Detection of Oscillations in a Type I X-Ray Burst of 4U 0614+091 with SVOM/ECLAIRs

TL;DR

This study analyzes a 2025 Type I X-ray burst from the ultracompact LMXB 4U 0614+091 using SVOM/ECLAIRs, performing time-resolved spectroscopy and a Doppler-aware burst oscillation search. The authors detect a coherent oscillation at $\bar{\nu}=413.674\pm0.002$ Hz with a steady drift $\dot{\nu}=(-4.7\pm0.3)\times10^{-3}$ Hz s$^{-1}$ over a 51 s interval, achieving a post-trial significance of $\sim4.6\sigma$ and a fractional amplitude of $\sim9.6\%$ rms in 4–40 keV. The drift may be explained by Doppler modulation from orbital motion, implying an orbital period $P_{\rm orb} \lesssim 20$ minutes for plausible neutron-star and companion masses and inclinations. If confirmed, this would place 4U 0614+091 among the most compact known LMXBs and demonstrates SVOM/ECLAIRs' capability to constrain binary parameters through burst timing, motivating follow-up observations to refine the orbital solution.

Abstract

On 2025 January 10, a thermonuclear (Type I) X-ray burst from the neutron star low-mass X-ray binary \textit{4U~0614+091} was detected with the ECLAIRs instrument on board the \textit{SVOM} mission. We present here a time-resolved spectroscopic analysis of the burst, along with the detection of burst oscillations within a 51-second interval during the decay phase. The oscillation frequency is measured to be $ν= 413.674 \pm 0.002\,\mathrm{Hz}$, consistent with previous reports. However, we detect a significant downward frequency drift over the burst duration, characterized by $\dotν = (-4.7 \pm 0.3) \times 10^{-3}\,\mathrm{Hz\,s^{-1}}$. This frequency evolution is atypical compared to those observed in similar burst oscillation sources. We tentatively attribute the observed drift to a Doppler shift induced by orbital motion. Under this interpretation, the inferred orbital period must be shorter than 20 minutes, placing \textit{4U~0614+091} among the most compact known low-mass X-ray binaries.

Figures (5)

• Figure 1: Time-integrated spectrum between $T=T_0$ and $T=T_0 +25$ s, with in red a fitted bbodyrad model with $kT = 2.04$ keV.
• Figure 2: Evolution of time-resolved spectroscopy with a bbodyrad model, with (a) the flux between 4 and 30 keV, (b) the temperature $kT$, (c) the emission radius, assuming a distance of 3 kpc, with a constant radius shown as a dashed line, and (d) the reduced chi square $\chi^2_r=\chi^2/17$ degrees of freedom.
• Figure 3: Significance contours of the oscillations as a function of time and frequency (left-hand axis), computed using a ${Z_1^2}$ test in a 20-second sliding window with 1-second steps. Significance levels are given as a color gradient. The burst light curve is shown as the equivalent on-axis count rate (right-hand axis) in the 4–40 keV energy band, with the persistent emission subtracted, binned in 1-second intervals.
• Figure 4: Left: Significance contours in the $\bar{\nu}$–$\dot\nu$ plane for the time window yielding the most significant signal [$T_0$+11 s, $T_0$+62 s]. Significance levels are given as a color gradient. Overlaid in red are the contours of a two-dimensional Gaussian fitted around the peak. The best-fit parameters, $\bar{\nu} = 413.674$ Hz and $\dot\nu = -4.7 \times 10^{-3}$ Hz/s, are used to fold the light curve in this interval. Right: Folded burst profile using the fitted parameters over the selected time window, in black. The best-fit sinusoidal model to the profile is shown in red
• Figure 5: Upper limit of the orbital periods set by the $\Delta\nu$ (dashed lines) and $\dot\nu$ (solid lines) as a function of $M_{\rm{WD}}$, at different inclinations $i$, and for $M_{\rm{NS}} = 1.2 ~ M_\odot$ (left) and $M_{\rm{NS}} = 2.0 ~ M_\odot$ (right). The areas of the parameter space incompatible with the hypothesis that both $\Delta\nu$ and $\dot\nu$ originate from orbital motion are represented in gray.