Modelling of isolated radio pulsars and magnetars on the fossil field hypothesis

Abstract

We explore the hypothesis that the magnetic fields of neutron stars are of fossil origin. For parametrized models of the distribution of magnetic flux on the main sequence and of the birth spin period of the neutron stars, we calculate the expected properties of isolated radio pulsars in the Galaxy using as our starting point the initial mass function and star formation rate as a function of Galactocentric radius. We then use the 1374‐MHz Parkes Multi‐Beam Survey of isolated radio pulsars to constrain the parameters in our model and to deduce the required distribution of magnetic fields on the main sequence. We find agreement with observations for a model with a star formation rate that corresponds to a supernova rate of 2 per century in the Galaxy from stars with masses in the range 8–45 M⊙ and predict 447 000 active pulsars in the Galaxy with luminosities greater than 0.19 mJy kpc2. The progenitor OB stars have a field distribution which peaks at ∼46 G with ∼8 per cent of stars having fields in excess of 1000 G. The higher‐field progenitors yield a population of 24 neutron stars with fields in excess of 1014 G, periods ranging from 5 to 12 s, and ages of up to 100 000 yr, which we identify as the dominant component of the magnetars. We also predict that high‐field neutron stars (log B > 13.5) originate preferentially from higher‐mass progenitors and have a mean mass of 1.6 M⊙, which is significantly above the mean mass of 1.4 M⊙ calculated for the overall population of radio pulsars.

Pulsars and Magnetars

“…The wide distribution of field intensities in magnetic white dwarfs has been attributed to the spread in the magnetic fields of their progenitors and a similar situation could apply to NSs [52,84]. The possible association of some AXPs/SGRs with clusters of massive stars seems to indicate that they descend from stars of mass above ∼40 M ⊙ .…”

The strongest cosmic magnets: soft gamma-ray repeaters and anomalous X-ray pulsars

“…The wide distribution of field strengths in magnetic white dwarfs is thought to result from the spread in the magnetic fields of their progenitors. Extrapolating this result to the more massive progenitors of neutron stars could explain the origin of magnetars [70]. On average, higher magnetic fluxes are expected in the more massive progenitors.…”

Warm and dense stellar matter under strong magnetic fields

“…(4)- (8) self-consistently at a given baryon density ρ = b ρ v b in the presence of a strong magnetic field. The energy density of stellar matter is given by…”

“…On the other hand, it is estimated that the interior field in neutron stars may be as large as 10 18 G [7]. Ferrario and Wickammasinghe [8] suggest that the extra strong magnetic field of the magnetars results from their stellar progenitor with a high magnetic field core. Iwazaki [9] proposed that the huge magnetic field of the magnetars is some color ferromagnetism of quark matter.…”