1. Pulsar Data II Single-Pulse Plots

2. What is a single-pulse plot?
Shows bright individual bursts of radio emission as a function of time and DM, over the whole observation
Pulses are not folded at a period
Finds irregularly emitting (but regularly spinning) pulsars and Fast radio Bursts
The PRESTO software creates single-pulse plots in addition to the plots we have been looking at for the past few weeks. For each pointing of the telescope, there are six single-pulse plots.
In the prepfold plots, the software added data together (folding it, like the piece of paper in the demonstration), to look for signals that repeat regularly at a specific period over the course of the whole observation.
However, sometimes we don't see a pulse for every rotation. While the actual rotation of a pulsar is extremely regular, its individual pulses can vary in strength (sometimes being too weak for us to see), or the pulsar may not emit radio waves for a length of time. We might see five and then none for 30 seconds and then ten more, or we might see very other pulse.
Single-pulse plots are created to find these irregularly emitting pulsars, as well as to find rapidly rotating radio transients (RRATs), which may emit a burst of radio waves all of a sudden and then nothing.
If we averaged the data together over the whole observation, we would miss these two kinds of pulsars in our search.

3. Plot Header
The header on a single-pulse plot contains much of the same information as the header in a PRESTO plot. You can find the right ascension (RA) and the declination (Dec), which give the location in the sky at which the telescope was looking during this pointing. You can also find the Mean Julian Date (MJD), which is a standardized way of measuring time,described in the first online session.
The first column in the header gives the source (which is always uknown since we are searching for new pulsars and not looking at unknown ones), the telescope with which the data was taken (GBT), and the instrument that took data (Spigot, which is the GBT's pulsar “backend”).
The third column tells the number of times the telescope sampled data (took data for a certain amount of time and then dumped it into a file), which is called Nsamples. The amount of telescope time each sample representsis called “sampling time.” The last entry is the center frequency of the receiver. For the drift scan, this is always 350 MHz.

4. The bottom subplot is the most important one in a single-pulse plot, and gives most of the information you need to find out if your plot contains a pulsar candidate.
On the y-axis of this plot is DM, which, you will recall, is a measurement of how many free electrons the pulse traveled through, how much its different frequencies were delayed, and how much it was “smeared” as a result.
The x-axis is time and will always go from seconds, for the full 2.5 minutes of each pointing.
Each black mark on the graph represents some amount of radio waves received by the telescope at that DM. You can't tell very well, but each mark is a circle. Each circle is located at a certain time and at a certain DM.
The PRESTO software is guessing a bunch of Dms, just like it did for the prepfold plots, but this time it is trying to fit a DM to each individual pulse. .
The smaller the circle, the weaker the signal-to-noise ratio for the pulse at that DM. The bigger the circle, the more the pulse rises above the noise (has a higher signal-to-noise ratio) when that DM is applied..
On this subplot, you are looking for two kinds of signals:
1. A signal that repeats at the same DM, which would show up as black circles in a horizontal line across the subplot. The circles will also have some vertical height, meaning that a range of DMs fit all right.
2. A strong signal that occurs once and has what we call the “characteristic shape.” The characteristic shape is wide in the middle and tapers on both ends, which means that at a particular DM, there are a lot of radio waves that are very well unsmeared; the tapering at each end means that the DMs slightly above and below the 'perfect' one unsmear fairly well, but not perfectly, just like on the DM subplot in the prepfold plots, where there's a peak in the DM curve, but it is still a curve, meaning that other DMs fit okay.
DM Subplot

5. S/N Histogram
On the database, students will also grade the signal-to-noise histogram , located at the top right.
This histogram shows much of the same information as the DM subplot, except not as a function of time. The x-axis shows dispersion measure, and the y-axis shows signal-to-noise.
You can read this plot by looking at a given dispersion measure and seeing which S/N level that corresponds to. For example, when the software guessed a DM of 20, the signal-to-noise ratio was about seven. However, when the software guessed a DM o 60, the S/N was more like 12, meaning that there is a lot of signal that was unsmeared into pulses by applying a DM of 60.
If you've found a pulsar, the DM with the largest S/N in the S/N histogram (the peak) will be the same DM at which you see pulses or a pulse in the DM subplot.

6. DM Histogram
You can read the DM Histogram basically the same way as the S/N Histogram, except that the y-axis is “number of pulses.”
The number of pulses you see at most Dms, if you go along the x-axis and see how high the black lines go, is between 1 and 3. However, around 60, the number of pulses jumps to 15. This means that the software found only a few pulses that were unsmeared by DMs lower and higher than 60, but many pulses that were unsmeared by DMs right around 60. This information agrees with the previous two subplots we looked at.

7. Number of Pulses Histogram
You will not be grading the number of pulses histogram, but we'll go over what it means anyway.
This histogram contains no DM information. It only shows how many pulses were received at a given S/N. There are usually lots of pulses with a low S/N, which means the histogram will be high toward the left-hand side, and fewer with a high S/N, which means it will step down as you look rightward.
This is because each of those tiny black circles on the graph counts as a pulse. There are many little black circles, but, as you know since they're small, they aren't strong. There are many fewer strong ones, but these are the ones we're interested in, as they're more likely to come from pulsars.
If you just saw a single large signal, you the histogram would go all the way down to 1 at a large S/N.

8. Does this look like a pulsar?
Yes!
Many pulses centered on the same DM (meaning that they went through the same amount of electrons in space, and that that amount was not zero).
Peaks in the DM and S/N histograms at the same DM as the center DM in the bottom plot.

9. Does this look like a pulsar?
NO!
This is RFI. The pulses in the bottom plot are fairly regular, but they start at a DM of zero. Any data that has signal at a DM of zero, even if it has signal above that DM, has to be tossed out as contaminated.
The top two histograms confirm the observation: their peaks are at a DM of zero.

10. This plot presents a common student pitfall: not looking at the y-axis of the bottom plot. Students assume that the bottom of a y-axis is zero, but that's not always true. PRESTO's plots show a range of different Dms, and some of them, like this one, start far above zero. This doesn't look like a pulsar, but is it RFI? Or is it noise?
We can't tell, because we can't see if the signal extends to a DM of 0. When this happens, you can look at the other single-pulse plots in the pointing and determine whether this is regular noise or RFI.