Astrophysical origins

Astrophysical origins - sources edit…


Pulsars can have rotation periods as short as a few milliseconds, although the ones detected with IACTs so far have periods of at least a few tens of milliseconds. The only four pulsars that have been detected with IACTs so far are the Crab pulsar, the Vela pulsar, the Geminga pulsar, and PSR~B1706-44. Their pulsation is highly regular, which makes it predictable, and allows stacking of signal from different periods, thus increasing the detected statistics. Additionally, these four pulsars are located in the Milky Way. This relatively close proximity significantly impairs the sensitivity of LIV tests performed on pulsar data.


Activ galactic nuclus (AGN)

Active galactic nuclei are persistent sources at distances comparable to GRB. During their flaring states, they emit signals abundant in VHE gamma rays, with flux variability timescales on the order of minutes. Although unpredictable, flares usually last longer than GRB. In addition, they emit stronger fluxes, with the most energetic photons reaching higher energies. All of this makes flares from AGN easier to detect with IACT compared to GRB.


Gamma-ray burst (GRB)

GRBs are powerful transient cosmic explosions, usually associated with collapses of massive stars into black holes (long GRB), or mergers of neutron stars (short GRB). Their light curves are variable on the timescales of a second. Unlike pulsars, GRB are completely unpredictable. Satellite-borne detectors with a large field of view, such as GMB  and LAT  onboard satellite on average detect one GRB almost every day. However, IACT with a rather small field of view (an order of few degrees) rely on alerts from satellite-borne detectors to trigger observations. Furthermore, because of their large distances, VHE gamma rays are strongly absorbed on the EBL. For these reasons, GRB are elusive and notoriously difficult to detect with IACT, with only four detected to date. However, due to their short variability timescales, combined with large distances, once a GRB is detected, the signal becomes a valuable asset for probing QG