Measuring the mass of a neutron star

Go here to read about a unique opportunity to measure the mass of a neutron star, and hopefully learn most about the state of matter in these incredible dense balls. For more information on this topic, listen to this interview with Tod Strohmayer of NASA, who works on RXTE - the satellite that discovered this system.

That Flipping Black Holes

No, I'm not using a euphemism for swearing. Black holes can change the direction in which they spin (i.e, flip), and this can have some impressive results. Go here to learn more!

Brightest GRB ever!

Until the next one, at least. Read more about it here, and learn more about the Swift satellite by listening to this.

Puff the Small Black Hole...

.. it can make such a big hole. Remember how a couple of days ago I linked to a post stating that astronomer now think that most of the X-ray emission from a black hole is generated by fast-moving material ejected (called "jets") which are from the disk of infalling material? Well, there is now evidence that these jets can clear out large cavities around the black hole, pushing the surrounding medium far away - as you can read here and here or in its full scientific glory here (library access required, sorry). This isn't new necessarily, evidence for such behavior from the supermassive black holes located in the center of galaxies has been around a for a while, but seeing it in a low-mass black hole that is nearby is new and suggests we may be able to study this process a lot better than before. Enjoy!

X-rays from Black Holes

Huh? Well, yes, black holes are "black" because their gravity is so strong that any light emitted from inside them can't escape. However, light can be emitted from outside black holes, and in fact it often is - it believed that many of the most luminous objects in the universe (quasars) are powered by material falling into a black hole. A recent study by RXTE suggests that the X-ray emitted by these objects is not from material falling into the black hole, but material flowing away from the black hole very rapidly in a narrow cone, called jets (go here for details). Very interesting indeed. To learn more about RXTE, listen to this interview I broadcast a while back.

The "Quiet" Black Hole

at the center of M31, has its own bursts, as revealed by Chandra. Go here to read more.

The Origin of Type Ia Supernovae

As you may have seen, last week a major discovery concerning the nature of Type Ia supernovae which are so important for cosmology. First, why are these supernovae so important? It is because this class of stellar explosions are believed to all be essentially the same - i.e., they all produce the same amount of energy. This means that, if you see one on the sky, you can use its brightness to get its distance (its "luminosity distance" to be overly technical). From its spectrum, you can get its redshift, and by comparing the distance vs. redshift for many of these, one derives the expansion history of the universe. This is pretty much how astronomers first inferred the existence of "dark energy" in the universe, a measurement that is supported by completely separate techniques which I'm not going to discuss now.

Okay, so what are these explosions? That has been the tricky part. Based on their similarity to each other, their optical spectra, how often they occur, and the type of galaxies in which they occur, type Ia supernovae have long been associated with the thermonuclear explosion created by the gravitational collapse of white dwarf. What's that? Well, white dwarfs are "stars" which are supported not by fusion in their core like our Sun, but by the fact that electrons can't get too close to each other ("electron degeneracy pressure"). However, if the mass of the white dwarf is too high, its own gravity is too strong to be balanced by this pressure and it collapses on its self, triggering an explosion which envelopes, burns, and then blows up the entire white dwarf. Since this maximum mass is pretty much the same for all white dwarfs (1.4 Solar Masses, the so-called Chandrasekhar limit since it was first derived by Prof. Chandrasekhar - easily one of the most brilliant astrophysicists of the 20th century and the person that Chandra is named after), it isn't too surprising that Ia supernovae are so similar.

Okay, but how does the white dwarf get so much material? There are two possibilities. The first is that a normal star is in a close orbit around the white dwarf. In some cases, the gravitational attraction of the white dwarf will be so strong that is will rip off the outer layers of the normal star and cause this material to fall onto the white dwarf, gaining mass. (This process is generically called "accretion") The second possibility is that two white dwarfs are orbiting each other, merge for whatever reason, and then the combined mass of the two white dwarfs is so high that it explodes. How can you distinguish them? Well, in the first scenario, the material which accretes onto the white dwarf is hot and shines brightly in the X-rays, while in the second scenario you would get no such emission before a Ia supernova. Is this difference actually observable? Looks like it is. To read more, go here and here among other places. The actual scientific article can be read here, but you might need a library or university IP address to read it for free (any decent public library will have the print version, which is easier to read anyway). Hope this made sense. Please leave questions below.

Speaking of stellar-mass black holes...

