Genuine giant pulses

Bright pulse activity from the Vela pulsar is variable over time (see Figure 7.3 on

page 145). At times of high activity, genuine giant pulses are emitted. Figure 5.4 on

page 87 shows the brightest pulse we found. Using our relative scale, the peak flux

density was just under 130, which equates to approximately 6000 Jy. To examine the

microstructure in further detail, we re-processed the original source file using 16384,

32768, and 65536 timing bins (instead of the usual 8192). This gave us a timing

resolution of 5.45, 2.73, and 1.36

µs respectively. See Figures 5.5 to 5.7 on pages 88–

89.

As can be seen, further microstructure appears as the resolution improves. In Figure 5.8

on page 90 we focused on the peak and reprocessed with 1048576 timing bins each

85.2 ns wide. There appear to be some very short

nanoshots

of very high intensity.

A similar pattern was also shown by Hankins et al. (2003) on giant pulses in the

Crab pulsar. Their peaks had peak fluxes of

≈

2000 Jy whereas Vela appears to be

≈

18000 Jy. An important difference is that their pulses rise up from a low baseline,

whereas Vela’s rises up from 6000 Jy, which means these could be just radiometer noise.

It should also be noted that Hankins et al. (2003) did have a much better resolution

of 2 ns. The only model that they concluded explains the cause of their results was a

“collapse of plasma-turbulent wave packets in the pulsar magnetosphere”.

As a comparison, in Figure 5.31 on page 104 we examined a giant pulse from the

Crab using 262144 timing bins (128.68 ns) which is nowhere near the resolution (2 ns)

described by Hankins et al. (2003).

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CHAPTER 5. BRIGHT AND GIANT PULSES

0

2

4

6

8

10

0

2

4

6

8

10

12

14

Peak Flux

Count (millions)

0

40

80

120

0

1

2

3

4

5

6

7

Peak Flux

log(Count)

0.0

0.4

0.8

0

2

4

6

8

10

12

14

log(Peak Flux)

Count (millions)

0.0

0.5

1.0

1.5

2.0

0

1

2

3

4

5

6

7

log(Peak Flux)

log(Count)

Figure 5.3: Histograms of pulse flux density (relative units) showing a nearly log-normal

distribution. Data is from 3.31×108

_{pulses with a bin width of 0.1.}

5.1. MOUNT PLEASANT OBSERVATIONS

87

Figure 5.4: The brightest pulse we found, sampled using 8192 bins, each 10.9

µs wide.

The X-axis spans 0.01 of a pulse period which is 893

µs.

Figure 5.9 on page 91 shows frequency versus arrival time (reprocessed using 1024

frequency bins) and clearly shows the range of frequencies due to dispersion measure.

Figures 5.10 to 5.17 on pages 92–95 show frames of a movie of the arrival of this

brightest giant pulse viewing the baseband recording in the frequency domain. The

X-axis shows frequency (in bins, highest frequency to the left) and the Y-axis shows

power using arbitrary units. Remember that Vela is highly linearly polarised and this

is shown by the different powers in each channel. The command to produce this movie

was:

fauto -n 1000 -N 20 -ymax 30 -s 233000000 PSR J0835-4510 092 014906.lba

As stated earlier, Katz (2017) proposed that FRBs could be an analogy to lightning

here on earth, and be a breakdown of the insulating vacuum gap that exists in the

pulsar magnetosphere. We have no FRB data to analyse, but it is clear that our

giant pulses have no resemblance to our lightning data discussed in Section 4.5.3 on

page 59.

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CHAPTER 5. BRIGHT AND GIANT PULSES

Figure 5.5: The brightest pulse we found, sampled using 16384 bins, each 5.45µs wide.

The X-axis spans 0.01 of a pulse period which is 893

µs.

Figure 5.6: The brightest pulse we found, sampled using 32768 bins, each 2.73µs wide.

The X-axis spans 0.01 of a pulse period which is 893

µs.

5.1. MOUNT PLEASANT OBSERVATIONS

89

Figure 5.7: The brightest pulse we found, sampled using 65536 bins, each 1.36µs wide.

The X-axis spans 0.01 of a pulse period which is 893

µs.

5.1.4

Consecutive bright pulses

In Palfreyman et al. (2011) (and discussed in Section 3.1.4 on page 36) it was shown

that sequences of 6 consecutive bright pulses appeared to occur with billions of times

the expected probability, assuming bright pulses were independent events. We have

discovered subsequent confirmation of long sequences of consecutive bright pulses as

shown in Figure 5.18 on page 96.∗

These consecutive pulses appear to drift in phase. Drifting sub-pulses have been anal-

ysed by Deshpande and Rankin (2001) in B0943+10 but have not been noted to appear

in the Vela pulsar. Figure 5.19 on page 96 shows what certainly could be described as

drifting sub-pulses. Figure 1 in Palfreyman et al. (2011) also shows this drift. However,

since the drifts appear to occur in both directions a rotating carousel could not explain

this. If such a carousel exists then it would have to be oscillating not rotating. With

the pulsar precessing (see Sections 6.1 to 6.4 on pages 133–142) emission zones would

move in and out of the sight-line as shown in Figure 5.22 on page 98.

