AGILE observations of PSR B1509-58: a new class of highly magnetized, 'soft' gamma-ray pulsars?

TL;DR

This paper reports AGILE observations of PSR B1509-58, revealing a very soft gamma-ray spectrum with a cutoff at 81 MeV, possibly indicating photon splitting in its strong magnetic field, and discusses implications for pulsar emission models.

Contribution

First detection of a remarkably soft gamma-ray spectrum with a cutoff at 81 MeV in PSR B1509-58, suggesting photon splitting effects in its strong magnetic field.

Findings

Gamma-ray spectrum well described by a power-law with photon index 1.87

Spectral cutoff at 81 +/- 20 MeV, the softest among gamma-ray pulsars

Light-curve features consistent with previous observations

Abstract

We present the results of 2.5 years AGILE observations of PSR B1509-58. The modulation significance of the light-curve above 30 MeV is at a sigma confidence level and the light-curve is similar to those found earlier up to 30 MeV by \comp: 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 pulsed flux 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 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 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.