Probing the detectability of electromagnetic signatures from Galactic isolated black holes

¹ Facultad de Ciencias Exactas, UNLP, Calle 47 y 115, CP(1900), La Plata, Buenos Aires, Argentina
² Instituto Argentino de Radioastronomía (CCT La Plata, CONICET), C.C.5, (1894) Villa Elisa, Buenos Aires, Argentina
³ Departament de Física Quántica i Astrofísica, Institut de Ciéncies del Cosmos (ICCUB), Universitat de Barcelona (IEEC-UB), Martí i Franquès 1, E-08028 Barcelona, Spain
⁴ Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque s/n, 1900 La Plata, Argentina
⁵ Department of Space, Earth and Environment, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden

^⋆ Corresponding author: jmartinez@iar.unlp.edu.ar

Received: 31 March 2025
Accepted: 18 June 2025

Abstract

Context. A large number of isolated stellar-mass black holes (IBHs) are expected to populate the Galaxy. However, only one has been confirmed by the analysis of a microlensing event, and no confirmed emission detection from an IBH has been reported so far.

Aims. We analysed the detectability of electromagnetic signatures from IBHs moving in the Galaxy.

Methods. We considered accretion from the interstellar medium onto an IBH and assumed the formation of an outflow. We then semi-analytically modelled the accretion process and the interaction of the outflow with the surrounding medium on large scales, including mechanical feedback on the accretion process. Furthermore, we also (semi-)analytically calculated the emission from three different regions: the accretion region, the thermal and the non-thermal radiation from the outflow-medium interaction structure, and the non-thermal emission of relativistic particles that diffuse in the surrounding medium.

Results. Our results show that multi-wavelength emission associated with Galactic IBHs can be detected in systems moving through a very dense medium such as the core of a molecular cloud. In particular, thermal emission from accretion could be observed in the mid-infrared and in hard X-rays with current and forthcoming observatories. Thermal and non-thermal emission from the outflow-medium shock could also be detected in the radio and millimetre ranges. Moreover, detection of the emission from particles diffusing in a dense medium could be feasible in γ-rays. Applying our model to the IBH associated with the gravitational microlensing event MOA-2011-BLG-191/OGLE-2011-BLG-0462, we inferred that radio and infrared detection of the IBH is plausible. Also, we derived that IBHs could be modest Galactic cosmic ray contributors, potentially reaching a ∼1% contribution at E ≳ 1 PeV. Finally, by extending our model to primordial black holes, we conclude that efficient leptonic acceleration in their outflow-medium interactions would rule them out as a major dark matter component.