A treasure hunt for the origin of gamma rays with very high energy

A treasure hunt for the origin of gamma rays with very high energy

What does the VHE sky look like?

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

Full sky map of the 4FGL catalog showing sources by class in galactic coordinates.  Many sources are gathered near the galactic plane, and blue dots representing AGNs are scattered isotropically across the sky.
Figure 2: Distribution of photons in the main catalog used in this article, 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 collected near b = 0, which shows the galactic sources in the catalog. This magazine is mainly interested in extragalactic sources, so they use only those sources with b> | 50 | in its analysis.

This article describes the choice of events in galactic coordinates, which is done because the sources of VHE gamma rays differ from the Milky Way and extragalactic sources. To look at extragalactic sources, the authors look at photons and sources that can be seen at galactic latitude (usually called b), b> 50 degrees (northern galactic hemisphere) and b <-50 degrees (southern galactic hemisphere).

a circular graph, with blue dots representing photons evenly distributed across the northern galactic pole.  Continuation in the next picture.
A circular graph, with blue dots isotropically scattered across the sky, representing photons.  There is a grouping of red dots on the right, which represents the southern Fermi bubble.
Figure 3: VHE photons for the analysis with b> | 50 |, after masking of known sources. The region in red is part of The South Fermi bubble and is also masked. The authors test the assumption that each of these photons comes from a single blazer source. (Picture 2 in the newspaper).

The authors test the hypothesis that a single VHE photon can be attributed to an astrophysical equivalent. To do so, they want to mask all clusters of VHE photons coming from bright gamma-ray sources, to avoid double-counting these multi-source clusters. After making this mask, the data sample used in both the northern and southern galactic regions can be seen in Figure 3.

The results: do VHE photons come from blazers?

With these methods, they establish an association radius, rassoc= 0.15 degrees, where for each photon a blazer from the catalogs is associated with that photon if it is within that radius. They then create 5,000 sham photo lists by shifting each data photo by a random amount between 0 and 5 degrees in a random direction. The number of matches in the real data can be compared with each of these pretend lists to get a statistical significance for the real data, which can be seen in Figure 4.

Plot showing the relationships between photons and blazer sources used.  There are 2 distributions, orange and blue, which look like Poisson functions, between 0 and 20 matches, which represent the matches for the pretend 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 blazer sources, compared to pretend lists of VHE photons where the photons are randomly offset 0-5 degrees from their actual source in a random direction. These pretend lists make matches that appear in the distributions on the left side of the plot, and can be equipped with a Poisson function to calculate the significance of the actual number of matches (vertical lines, marked NS and NN). (Figure 5 in the magazine)

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 – only 5 sigma means that there is a chance of one million that there is a random fluctuation). About 70% of these compounds are with a certain type of blazer, BL Lac objecteven though BL Lacs is only 28% of the main directory.

They are also considering completeness in their directory and check this result with other directories, including one that is more complete. Completeness is a measure of the proportion of sources of a certain type that are in a certain catalog. For example, if we could know that there are 1000 blazers in a certain part of the sky that are 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 that 27.3% of these VHE photons are associated with blazers.

Conclusions: what does this mean?

The authors can draw some different conclusions from this article. Their main question in this analysis is: Is 1 VHE extragalactic photon detection evidence for a blazer?

The answer seems to be No: they observe that 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 blazers, they still find that less than 30% of VHE gamma radiation can be said to come from blazers. Another interesting result from this paper is that almost 70% of their matches come from BL Lac objects, rather than other types of blazers.

These results are particularly exciting for future gamma-ray telescopes, which Cherenkov Telescope Array (CTA)which will be able to investigate these VHE gamma rays and hopefully provide more insight into the extragalactic diffuse gamma ray sky.

Astrobite edited by Evan Lewis

Selected image credit: edited image, combined tax card, Fermi-LATand NASA

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