... it turns out that their formation might help their parent explode after all. Black holes as black widows seems fairly fitting, don't you think? Go here to read about it. This is one of the very few times where we actually observed the supernova and measured the properties of the produced compact object - and the first time for the limited set of explosions that it appears a black hole and not a neutron star was produced. Very exciting indeed.

NASA's next satellite

NuStar is (hopefully) going to do something no satellite has ever done before - focus very high-energy photons. By doing so, it might be able to shed important light (pun quasi-intended) on how massive stars explode. Read this article for more information. Enjoy!

(Belated) Happy Birthday XMM

The largest X-ray telescope ever launched to space, ESA's XMM-Newton has just celebrated 10 years in orbit. Go here to read more about this telescope, the highlights of the previous decade, and its plan for the future. Here's hoping for (at least) one more decade of operations!

PS. Listen to this with Dr. Ann Hornschemeier of Goddard Space Flight Center about the next big X-ray satellite, current dubbed Constellation-X.

Happy 10th Birthday XMM-Newton

Launched ten years ago TODAY, XMM-Newton - the European Space Agency's X-ray satellite and the largest X-ray satellite ever (and likely to hold that title for at least 20 years) - is still going strong (for the most part). Go here for more info on the birthday celebration. I'm presonally hoping for at least 10 more years of XMM and Chandra...

Happy 10th Birthday Chandra X-ray Observatory!

The highest angular resolution X-ray observatory ever (and likely to hold this title for at least 20 years, maybe more) and first of the current, new generation of X-ray telescopes, the Chandra X-ray Observatory was launched into space onboard the space shuttle Columbia about 10 years ago. For more information on the wonderful science this satellite has helped accomplish, read this article. Chandra really is an amazing facility, and hopefully will last (at least) another 10 years!

Description of December 3rd Radio Show: Black Holes

Long available here, below is a description of the 2008 December 3rd episode of this radio show with focused on black holes. On this program, I discussed the following:

