## On the suspected timing error in Wilkinson microwave anisotropy probe map-making

Toruń Centre for Astronomy, Nicolaus Copernicus University,
ul. Gagarina 11, 87-100 Toruń, Poland

Received:
26
April
2010

Accepted:
27
May
2010

*Context. *It has recently been suggested that the compilation of the
calibrated time-ordered-data (TOD) of the Wilkinson Microwave Anisotropy Probe
(WMAP) observations of the cosmic microwave background (CMB) into
full-year or multi-year maps may have been carried out with a small
timing interpolation error.
A large fraction of the previously
estimated WMAP CMB quadrupole signal would be an artefact of
incorrect Doppler dipole subtraction if this hypothesis were
correct.

*Aims. *Since observations of bright foreground objects constitute part of
the TOD, these can be used to test the hypothesis.

*Methods. *Scans of an object in different directions should be shifted by the
would-be timing error, causing a blurring effect. For each of
several different timing offsets, three half-years of the
calibrated, filtered WMAP TOD are compiled individually for the four
*W* band differencing assemblies (DA's), with no masking of bright
objects, giving 12 maps for each timing offset. Percentiles of the
temperature-fluctuation distribution in each map at HEALPix
resolution *N*_{side} = 2048 are used to determine the
dependence of all-sky image sharpness on the timing offset. The
*Q*
and *V* bands are also considered.

*Results. *In the *W* band, which is the band with the shortest exposure times,
the 99.999% percentile, i.e. the temperature fluctuation in the
*≈*503-rd brightest pixel, is the least noisy percentile as a
function of timing offset. Using this statistic, the hypothesis that
a -25.6 ms offset relative to the timing adopted by the WMAP
collaboration gives a focus at least as sharp as the uncorrected
timing is rejected at 4.6*σ* significance, assuming Gaussian
errors and statistical independence between the maps of the 12
DA/observing period combinations. The *Q* and *V* band maps also reject the -25.6 ms offset hypothesis at high
statistical significance.

*Conclusions. *The requirement that the correct choice of timing offset must
maximise image sharpness implies that the hypothesis of a timing
error in the WMAP collaboration's compilation of the WMAP
calibrated, filtered TOD is rejected at high statistical
significance in each of the *Q*, *V* and *W* wavebands. However, the
hypothesis that a timing error was applied during *calibration * of the raw TOD, leading to a dipole-induced difference
signal, is not excluded by this method.

Key words: cosmology: observations / cosmic background radiation

*© ESO, 2010*