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Outburst Characteristics of Transient Low Mass X-ray Binary 4U 1608-52

Can Güngör

TL;DR

The paper addresses the diverse outburst morphologies of the transient NS-LMXB 4U 1608-52 and develops a systematic classification for fast rising exponential (FRED) events by leveraging long-term MAXI and RXTE/ASM X-ray light curves. It employs cross-instrument calibration, automated event detection, s’Bézier smoothing, and Gaussian Mixture Models to identify three FRED classes (long-high, short-medium, short-low) and analyzes the relationship between pre-outburst quiescent duration and outburst energetics, finding a positive, though scattered, correlation for peak and integrated emission. The work places these findings in the disk instability and irradiated-disk framework, discusses viscous time scales and reservoir mass as physical drivers, and compares results with Aql X-1 and reflection spectroscopy studies to support a state-dependent accretion geometry interpretation. Overall, the study provides a transferable classification scheme for NS-LMXB outbursts and reinforces the view that waiting-time driven mass buildup plays a key role in outburst energetics, with additional factors such as disk geometry contributing to the observed scatter.

Abstract

We present a deep study of the long-term X-ray light curve of 4U 1608-52 by investigating the fast rising exponential decay (FRED) outbursts, low intensity state (LIS) and quiescent intervals. By calibrating the onset times of the outbursts, we identify three distinct classes for the FRED-type events: (i) the long-high outbursts, exceeding ~50 d in duration with peak count rates above ~40 cnt/s; (ii) the short-medium outbursts, with durations of ~20 d and peak count rates of ~30-50 cnt/s; and (iii) the short-low outbursts, also lasting ~20 d but reaching only ~20-30 cnt/s at peak. We, furthermore, examine the relation between pre-outburst duration and the peak & integrated count rates of the upcoming outburst. We show that outbursts following longer quiescent periods tend to be more energetic.

Outburst Characteristics of Transient Low Mass X-ray Binary 4U 1608-52

TL;DR

The paper addresses the diverse outburst morphologies of the transient NS-LMXB 4U 1608-52 and develops a systematic classification for fast rising exponential (FRED) events by leveraging long-term MAXI and RXTE/ASM X-ray light curves. It employs cross-instrument calibration, automated event detection, s’Bézier smoothing, and Gaussian Mixture Models to identify three FRED classes (long-high, short-medium, short-low) and analyzes the relationship between pre-outburst quiescent duration and outburst energetics, finding a positive, though scattered, correlation for peak and integrated emission. The work places these findings in the disk instability and irradiated-disk framework, discusses viscous time scales and reservoir mass as physical drivers, and compares results with Aql X-1 and reflection spectroscopy studies to support a state-dependent accretion geometry interpretation. Overall, the study provides a transferable classification scheme for NS-LMXB outbursts and reinforces the view that waiting-time driven mass buildup plays a key role in outburst energetics, with additional factors such as disk geometry contributing to the observed scatter.

Abstract

We present a deep study of the long-term X-ray light curve of 4U 1608-52 by investigating the fast rising exponential decay (FRED) outbursts, low intensity state (LIS) and quiescent intervals. By calibrating the onset times of the outbursts, we identify three distinct classes for the FRED-type events: (i) the long-high outbursts, exceeding ~50 d in duration with peak count rates above ~40 cnt/s; (ii) the short-medium outbursts, with durations of ~20 d and peak count rates of ~30-50 cnt/s; and (iii) the short-low outbursts, also lasting ~20 d but reaching only ~20-30 cnt/s at peak. We, furthermore, examine the relation between pre-outburst duration and the peak & integrated count rates of the upcoming outburst. We show that outbursts following longer quiescent periods tend to be more energetic.
Paper Structure (6 sections, 3 figures)

This paper contains 6 sections, 3 figures.

Table of Contents

  1. Introduction
  2. Methodology and Results
  3. Classification of Outbursts
  4. Pre-outburst Duration vs Peak $\&$ Integrated Emission
  5. Discussion
  6. Conclusion

Figures (3)

  • Figure 1: The long-term light curve of 4U 1608--52
  • Figure 2: The smoothed light curves of the FRED type outbursts of 4U 1608--52 calibrated to their onsets. The inner panel shows the relation between the pre-outburst duration and the peak count rate of each outburst, illustrating the separation of the long-high, short-medium, and short-low classes as revealed by the GMM fits.
  • Figure 3: Relation between the pre-outburst duration and the peak count rate (upper panel) and the integrated count rate over the outburst (lower panel) for FRED-type events of 4U 1608--52. The red line represents the best-fit linear trend to the data.