrared Astronomical Satellite). The size of IRS16 is very small - it subtends an angle of only 1 arcminute which translates to 4 parsecs or 13 light years. The energy IRS16 outputs however, is prodigious and equals to that of 80 million suns ! Initially in the 1980s, IRS16 convinced astronomers that the black hole at Sagittarius A* is only of about 100 solar mass instead of millions of solar masses as thought, if indeed it exist. This came about through the below sequence of events.

Kuiper Airborne Observatory first discovered that the Sagittarius A complex is doughnut in shape with compact radio source Sagittarius A* and infrared source IRS16 very close to each other. Astronomers at Mauna Kea later detected the anticipated ring of dust around it and confirmed it’s expected rotation. The Anglo Australian Telescope went on to reveal that Sagittarius A* and IRS16 are in fact separate objects not quite in the same location - i.e. different energy sources. Thus any black hole at Sagittarius A* must be considered to be much smaller than earlier anticipated - this evidence suggest that our galaxy's black hole should be no more massive than 100 solar masses. Although prior evidence indicate that the central black hole should be for more massive, none of it is conclusive up to then. Earlier infrared observations of neon gas revealed velocities that would have caused the gas to escape the region unless the gravity of some 3 million solar masses were present. Astronomers argue that the gas is perhaps escaping and that all the heating necessary in the region can still be explained by the hot stars found in IRS16. They therefore suggest a model  whereby the galactic centre 1 parsec around Sagittarius A* contains a sphere of stars about 0.1 parsec in radius and a few clusters with newly formed B stars within their cores and an accreting black hole of no more than 100 solar masses.

More recently however, astronomers made new measurements of radial motions for more than 200 stars around Sagittarius A*. These measurements showed that stellar velocities increase with Kepler's law down to scale of 1 light week from Sagittarius A*. This new data makes a strong compelling case and reinforce the presence of a massive compact, dark mass of 2.6 million solar mass - very possibly a supermassive black hole. Some astronomers are however still not convinced. Astronomers today are still attempting to confirm this mass discrepancy. One method currently being used is the measurement of proper motion of Sagittarius A* with respect to background quasars using the VLBA. If Sagittarius A* is indeed a supermassive black hole on the order of millions of solar mass at the dynamical centre of our galaxy, then astronomers expect to find a peculiar motion for Sagittarius A* on the order of 1 km/s. This motion is due to gravitational perturbations on Sagittarius A* from close encounters with massive stars in the central star clusters. However, if Sagittarius A* is a much smaller mass system or black hole, then astronomers expect peculiar motions to be found on very much higher order.

Fig 3 : Chandra X-ray image of innermost 10 light years at the center of our galaxy. The image has been smoothed to bring out the X-ray emission from an extended cloud of hot gas surrounding supermassive black hole candidate Sagittarius A* (white dot at the center of the image). This gas glows in X-ray light because it has been heated to a temperature of millions of degrees by shock waves produced by supernova explosions and perhaps by colliding winds from young massive stars.

Sagittarius A is important to gamma ray astronomy for obvious reasons. Firstly, it is important to note that hard gamma-rays have so far mysteriously not been detected from Sagittarius A* as would be expected if Sagittarius A* is indeed a black hole. Whether the reasons are due to the insensitivity of present instruments or the environment around Sagittarius A or any other reasons, future gamma-ray studies, possibly through development of more sensitive detectors than those now employed on the INTEGRAL (International Gamma-Ray Astrophysics Laboratory) are needed to provide important answers. It should also be noted that many high density gamma-ray sources have in fact been detected in the Sagittarius A complex by the HEAO-3 (High-Energy Astronomical Observatory-3) and others. These sources are possibly related to interacting binaries but they puzzled astronomers by their constant variation in intensity. The amount of gamma-rays detected earlier by HEAO-3 can also be accounted for by the presence of a black hole no more than a hundred solar mass.