As we saw from our plot of the light curve, Quasars do have large scale variability over time. There are several competing models to try and explain the variability, but one fit the observed data exactly. A plot of the color variation of a quasar rather than just a single light curve could be potentially useful for explaining the observed variability. We'll use quasar 999508 again, with the data file found here:

4950833.dat

As before, we need to make use of a plotting program. The format for the datatset is shown in the table below:

Field (Column)	Format	Units	Label	Explanations
1	D	days	MJD_u	Modified Julian Date for u-band observation
2	F6.3	mag	u	Apparent u-band magnitude (1)
3	F5.3	mag	u_err	u-band error
4	D	days	MJD_g	Modified Julian Date for g-band observation
5	F6.3	mag	g	Apparent g-band magnitude (1)
6	F5.3	mag	g_err	g-band error
7	D	days	MJD_r	Modified Julian Date for r-band observation
8	F6.3	mag	r	Apparent r-band magnitude (1)
9	F5.3	mag	r_err	r-band error
10	D	days	MJD_i	Modified Julian Date for i-band observation
11	F6.3	mag	i	Apparent i-band magnitude (1)
12	F5.3	mag	i_err	i-band error
13	D	days	MJD_z	Modified Julian Date for z-band observation
14	F6.3	mag	z	Apparent z-band magnitude (1)
15	F5.3	mag	z_err	z-band error
16	D	deg	ra_median	Median Right Ascension in decimal degrees (J2000, 360° is subtracted from all RA values > 300°)
17	D	deg	decl_median	Median Declination in decimal degrees (J2000)

Note (1): The ugriz magnitudes are not corrected for Galactic absorption. Bad observations are printed as '-99.99'.

For IDL users, my readcol looks like the following:

readcol,'4950833.dat',mjdu,u,uerr,mjdg,g,gerr,mjdr,r,rerr,mjdi,i,ierr,mjdz,z,zerr,RA,DEC

To make our plot, we want to use the i band for our magnitude, and the g and i bands for the color. On the Y axis we want to plot the i band magnitude, while on the X axis we want to plot g-i for color. If you aren't sure how this equates to color, reading the background information on colors and magnitudes is suggested. I left lines connecting the data points to see if there were any trends in the variability tied to time of observatrion. After inputting our data, we should get a plot that looks like this:

From our plot, small variations in brightness tend to push the quasar towards the red, while large variations in magnitude cause the quasar to shift towards blue. Similar to the last plot of the lightcurve, this is likely because there is more than one mechanism at work. The starburst model, changes in accretion rate, and gravitional lensing can all cause changes in magnitude and color of a quasar. A combination of these mechanisms is a probable cause of the difference between the small and large scale variations.

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