AXPs and SGRs in the outer gap model: confronting Fermi observations

TL;DR

This paper tests the outer gap model's predictions for gamma-ray emissions from AXPs and SGRs, finding a discrepancy with Fermi-LAT observations that challenges the magnetar hypothesis.

Contribution

It calculates theoretical gamma-ray spectra for AXPs and SGRs under the outer gap model and compares them with observations, highlighting conflicts and alternative explanations.

Findings

Most AXPs and SGRs should emit detectable gamma-rays if they are magnetars.

Fermi-LAT did not detect gamma-rays from these sources, conflicting with model predictions.

Future observations could distinguish between magnetar and fallback disk models.

Abstract

Anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) are magnetar candidates, i.e., neutron stars powered by strong magnetic field. If they are indeed magnetars, they will emit high-energy gamma-rays which are detectable by Fermi-LAT according to the outer gap model. However, no significant detection is reported in recent Fermi-LAT observations of all known AXPs and SGRs. Considering the discrepancy between theory and observations, we calculate the theoretical spectra for all AXPs and SGRs with sufficient observational parameters. Our results show that most AXPs and SGRs are high-energy gamma-ray emitters if they are really magnetars. The four AXPs 1E 1547.0-5408, XTE J1810-197, 1E 1048.1-5937, and 4U 0142+61 should have been detected by Fermi-LAT. Then there is conflict between out gap model in the case of magnetars and Fermi observations. Possible explanations in the magnetar model are discussed. On the other hand, if AXPs and SGRs are fallback disk systems, i.e., accretion-powered for the persistent emissions, most of them are not high-energy gamma-ray emitters. Future deep Fermi-LAT observations of AXPs and SGRs will help us make clear whether they are magnetars or fallback disk systems.