Now let's go one step further. Let's revisit Cen X-3 15 years after the EXOSAT observation we just examined and see what Chandra can add to the picture. Okay, so we restart DS9, go to Analysis>Virtual observatory. Look at the primary MOOK. Here's our set of possible observations. And let's scroll down to obs ID 1943, the wind and accretion disk in Cen X-3. And now you can see what Cen X-3 looks like in Chandra. It looks very, very peculiar, almost like a solar eclipse here. That's because Cen X-3 is so bright, and we're using kind of a different detector for this observation than for a lot of the other Chandra observations. But our green regions are already set down here in order to capture most of the photons that are present or are coming from Cen X-3 into our satellite. So let's now look at our light curve. Let's first get our analysis programs set up in the way we usually do. And we will look at our light curve just like we did for the EXOSAT data. When we click on Light Curve, our computer chugs along and after a little bit of time, here we go. Here's our light curve. Also wildly varying up and down, very similar to what it looked like in the EXOSAT observation. Notice though, this is not as long an observation. Okay, this is about 50,000 seconds. And we don't see any hint of that eclipse; that's because we are in the cycle of Cen X-3 that part of the cycle that is not being eclipsed. So, let's do the same thing we did. Let's zoom in on our light curve region. Now we're getting a little bit closer to seeing what's going on. We'll zoom in again. We left-click and then release. And, oh, look at that! Boom, boom, boom, boom, boom. You can count the peaks here and count the amount of time. And sure enough, it looks like it's about 4.8 seconds again. But is it exactly? Let's go to our Power Spectrum and find out. We click on the Power Spectrum and here it is: 0.2 seconds per, se- se, 0.2 cycles per second. Let's zoom in on it, left click, make our box here. Here we go. Do it again. Left click and drag our box and there we have it. Look at this! Our frequency for our EXOSAT data was at about 0.207. There's absolutely nothing in the Chandra observation at point 207. It's moved! It's moved to 0.208 cycles per second. Look! The frequency is slightly different. We get a doppler shift range just like we did before,but now with Chandra, it is centered on 0.2080 seconds, instead of, 0.2071 seconds. So f for Chandra, it is centered on a frequency equal 0.2080 cycles per second, or a period of about 4.81 seconds instead of our frequency with EXOSAT, which was about 0.2071 cycles per second, for a period with EXOSAT of about 4.83 seconds. There's a clear difference here. It's a small amount, but look at the power spectra of these two satellite observations side by side. But it's so small, maybe it's insignificant, but just as we did a period fold with GK per we can do it with Cen X-3. I urge you to do this on your own. It's a lot of fun using DS9, but I will show you just the results here. And you can see that with a 4.81 second fold, the EXOSAT data Is just flatter than a pancake. And with a 4.83 second fold, the Chandra curve shows no variability. The effect is real. What this means is Cen X-3 has spun up in 15 years; it's going faster. And in fact, when you look at it over even longer time spans, it seems relentlessly and predictably gaining speed in a more or less linear fashion. What could be causing that? Well, it appears that the companion star feeds the x-ray source some material, and as that gas gets closer and closer to the neutron star, it gives the star a bit of a kick. It's quite similar to what happens when an ice skater does a spin and draws in his or her arms. They spin faster and faster and faster to conserve angular momentum. Pretty neat. One more interesting thing we can do is to find out the luminosity of this object in the x-rays. From the optical brightness and spectrum of Krzeminski's star, we have deduced that the distance to Cen X-3 is about 20,000 light years. Now, from the x-ray observations, we can find the average flux, or the amount of energy that passes through each square centimeter of our satellite's detector each second. To do this, we return one last time to DS9 and examine Cen X-3's energy spectrum. We go to Analysis, and now we're going to do our Chau Sherpa spectral fit. This is going to take all the photons in the observation and fit it to a particular model of radiation. There are lots of different models that you can try to fit your data to, and in this situation, it's not really all that important, because we just want to see the overall number, an amount of radiation that's passing through our satellite detectors independent of the actual way that it's actually radiating. So we're going to choose the McAll data fit, or the McAll spectral fit. But what we do want is to display the log, because we are going to be interested in the flux, the amount of energy passing through each square centimeter of our detector each second. So we click on Display Sherpa logs. And now we wait for our computer to do this analysis. It's fitting hundreds and hundreds of thousands of photons and it's going to take a little bit of time. But here we are, and now you can see this is the plot of the energy output of Cen X-3 superimposed with a little white line that's almost impossible to see. We don't have to worry about that. That's the actual model fit. What we're interested in is this number near the top of our log. If our model choice is more or less valid, we can use this flux to predict the intrinsic luminosity of the object. This is the flux that we would get from Cen X-3 if there was no absorbing gas and dust between Cen X-3 and the Earth. This is the number we want, it's about 2.4 times ten to the minus nine ergs per centimeter squared per second. The flux is 2.4 times 10 to the minus 9 ergs per square centimetre per second. This means that every square centimetre area at the distance of the Earth from Cen X-3 receives about 2.4 times 10 to the minus 9 ergs of x-rays each second. If Cen X-3 radiates isotropically, which is a highfalutin word for uniformly in all directions, that means we can take this number and multiply it by 4 pi r squared to find out the luminosity of Cen X-3, where r in this case is 20,000 light years, right? So, basically, what's happening is here we are near the Earth. Here's Cen X-3, and Cen X-3 is putting out light in all directions, a little bit of which gets to us. And if this is at a distance of 20,000 light years, let me put that distance in blue here. You can see that each second the light from Cen X-3 will fill up a ball whose surface area is 4 pi d squared. So all we have to do is take the flux, which represents one square centimeter of area and multiply it by all of these other square centimeters of which there are 4 pi r squared of them, where our satellite isn't. But which if we did have something to detect Cen X-3, we would see the same thing as we do here near the Earth. Thus, the luminosity is l equals four pi d squared times the flux. And if we do that, d is 20,000 light years. We multiply our flux by the centimeter equivalent of 20,000 light years, and if you do that, converting light years to centimeters, you get the luminosity of Cen X-3 is about 1.3 times 10 to the 37 ergs per second. This is about 3000 times the entire energy output of the Sun. And, all this from an object whose radius is no bigger than half the length of Manhattan Island. So we've come full circle. It appears that the precisely varying x rays we see in Cen x-3, GK Per and other compact x-ray binaries are rotation hotspots associated with the star's magnetic field. Over the years we have discovered many such sources, each with a unique set of orbital circumstances, and each with its own characteristic idiosyncrasies. Indeed, we have just scratched the surface of the wonderful world of x-ray binaries. And many surprises were in store for astronomers over the past 40 years, and many more undoubtedly will come in the future. But now it is time to move on, and explore an entirely different class of objects. So stay tuned as we examine our cosmic recycling centers, Supernova remnants, which are the products of the most dramatic and energetic explosions that our universe has to offer.
