''Super-sequence'': Emission Patches from PSR B0809+74 at the Low Frequencies

© Xiaoxi Song, Vlad Kondratiev, Anya Bilous

My summer project focused on single-pulse analysis of PSR B0809+74 using LOFAR Low Band Antennas. While analysing single-pulse spectra, we spotted a distinct 50-pulse sequence of emission patches. We dubbed it as a ''super-sequence''. The super-sequence snapshot (upper panel) shows the band-averaged profiles of pulses (right top subplot) with spectrum of all pulses superimposed (left top subplot, showing intensities with S/N > 3.5 only). For clarity, the pulses are divided in 5 groups represented by different colours. The apparent emission patch starts at frequency of about 15 MHz, drifting in phase and growing in spectral width towards high frequencies. It essentially drifts in both frequency and time.

We argue that the super-sequence is very unlikely to be pulsar-intrinsic. With the dispersion measure of 5.7 pc/cm^3, single pulses from B0809+74 are dispersed for about 100 s between 15 and 62 MHz. Thus, any broadband brightness variations across this 100-s time scale will present itself as ''patches'' in pulse spectra (see cartoon at the bottom of the image). At lower frequencies, smaller portion of pulse spectrum is affected. Thus, after de-dispersion we observe the sequence of patchy pulses with ''emission patch'' moving up in frequency and growing in spectral width. In addition, PSR B0809+74 exhibits the phenomenon of drifting subpulses, thus the patches move also in phase.

Observed broadband brightness variations can be potentially related to the apparent source position jitter due to the ionosphere. It is important to be aware of this non-pulsar-intrinsic spectrum modulation when carrying out single-pulse analysis at low frequencies. Similar phenomenon is also observed with High Band Antennas, where modulation is in turn caused by interplanetary scintillations (see daily image on 07-05-2014).