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Paper: Turn-Over in Pulsar Radio Spectra
Volume: 407, The Low-Frequency Radio Universe
Page: 341
Authors: Kijak, J.; Lewandowski, W.; Gupta, Y.
Abstract: Our recent observations using the GMRT (2005-2008) have revealed new results in pulsar spectra. We find a new class of pulsars that show a maximum flux in the spectrum above 1 GHz (the so called “peak frequency”). For these pulsars the emitted energy decreases below 1 GHz, showing a positive spectral index at low frequencies. The value of “peak frequency” appears to correlate with both pulsar age and dispersion measure. A possible relation with pulsar age is interesting, as millisecond pulsars, which are very old objects, show no evidence for spectral turn-over down to 100 MHz. On the other hand, some pulsars with turn-over at high frequencies have been shown to have very interesting interstellar environments. This could suggest that the turnover phenomenon is associated with the environmental conditions around neutron stars, rather than being related intrinsically with the radio emission mechanism. Although there are no earlier reports of such a connection, a more detailed study on a larger sample of pulsars is needed to address this idea more quantitatively. Additionally, if such objects turn out to be a significant sub-group of the pulsar population, we may need to re-evaluate (at least to some degree) our low frequency search strategies, as we may overestimate the low frequency flux of such sources, and thus overestimate the number of pulsars expected to be found in low-frequency surveys.
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