AGILE and Fermi Observations of the Soft Gamma-Ray Pulsar B1509-58

TL;DR

This study reports on 2.5 years of AGILE and Fermi gamma-ray observations of pulsar B1509-58, revealing a soft gamma-ray spectrum with a cutoff below 100 MeV and providing insights into its emission mechanisms.

Contribution

First detailed gamma-ray spectral and timing analysis of PSR B1509-58 combining AGILE and Fermi data, highlighting its unique soft spectrum and potential emission models.

Findings

Gamma-ray modulation detected at 5 sigma significance.

Spectrum shows a cutoff below 100 MeV, the softest among gamma-ray pulsars.

Light curve features two peaks with specific phase offsets.

Abstract

We present the results of 2.5 years of AGILE observations of PSR B1509-58 and of the same interval of Fermi observations. The modulation significance of AGILE light curve above 30 MeV is at a 5 sigma confidence level and the light curve shows a broad asymmetric first peak reaching its maximum $0.39 \pm 0.02$ cycles after the radio peak plus a second peak at$0.94 \pm 0.03$. The gamma-ray spectral energy distribution of pulsed flux is well described by a power-law (photon index $\alpha=1.87\pm0.09$) with a remarkable cutoff at below 100 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.