AGILE observations of the "soft" gamma-ray pulsar PSR B1509-58

We present the results of new AGILE observations of PSR B1509-58 performed over a period of similar to 2.5 years following the detection obtained with a subset of the present data. The modulation significance of the light curve above 30 MeV is at a 5 sigma confidence level and the light curve is similar to those found earlier by COMPTEL up to 30 MeV: a broad asymmetric first peak reaching its maximum 0.39 +/- 0.02 cycles after the radio peak plus a second peak at 0.94 +/- 0.03. The gamma-ray spectral energy distribution of the pulsed flux detected by COMPTEL and AGILE is well described by a power law (photon index alpha = 1.87 +/- 0.09) with a remarkable cutoff at E(c) = 81 +/- 20 MeV, representing the softest spectrum observed among gamma-ray pulsars so far. The pulsar luminosity at E > 1 MeV is L(gamma) = 4.2(-0.2)(+0.5) x 10(35) erg s(-1), assuming a distance of 5.2 kpc, which implies a spin-down conversion efficiency to gamma rays of similar to 0.03. The unusual soft break in the spectrum of PSR B1509-58 has been interpreted in the framework of polar cap models as a signature of the exotic photon-splitting process in the strong magnetic field of this pulsar. In this interpretation, our spectrum constrains the magnetic altitude of the emission point(s) at 3 km above the neutron star surface, implying that the attenuation may not be as strong as formerly suggested because pair production can substitute photon splitting into regions of the magnetosphere where the magnetic field becomes too low to sustain photon splitting. In the case of an outer-gap scenario or the two-pole caustic model, better constraints on the geometry of the emission would be needed from the radio band in order to establish whether the conditions required by the models to reproduce AGILE light curves and spectra match the polarization measurements.