Anomalous X-ray pulsars (AXPs) are thought to be magnetars which are young isolated neutron stars with extremely strong magnetic fields of >1014 Gauss. Their tremendous magnetic fields inferred from the spin parameters provide a huge energy reservoir to power the observed X-ray emission. High-energy emission above 0.3 MeV has never been detected despite intensive search. Here, we present the possible Fermi Large Area Telescope (LAT) detection of γ-ray pulsations above 200 MeV from the AXP, 1E 2259+586, which puts the current theoretical models of γ-ray emission mechanisms of magnetars into challenge. We speculate that the high-energy γ-rays originate from the outer magnetosphere of the magnetar.
Neutron stars are now known to have many different manifestations besides rotation-powered and accretionpowered pulsars. While pulsars typically have a surface magnetic field of ~1012 G, it has been suggested that neutron stars can possess magnetic fields with a strength as high as ~1015 G (Duncan et al. 1992). These highly magnetized neutron stars are called magnetars. The existence of magnetars provides a unique laboratory for exploring the physics of compact objects in the presence of a ultrastrong magnetic field. Based on current observations and theories, the emissions from magnetars mainly emerge in X-ray energy bands; their broad band spectral shapes can be well described by a blackbody component (with a hard tail probably due to Compton scattering) below 10 keV, which is likely from the magnetar’s surface, plus a nonthermal component dominating above 10 keV, which is from the magnetosphere (Thompson et al. 2002). On the basis of theoretical models of high-energy emission from magnetars, it is not expected to detect emission above ~1 MeV (Thompson et al. 1995). Although Castro et al. (2012) have report the
For the spectral analysis, we used the LAT data between 2008 August 04 and 2011 November 09 (3.5 years of data). To reduce and analyze the data, the Fermi Science Tools v9r23p1 package, available from the Fermi Science Support Center, was used. We used Pass 7 data and selected events in the Source class (i.e. event class 2) only. In addition, we excluded the events with zenith angles larger than 100o to greatly reduce the contamination by Earth albedo gammarays. The instrumental response functions (IRFs) P7SOURCE V6 were adopted throughout the study. Fig. 1 shows the
binned energy spectrum of CTB 109/1E 2259+586.
As 1E 2259+586 is known to have frequent glitches that are sudden increases in its spin frequency, we only used data taken after the last glitch seen on 2009 February 18 to search for
Pulsed
If the detection of the pulsed