Ultrahigh-Energy Gamma-ray Emission Associated with Black Hole-Jet Systems

TL;DR

This work demonstrates that accreting stellar-mass black holes with jets can accelerate particles to ultrahigh energies, as shown by LHAASO detections of UHE gamma rays from five Galactic microquasars (SS 433, V4641 Sgr, GRS 1915+105, MAXI J1820+070, Cygnus X-1), with several sources reaching or exceeding $E \approx 100~\mathrm{TeV}$ and one extending to $800~\mathrm{TeV}$. In SS 433, the UHE emission is spatially associated with a giant atomic cloud, indicating a hadronic origin and a multi-component emission scenario that cannot be explained by leptonic processes alone. These results imply that microquasars can act as PeVatrons, potentially contributing to Galactic cosmic rays near the knee, and suggest a population-wide PeV proton injection of order $\sim 10^{39}~\mathrm{erg~s^{-1}}$. The findings motivate future multiwavelength and temporal studies to pinpoint acceleration sites and mechanisms within BH–jet systems and to assess their role in the Galactic CR budget.

Abstract

Black holes (BH), one of the most intriguing objects in the universe, can manifest themselves through electromagnetic radiation initiated by the accretion flow. Some stellar-mass BHs drive relativistic jets when accreting matter from their companion stars, forming microquasars. Non-thermal emission from the radio to tera-electronvolt (TeV) gamma-ray band has been observed from microquasars, indicating the acceleration of relativistic particles. Here we report detection of four microquasars (SS 433, V4641 Sgr, GRS 1915+105, MAXI J1820+070) of spectrum extending to the ultrahigh-energy (UHE; photon energy $E>100$ TeV) band and one microquasar (Cygnus X-1) of spectrum approaching 100 TeV, using the Large High Altitude Air Shower Observatory (LHAASO). Notably, the total emission associated with SS 433 cannot be interpreted with a single leptonic component. In the UHE band, its emission is in spatial coincidence with a giant atomic cloud, which is consistent with a hadronic origin. An elongated source is discovered from V4641 Sgr with the spectrum continuing up to 800 TeV. The detection of UHE gamma rays demonstrates that accreting BHs and their environments can operate as extremely efficient accelerators of particles out of 1 peta-electronvolt (PeV), suggesting microquasars to be important contributors to Galactic cosmic rays especially around the `knee' region.

Figures (3)

• Figure 1: Significance maps and spectral energy distribution (SED) of SS 433 measured by LHAASO, with surrounding sources being subtracted. (a - c): SS 433 at energy 1-25 TeV, 25-100 TeV and above 100 TeV. In top three panels, the green cross marks the position of the BH of SS 433. In (a), the blue diamonds shows the position of H.E.S.S. detected gamma-ray emission above 10 TeVHESS24_SS433. In (a) and (b), black crosses indicate the position of resolved two point-like sources at 1-25 TeV and 25-100 TeV. In (c), the white contour indicates the H i atomic clouds at consistent distance of SS 433 (see Supplementary Materials). The cyan contour show the X-ray emission of the two lobes. The green circle in (c) exhibit 68% containment radii of the LHAASO source. The dashed white line indicate the direction of the Galactic plane with $b=-2^\circ$. The yellow circles show the corresponding 68% containment radii of LHAASO PSF at the corresponding energy range. (d) shows spectra of two point-like sources associated with the east and west lobes of SS 433 with blue circles and red squares respectively. The spectrum of the central extended source is shown with the black dots; (e) compares the total measured spectrum (with the fluxes associated with the two lobes summed up) and the prediction of models. The red solid curve showcases the best-fit spectrum based on multiwavelength data with a single leptonic component (see SM for details), where the red band represents the $1\sigma$ uncertainty. The best-fit value of the high-energy spectral cutoff energy is about $E_{\rm e, max}=10$ PeV. The red dashed curve shows the predicted spectrum with a conservative $E_{\rm e,max}=200\,$TeV with other parameters unchanged. The target photon fields of IC radiation include the cosmic microwave background and interstellar radiationPopescu17. The solid blue curve shows an additional hadronic component and the solid black curve is the sum of the hadronic component and leptonic component with $E_{\rm e, max}=200\,$TeV. In (d) and (e), error bars represent the $1\sigma$ uncertainties of fluxes and bars with downward-pointing triangles are one-tailed 95% upper limits of the flux. In (d), The vertical brackets showcase the $1\sigma$ uncertainties of the flux including the systematic errors.
• Figure 2: Significance maps of four other LHAASO measured microquasars besides SS 433, (a) V4641 Sgr, (b) GRS 1915+195, (c) MAXI J1820+070 and (d) Cygnus X-1 at above 25 TeV, with surrounding sources being subtracted. In each panel, the green cross marks the position of the BH of each microquasar. The green circles in (a) and (b) exhibit 68% containment radii of the LHAASO sources, whereas no green circles are shown in (c) and (d) because of the point-like nature of associated LHAASO sources. In (a), Fermi-LAT 4FGL-DR4 GeV gamma-ray sources within 3 $\sigma$ significance region of V4641 Sgr are shown with white crosses. In (b), other possible counterparts to the observed TeV emission are shown with cyan crosses. The black cross represents the hot spots observed by ALMA In panel (c), the black arrow represents the propagation direction of the receding ejecta. The black arc in (d) represents the bow-like radio structure inflated by the jet of Cygnus X-12005Natur.Cygnus.X1. The yellow circle in each panel shows the corresponding 68% containment radii of LHAASO PSF. The dashed white lines indicate the direction of the Galactic plane.
• Figure 3: Spectra of the LHAASO sources associated with four microquasars. (a) V4641 Sgr; (b) GRS 1915+105; (c) MAXI J1820+070; (d) Cygnus X-1. In each panel, error bars represent the $1\sigma$ statistical uncertainties of measured fluxes and bars with downward-pointing triangles (if present) are one-tailed 95% upper limits of the flux. Vertical brackets showcase the $1\sigma$ uncertainties of fluxes including the systematic errors. Red solid lines represents the best-fit spectrum with a power-law function and shaded regions showcase uncertainties.