The 2024 outburst of the neutron star LMXB EXO 0748-676: an investigation of bursts and eclipses with AstroSat

TL;DR

This study analyzes the 2024 outburst of the neutron-star LMXB EXO 0748-676 using AstroSat's LAXPC, SXT, and UVIT to investigate three Type-I bursts and six eclipses. Time-resolved spectroscopy and joint LAXPC+SXT analyses reveal PRE in two bursts, a distinct soft secondary peak in one burst, a hard X-ray deficit with a ~4 s lag due to coronal cooling, and a PRE-based distance of $7.42\pm0.53$ kpc; the bursts suggest mixed H/He fuel and partial surface burning with flame-front confinement. Eclipse timing and profile modeling show six full eclipses with no strong energy dependence in 4–18 keV and a stable orbital period, offering insights into the binary geometry and environment. Multiwavelength observations, including UV reprocessing of bursts, provide a comprehensive view of burst ignition, flame propagation, burst–accretion interaction, and the evolving environment of this well-studied high-inclination LMXB.

Abstract

We present a detailed analysis of the Type-I (thermonuclear) X-ray bursts and eclipses observed from the neutron star low-mass X-ray binary (LMXB) EXO 0748--676 with AstroSat during the second known outburst of the source following a 16-year-long quiescence period. We detect three thermonuclear X-ray bursts, with two displaying simultaneous coverage in the soft X-rays. Simultaneous UV observations show evidence of reprocessed burst emissions in the far-ultraviolet band. The time-resolved spectral analysis reveals the photospheric radius expansion (PRE) nature for two bursts. We estimate the distance to the source to be $7.42\pm0.53$ kpc using the peak flux of PRE. Notably, one of the bursts exhibited a secondary peak, $\sim30$ s after the primary, particularly dominating in the softer X-rays, which reveals a correlation with the evolution of burst hotspot radius with no temperature dependence. The burst properties and corresponding flux values suggest that mixed H/He burning may have fueled the bursts. We also detect evidence of a soft excess during one burst, likely arising from the interaction of the burst photons with the surroundings. We uncover evidence for a hard X-ray deficit during the peak of all bursts and a hard lag of $\sim4$ s, which can be attributed to the Compton cooling of the corona by the burst photons. We also probe the temporal evolution and the energy dependence of the eclipses, which offer insights into the binary environment. Our study helps gain deeper insight into the physics of burst ignition, flame propagation, the burst-accretion interaction, and the evolution of LMXBs.

Figures (12)

• Figure 1: The light curves binned at 1 s are shown for LAXPC (top panel), SXT (middle panel), and UVIT (bottom panel) aboard AstroSat. The red and blue vertical lines indicate the times corresponding to the bursts and full eclipses detected in the LAXPC data. The zero second in the light curves corresponds to MJD = 60515.34911006 (2024-07-24,08:01:03), which is the start time of the LAXPC observation of EXO 0748$-$676.
• Figure 2: 1 s binned X-ray light curve of B1 (left), B2 (middle), and B3 (right) respectively observed during the 2024 outburst of EXO 0748. The energy range 4.0-6.0 keV, 6.0-8.0 keV, 8.0-10.0 keV, 10.0-14.0 keV, and 14.0-18.0 keV are shown in light blue, orange, green, red, and purple, respectively. All light curves begin 5 s before the burst onset and last till the total burst duration mentioned in Table \ref{['tab:burst_char']}. Zoomed-in plots in B1 and B3 show a hump-like structure and a secondary peak during the decay phase of the respective bursts. In $4.0-6.0\rm \:keV$ light curve of B3, it is evident that the secondary peak is comparable to the intensity of the primary peak.
• Figure 3: Simultaneous bursts of EXO 0748 occurred across different detectors of AstroSat for B1 (upper) and B3 (lower). All the light curves are 2.81 s binned. LAXPC and SXT cover an energy range of 3-80 keV and 0.7-8 keV, respectively, and UVIT works in 130-600 nm. All the light curves begin 20 s before the burst onset and continue until 100 s after the burst starts. B1 was detected in LAXPC and SXT, and the decay portion was found in UVIT. It is the first ever detection of a burst in AstroSat/UVIT. B3 simultaneously occurred in LAXPC and SXT. The secondary peak of B3 is clearly visible in the SXT light curve, reaching an intensity of two-thirds of the primary peak.
• Figure 4: The light curves for B1 (left), B2 (middle), and B3 (right) are shown in two panels: the upper panel displays the low-energy light curves (4-18 keV), while the lower panel presents the simultaneous high-energy light curves. Vertical red lines indicate the burst's start and peak on the 1 s binned low-energy light curve. The high-energy light curves are coarsely binned for better representation. We have taken the coarse bin sizes as 3 s, 3.5 s, and 3 s, for B1, B2, and B3, respectively. We have chosen the high energy range and the coarser binning for the illustrative purposes. Horizontal blue lines represent the persistent count rate in the higher energy range, measured 100 s prior to the start time of the burst. All light curves begin 30 s before the onset of the burst and continue till the duration of the burst.
• Figure 5: The variation of the best-fit spectral parameters during the thermonuclear bursts B1 (left), B2 (middle), and B3 (right) for the observation of EXO 0748--676 is presented. The first panel illustrates the evolution of the source flux in the 4-18 keV energy range. The second panel shows the change in blackbody temperature (kT). The third panel shows the blackbody normalization, which is given by $R^{2}_{km}/D^{2}_{10\text{kpc}}$, where $R_{km}$ represents the hotspot radius in kilometers and $D_{10\text{kpc}}$ is the distance to the source, in scale of 10 kpc. The fifth panel represents the reduced chi-squared ($\chi^{2}_{\nu}$) of the fit, while the sixth panel shows the count rate. The spectra are dynamically binned, and the count rate is presented to highlight the position of the hump and the secondary burst. Bursts B1 and B3 show a double peak temperature profile, indicating the Photospheric Radius Expansion (PRE). Sudden expansion of the Normalization value can be seen during the secondary peak of B3, and the Normalization value stays constant for the hump-like feature during B1, both without changing the temperature.