# A Understanding MASERs and their properties

1. Dec 24, 2016

I've been reading Elitzur although with a number of questions, some likely too obvious that they aren't spelled out in the literature based on my searching. To ensure I understand some key details of masers, I thought I'd ask a few (simple) questions if that's all right.

1. Is there a maximum suspected density for masing regions? The largest I've read about is $10^{10}$ atoms/m$^3$. But the only real limit as far as I can tell is that atomic collisions should be sufficiently low as to not impede the emission process (i.e. dipole interactions should be sufficiently weak). And furthermore, are these masing regions typically assumed to be homogeneous in density for any particular reason?

2. Is there a maximum length on masers? I have read they can be about a parsec long, but did not read anything regarding constraints on this length. Although I did read velocity coherence is a necessary condition for masers; and Elitzur says "observations show indeed that maser sources are comprised of many emission spots, each with its own well defined velocity." I'm sure they can vary, but do individual emission spots have a typical length (i.e. are they on the order of parsecs in length still)?

3. If masers can be on the order of light years long, what exactly is the physical meaning behind pulses that are shorter in temporal width than a year? How exactly do photons from the stimulated emission process combine such that the observed signals are less than the light-propagation time through the maser? This seems to be observed in other forms of coherent radiation where the inverted sample itself is a few metres long yet the observed pulse has a width of femtoseconds. I believe these are entangled quantum systems, and perhaps there is more going on than I am aware, but is there any physical meaning behind how these pulses are smaller than the light propagation time through the inverted sample? Does light propagation still occur as normal, just with the radiating photons interacting in such a way that the width of the pulse is decreased?

4. What is the exact meaning of flux versus velocity graphs? For example, Figure 2 in this paper shows the observed flux density and it is plotted against velocity. Does the author simply assume the "true" frequency is a given value and then use their measured frequencies as velocities based on simply applying a (relativistic) Doppler shift equation? But if that is the case, we are only assuming the masing region is travelling at those velocities in the line of sight to us, correct? It appears there is a systemic velocity so I am likely completely wrong...

5. I have seen a lot of flux measurements of masers in the literature be made in Jansky which has units of $\frac {\text{W}}{\text{m}^2 \text{Hz}}$. To convert this to the more familiar unit of watts, is it fine to simply multiply the flux by the surface area of the observing radio telescope (e.g. Parkes Radio Telescope, nearly 4000 m$^2$), and then integrate this over the frequencies measured based on the signal's spectrum?

Any answers you may know and/or references would be greatly appreciated.

Last edited: Dec 24, 2016
2. Dec 26, 2016