Detection of a Molecular Cloud toward the Heartbeating Gamma-ray Source near the Microquasar SS 433

Abstract

We report the detection of a molecular cloud, CO+40.05-2.40, positionally coincident with the "heartbeating" GeV source Fermi J1913+0515 at the northern boundary of the SS 433/W50 system. Millimeter and submillimeter spectroscopy with the Nobeyama 45 m telescope and the James Clerk Maxwell Telescope shows that the cloud has physical properties typical of quiescent dark clouds in the Galactic disk, with no evidence of shock heating or enhanced excitation. We examine possible high-energy emission mechanisms and find that the observed GeV luminosity cannot be accounted for by electron bremsstrahlung or hadronic interactions driven by relativistic particles originating from SS 433 under reasonable energetic assumptions. As an alternative, we propose that the gamma-rays may arise from a compact object embedded within the cloud and powered by Bondi-type accretion. In this framework, the reported heartbeat-like variability may reflect periodic modulation of the accretion flow by density waves induced by the precessing equatorial outflow of SS 433.

Figures (3)

• Figure 1: (a) Radio continuum and hard X-ray images of the SS 433/W50 system. The black-blue-white colormap shows the JVLA L-band image Sakemi_2021, while the black-magenta-white colormap represents the Chandra$0.5\hbox{--}7$ keV image Tsuji_2025. Gray contours indicate the 1.4 GHz continuum flux observed with the Effelsberg 100 m telescope Reich_1982Reich_1986. The magenta cross marks the position of SS 433. The green circle denotes the 95% positional error circle of Fermi J1913+0515, and the white square outlines the area shown in panel (b). (b) Velocity-integrated map of CO J=3--2 emission obtained with ASTE. The green circle is the same as in panel (a), and the magenta rectangle denotes the mapping area covered by the NRO 45m observations in 2023.
• Figure 2: (Top) Maps of velocity-integrated emission of (a) CO J=1--0, (b) $^{13}$CO J=1--0, and (c) CO J=3--2 lines. The integration velocity range is $\hbox{$V_{\rm LSR}$}\!=\!34\sim38\,\hbox{km s$^{-1}$}$. (Bottom) Longitude velocity maps of (a) CO J=1--0, (b) $^{13}$CO J=1--0, and (c) CO J=3--2 lines. The integration latitude range is $b\!=\!-2\fdg 46\sim -2\fdg 35$.
• Figure 3: Schematic illustration of the mechanism for producing periodic variability in the accretion scenario.