A treasure hunt for the origins of very high energy gamma rays


What does the VHE sky look like?

These authors look for VHE gamma-rays from blazars collected by several catalogs, including 4FGL-DR3 from Fermi-LAT called (which can be seen in Figure 2), Roma-BZCat, as well as other catalogs of blazars. The extragalactic sky is dominated by blazars, as can be seen in the blue crosses in Figure 2.

Figure 2: Distribution of photons in the main catalog used in this paper, in galactic coordinates (from Fermi-LAT Collaboration Astrophys.J.Suppl. 247 (2020) 1, 33., figure 14) and the sources they are associated with. Most of the sources in red are clustered near b=0, showing the galactic sources in the catalog. This paper is primarily interested in extragalactic sources, so they use only those sources with b>|50| in their analysis.

This paper describes its event selection in galactic coordinates, which is done because the sources of VHE gamma rays are different from the Milky Way versus extragalactic sources. To look at extragalactic sources, the authors look at photons and sources that can be seen in galactic latitude (usually called b), b>50 degrees (Northern galactic hemisphere) and b<-50 degrees (Southern galactic hemisphere).

a circular graph, with blue points representing photons evenly spread across the northern Galactic pole.  Continued in next image.
A circular graph, with blue points isotropically spread over the sky, representing photons.  There is a grouping of red points on the right side, representing the Southern Fermi Bubble.
Figure 3: VHE photons for the analysis with b>|50|, after masking out known sources. The region in red is part of the Southern Fermi bubble and is also masked out. The authors test the assumption that each of these photons comes from an individual blazar source. (Figure 2 in the paper).

The authors test the hypothesis that a single VHE photon can be attributed to an astrophysical counterpart. To do that, they want to mask out all the clusters of VHE photons coming from bright gamma ray sources, to avoid double counting these clusters with multiple sources. After doing this masking, the data sample being used in both the Northern and Southern galactic regions can be seen in Figure 3.

The results: do VHE photos come from blazars?

Using these methods, they establish an association radius, rasoc=0.15 degrees, where for each photon, a blazar from the catalogs is associated with that photon if it is within that radius. They then create 5000 mock photon lists, by displacing each data photon by a random amount between 0 to 5 degrees in a random direction. The number of matches in the real data can be compared to each of these mock lists to get a statistical significance for the real data, which can be seen in Figure 4.

Plot showing the associations between the photons and blazar sources used.  There are 2 distributions, orange and blue, which look like Poisson functions, between 0 and 20 matches, representing the matches for the mock lists.  There are 2 vertical lines for these distributions - Southern is orange at 69 matches, and Northern is blue with 114 real matches.
Figure 4: Number of matches between VHE equivalent photons and known blazar sources, compared to mock lists of VHE photons where the photons are randomly displaced 0-5 degrees away from their actual source in a random direction. These mock lists make matches as shown in the distributions on the left side of the plot, and can be fitted with a Poisson function to calculate the significance of the actual number of matches (vertical lines, labeled NS and NN). (Figure 5 in the paper)

They find that 22.8% of the extragalactic VHE photons can be associated with blazars at a significance level of 40.3 sigma combined between the two hemispheres (this is extremely high significance – just 5 sigma means that there is a one in one million chance that it’s a random fluctuation). About 70% of those associations are with a particular type of blazar, BL Lac objects, although BL Lacs are only 28% of the main catalog.

They also consider the completeness of their catalog, and check this result with other catalogs, including one that is more complete. Completeness is a measure of what percentage of sources of a certain type are present in a particular catalog. For example, if we were able to know that there are 1000 blazars in a particular region of the sky covered by a catalog, but only 500 of them are in the catalog, then that catalog is 50% complete. With this more complete catalog (called WISECATS), they find 27.3% of these VHE photons are associated with blazars.

Conclusions: what does this mean?

The authors are able to draw a few different conclusions from this paper. Their main question in this analysis is: Is 1 VHE extragalactic photon detection evidence for a blazar?

The answer seems to be No: they observe 22.8% of the VHE photons in their sample can be associated with blazars, which means that more than 75% of VHE gamma rays have no clear origin. Even when using a catalog with more blazars, they still find less than 30% of the VHE gamma rays can be said to come from blazars. Another interesting result from this paper is that almost 70% of their matches are from BL Lac objects, rather than other types of blazar.

These results are especially exciting for upcoming gamma-ray telescopes, like the Cherenkov Telescope Array (CTA), which will be able to investigate these VHE gamma rays and hopefully give more insight into the extragalactic diffused gamma-ray sky.

Astrobite edited by Evan Lewis

Featured image credit: edited image, combined treasure map, Fermi-LATand NASA

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