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 center 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.Comment: 13 pages, 12 figures, accepted to MNRA

Radiatively inefficient accretion in short-period black hole low mass X-ray binaries

There is statistical evidence for a dearth of short-period (Porb < 4h) black hole (BH) low mass X-ray binaries (LMXBs) in the Galaxy. At short periods accretion onto the central object (be it a BH) may become inefficient because the cooling timescale of the gas is greater than the accretion timescale (this is the well known ADAF model). The nature of the switch is important in terms of the outburst timescales of transient sources. The switch may be sharp or occur smoothly over time. I show that the dearth can be explained if the switch to inefficiency occurs sharply at some fraction of the Eddington luminosity of the BH (fLEdd )

Accretion and jets from stellar-mass to supermassive black holes

Accretion and jets occur in many astrophysical systems across a multitude of size and mass scales, and environments. As such, the study of accretion and jet physics has for decades been, and still remains, a hot topic in astrophysics. Accretion onto black holes has particular significance for many reasons, not least because supermassive black holes likely exist at the centres of every galaxy in the universe. The energetic impact of black hole accretion is therefore key to furthering our understanding of the universe as a whole. Over the past few decades we have learned that black hole accretion seems to be a scale-invariant process: despite the orders of magnitude difference in black hole mass, and the diversity of astrophysical environments accretion occurs within, the efficiency and power budget associated with the accretion process appears ignorant to these variables. However, the physical explanation for this scaling is lacking, due to the degeneracy in broadband spectral modelling. In this thesis I show work I have done, alongside many collaborators, to address these modelling degeneracies. I focus on the outflow-dominated states of accreting black holes, from the stellar-mass components of X-ray binaries in the Milky Way, to their supermassive analogues in low-luminosity active galactic nuclei. In addition, I show how this concept of scale-invariance can be used to make predictions about the existence of the more controversial primordial black holes