Expected imprints of the carousel in multi-frequency pulsar observations and new evidence for multi-altitude emission

Subpulse modulation has been regarded as one of the most insightful and intriguing aspects of pulsar radio emission. This phenomenon is generally explained by the presence of a carousel of sparks in the polar acceleration gap region that rotates around the magnetic axis due to the E × B drift. While there have been extensive single pulse studies, geometric signatures of the underlying carousel, or lack thereof, in simultaneous multi-frequency observations have remained largely unexplored. This work presents a theoretical account of such expected signatures, particularly that of a geometry induced phase-offset in subpulse modulation, including various formulae that can be readily applied to observations. A notable result is a method to resolve aliasing in the measured subpulse modulation period without relying on knowledge of the viewing geometry parameters. It is also shown in detail that the geometry induced phase-offset enables critical tests of various observed phenomena as well as proposed hypotheses, e.g., multi-altitude emission, magnetic field twisting, pseudo-nulls, etc., in addition to that of the carousel model itself. Finally, a detailed analysis of a 327 MHz pulse sequence of PSR B1237+25 is presented as a case study to test the single frequency multi-altitude emission scenario. The analysis provides firm evidence of the inner and outer conal components of this pulsar to have originated from the same carousel of sub-beams and emitted at different heights.

Note:

Accepted for publication in ApJ

pulsars: general
pulsars: individual
radiation mechanisms: non-thermal
stars: neutron

References(43)

Figures(10)

[1]

A formulation has been presented for the expected change in the modulation phase under a pulse component observed simultaneously at two or more frequencies. The phase-offset is illustrated using simulated pulse sequences at two frequencies, and expected variations with the viewing geometry are demonstrated with a reasonable assumption about the frequency dependence of the emission cone size

[2]

It is shown that the above phase-offset can be used to resolve the aliasing issue in the measured {P}_{3} without relying on knowledge of the viewing geometry of the pulsar. For a known emission geometry, the phase-offset can also be used to determine various physical parameters associated with the carousel, e.g., the average velocity of the sparks and the carousel circulation period

[3]

The phase-offset between the subpulse modulations observed simultaneously at two or more frequencies has important implications in probing several phenomena (e.g., single frequency multi-altitude emission, field twisting, orthogonal polarization modes emission sites, etc.) as well as scrutinizing the carousel model and related phenomena (e.g., the pseudo-nulls)

[4]

A detailed analysis of a 327 MHz pulse sequence from B1237+25 has been presented uncovering the modulation even under the inner conal component on the trailing side. The phase-offsets between modulations under the inner and outer conal components are presented as strong evidence for single frequency multi-altitude emission in B1237+25. Within this multi-altitude scenario, it is shown that the measured {P}_{3} of 2.7-2.8 P is definitely not aliased