LONG – TERM ROTATIONAL STABILITY OF SOME ISOLATED RADIO PULSARS
ABSTRACT
We re-examined the phenomenon of rotational instability in radio pulsars on decadal timescales using a sample of 315 radio pulsars whose spin-rates [rotation period (P) and spin-down rate ( Pı) ] have been measured on, at least two different epochs. The first stage of our analysis involves a direct comparison of the measured values of P and Pı at two epochs that are ~ 10 – 20 years apart in order to estimate the fluctuation in the parameters. The results show that the fluctuation in the objects’ rotational period (DP) is significant in 293 pulsars, while it is at the level of measurement uncertainties in the remaining 22 pulsars. For 293 objects, the fractional change in P ( D/P) was found to have absolute values in the range of ~ 10-11 -10-1 . Similarly, the observed fluctuation in spin-down rate (∆ Pı) was considered significant in only 178 pulsars in the sample. In this case, the fractional change in the parameter ( D ıPı ), irrespective P of sign, has values in the range of ~ 10-5 -102 . Further analysis of our sample reveals a moderate correlation ( with r ~ 0.6) between DP and ∆ Pı. On the other hand, only weak correlations ( with |ı| < 0.3 ) were found between DP / D Pı and the pulsar spindown parameters, which suggests that they are unrelated. For seven radio pulsars, whose spin-rates (P and ∆ Pı) have been measured at five different epochs, we found strong evidence for non-stationarity of the objects’ spin-down rates. The braking indices (n) of the 315 radio pulsars were extensively measured using three different methods that involve: (i) integration of the spin-down law (n ) , differentiation of the spin-down law (n ) and direct measurement of pulsar period second derivative ( Pı) using measurement of Pı at two epochs ~ 10 – 20 years apart (n ) . The three methods produced braking indices of anomalous values with magnitudes in the range of ~ 0.1 -1011 . Irrespective of methods of measurement, the absolute values of n were found to be strongly correlated ( with r ~ 0.7 – 0.8) with the pulsar spin-down age (t ) and anti- correlated ( with r ~ 0.6 – 0.8) with objects’ spin-down rate ( Pı) and surface magnetic field (Bsurf ).