Emission heights and possible radius-to-intensity mapping in pulsar magnetospheres

Abstract

Pulsars are highly magnetized, rapidly spinning neutron stars. One of their most notable characteristics is that they emit narrow beams (in the form of a cone) of electromagnetic (EM) radiation from the space above the magnetic poles defined by the last open magnetic field lines. Their magnetic and rotational axes are not aligned, causing a “lighthouse ef- fect” whenever one of the beams crosses our line of sight. Every time one of the beams crosses our line of sight we see a “pulse” of EM radiation. Soon after discovery of the first pulsar it was found that average profiles, obtained by integrating many single pulses synchronously with the period of the pulsar, depend strongly on the observing frequency. This effect is known as Radius-to-Frequency Mapping (RFM). The simplest explanation is that higher frequencies are generated in the narrow part of the emission cone, closer to the surface of the star, and lower frequencies are produced in the wider part of the cone, which is further away from the star. While the scientific community agrees that RFM generally is working, it is not quite clear how thick a layer producing one radio frequency may be, and if it is possible that two or more radio frequencies could originate from the same layer of the magnetosphere. Previous results also show that not only average profiles depend on frequency, but also on intensity. We studied the intensity dependence of the average pro- files of 7 pulsars, 5 of them at two different frequencies. For each frequency we integrated pulses in 10 intensity bins to produce 10 average profiles, one for each frequency. We found that, in general, the width of the profiles are intensity dependent. The profiles of the strongest pulses were always narrower than those of the weakest. Additionally, we found that in most pulsars, component peak-to-peak separation decreased with increasing inten- sity. We interpret this as different intensities being emitted at different emission heights, where the highest intensities (for one frequency) are emitted closer to the star and lower intensities further away, we call this Radius-to-Intensity Mapping (RIM). We calculated emission heights by measuring the width of the average profiles at different intensities at 1% of the maximum level. For the 5 pulsars studied at more than one frequency, 4 have overlapping regions, which challenges conventional views that each frequency is produced in a separate layer of the beam emitting the radiation.