Vela being a young pulsar would not be expected to have conal emission - only core

emission (Rankin 1990) however it could be possible to have some core outriders

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CHAPTER 5. BRIGHT AND GIANT PULSES

Figure 5.8: The brightest pulse we found, zoomed into the highest peak, sampled using

1048576 bins, each 85.2 ns wide. A filterbank of only 4 frequency channels could be

utilised due to computer memory constraints.

5.1. MOUNT PLEASANT OBSERVATIONS

91

Figure 5.9: Frequency vs Phase of the brightest pulse we found, processed using 4096

timing bins, and 1024 frequency channels. Note that this pulse took about 15 ms

to arrive over the 64 MHz bandwidth and the power is evenly spread across those

frequencies. The filename is different from previous plots because due to the file’s size,

processing was commenced part way through the file.

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CHAPTER 5. BRIGHT AND GIANT PULSES

Figure 5.10: Brightest pulse arrival in real time. Frame 1 of 8. The signal at the centre

is artificial.

5.1. MOUNT PLEASANT OBSERVATIONS

93

Figure 5.12: Brightest pulse arrival in real time. Frame 3 of 8.

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CHAPTER 5. BRIGHT AND GIANT PULSES

Figure 5.14: Brightest pulse arrival in real time. Frame 5 of 8.

5.1. MOUNT PLEASANT OBSERVATIONS

95

Figure 5.16: Brightest pulse arrival in real time. Frame 7 of 8.

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CHAPTER 5. BRIGHT AND GIANT PULSES

Figure 5.18: The brightest 6 consecutive bright pulses we’ve discovered so far.

5.1. MOUNT PLEASANT OBSERVATIONS

97

Figure 5.20: Drifting sub-pulses in opposite directions at the same time. Note the “λ”

shape at

t

≈5 s.

(Rankin, private communication).

In Figure 5.20 we see a very interesting phenomenon. Note the “λ” shape in the lower

third of the diagram. We’ve seen Vela pulses drift in both directions, but this shows

pulses drift in opposite directions

at the same time.

Cordes and Shannon (2008) discuss the possibility of supernova fallback material en-

tering the light cylinder. Brook et al. (2014) examine pulse profile shape changes and

postulate the cause is an asteroid or in-falling debris, and Kotera et al. (2016) also

discuss this possibility.

An intriguing pattern has also revealed itself with regard to consecutive giant pulses.

Often the consecutive pulses have the same “shape”. Normally the shape is typically

Gaussian in appearance but occasionally this is not the case. An excellent example

is shown in Figures 5.23 to 5.24 on page 99: two consecutive wide triple-peaked giant

pulses. Even though the amplitudes change, the locations of the peaks are similar. This

supports the notion inferred in Palfreyman et al. (2011) that we are seeing multiple

passes of a single emission zone (in this case multi-peaked) passing through the line-

of-sight.

As stated earlier, Vela being a young pulsar should have only core emission and not

conal emission (Rankin 1990) and certainly not a rotating carousel of emission zones

like B0943+10, as shown in Deshpande and Rankin (2001). The above results appear

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CHAPTER 5. BRIGHT AND GIANT PULSES

Figure 5.21: A large curving drifting sub-pulse which could not occur if there were

emission zones rotating around the magnetic pole. Epicycles within the major rotation

could cause this effect.

Figure 5.22: Simple graphic of potential core emission with some outriders. The two

sight-lines in this diagram show how precession could change the pulse profile and why

bright pulses only occur at the leading edge.

5.1. MOUNT PLEASANT OBSERVATIONS

99

Figure 5.23: The first pulse of two consecutive bright pulses. Note the unusual shape

in both pulses.

Figure 5.24: The second pulse of two consecutive bright pulses. Note the unusual shape

in both pulses.

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CHAPTER 5. BRIGHT AND GIANT PULSES

to contradict this, but Vela could just have emission zones that are “wobbling” into

the line-of-sight due to precession or some other cause.

Another intriguing sequence is shown in Figure 5.21 on page 98. This large curving

stream of pulses could not be explained by the rotating carousel of Deshpande and

Rankin (1999). This pattern could be explained if such a carousel had emission zones

of rotating epicycles.

In document A long term single pulse study of the Vela pulsar (Page 108-123)