Indication of rapid magnetic field decay in X-ray Dim Isolated Neutron Star RX J0720.4-3125

TL;DR

This paper investigates the magnetic field decay in the neutron star RX J0720.4-3125 by analyzing its spin period derivative and thermal emission, suggesting a rapid decay timescale of about 10,000 years.

Contribution

It provides the first evidence linking the spin period derivative evolution with magnetic energy dissipation in this neutron star.

Findings

Magnetic field decay timescale is approximately 10^4 years.

Spin period derivative evolution correlates with magnetic energy dissipation.

Thermal emission excess indicates rapid magnetic field decay.

Abstract

Magnetic field evolution of neutron stars is a long-standing debate. The rate of magnetic field decay for isolated, non-accreting neutron stars can be quantified by measuring the negative second derivative of the spin period. Alternatively, this rate can be estimated by observing an excess of thermal emission with respect to the standard cooling without additional heating mechanisms involved. One of the nearby cooling isolated neutron stars -- RX J0720.4-3125, -- offers a unique opportunity to probe the field decay as for this source there are independent measurements of the surface X-ray luminosity, the second spin period derivative, and magnetic field. We demonstrate that the evolution rate of the spin period derivative is in correspondence with the rate of dissipation of magnetic energy of the dipolar field if a significant part of the released energy is emitted in X-rays. The instantaneous time scale for the magnetic field decay is $\sim 10^4$ years.