The Fermi LAT team became aware (*) of an error in the preparation of the radio timing model used to calculate the rotational phase of GeV gamma-ray photons from the Crab. The main gamma-ray peak ("P1") precedes the 1400 MHz radio peak by 0.0044 +/- 0.0006 +/- 0.0006 in phase, as compared with the published value of 0.0085 +/- 0.0005 +/- 0.0006 (Abdo et al, ApJ 708, 1254-1267, 2010 January 10, hereafter "our Crab paper").

The error is briefly described below. The rotation ephemeris after correcting the error is in the file NewEphem_0534p2200_ApJ_708_1254_2010.par .

A corrected version of Figure 1 from our Crab paper is in the file Figure1.png  .
The phase histogram bin contents are provided in the ascii file CrabPulsar_LightCurve_Fermi_newephemeris.txt .

Results of fits of the corrected phase histograms (using two half-Lorentziens, as in our Crab paper) :
Peak P1:  0.9956 +/- 0.0006
Peak P2:  0.3940 +/- 0.0021

Thus, the gamma-ray peak leads the radio peak by 138 us, or roughly half the published value. The uncertainties are essentially unchanged.

The separation between P1 and P2 remains delta = 0.398

Gamma-ray phase values cited in the Tables, Figures, and text of our Crab paper should be increased by 0.004 .

The corrected Dispersion Measure is DM = 56.7856 +/- 0.0002 cm/pc^3 (as compared to 56.7037 +/- 0.0003 cm/pc^3 in our paper). The first time derivative of the Dispersion Measure, DM1, remains unchanged, within uncertainties. The DM epoch is 54781.6, the same as the period epoch.

The cause of the error was in determining DM, combined with how we found the overall time offset (the "JUMP" parameter in TEMPO2) between the Jodrell Bank and Nançay data. To correct the ephemeris we first used the large frequency lever arm provided by the JBO data (600 and 1400 MHz data) to accurately determine DM ; only then did we fit for the JUMP value needed to include the high precision Nançay times-of-arrival.

(*) We thank Nepomuk Otte, UC Santa Cruz, for pointing out the anomaly that he noticed while working on VERITAS' pulsed Crab signal at TeV energies (Science, in press).