Diego Altamirano

What are my research interests? (in Layman's terms!)

 

 

On Low-Mass X-ray Binaries

 

Neutron stars (NSs) and black holes (BHs) are compact objects. The tiny NSs being no more than 20-30 km in diameter but containing masses of 1 to 3 times the Sun, represent extremes of gravity, pressure and density, making them the only stars where matter burns on the outside. While NSs are the densest objects we know in nature from which we can still see the surface, BHs are denser but invisible behind an event horizon. Fortunately, some of these compact objects have close companion stars; gas from these stars, attracted by the compact object strong gravity, funnels and spirals towards it, forming an accretion disk. These systems are called low-mass X-ray binaries (LMXBs); the most powerful phenomena we observe from them are directly related to these accretion disks, as a large amount of gravitational energy is released when the matter approaches the compact object. This causes the inner accretion disk to reach temperatures as high as 107 Kelvin and therefore to emit the bulk of the energy in the X-ray band of the spectrum. It is the flow of this accreting plasma onto the compact objects which provides one of the very few opportunities to directly observe the properties of the tiny (few km) regions of the most strongly curved spacetime known to exist in Nature and, additionally see General Relativity (GR) effects in action in otherwise inaccessible regimes.

 

Lately all my efforts go to black holes:

 

Black holes in our Galaxy have masses between 5 and 15 times that of our Sun, and are formed when a massive star explodes in a Supernova. A black hole is so massive and confined in such a small volume in space, that not even light can escape its gravitational attraction. Fortunately, some black holes are in binary systems with a star similar to, or smaller than our Sun. If the black hole and  the companion star are close enough, the strong gravity produced by the black hole will slowly ``suck'' gas from its companion, deforming it into a pear-shape star. The gas pulled off by the black hole does not fall directly into it, but swirls in like bath water around a plug-hole, forming a disk of gas which astronomers call accretion disk. This disk can be described as a set of thin rings of gas, one inside each other, all rotating at a speed that depends on the distance to the black hole: the closer to it, the faster the ring rotates. Contiguous rings rub against each other, becoming hot due to friction. At a few km from the black hole, the speeds are so high and the friction is so strong, that the disk reaches more than 10 million degrees (2000 times hotter than the surface of our Sun!!), emitting all the energy in X-rays. It is the information transmitted in those X-rays that, similarly to a ``feel Box'' game, astronomers use to
comprehend and test the complex theoretical models which describe the exotic properties of black holes. During my project, I plan to use today's best X-ray telescopes, together with today's largest radio antennas and most sensitive optical telescopes to study in detail the two most fascinating black hole systems in our Galaxy. Named IGR J17091-3624 and GRS 1915+105 after the astronomical coordinates of its sky position, these two systems are engulfing their accretion disks at the highest rate allowed by physical laws. The rate is actually so high, that these systems find themselves in a bottleneck, which in turn produces remarkable effects, such as extraordinary variations in their X-rays brightness and, hurricane-like disk winds which have never been seen before in others systems.

Among the many variations we see, there is one nicknamed a "heartbeat", because of its resemblance to an electrocardiogram. Inthis webpage

 

http://www.nasa.gov/topics/universe/features/black-hole-heartbeat.html

together with NASA, I explain with an animation how GRS 1915+105 and IGR J17091-3624 look like, as well as how the heartbeats differ between each other. By understanding these differences, I will be able to understand which are the main physical factors which control the production of these black-hole heartbeats, which, in turn, will help me to understand some of the exotic properties of their black hole. I will then use this information to test current theoretical ideas on other Galactic black hole systems where gas swirls-in at much lower rate. As a final part of my project, I will extrapolate my new knowledge to what is known about supermassive black holes, i.e., about black holes with masses more than a million times higher than that of our Sun, and which are located at the center of galaxies.

 

Interested about the hurricane-like disk winds in IGR J17091-3624?

http://chandra.harvard.edu/photo/2012/igr

More about the physics of the heartbeats in GRS 1915+105:

http://chandra.harvard.edu/press/11_releases/press_011211.html

What are supermassive black holes?  

http://imagine.gsfc.nasa.gov/docs/science/know_l2/active_galaxies.html