Tiger, Tiger Burning Bright…

Now that we have a distance to this puzzling object, let’s find out its true luminosity. Open DS9, connect to the Virtual Observatory, and load the image of 3C273 from the Chandra-ed page. Note how different this source appears from Cas-A! It appears much smaller, and there is a little jet-like protrusion coming out from the lower right hand side of the object. Also, it seems to be black in the center. This is not really the way 3C273 is in the sky. It is an artifact of the satellite and occurs because the object is so bright in x-rays that Chandra’s counters get saturated. We call this phenomenon “pileup”, and it is similar to overexposure in a photographic image. But the x-rays are still there; they are just spread out along a column of the detector. See if you can adjust the contrast and brightness in DS9 to see the line of radiation. (If you can’t get a good look at this, select the “bb” color scheme, go to the color menu, click on contrast/bias, and set the contrast to 1.5 and the bias to 0.10). Now, load the analysis commands, and let’s roll up our sleeves and get to work!

Project 3: Find the luminosity of 3C273.

(3.1) Enclose the image of 3C273 and its jet with a circular region. We will be excluding some of the “pile-up” photons, but we are just interested in an “order of magnitude” estimate of the energy output from the object. Make a light curve, using 1000 second bin widths, normalized by time. (I.e. in the drop-down menu for light curve plots, enter 1000 in the box, and check both the bin width and normalize by time options). When you get the results, you should see a plot that has a y-axis value of about 1 count/sec.

What this means is that Chandra has received about one x-ray each second from the region of the sky near 3C273.Since this is the only strong source in the field of view of the satellite, we can say that this represents roughly the x-ray energy received from 3C273 in the energy band that Chandra is sensitive to. But think for a moment; 3C273 is pouring out these photons everywhere in the sky. Chandra only picks up a very tiny percentage of them. The rest keep streaming out into space, where no x-ray satellite is there to see them. In fact, we can imagine a huge ball, centered at 3C273, whose radius is equal to the distance from the source to the Earth. The tiny satellite’s area must be multiplied by the area of the ball (4pd², where d= distance from 3C273 to the Earth) to get the amount of x-radiation that 3C273 is giving off into space.

(3.2) It turns out that each count per second for the ACIS detector on board Chandra corresponds to about 10^-11 erg/sec of energy crossing each cm² of surface at the distance of the Earth. (Later on, we shall see a simple way to determine this more accurately.) So what is the x-ray output of 3C273? [ 1 ct/sec= 10^-11 erg/sec/cm². So L_x = 10^-11 x 4pd², where d = 800 Mpc = 800 x 10⁶ parsecs x 3 x 10¹⁸ cm/pc = 2.4 x 10²⁷ cm. Thus, L_x = 6 x 10⁴⁴ erg/sec. ]

This is almost one trillion times the entire energy output of our Sun, and 100 times the luminosity of our entire galaxy. Finding a mechanism to produce this much energy would be difficult under any circumstances. But the quasars present an even more difficult puzzle. These objects fluctuate in brightness, and because of this, they must be rather small. To see why this is so, let us move onward….