Precision measurement of the scalar perturbation spectral index , n _ { s } , from the Wilkinson Microwave Anisotropy Probe temperature angular power spectrum requires the subtraction of unresolved point source power .
Here we reconsider this issue , attempting to resolve inconsistencies found in the literature .
First , we note a peculiarity in the WMAP temperature likelihood ’ s response to the source correction : Cosmological parameters do not respond to increased source errors .
An alternative and more direct method for treating this error term acts more sensibly , and also shifts n _ { s } by \sim 0.3 \sigma closer to unity .
Second , we re-examine the source fit used to correct the power spectrum .
This fit depends strongly on the galactic cut and the weighting of the map , indicating that either the source population or masking procedure is not isotropic .
Jackknife tests appear inconsistent , causing us to assign large uncertainties to account for possible systematics .
Third , we note that the WMAP team ’ s spectrum was computed with two different weighting schemes : uniform weights transition to inverse noise variance weights at l = 500 .
The fit depends on such weighting schemes , so different corrections apply to each multipole range .
For the Kp2 mask used in cosmological analysis , we prefer source corrections A = 0.012 \pm 0.005 \mu K ^ { 2 } for uniform weighting and A = 0.015 \pm 0.005 \mu K ^ { 2 } for N _ { obs } weighting .
Correcting WMAP ’ s spectrum correspondingly , we compute cosmological parameters with our alternative likelihood , finding n _ { s } = 0.970 \pm 0.017 and \sigma _ { 8 } = 0.778 \pm 0.045 .
This n _ { s } is only 1.8 \sigma from unity , compared to the \sim 2.6 \sigma WMAP 3-year result .
Finally , an anomalous feature in the source spectrum at l < 200 remains , most strongly associated with W-band .