How has the Rossi X-ray Timing Explorer helped us to see the difference between neutron stars and black holes?

Neutron stars and black holes have a lot in common.  Both are massive, exotic objects thought to be formed when a star reaches the end of its life and explodes in a supernova.  Theoretical calculations show that any star more than a few times the mass of our Sun will collapse and explode when its nuclear fuel runs out.  If enough mass falls into the core in the process of this explosion, then a black hole will be formed.  If less mass is left in the core, it will become a neutron star.  Sometimes, though, even a star that has lots of mass to start with does not have enough left in the core to form a black hole (see http://www.space.com/scienceastronomy/051102_neutron_star.html   for an example).

How do we tell the difference between a neutron star and a black hole?  If it is part of a binary system, then it is often possible to estimate the mass.  If the mass is high enough, the object is assumed to be a black hole. Measuring the mass is therefore one way to tell these things apart, as least to the extent that we believe the theory about how much mass is needed to guarantee a black hole.

Other than mass, an important difference between a neutron star and a black hole is that a neutron star has a solid surface but a black hole does not.  A black hole instead has an "event horizon," a location near the black hole that marks the "point of no return."  Anything, even light, that crosses the event horizon falls into the black hole and is gone.

This distinction of having a solid surface is where the Rossi X-Ray Timing Explorer (RXTE) comes in.  As its name implies, RXTE measures time variations of X-rays from astrophysical sources.  Some X-ray sources produce bursts of X-rays whose properties seems to be best explained by gas falling onto a star surface and accumulating until it produces a thermonuclear flash.  Those sorts of bursts should not occur if a source is a black hole and has no surface.  The falling gas would just disappear into the black hole.  RXTE recently celebrated its tenth birthday. Astronomers used the data RXTE accumulated over those ten years to compare sources thought to be neutron stars with those thought to be black holes, based on mass estimates.  In all cases, X-ray bursts were seen only from the neutron-star-type sources, exactly what would be expected.  These observations help support the idea that black holes really do have event horizons.

Here is a Web site with more information about the RXTE result.  Please note that the links in this note are not NASA sites. This listing does not constitute NASA endorsement.

http://www.space.com/scienceastronomy/060109_event_horizon.html

This week's question comes from Dr. Dave Thompson. Dr. Thompson is an astrophysicist who studies gamma rays in the Exploration of the Universe Division. He helped build, test, and analyze data from EGRET on the Compton Gamma Ray Observatory, and he is now helping build part of the Gamma Ray Large Area Space Telescope (GLAST), scheduled for launch in 2007. His particular scientific interest is gamma-ray pulsars.