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Microquasar remnants as reservoirs of PeV cosmic rays

Leandro Abaroa, Gustavo E. Romero, Valentí Bosch-Ramón

Abstract

The Large High Altitude Air Shower Observatory (LHAASO) has revealed a population of Galactic gamma-ray sources radiating beyond 100 TeV, but the nature of several of them is still uncertain. In this contribution, we explore the idea that some of these ultrahigh-energy emitters are not powered by currently active accelerators, but by the fossil remains of microquasars (MQs). We consider systems in which mass transfer onto the stellar-mass black hole has already stopped, so that the central engine and its jets are permanently quenched. During the active phase, powerful transrelativistic jets inflate a hot cocoon whose interior is filled with cosmic rays (CRs) accelerated at the jet termination shocks. Once the jets switch off, the cocoon enters a long afterlife stage in which it behaves as a large reservoir of PeV CRs. If the remnant lies in or near a star-forming region, these relic CRs can still interact with dense clumps and molecular clouds, inside the cocoon or in the surrounding interstellar medium, leading to delayed gamma-ray emission via inelastic pp collisions and the subsequent decay of neutral pions. We present a time-dependent model for the jet-cocoon system, follow the evolution of the CR population during and after the MQ phase, and discuss the conditions under which the resulting microquasar remnants can account for some of the unidentified LHAASO sources.

Microquasar remnants as reservoirs of PeV cosmic rays

Abstract

The Large High Altitude Air Shower Observatory (LHAASO) has revealed a population of Galactic gamma-ray sources radiating beyond 100 TeV, but the nature of several of them is still uncertain. In this contribution, we explore the idea that some of these ultrahigh-energy emitters are not powered by currently active accelerators, but by the fossil remains of microquasars (MQs). We consider systems in which mass transfer onto the stellar-mass black hole has already stopped, so that the central engine and its jets are permanently quenched. During the active phase, powerful transrelativistic jets inflate a hot cocoon whose interior is filled with cosmic rays (CRs) accelerated at the jet termination shocks. Once the jets switch off, the cocoon enters a long afterlife stage in which it behaves as a large reservoir of PeV CRs. If the remnant lies in or near a star-forming region, these relic CRs can still interact with dense clumps and molecular clouds, inside the cocoon or in the surrounding interstellar medium, leading to delayed gamma-ray emission via inelastic pp collisions and the subsequent decay of neutral pions. We present a time-dependent model for the jet-cocoon system, follow the evolution of the CR population during and after the MQ phase, and discuss the conditions under which the resulting microquasar remnants can account for some of the unidentified LHAASO sources.
Paper Structure (9 sections, 3 equations, 2 figures)

This paper contains 9 sections, 3 equations, 2 figures.

Table of Contents

  1. Introduction
  2. From active microquasars to fossil cocoons
  3. Pathways to microquasar remnants
  4. Jet launching and termination
  5. Growth and fossil evolution of the cocoon
  6. Production and transport of cosmic rays
  7. Injection at the reverse shock
  8. Transport in the jet termination region and the cocoon
  9. Discussion and conclusions

Figures (2)

  • Figure 1: Total number of protons as a function of time in the jet termination region (left) and in the cocoon (right). The color scale encodes the elapsed time since the onset of the MQ phase. Particles quickly accumulate in the cocoon, which retains a substantial CR population long after the jets have switched off.
  • Figure 2: Spectral energy distributions of the illuminated cloud at 50 pc from the MQR, at different times. Leptonic emission is produced by secondary pairs created in the cloud via $pp$ interactions. Thick dashed lines show the sensitivity curves of Fermi (10 yr, gray), SWGO (5 yr, light blue), and LHAASO (1 yr, green) for a Galactic MQR at 3 kpc.