Hi gang and welcome back to analyzing the the Universe. Today we begin our exciting in depth source by source analysis of some of the most interesting objects in the sky. X-ray astronomy has been a curious mix of finding unusual behavior in previously known objects. Plus the startling discovery of hither to completely unknown sources. The latter objects in the early days meaning the 1960s were designated by the constelation in which they resided appended with x1, x2, x3. Etc., in order of their discovery. Thus we had a mixture of names such as AM-Her. Where Her is short for the constellation Hercules, and Sco X1. Or Cen X3. Where Sco stands for Scorpio, and Cen indicates Centaurus. We will begin our source by source saga, with a well known object, GK Per, a double star in the constellation of Perseus. Which has a white dwarf as a member of the pair. The nomenclature is pretty arcane but any star with one or two capital letters at the beginning of its designation means that the star is variable in light. And indeed, late in the evening of February 21st, 1901, a Scottish clergyman, Thomas Anderson, was working home in Edinborough where he saw a brilliant star in Perseus when none was present before. The next day, he notified the Greenwich Observatory and found out that he was the discoverer of Nova Persei 1901, the first new star of the 20th century. Back at the Harvard College Observatory, it was found that this star had been seen before on photographic plates, but at a magnitude 13, about 500 times fainter than the faintest star visible to the naked eye. A number of photographs revealed that the star had small fluctuations in light. But never anything like the factor of 10,000 that Anderson's observation implied. Indeed, at maximum light, two days later, Nova Persei was briefly emitting about 400,000 times more light than it had done previously. Fortuitously, one of the Harvard photographs was taken just two days before the outburst. Thus, in less than 4 days, the star increased its luminosity by 400,000. Now, that's what I call some explosion. In the light curve displayed here, you can see that six days after its maximum, the nova had faded by about a factor of 6. And it continued to fade smoothly until after about two weeks, when a series of oscillations set in with a period of about four days. These fluctuations lasted for several months, while the star faded some more until at last 11 years after its outburst it was back to its original brightness. Apparently oblivious of its former glory. However, decades after it's spectacular outburst, Nova Persei 1901, also designated GK Per, began in 1966 to display smaller outbursts of about three magnitudes in amplitude occurring about every three years. You can see them in this long-term light curve shown here. It was during one of these outbursts in 2002 that NASA's Chandra satellite looked at the object to see if any x-rays were being emitted. Let's go to DS9 and check it out. Alright, so we open DS9. We go to analysis. Go down to the virtual observatory and remember to connect using the web proxy. Click on the Rutgers primary MOOC. Here we have a list of all the observations that we can access. Let's look for the GK Per observation. And here it is. Chandra HETG Spectroscopy of GK Per in Outburst. We click on the title and voila, there it is. Let's minimize this, minimize this, because we don't need it anymore. Notice there are three regions here. There's a circle surrounding this point-like object, which in fact, is the star, and two long, skinny rectangles extending diagonally across. These rectangles are there because this particular observation was made with a detector known as a transmission grading, which acts like a prism to disperse the different energy X-rays into the regions of these rectangles. The rectangles assure us of collecting all the photons, but the actual position of the object is the little dot in the center. You can see the RA and deck in the information box over here. Let's see if there's anything there in the optical or visible part of the spectrum. We go to Analysis, we go to Image Servers, we go to SAODSS. That will bring up a little box. And you'll see that the array and deck are already in there for us. We're going to retrieve it. And it's going to get all of that optical data, and with a little bit of luck, It seems to be hung, nope we finally got it. Okay, well that shows you again that you really have to be patient sometimes when you are doing some of these analysis tools and don't be too quick to try to think that there is some problem there. But let's look at that. Look at this is a very interesting looking object. Let's minimize our SAO box here. And let's lock the two frames together. Notice again this little blue border around this particular observation. We want that blue border to be around our x-ray box, so we'll make everything the size of the x-ray observation instead of vice versa. So we just click anywhere in the left frame Now we go to frame. Match. Frame. WCS. And now you see that the optical observation has been magnified so that it is in exactly the same proportional size as the X-ray image. Now we go to Edit And we change our pointer to a cross hair, and now we can lock the cross hairs. We go to Frame>Lock>Cross Hairs>WCS, and now we can grab. Any one of those crosshairs and cruise around. In particular, let's grab this crosshair, and see there's this very weird looking blob, that seems to be surrounding this point like object. That in the x-rays seems to be pretty bright. It turns out that even though it looks like there's nothing except at the center of where this object seems to be located, There doesn't seem to be any 'x' rays, there are tiny amounts of them, we can't just, we just can't see it in our frame over here but the majority of the 'x' rays is coming from our little tiny point like object Which is the position of GK Per. But what is this blob? Well, six months after its fantastic explosion, the French astronomer Flammarion announced that they had photographed a luminous shell surrounding the nova. This was simply astonishing. Typically it takes years to see this show develop. And its subsequent expansion seemed to imply, that the object was expanding faster than the speed of light. Clearly, this was a crisis in the making. Let's go back to the blackboard and see if we can find out what the explanation of this phenomenon is. What Flamarion saw, we think, was an example of a light echo. The way it works is as follows. Imagine a flash of light which is what the explosion must have looked like when it happened. As the flash expanded, it looked like a light bubble. Now imagine a sheet of dusty material between the nova and the Earth. When the bubble reached the sheet, the light interacted with it, much like sunlight bounces off of clouds in the sky, and sent light from the interaction towards the Earth. As the bubble expanded, other parts of it finally intersected the sheet. Notice that the additional distance that the light bubble actually traveled, shown here in red. Was actually very small, so it occurred soon after the initial contact. But the apparent radius of the deflected rays, shown here in blue, could be quite large, and thus the light echo appeared to be super luminal, traveling faster than light. But it really wasn't. The phenomenon was saved. This is typical in science. We have an idea that we are loath to give up because it's been so useful in the past. Such things as conservation of energy are such a useful part of our analytical arsenal. That it be behooves us to look for alternative explanations when faced with an odd apparently inexplicable phenomenon. This is not to say that such beliefs never change indeed the existence of the idea that no information can be transmitted faster than the speed of light came about because of the necessity to abandon other Quite cherished beliefs, concerning the nature of space and time itself. 15 years after this light echo was observed, a material shell did appear surrounding the star. And it is still expanding. Doppler measurements which we will study a bit later, showed that the velocity of this shell was 1200 km per second. Observations over several years show that it was expanding at the rate of zero point. Five, four arc seconds per year. You can actually see the expansion in this video which steps you through from 1953 to the present. But beyond being a pretty movie, this provides us with a spectacular way to get an accurate distance to GK Per. The idea is this - in one year the distance the shell expands is D equals. Velocity times the time. Or, 1200 kilometers per second, times 3 times 10 to the seven Seconds, which is the number of seconds, approximate number of seconds, in one year. This works out to about 3.6 times 10 to the 10 kilometers. This distance Which we can represent just by this little line. Is therefore corresponding to an angle Theta of 0.54 arc seconds, so we know this distance, we know this angle and we can calculate through Simple trigonometry, what the distance is. It turns out to be about 470 par secs, or about 1,500 light years. Pretty neat, huh? Now that we have explored the appearance and evolution of GK Per, In our next lecture, we will return to DS9 and look at what GK Per is telling us in the realm of the X-rays.
