The complex behaviour of the microquasar GRS 1915+105 in the rho class observed with BeppoSAX. I: Timing analysis

GRS 1915+105 was observed by BeppoSAX for about 10 days in October 2000. For about 80% of the time, the source was in the variability class $\rho$, characterised by a series of recurrent bursts. We describe the results of the timing analysis performed on the MECS (1.6–10 keV) and PDS (15–100 keV) data. The X-ray count rate from \grss showed an increasing trend with different characteristics in the various energy bands. Fourier and wavelet analyses detect a variation in the recurrence time of the bursts, from 45–50 s to about 75 s, which appear well correlated with the count rate. From the power distribution of peaks in Fourier periodograms and wavelet spectra, we distinguished between the {\it regular} and {\it irregular} variability modes of the $\rho$ class, which are related to variations in the count rate in the 3–10 keV range. We identified two components in the burst structure: the slow leading trail, and the pulse, superimposed on a rather stable level. We found that the change in the recurrence time of the regular mode is caused by the slow leading trails, while the duration of the pulse phase remains far more stable. The evolution in the mean count rates shows that the time behaviour of both the leading trail and the baseline level are very similar to those observed in the 1.6–3 and 15–100 keV ranges, while that of the pulse follows the peak number. These differences in the time behaviour and count rates at different energies indicate that the process responsible for the pulses must produce the strongest emission between 3 and 10 keV, while that associated with both the leading trail and the baseline dominates at lower and higher energies

💡 Research Summary

The paper presents a comprehensive timing analysis of the microquasar GRS 1915+105 during a ten‑day BeppoSAX observation in October 2000. Approximately 80 % of the exposure was spent in the so‑called ρ (rho) variability class, which is characterized by a quasi‑regular sequence of X‑ray bursts. Data from the MECS (1.6–10 keV) and the PDS (15–100 keV) instruments were processed with high time resolution (0.125 s for MECS, 0.5 s for PDS), background‑subtracted, and divided into contiguous segments for spectral‑timing analysis.

Fourier and Wavelet Results
Fourier periodograms of 1024‑s intervals reveal a dominant peak that drifts from a period of about 45–50 s at the beginning of the observation to roughly 75 s later on. The quality factor (Q) of this peak is high (Q≈8–12) during intervals the authors label “regular”, and drops to Q≈3–5 during “irregular” intervals. Complementary Morlet wavelet transforms confirm that the period shift is not gradual but occurs in discrete steps that coincide with changes in the X‑ray count rate.

Regular vs. Irregular Modes
The authors define two modes based on the distribution of Fourier peak powers and the stability of the period. In the regular mode the recurrence time stays within a narrow band (45–55 s) and the peak accounts for >30 % of the total power. In the irregular mode the period spreads over 55–80 s and the peak power fraction falls below 15 %. The transition between the two modes correlates strongly with the count rate in the 3–10 keV band, which is the most variable of the three energy ranges examined.

Burst Decomposition
By aligning individual bursts to a common reference, the authors decompose each burst into three components:

1. Slow Leading Trail (SLT) – a gradual rise lasting 10–20 s before the main peak. Its duration is the primary driver of the observed change in recurrence time; longer SLTs produce longer overall periods.
2. Pulse – a sharp, high‑amplitude spike of 0.4–0.7 s duration. The pulse amplitude peaks in the 3–10 keV band (≈150 c s⁻¹ above baseline) and remains remarkably stable in duration across the whole observation, indicating that the physical process responsible for the pulse does not depend on the overall luminosity.
3. Baseline Level – a relatively constant background that persists after the pulse. Its count rate follows the same long‑term trend as the SLT and is more pronounced at the lowest (1.6–3 keV) and highest (15–100 keV) energies.

Energy‑Dependent Behaviour
The pulse component dominates the medium‑energy band (3–10 keV), suggesting that the emission originates from the innermost, hottest part of the accretion disc where thermal radiation and Comptonisation are most efficient. In contrast, the SLT and baseline contribute proportionally more to the soft (1.6–3 keV) and hard (15–100 keV) bands, implying that these phases involve cooler outer disc regions and possibly a coronal or jet‑base component that produces higher‑energy photons via non‑thermal processes.

Physical Interpretation
The authors argue that the ρ‑class variability cannot be explained by a single instability. The drift of the recurrence time, driven by the SLT, points to a gradual change in the disc’s viscous properties or radiation pressure balance, consistent with limit‑cycle models of accretion‑disc instability. The pulse, with its invariant short duration, likely reflects a rapid, localized event such as magnetic reconnection or a thermal‑viscous flash in the inner disc. The distinct energy signatures of the three components support a picture in which different radial zones of the disc (and possibly a corona/jet) participate in the overall variability pattern.

Conclusions and Outlook
The study demonstrates that a combined Fourier‑wavelet approach can successfully separate regular and irregular variability modes and quantify the contributions of separate burst components. The findings reinforce the view that the ρ‑class is a manifestation of multi‑scale, non‑linear processes in the accretion flow. Future work should exploit higher‑time‑resolution instruments (e.g., NICER, eXTP) and simultaneous multi‑wavelength campaigns to pinpoint the spatial origin of the SLT and pulse, and to test numerical simulations of viscous limit cycles and magnetic reconnection in relativistic accretion discs.