RADIO ASTRONOMY AND MOONBOUNCE COMMUNICATIONS
AT K5SO
B0531+21 (J0534+2200) single giant pulse detection
(detected during a 3 hour observation)
Flux density (S1400) = 8.9 mJy
5 Sept 2020
(1400 MHz, 25 MHz bandwidth, dual polarization chnls, V & H)
It is known that the Crab pulsar occassionally emits single giant pulses in the radio frequency spectrum, occasional single pulses that are many times stronger than the typical pulses from the pulsar. One of these giant pulses was detected by our telescope during a 3 hour observation designed to look for such an event. The details of the giant pulse detection are shown in the graphics below.
The primary method of distinguishing a giant pulse from a pulsar from a radio frequency interference (RFI) pulse that has been generated on earth or by a satellite in orbit is to observe that the pulsar pulse has undergone frequency dispersion as a result of passing through the ionic interstellar medium (ISM) between the pulsar and the telescope whereas RFI pulses do not exhibit frequency dispersion because they have not traveled through the ISM.
Frequency dispersion results in the effect that higher frequency components of a pulse pass through the ISM slightly faster than do lower frequency components of the pulse. This effect results in a time delay between the arrival times of low frequency components and the arrival times of high frequency components and in many cases can be measured directly. This is the case for this particular pulsar with the known dispersion measure (DM), and the radio telescope being used.
The delay between the relative arrival times of the low frequency component relative to a higher frequency component can be calculated if the DM for the pulsar is known and the low and high frequencies are specified.
Namely,
dt = DM * ((1/f2^2) - (1/f1^2)) / 2.4 (Eqn. 1)
where dt is in seconds, DM is in pc-cm^-3, and f1 and f2 are the low/high frequencies in units of 100 MHz.
In the case of the Crab pulsar the DM value is well known and is:
DM = 56.78 pc-cm^-3.
For this observation, a 25 MHz bandwidth was used centered on 1400 MHz, with the high-frequency limit of the bandwidth f1 = (1400 MHz + 12.5 MHz) / 100 = 1412.5 and the low -frequency limit of the bandwidth f2 = (1400 MHz - 12.5 MHz) / 100 = 13.875. Using these values in Eqn 1 above,
dt = 0.0043 seconds = 4.3 mSecond
This value of frequency dispersion is in fact observed in the images below, which are graphic outputs of the PRESTO exploredat, single_pulse_search.py, and PSRCHIVE dspsr/psrplot programs, frequently used by professional pulsar astronomers. The 4 mSec delay between the f1 and f2 limits of the bandwidth in the last graphic below confirms that this pulse is in fact a giant pulse from the Crab pulsar.
Above: A PRESTO "exploredat" plot of the 6 second interval containing the giant pulse, showing the pulse in time domain.
Above: A PRESTO "single_pulse_search.py" graphic output of the 1 second interval containing the giant pulse, showing the expected peak in the Signal-to-noise vs DM plot for a giant Crab pulsar pulse.
Above: A PSRCHIVE dspsr/psrplot graphic output of a 100 milli-second interval containing the giant pulse showing a sloped line for the pulse, indicating that the pulse exhibits frequency dispersion, as expected for a giant pulse from the Crab pulsar passing through an ionic ISM on its way to the telescope.
Below: A PSRCHIVE dspsr/psrplot graphic output of a 20 milli-second interval containing the giant pulse, showing a delay time of 4 mSec of the lowest-frequency component of the giant pulse within the 25 MHz bandwidth relative to the highest-frequency component of the bandwidth, consistent with the pulse passing through an ISM region where DM = 56. This confirms that the detected pulse is a giant pulse from the Crab pulsar.