The prototype X-ray binary GX 339-4: using TeV γ-rays to assess LMXBs as Galactic cosmic ray accelerators

Since the discovery of cosmic rays (CRs) over a century ago, their origin remains an open question. Galactic CRs with energy up to the knee (10(15) eV) are considered to originate from supernova remnants, but this scenario has recently been questioned due to lack of TeV gamma-ray counterparts in many cases. Extragalactic CRs, on the other hand, are thought to be associated with accelerated particles in the relativistic jets launched by supermassive accreting black holes at the centre of galaxies. Scaled down versions of such jets have been detected in X-ray binaries hosting a stellar black hole (BHXBs). In this work, we investigate the possibility that the smaller scale jets in transient outbursts of low-mass BHXBs could be sources of Galactic CRs. To better test this scenario, we model the entire electromagnetic spectrum of such sources focusing on the potential TeV regime, using the 'canonical' low-mass BHXB GX 339-4 as a benchmark. Taking into account both the leptonic radiative processes and the gamma-rays produced via neutral pion decay from inelastic hadronic interactions, we predict the GeV and TeV gamma-ray spectrum of GX 339-4 using lower frequency emission as constraints. Based on this test-case of GX 339-4, we investigate whether other, nearby low-mass BHXBs could be detected by the next-generation very-high-energy gamma-ray facility the Cherenkov Telescope Array, which would establish them as additional and numerous potential sources of CRs in the Galaxy.

Automated summary

This study investigates the possibility of smaller scale jets in transient outbursts of low-mass black hole X-ray binaries (BHXBs) being sources of Galactic cosmic rays (CRs). By modeling the electromagnetic spectrum and using constraints, the predicted gamma-ray spectrum of these BHXBs is investigated, and the potential detection of other low-mass BHXBs by future gamma-ray facilities is explored.