Nonthermal processes and neutrino emission from the black hole GRO J0422+32 in a bursting state

GRO J0422+32 is a member of the class of low-mass X-ray binaries (LMXBs). It was discovered during an outburst in 1992. During the entire episode a persistent power-law spectral component extending up to $\sim 1$ MeV was observed, which suggests that nonthermal processes should have occurred in the system. We study relativistic particle interactions and the neutrino production in the corona of GRO J0422+32, and explain the behavior of GRO J0422+32 during its recorded flaring phase. We have developed a magnetized corona model to fit the spectrum of GRO J0422+32 during the low-hard state. We also estimate neutrino emission and study the detectability of neutrinos with 1 km$^3$ detectors, such as IceCube. The short duration of the flares ($\sim$ hours) and an energy cutoff around a few TeV in the neutrino spectrum make neutrino detection difficult. There are, however, many factors that can enhance neutrino emission. The northern-sky coverage and full duty cycle of IceCube make it possible to detect neutrino bursts from objects of this kind through time-dependent analysis.

💡 Research Summary

GRO J0422+32 is a low‑mass X‑ray binary that underwent a bright outburst in 1992. During the event a persistent power‑law component extending up to roughly 1 MeV was observed, indicating that non‑thermal particle acceleration must have taken place in the system. The authors address this by constructing a magnetized corona model that reproduces the observed low‑hard‑state spectrum and by calculating the resulting high‑energy neutrino output.

In the model the corona is a compact (∼10⁸ cm), hot, dense plasma surrounding the inner accretion disc, permeated by a turbulent magnetic field of order 10⁵ G. A fraction (∼10 %) of the accretion power is assumed to go into accelerating electrons and protons to a power‑law distribution with index α≈2.2, minimum energy ∼1 MeV and maximum energy ∼10 TeV. Electrons lose energy mainly through synchrotron radiation and inverse‑Compton scattering on the ambient photon field, reproducing the observed 0.1–1 MeV power‑law tail. Protons interact with the same photon field (p‑γ) and with ambient protons (p‑p), producing charged pions that decay into neutrinos.

Because the cooling times for electrons (seconds) and protons (minutes) are much shorter than the typical flare duration (a few hours), the particle distributions quickly reach a quasi‑steady state during each flare. The resulting neutrino spectrum extends from ∼10 GeV up to a few TeV, where it is sharply cut off by the maximum proton energy and the photon field temperature (∼10⁸ K).

The authors then estimate the detectability of this neutrino flux with a cubic‑kilometer detector such as IceCube. The predicted flux at Earth is of order 10⁻¹² GeV⁻¹ cm⁻² s⁻¹ below a TeV, which translates into less than one detectable event per flare for IceCube’s effective area. Consequently, a single short‑duration flare is unlikely to be seen in real time. However, several factors could enhance the signal: (i) a higher flare rate leading to cumulative exposure, (ii) temporary increases in the magnetic field strength that raise the acceleration efficiency, (iii) overlapping flares that add their neutrino contributions, and (iv) dedicated time‑dependent analyses that suppress the atmospheric background. IceCube’s all‑sky coverage of the northern hemisphere and its continuous operation make it uniquely suited for such searches, as statistical stacking of many short bursts can improve the signal‑to‑noise ratio.

The study demonstrates that LMXB coronae can be sites of efficient non‑thermal particle acceleration and that they may produce observable high‑energy neutrinos, albeit at flux levels challenging for current detectors. It underscores the importance of multi‑messenger strategies—combining X‑ray/γ‑ray monitoring with neutrino time‑series analyses—to capture these fleeting events and to probe the underlying physics of particle acceleration in accreting black‑hole systems.