What is an electromagnetic pulse? Is an electromagnetic pulse a plain electromagnetic wave with one peak?
 A: It could also be a chirp.

A chirp is a signal in which the frequency increases ('up-chirp') or
  decreases ('down-chirp') with time. In some sources, the term chirp is
  used interchangeably with sweep signal.1 It has also been called
  quadratic-phase signal.[2] It is commonly used in sonar and radar

A: Electromagnetic wave solutions to Maxwell's equations have the form $\vec{E} = \vec{E_0} f (\vec{k}\cdot \vec{r} - \omega t)$, where $f$ can be any mathematical function and $\vec{E_0} \cdot \vec{k} = 0$.
In the case of an electromagnetic pulse, a plausible function that could represent a Gaussian pulse propagating along the x-axis would be:
$$ \vec{E} = E_{0} \exp [(kx -\omega t)^2/2\sigma^2]\ \vec{j},$$
where $\sigma$ would represent the dispersion of the pulse.
Other suitable functions could be obtained to suit the geometry/situation at hand. The wiki page on electromagnetic pulses illustrates some other, more complex, possible functions.
A: " An electromagnetic pulse (EMP), also sometimes called a transient electromagnetic disturbance, is a short burst of electromagnetic energy. Such a pulse may occur in the form of a radiated electric or magnetic field or conducted electrical current depending on the source, and may be natural or man-made. 
An electromagnetic pulse is a short burst of electromagnetic energy. Its shortness means that it will always be spread over a range of frequencies. Pulses are typically characterised by:
The type of energy (radiated, electric, magnetic or conducted).
The range or spectrum of frequencies present.
Pulse waveform: shape, duration and amplitude.
The last two of these, the frequency spectrum and the pulse waveform, are interrelated via the Fourier transform and may be seen as two different ways of describing the same pulse.!Here are some examples:   
Types of EMP
Lightning electromagnetic pulse (LEMP). The discharge is typically an initial huge current flow, at least mega-amps, followed by a train of pulses of decreasing energy.
Electrostatic discharge (ESD), as a result of two charged objects coming into close proximity or even contact.
Meteoric EMP. The discharge of electromagnetic energy resulting from the passage of a meteoroid (known specifically as an electrophonic bolide) through the Earth's atmosphere. " Source: Wikipedia
A: According to mainstream theory,a single photon is a plain (one peak) wave.If this is true,then an EM pulse (usually produced by a nuclear explosion) will be a large single photon.But it is not.It is a big number of photons released within a short time period.The visible part of them makes the flash.Pulse is figured with one peak.The area under the curve corresponds to the amount of photons.
