The brightening of the pulsar wind nebula of PSR B0540−69 after its spin-down-rate transition

Abstract

It is believed that an isolated pulsar loses its rotational energy mainly through a relativistic wind consisting of electrons, positrons and possibly Poynting flux 1-3 . As it expands, this wind may eventually be terminated by a shock, where particles can be accelerated to energies of X-ray synchrotron emission, and a pulsar wind nebula (PWN) is usually detectable surrounding a young energetic pulsar 1-3 . However, the nature and/or energetics of these physical processes remain very uncertain, largely because they typically cannot be studied in a timeresolved fashion. Here we show that the X-ray PWN around the young pulsar PSR B0540-69 brightens gradually up to 32±8% over the mean previous flux, after a sudden spin-down rate (ν) transition (SRT) by ∼ 36% in December 2011, which has very different properties from a traditional pulsar glitch 4 . No evidence is seen for any change in the pulsed X-ray emission. We conclude that the SRT results from a sudden change in the pulsar magnetosphere that increases the pulsar wind power and hence the PWN X-ray emission. The X-ray light curve of the PWN suggests a mean life time of the particles of 397 ± 374 days, corresponding to a magnetic field strength of 0.78 +4.50 −0.28 mG in the PWN.

Atacama Compact Array observations of the pulsar-wind nebula of SNR 0540-69.3

“…If one assumes that this break is due to synchrotron cooling of relativistic electrons, one can estimate the magnetic field strength, B, where the electrons are being injected. Equalling the lifetime of synchrotron-emitting electrons, τ synch = 6 × 10 11 B −3/2 ν −1/2 break s (Pacholczyk 1970;Brantseg et al 2014), to the age of the remnant, assumed to be 1100 yr (Reynolds 1985), this translates into B = (3.9 +1.1 −0.7 ) × 10 −4 G. This is higher than the findings of Manchester et al (1993b) and Brantseg et al (2014), who both estimated B ∼ 2.5 × 10 −4 G, but somewhat lower than a recent value derived from X-rays, B = (7.8 +45 −2.8 ) × 10 −4 G (Ge et al 2019). The spectral break between the low-and high-frequency parts of the synchrotron spectrum gives a steepening in power-law index by = 0.70 +0.08 −0.10 , which is slightly larger than the expected standard change of 0.5 due to synchrotron losses in a homogeneous medium.…”

“…The observed steepening in power-law index by 0.52 ± 0.11 to F ν ∝ ν −0.87 +0.10 −0.08 for ν > ∼ 2 × 10 13 Hz could be due to synchrotron cooling, as in the Crab. For a magnetic field strength of B ≈ 8 × 10 −4 G, as indicated by recent X-ray studies (Ge et al 2019), the synchrotron cooling time is ∼190 yr. For ACA Band 4, we clearly detect the SNR shell of SNR 0540-69.3, and we can constrain the spectrum of its synchrotron emission. We find that its spectral slope is steeper than that for the PWN.…”

Evolution of Neutron Star Magnetic Fields

“…Another candidate is PSR B0540-69. This pulsar demonstrates a very complicated behaviour: after a long period of stable behaviour [139], it changed its braking index from ∼2.1 down to 0.03, and then in a few years, rose up to 0.9 [140]. One interpretation of such an evolution is via field modification, due to instabilities in the NS interior [141].…”

Comparison of simulated backgrounds with in-orbit observations for HE, ME, and LE onboard Insight-HXMT

“…Based on this spectrum, the expected contribution of Cas A to HE at 20-300 keV is ∼3 counts per second. For PSR B0540-69, the spectrum of the pulsar plus its wind nebula observed by NuSTAR could be fitted with an absorbed power law with an index of 1.99 ± 0.01 (Ge et al 2019). A rate of ∼2 counts per second is obtained by extrapolating this spectrum to the energy band of HE.…”