Strength Under Tension

Russell Bainer 1,2 and Valerie Weaver 1,2, 3, 4, 5, 6 Mechanical tension on a cell from its environment alters the expression of a protein that changes the physical properties of the nucleus. and accreted onto the black hole ( 6). Summing over the total wind loss from the surrounding stars, this gives an accretion rate of 10 solar masses per year ( 7). The mysterious thing is, this accretion rate should make Sgr A* more than a million times brighter than it actually is. So either the vast majority of the gas never reaches the black hole, or it is somehow accreting so ineffi ciently that it never gives off any energy on its way to the horizon. It is as though you fi lled up your car’s gas tank, drove 2 feet, and ran out of gas. Is there a massive leak in the fuel line, or is the mileage really that horrible? With the unprecedented spatial and energy resolution of these new Chandra observations, Wang et al. can infer the temperature and density profi le of the gas cloud surrounding Sgr A*. They show that over 99% of the gas never reaches the central black hole, but rather is ejected from the system, consistent with the predictions of a popular class of theoretical models called RIAFs (radiatively ineffi cient accretion fl ows) ( 8). By measuring the relative strength of various emission lines, they can also rule out a recent proposal that suggests that the diffuse x-ray emission is coming from the superposition of thousands of coronal fl ares from rapidly rotating stars around Sgr A* ( 9). Another major challenge with observing Sgr A* has been its inherently low luminosity, orders of magnitude below its theoretical potential. And yet the future looks bright, quite literally. In the next few months, a large cloud of gas is on course to collide with the black hole ( 10), which could potentially brighten by a factor of a million or more, greatly enhancing our ability to observe this unique region where gravity rules supreme.