GeV emission around SS 433 with 17 years Fermi-LAT observation

Based on 17 years of Fermi-LAT observations, this study identifies multiple GeV sources around the microquasar SS 433, including distinct East and West excesses, and provides the first observational evidence suggesting the acceleration of cosmic-ray protons in the large-scale outflows of Galactic microquasars.

The Gist

Imagine the universe as a giant, chaotic construction site. In the middle of this site sits SS 433, a cosmic powerhouse. It's a "microquasar," which is basically a hungry black hole eating a nearby star. As it eats, it spits out two massive, high-speed jets of particles, like a firehose spraying water in opposite directions. These jets smash into the surrounding gas, creating huge, glowing bubbles (lobes) that we can see in X-rays and other wavelengths.

For a long time, astronomers have been trying to figure out exactly what kind of "light" these jets are producing in the GeV range (a specific type of high-energy gamma-ray light). They've been watching this site with a giant space telescope called Fermi-LAT for 17 years.

Here is what the researchers found, explained simply:

1. The Four "Lanterns" in the Dark

When the team looked at the data, they didn't just see the main jets. They spotted four distinct sources of GeV light, like four lanterns glowing in the dark:

• The "Newcomer" (PS J1910+0550): A bright spot far away from the main action. The team decided to ignore this one because it's too far from the jets to be part of the SS 433 family.
• The "Famous Neighbor" (J1913+0512): A bright spot that other astronomers had seen before.
• The "East Excess": A glow on the eastern side of the jets.
• The "West Excess": A glow on the western side of the jets.

The paper focuses on the three that are actually part of the SS 433 system: the Famous Neighbor, the East, and the West.

2. The Mystery of the "Beating Heart"

In a previous study, scientists thought the "Famous Neighbor" (J1913+0512) was pulsing or "beating" like a heart every 160 days. They thought this rhythm matched the way the jets from SS 433 wobble (precess).

The New Finding: The team looked at 17 years of data (almost double the time of the previous study) to check this heartbeat.

• Result: The heartbeat is gone. When they looked at the full 17-year dataset, the rhythm disappeared. It turns out the "heartbeat" was likely just a random glitch or a fluke in the shorter 10-year data. So, we can't be sure this "Famous Neighbor" is actually connected to SS 433 anymore, though it might still be related for other reasons.

3. The Tale of Two Lobes: East vs. West

The most interesting part of the paper is comparing the East Excess and the West Excess. Even though they are on opposite sides of the same system, they are behaving very differently.

• The East Excess (The "Hard" Light):

  • What it looks like: It has a "hard" spectrum. Think of this like a high-pitched, sharp whistle.
  • The Cause: This light is likely made by electrons (tiny charged particles) that have been sped up to near the speed of light. As they fly through space, they bump into low-energy light (like the Cosmic Microwave Background) and boost it up to high-energy gamma rays. This is called "Inverse Compton scattering."
  • Location: It's located slightly away from the brightest X-ray spots, which fits the theory that these fast electrons have traveled a bit before glowing.

• The West Excess (The "Soft" Light):

  • What it looks like: It has a "soft" spectrum. Think of this like a low, deep rumble.
  • The Location: It is shifted away from where the X-rays and TeV (very high energy) light are usually seen. It's like finding a campfire in a different room than the smoke.
  • The Cause: Because it's "soft" and in a weird spot, the electron theory doesn't work well here. Instead, the authors suggest this light comes from protons (the heavy nuclei in atoms) crashing into dense clouds of gas.
  • The Analogy: Imagine a cannonball (the proton) flying through a thick fog (the gas cloud). When it hits the fog, it creates a flash of light. This is called a "hadronic" process.

4. The Big Picture: Cosmic Protons

The most exciting conclusion of the paper is about the West Excess (and potentially the "Famous Neighbor").

The authors suggest that SS 433 isn't just accelerating electrons; it's also accelerating protons (which are the building blocks of cosmic rays). These protons are like heavy, invisible bullets. They get shot out of the jets, travel far away from the main explosion, and then crash into pockets of dense gas hidden in the surrounding area.

• Why this matters: If this is true, it's the first time we have strong evidence that microquasars (small black holes) can act as factories for cosmic-ray protons on a large scale. It's like finding out that a small backyard fireworks show is actually launching heavy artillery into the neighborhood.

Summary

• 17 Years of Data: The team watched SS 433 for a long time.
• No Heartbeat: The suspected 160-day pulse of the "Famous Neighbor" was a false alarm.
• Two Different Glows: The East side glows because of fast electrons; the West side glows because protons are hitting gas clouds.
• The Discovery: This suggests that SS 433 is a factory for high-energy protons, which could be a major source of the cosmic rays that constantly rain down on Earth.

