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I am unable to interpret this graph. What is the time on the x-axis? Due to dispersion why is pulse spreading across frequency rather than time if dispersion causes the delay? And how is that top graph for flux obtained?enter image description here

Image source: Lorimer et al. 2007, now also known as FRB 010724

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  • $\begingroup$ Thanks for the reminder. $\endgroup$
    – Parsec
    Jan 31, 2022 at 23:19

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The bottom portion is what's known in this context as a waterfall plot, which plots the intensity of the signal as a function of observing frequency and time (a "dynamic spectrum"). It's a useful tool in a number of ways, including showing radio frequency interference (RFI); those two brown horizontal bands are frequency channels that exhibited significant interference during the observation and were therefore removed ("zapped") from the analysis. If they'd been left in, there would presumably be very strong mission at those frequencies, possibly exceeding the intensity of the burst itself.

Fast radio bursts are what we call broadband signals, meaning that they can be detected across a wide range of frequencies, unlike, for instance, spectral lines (which do have non-zero widths due to various effects but are usually quite narrow in comparison). This observation spanned a frequency range within the L band, and it's not surprising that the FRB was detectable over a bandwidth of ~300 MHz. In other words, the fact that the burst is visible across the entire frequency range isn't due to dispersion; it's an inherent characteristic of FRBs and other broadband sources.

The top plot shows the burst averaged across all frequencies within the band. Each frequency channel has been dedispersed according to its central frequency, and then the channels have been added together. This isn't perfect - the non-zero width of a channel means that there is some dispersive smearing within it - but it's pretty good all the same.

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