Timing of young radio pulsars – II. Braking indices and their interpretation

Abstract

In Paper I of this series, we detected a significant value of the braking index (n) for 19 young, high-E radio pulsars using ∼ 10 years of timing observations from the 64-m Parkes radio telescope. Here we investigate this result in more detail using a Bayesian pulsar timing framework to model timing noise and to perform selection to distinguish between models containing exponential glitch recovery and braking index signatures. We show that consistent values of n are maintained with the addition of substantial archival data, even in the presence of glitches. We provide strong arguments that our measurements are unlikely due to exponential recovery signals from unseen glitches even though glitches play a key role in the evolution of a pulsar's spin frequency. We conclude that, at least over decadal time scales, the value of n can be significantly larger than the canonical 3 and discuss the implications for the evolution of pulsars.

Braking indices of young radio pulsars: theoretical perspective

“…According to Kochanek et al (2019) we are left with small (but non-negligible) fraction of such systems, few per cent at most. For three pulsars in the sample (PSR J0857-4424, PSR J1637-4642, PSR J1830-105) presence of either stellar or planetary companions have been discussed (Parthasarathy et al 2020), but detailed analysis did not confirm this hypothesis.…”

“…If all large braking indices are caused by presence of a companion on a wide orbit, we would expect random orientation of this companion's orbit in respect to the line of sight and random orbital phase, thus the gravitational acceleration (and braking index) should be positive in approximately half of the cases ( = 0.5) and negative in the other half (see eq. 19 in Parthasarathy et al 2020 which depends on the orbital phase as cos ). We plot oscillations of braking index for parameters of PSR J0857-4424 estimated by Parthasarathy et al (2020) (mass of unseen companion = 1 M ⊙ , orbital period 130 years, inclination = 30 • ).…”

“…Recently, Parthasarathy et al (2019Parthasarathy et al ( , 2020 measured braking indices, = 2 − ( )/( 2 ), for 19 young radio pulsars. Parthasarathy et al (2020) argued that this result is robust against uncertainties in the irregular (stochastic) component of pulsars' timing noise and reflects a regular pulsar evolution at least at the timescales of tens of years. The obtained values for eleven of them are in the range ∼ 10 -100, two are negative, two are above 100 and only four are compatible with the classical value = 3 predicted by the magneto-dipole formula for constant field and obliquity angle.…”

“…For most of them only upper limits on the braking indices (in the range 10 -2000) were obtained. Parthasarathy et al (2020) noted that braking indices are measured in 12 out of 19 radio pulsars with spin-down age less than 120 Kyr which is a fraction of ≈ 60 per cent. For older pulsars this fraction is much smaller 7/66 ≈ 11 per cent.…”

“…For older pulsars this fraction is much smaller 7/66 ≈ 11 per cent. Parthasarathy et al (2020) mentioned several hypotheses to explain large values of the braking index. In the present article we test these hypotheses and aim at identifying the physical cause for these measurements.…”