Technical Summary

Technical Summary: GeV emission around SS 433 with 17 years Fermi-LAT observation

Problem and Motivation
Recent detections of photons above 20 TeV from Galactic microquasars by LHAASO have established that accreting stellar-mass black holes can accelerate particles into the PeV regime. SS 433, a prototypical microquasar with precessing jets and extended X-ray lobes embedded in the radio nebula W50, is a primary candidate for studying these acceleration mechanisms. While previous Fermi-LAT analyses (e.g., Bordas et al. 2015; Fang et al. 2020; Li et al. 2020) reported GeV emission features around SS 433, including a source designated J1913+0512 and tentative evidence for a 160-day periodic modulation, the results remain controversial. Key issues include the spatial relationship between GeV, X-ray, and TeV emissions, the physical origin of the GeV excesses (leptonic vs. hadronic), and the validity of reported periodicities. This study aims to resolve these ambiguities using the full 17-year Fermi-LAT dataset to constrain particle acceleration and radiation mechanisms.

Methodology
The authors analyzed 17 years of Fermi-LAT data (up to September 2025) within a $2.5^\circ \times 2.5^\circ$ region of interest centered on SS 433. To mitigate contamination from the nearby bright pulsar PSR J1907+0602, a pulsar-gating method was employed, utilizing off-pulse phases (0–0.136 and 0.697–1) which retain 44% of the total exposure. The analysis utilized the P8R3 Source v3 instrument response function, a binned maximum likelihood fit with a spatial bin size of 0.08 degrees, and standard Galactic and isotropic diffuse emission models. Spectral and morphological analyses were conducted in the 1–100 GeV and 3–100 GeV bands. Periodicity searches were performed using the Lomb-Scargle periodogram on exposure-corrected, event-weighted light curves. Multiwavelength modeling involved a one-zone leptonic model to fit X-ray, GeV, and TeV data simultaneously, and estimates of particle confinement times and cooling breaks were derived to test Inverse Compton (IC) scenarios against hadronic (proton-proton) and bremsstrahlung models.

Key Results

1. Source Detection: Four GeV sources were identified in the region:

   • PS J1910+0550: A new source located $\sim 1^\circ$ north of SS 433, outside the W50 radio shell, with a significance of $\sim 5\sigma$. It is not considered physically associated with SS 433.
   • J1913+0512: A previously reported source outside the X-ray jets, detected with a significance of $\sim 7\sigma$ (TS $\approx$ 46) in the 1–100 GeV band.
   • East Excess: An emission feature associated with the eastern X-ray lobe, detected with $\sim 4\sigma$ significance (TS $\approx$ 16).
   • West Excess: An emission feature associated with the western X-ray lobe, detected with $\sim 4\sigma$ significance (TS $\approx$ 18).

2. Spectral Properties:

   • J1913+0512: Exhibits a soft spectrum with a photon index $\Gamma \approx 2.9$.
   • East Excess: Shows a hard spectrum with $\Gamma \approx 1.7$.
   • West Excess: Displays a significantly softer spectrum with $\Gamma \approx 2.6$.
   • The East excess is spatially displaced from the bright X-ray region "e2" and the bulk of the TeV emission, while the West excess is offset from the X-ray jet and TeV emission sites ("w1", "w2").

3. Periodicity Analysis:

   • The study re-examined the $\sim 160$-day periodic modulation previously reported for J1913+0512. While a low-significance peak appeared in a 10-year subset (consistent with prior work), the full 17-year dataset shows no significant periodicity. The signal at 160 days in the 17-year data is reduced by a factor of $\sim 2.7$ and is indistinguishable from background fluctuations.

4. Physical Modeling:

   • East Excess: The hard spectrum and spatial offset from the TeV peak are consistent with an Inverse Compton (IC) origin from relativistic electrons, where the GeV emission represents the cooling break of the electron spectrum.
   • West Excess & J1913+0512: The soft spectra and spatial offsets from the primary acceleration sites (X-ray/TeV lobes) are inconsistent with a standard one-zone IC model. The authors propose that these emissions arise from GeV particles (protons or electrons) accelerated in the system but distributed throughout the source volume, interacting with localized dense gas targets (molecular clouds). Both proton-proton (pp) and bremsstrahlung scenarios are viable, though the pp scenario is favored as it naturally accommodates a larger fraction of jet power transferred to relativistic protons compared to electrons.

Significance and Claims
The paper claims that the updated 17-year analysis provides robust constraints on the morphology and spectra of GeV emission around SS 433, independent of other instruments. By refuting the previously reported periodicity, the study weakens the case for a direct physical connection between J1913+0512 and the precessing jets of SS 433, though a spectral association remains possible.

Crucially, the distinct spectral and morphological properties of the East and West excesses suggest different physical origins. The authors posit that the detection of the West excess and J1913+0512, explained by particles interacting with dense gas targets outside the main lobes, may represent the first observational evidence for the acceleration of cosmic-ray protons in large-scale outflows from Galactic microquasars. This finding supports the hypothesis that microquasars can contribute to the Galactic cosmic-ray population, particularly around the 'knee' region, by accelerating hadrons that escape the immediate jet environment and interact with the surrounding interstellar medium.