• Black Holes: Black Holes are objects believed to be so dense that light can not escape if it gets to close (i.e., past the "event horizon"). Additionally, its gravity is so strong that material that makes up a black hole can not arrange itself into a structure that can withstand its own gravity, so this material is thought to collapse into a point. But if one can't see too close to a black hole, how does one know? Well, it the above is correct, that black holes should not have a surface, unlike a neutron star. If a neutron star or a black hole is close enough to a normal star, its gravity is so strong that it will rip material off the surface of the normal star and cause it to fall on itself. This process is called "accretion", and as discussed previously on this radio show by Dr. Tod Strohmayer, this has process have been observed for many neutron stars and black hole candidates. In the case of a neutron star, this material will pile up on the surface of the neutron star, and this pile will got hotter and denser until it is so hot and dense that the hydrogen in this material fuses into helium, releasing a burst of heat and light which is observable by X-ray telescopes such as the Rossi X-ray Timing Explorer. One would not expect such bursts from material falling onto a black hole - which is more massive than a neutron star - since their is no surface for the material to collect. These X-ray bursts have been detected from neutron stars (for example this link), but not from any black hole candidates. Therefore, even there seems to be evidence that black holes indeed do not have surfaces. Additionally, black holes appears to have a mass either a few times that of the Sun (called "stellar-mass black holes"), or millions to billions times that of the Sun - like the black hole believed to reside in the center of our galaxy, the Milky Way (called "super-massive black holes"). Why there seems to be a lack of black holes with a mass between these two extremes (for example, a thousand solar masses) isn't known. One possibility is that such black holes exist, but reside in the middle of globular clusters where they would be hard to detect. A recent survey of globular cluster RZ2109 did not find such a black hole, suggesting that if this was correct they are extremely rare (link). Studies of the super-massive black holes suggest that their may be an upper-limit to how massive they can be, around 10 billion times that of the Sun (link). Since black holes do not have any structure, they are often thought to be some of the simplest objects in the universe (really!) - according to Einstein's theory of General Relativity, in order to completely describe the properties of a black hole you need to know its mass, its spin, and its electric charge (and real black holes in the universe and expected to have zero electrical charge). If so, stellar-mass black holes and super-massive black holes should accrete matter the same way - as observed for black holes in the nearby galaxy M81 (link). As mentioned before, it a normal star passes too close to a black hole, the black hole's gravity will cause material from the star to fall towards, and eventually fall inside, the black hole. As this material falls towards the black hole, it gets hotter - so hot that it produces a lot of X-rays. By looking at the periodic flickering of this light, it is possible to estimate the mass of the black hole. This was recently done for one black hole in the Milky Way, at they estimate a mass of just 3.8 times that of the Solar Mass, the lowest mass black hole known (link). Periodic flickering has also been observed from the super-massive black holes in the centers of other galaxies, and can be used to estimate their mass as well. This was done for galaxy RE J1034+396, which has a mass a million times that of the Sun (link). What causes the light from gas falling into a black hole to flicker is not known, but a recent study of the visible light flickering and X-ray light flickering from a Galactic black hole suggests that its magnetic field plays an important role (link). As mentioned before, there is a super-massive black hole in the center of our galaxy called Sgr A*, and material falling into this black hole also produces regular flares of light (link). In fact, there is evidence that, around 300 years ago, it produced a flash of light so bright that today we are seeing some of the light reflecting off molecular clouds near the Galactic center (link). A major goal of astronomers today is to make an image of light coming from the event horizon around a black hole. The best chance to do this is using radio telescopes to observe Sgr A*, and radio astronomers have gotten down to only 3 times the expected size of the event horizon (link 1, link 2). It is possible to use to orbits of objects around black holes to test general relativity (GR), just as one does this with neutron stars. This was recently done for the two black holes believed to be orbiting each other in center of galaxy OJ 287, and the time between the closest approach of these two black holes agreed with what GR predicts. The material that flows into a black hole is believed to form a disk (called an "accretion disk") around the black hole before it passes the event horizon. Quasars are believed to be galaxies where the optical light of this disk shines much, much brighter than all the stars in the rest of the galaxy. If so, the spectrum of the quasar - how bright the quasar is at different colors - should resemble that of a disk. This is hard to measure in great detail since the light at some colors is absorbed by material between the Earth and the galaxy, but a recent study suggests that it does (link). Additionally, for some unknown reason, the presence of gas flowing into the black hole is often associated with the presence of gas flowing AWAY from the black hole with very high velocities (>60 million mph). Astronomers at UCSC and University of Florida recently observed this outflow turn on around a quasar (link). Sometimes this very collimated outflow (called a "jet) is pointed directly at the Earth, in which case the galaxy is called a "blazar." Recent radio observation of a blazar suggest that these jets are powered by the magnetic field of the black hole (link). Quasars come in very different varieties, with one class recently observed to produce many more X-ray than previously thought to be possible (link). The presence of very, young stars around super-massive black holes like Sgr A* strongly suggest that stars can form in the accretion disk around them (article, link 1). Optical observations of these accretion disk have also detected weird optical filaments around them, suggesting this gas is confined by a weak magnetic field possibly generated by the black hole (link). While all galaxies are currently thought to have a super-massive black hole in their center, very few galaxies are quasars. Why some galaxies are quasars are most are not is unknown, but it is though that quasars are the result of two galaxies recently merging together.
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Supernova Remnant Movie

Using data taken by the Chandra X-ray Observatory, astronomers have been able to observe both the expansion of the Cas A supernova remnant as well as determine its three-dimensional structure. Go here to see the expansion and time variability of this remnant, and here to see its 3D structure. Enjoy!

Pick your favorite X-ray image

The Chandra X-ray Center is having trouble picking its favorite Chandra image. You can cast you vote here until December 1st. It's a really hard choice, they all look so nice...

Neat Movie of Solar Activity

Located here is a pretty amazing movie showing plumes of hot gas moving on the Sun. Go here to read an article as to what astronomers think is going on. Enjoy!

Interview with Dr. Ann Hornschemeier now online

Available here is the interview I did with Dr. Ann Hornschemeier of Goddard Space Flight Center, who is Deputy Project Scientist of Constellation-X, the leading candidate for NASA's next generation X-ray satellite, the successor to the Chandra X-ray Observatory. Dr. Hornschemeier was kind enough to talk to about the science goals of Constellation-X, how it is being designed to achieve them, and the technology required to do this. I, personally, am very excited about this mission and, for more information on this project, please check out this webpage. As always, please email me or leave below any questions, comments, or concerns you might have.

This is the last interview on research at Goddard Space Flight Center, though I hope to have on as guests later others researchers at Goddard since this series really only covered the tip of the iceberg on all the work that goes on there. I'll post a summary of the interviews later, but I really hope you enjoyed this series. I had a lot of fun putting it together since it gave me an opportunity to speak to a lot of scientists whose papers and presentations I've enjoyed discussing projects I hope you found as interesting and exciting as I did. Thank you for listening.