🔭 astrophysics

Temporal evolution of the periodic GeV signal from 4FGL J1913.2+0512 and analysis of the SS 433 / W50 lobes

This study analyzes 16 years of Fermi-LAT data to confirm a GeV source and a western lobe excess in the SS 433/W50 system, revealing that the periodic ~162-day GeV modulation of 4FGL J1913.2+0512 was prominent during the first decade of the mission but has since diminished, suggesting evolving gamma-ray production efficiency or geometry in the microquasar environment.

Original authors: Ömer Faruk Çoban, Diego F. Torres, Jian Li, Daniela Hadasch, Agnibha De Sarkar, Matthew Kerr

Published 2026-05-18

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Ömer Faruk Çoban, Diego F. Torres, Jian Li, Daniela Hadasch, Agnibha De Sarkar, Matthew Kerr

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Cosmic "Lighthouse" That Flickers On and Off

Imagine a cosmic lighthouse in our galaxy. This isn't a lighthouse on a beach, but a tiny, super-dense object (a black hole) eating a giant star next to it. This system is called SS 433.

As the black hole eats, it shoots out two powerful beams of particles (jets) in opposite directions, like a garden hose spraying water. But here's the twist: these jets don't point in a straight line. They wobble and spin around in a cone shape, tracing a spiral path through space, much like a sprinkler head rotating. This spinning happens on a very precise schedule: every 162 days.

The Mystery Signal

Astronomers have been watching this system with a giant space telescope called Fermi, which sees high-energy light (gamma rays) that our eyes can't see. For a long time, they were confused. They saw a bright, steady source of gamma rays nearby called 4FGL J1913.2+0512, but they couldn't figure out if it was part of the SS 433 system or just a random background star.

The big problem was that there was a very bright "neighbor" pulsar (a spinning neutron star) nearby that was blinding the telescope, making it hard to see the faint signals from SS 433. It was like trying to hear a whisper at a rock concert; the loud music (the pulsar) drowned out the whisper (the jets).

How They Solved the Puzzle

The authors of this paper acted like sound engineers. They used a technique called "pulsar gating." Imagine they put on noise-canceling headphones that only let them hear the music when the pulsar is not spinning toward them. By blocking out the loud neighbor, they finally got a clear view of the SS 433 system.

With this clearer view, they looked at 16 years of data (from 2008 to 2024) and found something amazing:

The Signal is Real: They confirmed that the source 4FGL J1913.2+0512 is indeed connected to the SS 433 system.
The Rhythm: They found that the gamma-ray light from this source pulses with the exact same 162-day rhythm as the spinning jets. It's like the lighthouse is flashing in sync with the sprinkler.
The Twist (The "Flicker"): Here is the most surprising part. This rhythm wasn't there the whole time.
- Years 1–10: The signal was strong and rhythmic. The lighthouse was flashing clearly.
- Years 11–16: The rhythm disappeared. The light didn't stop, but it stopped flashing in time with the jets. It became a steady, non-pulsing glow.

What Does This Mean?

The authors suggest that the "engine" creating these gamma rays is changing. Imagine a sprinkler that sprays water in a perfect spiral for a while, but then the nozzle gets clogged or the water pressure changes, and the spray becomes a steady stream instead of a spiral.

The paper concludes that the efficiency or the shape of how these particles produce gamma rays is evolving over time. For a decade, the geometry was just right to create a rhythmic signal. Then, something changed, and the rhythm was lost.

Other Discoveries

While studying this, they also found:

A "West" and "East" Lobe: They confirmed gamma rays coming from the western end of the jets (where the water hits the wall) and saw a faint hint of activity on the eastern side.
A New Neighbor: They found a brand new gamma-ray source nearby that wasn't on any previous maps.
The Particle Recipe: They tried to figure out what kind of particles are making the light. They tested two recipes: one using protons (heavy particles) and one using electrons (light particles). The "electron" recipe fit the data better, especially when combined with X-ray observations, suggesting the light comes mostly from fast-moving electrons.

The Bottom Line

This paper tells us that the universe is dynamic. Even the most stable-looking cosmic objects can change their behavior over time. The SS 433 system gave us a rhythmic signal for a decade, and then it stopped. It's a reminder that in space, things are rarely static; they are constantly evolving, turning on and off, and changing their dance steps.

Technical Summary: Temporal evolution of the periodic GeV signal from 4FGL J1913.2+0512 and analysis of the SS 433 / W50 lobes

Problem and Context
SS 433 is a microquasar embedded in the supernova remnant W50, characterized by relativistic jets that precess with a period of approximately 162 days. While Very-High-Energy (VHE) gamma-ray emission from the jet lobes has been detected by HAWC, H.E.S.S., and LHAASO, the GeV emission landscape remains complex. Previous studies using Fermi Large Area Telescope (LAT) data yielded inconsistent results regarding GeV emission from the system. Specifically, Li et al. (2020) reported a hint of a GeV excess coincident with the western jet lobe and a nearby source, 4FGL J1913.2+0512, which exhibited a periodicity compatible with the jet precession. However, the persistence of this signal over longer timescales and the nature of the emission mechanisms remained open questions. Furthermore, the region is heavily contaminated by the bright nearby pulsar PSR J1907+0602, necessitating rigorous data cleaning to isolate potential signals from the SS 433 system.

