Effect of Interstellar Medium on Radio Pulsar Spin-Down Properties

Effect of Interstellar Medium on Radio Pulsar Spin-Down Properties


A statistical analysis of a large sample of 668 radio pulsars was undertaken in other to investigate the possible dependence of interstellar medium (ISM) parameters [dispersion measure (DM) and rotation measure (RM)] on pulsar spin-down parameters [rotation period ( ) and spin-down rate ( )]. The existence of such relationship will have a far reaching implication on the theories of pulsar birth and evolution. A simple descriptive analysis of the data reveals that the sample is quite heterogeneous, consisting of normal and recycled millisecond pulsars. Specifically, the published values of, DM and RM for objects in the sample were found to vary over a wide range: ~ 0.002 – 9 s, 3 × 10-21 – 2 × 10-11 ss-1, 2 – 1100 cm-3pc and -3000 – 2400 radm-2, respectively, with the corresponding mean values of ~ 0.69 ± 0.02 s, (9.94 ± 0.05) × 10-14 ss-1, 181 ± 7 cm-3pc and 157 ± 10 radm-2. Scatter plots of DM and RM (irrespective of sign) against reveal large amplitude (~ 3 orders of magnitude) scatter in the ISM parameters superimposed on a striking trend in which pulsars with large values of (young pulsars), on average, are characterized by large DM and |RM| values. A simple regression analysis of the DM/|RM| – data yields a moderate correlation (with correlation coefficient r ~ 0.5). A simple interpretation of the moderate DM/|RM| ‒ relationship is that objects with large (corresponding to young pulsars) are, on average, located in a region of ISM with high electron density content (in this case, the galactic plane). On the other hand, smaller values of , DM and |RM| correspond to relatively older pulsars located in regions farther away from the galactic plane (with low electron density content). The DM/|RM| ‒ correlation increased significantly (with correlation coefficient r ~ 0.95), when mean values of the parameters were employed in the analysis. The observed large scatter in the ISM data highlights the complex nature of the electron content distribution in the ISM and the large dispersion in both the magnitude and direction of pulsar space velocities. Similar analysis did show any appreciable dependence of both DM and RM on the pulsar rotation period. Our analysis also reaffirms the existence of a strong correlation (r ~ 0.7) between the DM and RM parameters, which are used to characterize the ISM.

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