The many tones of accretion - to the memory of Tomaso Belloni

Flip-Flops and QPO switching in GX 339–4 during the 2021 and 2024 NICER outbursts

8 Sept 2026, 10:15

15m

Cefalù

Talk Timing (Belloni Session). Session 2.1: QPOs

Speaker

Mr Federico Ferretti (Università degli Studi Roma Tre, Universitat Politecnica de Catalunya)

Description

GX 339–4 is a low-mass X-ray binary often used as an archetype, showing typical outburst behaviour. The source undergoes a full outburst every 2–3 years, passing through all known accretion states. We used NICER data to study the spectral-timing properties of the 2024 outburst of GX 339–4 and to compare them with the 2021 outburst. In particular we focused on rapid “flip-flop” transitions, i.e. step-like changes in X-ray flux that often come with changes
in the timing properties. We used Monte Carlo simulations to assess the statistical significance of candidate QPO peaks in the power spectra, testing for the short-timescale appearance and disappearance of Type-B QPO signatures. We used this statistically defined classification to compare variability across flip-flop levels, to extract energy spectra for intervals with and without QPO signatures, and to study relations between QPO peak frequency and other variability properties. We will present detailed
results and their implications.