| # # Arrival Time
| Photons of visible light enter the telescope, reflect off parabolic mirror surface and are collected at the focus. Many photons enter the telescope at the same time. Images created from the photon data are from an Advanced CCD Imaging Spectrometer (ACIS) observation and has a binning time of 3.2401 seconds. Any photon that is detected within that interval will be assigned the same arrival time. Any photon that is detected within that interval will be assigned the same arrival time. We therefore must decide how to divide up our data. We can choose to do this in a number of ways. The most informative is to choose a bin width that has a reasonable number of photons in it, and to normalize it as a function of time, so that we can see the count rate, i.e. the number of counts each second that the satellite detects. We multiply the binning time of 3.2401 seconds by 10 so that there are enough photons in each bin so we can see them. Data collected by observations with Chandra are stored on a recorder for later transmission to the ground every eight hours during regularly scheduled Deep Space network contacts. The data is then transmitted to the Jet Propulsion Laboratory and then to Operations Control at the Chandra X-ray Center (CXC) in Cambridge, MA for processing and analysis by scientists. Now let's try to analyze some data like the scientists at the CXC. Timing Analysis Access the Cas A image and make a circular region over the entire remnant. Like the one below. In the analysis drop down menu, select "light curve plot", as shown below. Place a value of 32.4104 in the upper part of the dialog box, and check both other boxes. A light curve is a one-dimensional histogram of the number of x-ray photons that fall into discrete time bins. The number of photons is on the y-axis and the time bins is in the x-axis. In the light curve plot, select view--> axis range... and change the Y axis to read from 0 to the upper value on curve. Deselect the auto box. Hit OK . For example, the light curve plot below varies rapidly from about 600 to 650 photons. If you change the value of the axis range from 0-650 the lower graph will result, which makes the curve look like it is changing less rapidly. Now make the size of the circular region smaller, and choose a different area on the remnant (a bright spot). Compare the light curve from this region to the previous area. What happens to the number of photons? increases decreases remains the same Repeat this process choosing an area that is less bright. Do not change the size of the circle. What happens to the number of photons? increases decreases remains the same Explain these observations. What does this all mean? Light curves (which present the brightness of the source as a function of time) can be very exciting and make the objects look like real "clocks in the sky". And indeed, at the time of its explosion, the light curve of Cas-A must have been incredibly rich and variable, marking the brightening and subsequent dimming of the central object. The bright, almost circular "ring" that we see in Cas-A is the current position of the expanding debris from the explosion. In reality, it is a large hollow shell with very little material in the interior region near the pulsar, since the explosion has swept up the material much like a snowplow does when it drives through snow. Because the material is moving so rapidly, a shock wave forms. We see this as a faint outer shell outside the main ring of ejecta. The jet-like structure visible on the left side of the remnant may indicate higher velocity material rushing outward through a rarified part of the interstellar medium.