The Magnetar Nature and the Outburst Mechanism of a Transient Anomalous X-ray Pulsar

TL;DR

This study spectroscopically confirms the magnetar nature of the AXP XTE J1810-197 by measuring its surface magnetic field at approximately 2.72×10^{14} G, and investigates the thermal and magnetic changes during its outburst.

Contribution

First direct spectroscopic measurement of an AXP's surface magnetic field confirming its magnetar status and analyzing its thermal evolution during outburst.

Findings

Surface magnetic field measured at (2.72±0.03)×10^{14} G

Magnetic field remains nearly constant during flux variations

Surface temperature declines steadily after outburst

Abstract

Anomalous X-ray Pulsars (AXPs) belong to a class of neutron stars believed to harbor the strongest magnetic fields in the universe, as indicated by their energetic bursts and their rapid spindowns. However, an unambiguous measurement of their surface field strengths has not been made to date. It is also not known whether AXP outbursts result from changes in the neutron star magnetic field or crust properties. Here we report a spectroscopic measurement of the surface magnetic field strength of an AXP, XTE J1810$-$197, and solidify its magnetar nature. The field strength obtained from detailed spectral analysis and modeling, B = (2.72$\pm$0.03)$\times10^{14}$ G, is remarkably close to the value inferred from the rate of spindown of this source and remains nearly constant during numerous observations spanning over an order of magnitude in source flux. The surface temperature, on the other hand, declines steadily and dramatically following the 2003 outburst of this source. Our findings demonstrate that heating occurs in the upper neutron star crust during an outburst and sheds light on the transient behaviour of AXPs.