An Introduction to Cosmic Flares: Gamma-Ray Bursts (II)
In the previous article, we’ve seen 3 potential sources that could have the power to generate such massive bursts. But, to narrow down our uncertainty line, we must look for more and more data. And that’s what scientists did. Soon after the detection of the bursts by the VELA satellite, astronomers launched the Compton Gamma Ray Observatory (BATSE) to detect more of these bursts to precise details and location. In fact, BATSE had such sensitivity, that it could detect as many as 1 burst each day! It revealed to astronomers 3 major clues about the distribution of Gamma-Ray Bursts. Namely:
i. The bursts were coming from every direction of the sky without any pattern whatsoever.
ii. A plot of burst fluence with its average timescale reveals that they come mainly in 2 types.
iii. Enough information to determine the plot of the Number of Bursts with flux.
Uniformity of Burst direction: Based upon the data collected by BATSE, the distribution of the Bursts look something like this:
Here, it is clearly evident that we don’t actually see any pattern whatsoever. They are quite random and possess uniform distribution. The bluer ones are the ones of greater energy while the redder possess energy at low values. If these bursts were coming from the Milky Way, we would’ve expected a thick accumulation of bursts at the middle, extending from +180 to -180. So, it sort of hints that they are somewhere from even deeper parts of the space.
Burst Fluence versus Average Timescale:
A plot of Burst fluence with the timescale of gamma-ray bursts looks like the following:
Here, a first look at the graph tells us that in ‘general’ the higher the energy, the lower its timescale. So, the strongest ones seem to explode all the radiation at a time. However, a second, closer look reveals something interesting. It looks as if, the points are densely populated at 2 separate regions on the graph. The separation isn’t at all homogenous, but clearly, it hints that there are 2 types of Gamma-Ray Bursts. One class of Bursts has extremely high energy but lasts for a very short time while the other type of bursts has low fluence but it lasts 100s of times longer. In astronomy, we classify them as:
i. Short Hard Bursts
ii. Long Soft Bursts
The 3rd Interesting Clue that BATSE satellite revealed looks like:
A graphical plot between the Number of Bursts vs. the flux received from the burst requires us to be able to appreciate the relationship that the number of Bursts received surveying the entire sky is expected to be proportional to the negative 3/2 of the fluence received from the bursts given that the energy from the Bursts are more or less the same and they are dispersed evenly across space. This basically means the following:
Let’s say we take 2 basic assumptions:
i. The GRBs are uniformly distributed in space
ii. They have more or less the same luminosity
If we assume (i) to be correct, then the flux we receive from any GRB to be proportional to the inverse of the square of the distance.
i.e. F α R^-2 — — — — — — — (i) (Where F defines Flux and R defines the distance.
Again,
If we assume (ii) to be correct,
The number of GRBs is proportional to the volume (V) contained within the distance R. So, the number of GRBs must be proportional to R³.
i.e. n α R³ — — — — — — (ii) (Where n is the number of GRBs within a sphere of radius R.
Equating (i) and (ii) gives us,
n α F^-3/2 — — — — — — (iii)
But, from the data, we see that the line plotted deviates from the line that follows our expected relationship. Therefore, one of our two assumptions must be wrong.
If we assume that the 2nd assumption is faulty, this would only change the coefficient of R^-2, This wouldn’t seriously affect our data as we would not interfere with the power of the indices. We can’t however, change the power of R against Flux because the inverse square law is inherent to the laws of Electromagnetic Rotation.
However, changing the 1st assumption will change the number count to flux relation drastically. This is because we assumed that the bursts are uniformly distributed out in space and that is proportional to the cube of the distance. However, the cubic relation changes as soon as we assume inhomogeneity in the distribution of the GRBs and produce a drastic change in the number count of flux relation. Therefore, we can safely assume that the Gamma-Ray Bursts are not uniformly distributed in space.
In a summary, the 3 results that we received from BATSE are:
i. They occur randomly at any patch of the sky which means that they are somewhere deep from space.
ii. They come in two types
iii. They are not distributed uniformly through space.
To be continued…
