Abstract
The search for Galactic pevatrons is now a well-identified key science project of all instruments operating in the very-high-energy domain. Indeed, in this energy range, the detection of gamma rays clearly indicates that efficient particle acceleration is taking place, and observations can thus help identify which astrophysical sources can energize particles up to the ~PeV range, thus being pevatrons. In the search for the origin of Galactic cosmic rays (CRs), the PeV range is an important milestone, since the sources of Galactic CRs are expected to accelerate PeV particles. This is how the central scientific goal that is ’solving the mystery of the origin of CRs’ has often been distorted into ’finding (a) pevatron(s)’. Since supernova remnants (SNRs) are often cited as the most likely candidates for the origin of CRs, ’finding (a) pevatron(s)’ has often become ’confirming that SNRs are pevatrons’. Pleasingly, the first detection(s) of pevatron(s) were not associated to SNRs. Moreover, all clearly detected SNRs have yet revealed to not be pevatrons, and the detection from VHE gamma rays from regions unassociated with SNRs, are reminding us that other astrophysical sites might well be pevatrons. This short review aims at highlighting a few important results on the search for Galactic pevatrons.
Highlights
One strong argument for the Galactic origin comes from the detection of gamma rays: observations of the Galaxy revealed that the Galactic disk is gamma-ray bright, which is a clear indication of the interactions of cosmic rays (CRs) with interstallar medium (ISM) material and Galactic magnetic field in the disk, and that gamma rays roughly scale with the amount of material in the disk
While the multiple observations in VHE range with Imaging Atmospheric Cherenkov Telescopes (IACTs) of supernova remnants (SNRs) shells were revealing that all known SNRs seem to not be pevatrons, observations with H.E.S.S. were objectifying that gamma rays up to ∼100 TeV seem to originate from the Galactic center region around Sagittarius A*, a source called HESS J1745-290, thereby indicating the presence of a pevatron in the Galactic center
HAWC has reported on the detection of gamma rays around ∼200 TeV from J1825-134 [68] and TibetASγ on the detection of gamma rays 100 TeV potentially associated to the region around SNR G106+2.7 or PSR J2229+6114, as previously discussed [69], and the LHAASO collaboration has reported on the detection of gamma rays of 1 PeV around the source J2032+4102, or from the Crab Nebula [116]
Summary
The term Pevatron, referring to an object capable of accelerating particles up to the PeV (=1015 eV) range, is widely used in the scientific programs of all major instruments investigating particle acceleration in astrophysical sources. Other observatories, using different techniques, relying on shower front detectors (or “air shower detectors”), such as HAWC [40], Tibet ASγ [41] and LHAASO [28], operating up to and above the ∼100 TeV gamma-ray domain reported on the detection of several Galactic pevatrons All these observations indicated that most of the pevatron candidates seem to not be associated to SNRs. All these observations indicated that most of the pevatron candidates seem to not be associated to SNRs In this short review, we intend to especially discuss the case of SNRs, since they have been, for long, at the center of the debate on the origin of Galactic CRs, and that the question of whether they can accelerate PeV particles is still open. For updated reviews discussing recent results and advances on the problem of the origin of CRs, we refer the reader to Blasi [44], Gabici et al [45]
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