Methodology
This study presents a comprehensive analysis of 16 years of Fermi-LAT data (August 2008 – September 2024), covering the 0.3–300 GeV energy range. The analysis employs several key methodological advancements over previous works:

Pulsar Gating: To mitigate contamination from PSR J1907+0602 (located 1.4° away), the authors applied a pulsar gating technique. Photons were assigned rotational phases using an updated ephemeris, and only off-peak phases ( $\phi = 0.0-0.132$ and $0.666-1.0$) were retained. This reduced the pulsar contribution by a factor corresponding to the phase window fraction (0.466).
Data Processing: The analysis utilized Pass 8 Release 3 (P8R3) SOURCE class data with updated instrument response functions (IRFs) and a newer source catalog (4FGL-DR4). A Region of Interest (ROI) of 15° centered on SS 433 was analyzed using binned maximum-likelihood methods.
Temporal Analysis: Exposure-corrected Lomb-Scargle (LS) periodograms were generated to search for periodicity. The dataset was segmented into various intervals (5-year blocks, 10-year blocks, and the full 16-year span) to assess the temporal stability of any detected signals.
Phase Folding: Events were folded using the precessional period ( $P_{prec} = 162.25$ days) and a reference epoch ( $T_0$ ) derived from optical line separation. Light curves and Test Statistic (TS) maps were constructed for precessional phases (0.0–0.5 and 0.5–1.0).
Spectral Modeling: Spectral Energy Distributions (SEDs) for the western lobe were modeled using the GAMERA code, testing hadronic (proton-proton collisions), leptonic (inverse Compton and synchrotron), and hybrid scenarios against multiwavelength data (radio to TeV).

Key Results

Source Detection: The analysis confirms the detection of 4FGL J1913.2+0512 with a significance of $TS = 45$ and a power-law spectrum (photon index $\Gamma = 2.61 \pm 0.08$ ). A GeV excess is confirmed at the western lobe ("West Excess") with $TS = 17$. A hint of emission is also found at the eastern lobe, though with low significance ($TS=7$). Additionally, a new source, Fermi J1909.6+0552, is revealed outside the SS 433 system ($TS = 20$, rising to 28 over 0.1–300 GeV).
Temporal Evolution of Periodicity: The ~162-day modulation in 4FGL J1913.2+0512 is not constant over the mission lifetime.
- 2008–2018 (First 10 years): The periodic signal is prominent, with a false alarm probability (FAP) < 1%. The flux is concentrated in the precessional phases 0.0–0.5, with the source appearing essentially "off" during phases 0.5–1.0.
- 2018–2024 (Last 6 years): The periodic modulation disappears. The source is detected with lower significance, and the LS periodogram shows no peak at 162 days.
- Full Dataset: When analyzing the full 16 years, the periodicity remains detectable but with reduced significance, attributed to the dilution caused by the non-modulated epoch in the later years.
Systematic Checks: The authors ruled out instrumental or mission-level systematics (e.g., spacecraft orbit, scanning strategy) as the cause of the periodicity. Window-function analyses and exposure history checks showed no power near 162 days. Nearby sources in the same ROI did not exhibit similar modulation.
Multiwavelength Modeling:
- Hadronic Scenario: A model involving proton acceleration and $\pi^0$ -decay can reproduce the GeV–TeV SED of the western lobe, requiring a proton luminosity of $\sim 2.5 \times 10^{38}$ erg s $^{-1}$ . However, this scenario fails to explain the observed X-ray and radio emission.
- Leptonic Scenario: A model where relativistic electrons produce synchrotron (X-ray/radio) and inverse Compton (GeV/TeV) emission provides a consistent fit to all multiwavelength data. This requires an electron luminosity of $\sim 6.5 \times 10^{35}$ erg s $^{-1}$ and a magnetic field of $36 \pm 3$ $\mu$ G.
- Conclusion on Mechanism: The authors favor the leptonic scenario as the more viable description of the western lobe emission, though a hybrid contribution is not excluded.

Significance and Claims
The paper claims to provide the most comprehensive analysis of the SS 433 region to date, extending the observation baseline by 50% compared to previous studies. The primary significance lies in the characterization of the temporal instability of the GeV periodic signal. The authors demonstrate that the 162-day modulation is a transient phenomenon, appearing strongly in the first decade of the Fermi mission but vanishing in the subsequent years.

The authors interpret this evolution as evidence that the efficiency and/or geometry of gamma-ray production in the SS 433 environment changes on multi-year timescales. They suggest that whatever mechanism generates the emission (likely leptonic based on their modeling) is subject to physical or geometric changes that disrupt the coherence of the precessional modulation over time. The study reinforces the reality of the periodic signal in 4FGL J1913.2+0512 while highlighting its non-stationary nature, offering a new constraint for theoretical models of jet-interstellar medium interactions. The paper does not claim to have identified the specific physical mechanism causing the signal's disappearance, noting that current understanding of the system is insufficient to draw strong conclusions without specific modeling of the evolution.