Is a pulsar EM-radiation affected by diffraction? Is a pulsar EM-radiation affected by diffraction? As I understand gaussean beam theory there should exist different diffraction or if You want diffusion angles for different wavelengths, for example blue and red.So if the angle for red is wider should we see at first a red tone and after a while the blue pulse from that pulsar or could the higher frequency have even a small probability to be observed as its radiation diffraction cone covers a smaller cross-section area?
 A: Yes, absolutely . The electromagnetic radiation from pulsars is affected by diffraction at many levels. Pulsars are neutron stars that emit directional radiation (mostly at radio frequencies). Just like you would expect for any kind of electromagnetic radiation, the Maxwell's equations apply. So does the diffraction phenomenon.
Just as the aperture size of a telescope limits its resolution, the size of the emitting region of a pulsar limits the collimation of the beamed radio emission.
For a monochromatic spherical wave solution $e^{ikr}/r$ propagating in the $z$-direction, the gaussian model that describes the beam width $w$ can be derived via the substitution $x \rightarrow x $, $y \rightarrow y $, $z \rightarrow z - iz_0 $. It yields :
\begin{equation}
w(z) = w_0 ( 1 + z^2 / z_0^2 )^2
\end{equation}
Where $z_0= \pi w_0^2 / \lambda$.
You can already see from the dependance on $\lambda$ that the beam width is chromatic (ie it depends on the frequency). This appears clearly if we determine the asymptotic angular spread of the beam (or diffraction angle $\theta_\mathrm{D}$):
\begin{equation}
\mathrm{tan}(\theta_\mathrm{D}) = \mathrm{lim}(z\rightarrow \infty) \frac{w(z)}{z} = \frac{\lambda}{\pi w_0}
\end{equation}
You can see on this figure from Hopf et al. 1975 the theoretical shape of the beam expected for a pulsar:

Thus, you are completely right that the angle of "red" radiation would be greater than the one from the "blue" radiation (in fact, we are talking about radio emission, so "red" and "blue" would be, for example, $\lambda=2$ cm v.s. $\lambda=1$ cm). In fact, Hopf et al. 1975 present a figure in which they show the spectral energy variation with respect to the frequency for different angles. You can see a sharp high-frequency cutoff for $\theta_0 > \theta_\mathrm{N}$:

I must add that the intrinsic broadening of the beam and modification of the spectrum is accompanied by many diffraction and refraction effects that occur along the line of sight between the pulsar and the observer.
Interstellar scintillation of pulsars is a well known effect, it is similar to the scintillation of visible stars (due to atmospheric effects). The scintillation of pulsars is due to small-scale irregularities of the turbulent, ionized interstellar plasma upon which radio electromagnetic waves are scattered.
Interstellar diffraction effects include (time scale ~minute):

*

*Angular broadening

*Temporal broadening

*Intensity scintillations in time and frequency

Interstellar refraction effects include (time scale ~weeks):

*

*Angular wandering of the apparent sources position

*Dispersive and geometric time-of-arrival variations

*Slow intensity variations

*Modification of the apparent brightness distribution

These diffractive and refractive scintillation effects are very convenient for astrophysicists, because they allow them to probe the electron density spectrum of the interstellar medium with pulsars.
source: Cordes 1986
