# EpochFolding.jl

#### Epoch folding, in Julia.

This package implements epoch folding in pure Julia. Epoch folding is a technique used to determine the periodicity of pulsed emission, as seen from sources like pulsars, magnetars, and others. Quoting from Bachetti et a. (2021):

Let $(t _{j}, X _{j}) \forall j = 1, \dots N$ be pairs of flux measurements $X _{j}$ at times $t _{j}$. If we define $f$ as the candidate, and $\dot f, \ddot f, \dots$ as the frequency derivatives measured at a reference time $t _{\mathrm{ref}}$, we calculate the pulse phase of each $X _{j}$ as:

$$\phi _{j}(t) = \phi _{0} + f(t _{j} - t _{\mathrm{ref}}) + 0.5 \dot{f} (t _{j} - t _{\mathrm{ref}})^{2} + \cdots ,$$

where $\phi _{0}$ is the pulse phase at $t _{\mathrm{ref}}$, which is set to zero for unknown or candidate pulsars. [...] The folded or pulsed profile is then a histogram of these phases falling into $N _{\mathrm{bin}}$ equal phase bins between zero and $2\pi$, weighted by the flux in each sample:

$$p _{i} = \sum _{j = 1} ^{N} X _{j} \theta (\phi _{j} - \phi _{\mathrm{mid}, i}), \quad i = 1, \dots, N _{\mathrm{bin}},$$

where

$$\phi _{\mathrm{mid}, i} = 2\pi \frac{i - 0.5}{N _{\mathrm{bin}}}$$

is the phase corresponding to the middle of bin $i$ and

$$\theta (x) = \begin{cases} 1, & \text{if}\ |x| < 0.5 / N _{\mathrm{bin}} \ 0, & \text{otherwise.} \end{cases}$$

While the approach described above is the most common way to fold radio data, some software packages like PRESTO use a slightly different approach: they assume that each sample is finite in duration and "drizzle" it over the appropriate pulse phase bins. Both approaches are implemented in this package; in fact, the code for both approaches is almost directly inspired from the code in PRESTO, with the occasional change made for Julia. Like PRESTO, this code also calculates and returns the statistics associated with the folded profile, including the $\chi^{2}$ statistic (Leahy et al. 1983).

Here is an example of using EpochFolding.jl to fold data for the pulsar B1929+10:

Install it by typing and running:

] add EpochFolding


in the Julia REPL.