Thermonuclear Type-I X-ray Bursts and Burst Oscillations from the Eclipsing AMXP Swift J1749.4-2807

Abstract

Swift J1749.4-2807 is the only known eclipsing accreting millisecond X-ray pulsar. In this paper, we report on 7 thermonuclear (Type-I) X-ray bursts observed by NICER during its 2021 outburst. The first 6 bursts show slow rises and long decays, indicative of mixed H/He fuel, whereas the last burst shows fast rise and decay, suggesting He-rich fuel. Time-resolved spectroscopy of the bursts revealed typical phenomenology (i.e., an increase in black body temperature during the burst rise, and steady decrease in the decay), however they required a variable $N_{\mathrm{H}}$. We found that the values of $N_{\mathrm{H}}$ during the bursts were roughly double those found in the fits of the persistent emission prior to each burst. We interpret this change in absorption as evidence of burst-disc interaction, which we observe due to the high inclination of the system. We searched for burst oscillations during each burst and detected a signal in the first burst at the known spin frequency of the neutron star (517.92 Hz). This is the first time burst oscillations have been detected from Swift J1749.4-2807. We further find that each X-ray burst occurs on top of an elevated persistent count rate. We performed time-resolved spectroscopy on the combined data of the bursts with sufficient statistics (i.e., the clearest examples of this phenomenon) and found that the black body parameters evolve to hotter temperatures closer to the onset of the bursts. We interpret this as a consequence of an unusual marginally stable burning process similar to that seen through mHz QPOs.

accepted for publication in MNRAS, 12 pages, 11 figures

References in corpus (13)

• Thermonuclear (type-I) X-ray bursts observed by the Rossi X-ray Timing Explorer
• An Empirical Background Model for the NICER X-ray Timing Instrument
• Evidence for enhanced persistent emission during sub-Eddington thermonuclear bursts
• Accretion disks and coronae in the X-ray flashlight
• Millihertz Oscillation Frequency Drift Predicts the Occurrence of Type I X-ray Bursts
• A connection between accretion state and $\text{Fe K absorption}$ in an accreting neutron star: black hole-like soft state winds?
• Characterizing the Evolving X-Ray Spectral Features During a Superburst from 4U 1636-536
• The accretion rate dependence of burst oscillation amplitude
• The X-ray bursts of XTE J1739-285: a NICER sample
• Outflows and spectral evolution in the eclipsing AMXP SWIFT J1749.4-2807 with NICER, XMM-Newton and NuSTAR
• An unusual precursor burst with oscillations from SAX J1808.4-3658
• On the peculiar long-term orbital evolution of the eclipsing accreting millisecond X-ray pulsar SWIFT J1749.4-2807
• Drifts of the marginally stable burning frequency in the X-ray binaries 4U 1608--52 and Aql X--1

Cited by in corpus (1)

• Simultaneous Optical and X-ray Detection of a Thermonuclear Burst in the 2024 Outburst of EXO 0748-676