An ultraviolet burst oscillation candidate from the low-mass X-ray binary EXO 0748-676

TL;DR

This study searches for ultraviolet burst oscillations in the neutron-star LMXB EXO 0748--676 by combining archival HST-STIS time-tagged FUV data with simultaneous RXTE X-ray observations from 2003. A timing analysis of the FUV burst rise reveals a candidate oscillation at $552.39$ Hz with Leahy power $P=23.17$, corresponding to a chance probability of about $3.7\times10^{-4}$ (robust to simulations), though the overall significance is limited by overlapping trials. A comprehensive search for persistent FUV pulsations yields no detections with an upper limit of $<1.7\%$ pulsed amplitude in the 545–565 Hz range. Interpreting the candidate within current optical/UV emission models fails to reconcile the observed pulsed fraction and luminosity with reprocessing or surface emission, even allowing for bright, high-temperature scenarios; this hints at a potentially unknown coherent emission process or beamed radiation if confirmed. The result motivates further UV/optical studies of burst oscillations in LMXBs to uncover the physical mechanism behind such extreme variability and to assess their diagnostic power for neutron-star physics.

Abstract

X-ray burst oscillations are quasi-coherent periodic signals at frequencies close to the neutron star spin frequency. They are observed during thermonuclear Type I X-ray bursts from a number of low-mass X-ray binaries (LMXBs) hosting a fast-spinning, weakly magnetic neutron star. Besides measuring the spin frequencies, burst oscillations hold the potential to accurately measure neutron star mass and radius, thus providing constraints on the equation of state of matter at nuclear densities. Based on far-ultraviolet (FUV) observations of the X-ray binary EXO 0748-676 taken with the Hubble Space Telescope in 2003, we report a possible indication of ultraviolet burst oscillations at the neutron star spin frequency ($\sim$552 Hz), potentially the first such case for an LMXB. The candidate signal is observed during an $\sim$8 s interval in the rising phase of an FUV burst, which occurred $\sim$4 s after a Type I X-ray burst. Through simulations, we estimated that the probability of detecting the observed signal power from pure random noise is 3.7$\%$, decreasing to 0.3$\%$ if only the burst rise interval is considered, during which X-ray burst oscillations had already been observed in this source. The background-subtracted folded pulse profile of the candidate FUV oscillations in the (120-160 nm) band is nearly sinusoidal with a $\sim$16$\%$ pulsed fraction, corresponding to a pulsed luminosity of $\sim$8$\times$10$^{33}$ erg/s. Interpreting the properties of this candidate FUV burst oscillations in the light of current models for optical-ultraviolet emission from neutron star LMXBs faces severe problems. If signals of this kind are confirmed in future observations, they might point to an unknown coherent emission process as the origin of the FUV burst oscillations observed in EXO 0748-676.

Figures (3)

• Figure 1: The red and black lines represent the FUV burst light curve observed by HST and the X-ray burst light curve detected by RXTE, respectively. The burst count rate is normalized for each instrument at the maximum count rate (312 counts s$^{-1}$ for HST and 2949 counts s$^{-1}$ for RXTE). The reference epoch is $T_{ref} = 52689.0772718$ MJD. The yellow box indicates the time interval of the BO candidate. The box size corresponds to the duration of the time window and the frequency resolution.
• Figure 2: Fourier power density spectrum from the FUV (120--160 nm) light curve collected with STIS on board HST during an 8-s observation chunk in the interval of (5--13) s since $T_{ref} = 52689.0772718$ MJD during the rise of the Type I burst. The light curve was rebinned to 250 µ s, yielding a Nyquist frequency of 2 kHz. The strong peak is at a frequency of $\sim$ 552.392 Hz. In the inset plot, we show the normalized pulse profile obtained by folding the light curve in the same time interval adopted to extract the power density spectrum using 14 phase bins per period cycle. The time series is folded at the frequency of 552.392 Hz. The blue dashed line shows the best-fit decomposition with one harmonic. The background-subtracted pulsed fraction is (16.1$\pm$5.4)% at 1$\sigma$ confidence level
• Figure 3: Spectral energy distribution for the total and pulsed emission of EXO 0748--676 corrected for the interstellar extinction. The X-ray spectrum is extracted during the thermonuclear burst in the time interval (5–13) s since $T_{ref}$ (TDB), while the FUV spectrum is extracted during the first 80 s of the burst. The X-ray spectrum acquired with RXTE in the 3--20 keV energy band is plotted in blue. The X-ray upper limit on pulsed emission is marked by a light blue arrow. The FUV fluxes measured with HST are shown in orange. The FUV pulsed flux with the corresponding error bars is plotted in red. The magenta line represents the fit to the X-ray spectrum using the burstatmos model Suleimanov, corrected for interstellar extinction and extrapolated down to the FUV band. The green dashed line shows the fit using the bbodyrad model in XSPEC, also corrected for interstellar extinction and extrapolated to the FUV range. The blackbody has a temperature of $\sim 2.2$ keV and an emitting radius of $\sim 4$ km (assuming a source distance of 7.1 kpc; see text).