Exploring the connection between compact object mergers and fast X-ray transients: The cases of LXT 240402A and EP250207b

TL;DR

The paper investigates two extragalactic fast X-ray transients, LXT 240402A and EP250207b, to test whether a subset of FXRTs arise from compact binary mergers. It employs precise Chandra localisations, deep optical/IR imaging, and early X-ray monitoring to constrain kilonova emission and host environments, finding results broadly compatible with merger scenarios but not definitive. The study highlights the challenge of securing distances and kilonova detections for optically faint events, and argues that high-resolution X-ray spectroscopy with future facilities like NewAthena could provide robust redshift measurements and tighter progenitor constraints. Overall, the work suggests that some FXRTs in the local universe may trace merger remnants with faint kilonovae, and emphasizes the potential of X-ray spectroscopy to revolutionize distance measurements for optically dim transients.

Abstract

The connection between compact object mergers and some extragalactic fast X-ray transients (FXRTs) has long been hypothesized, but never ultimately established. In this work, we investigate two FXRTs, the LEIA X-ray Transient LXT 240402A and the Einstein Probe EP250207b, whose precise positions lie close to nearby ($z\!\lesssim\!0.1$) quiescent galaxies with negligible probability of chance coincidence, identifying them as particularly promising cases of merger-driven explosions in the local Universe. We used Chandra to derive accurate localizations for both events and secure otherwise ambiguous associations with their optical counterparts. Deep optical and near-infrared observations with VLT, GTC, and LBT were performed to characterize the surrounding environment and search for kilonova emission, the hallmark of neutron star mergers. Complementary early-time X-ray monitoring with Swift and Einstein Probe was used to constrain the non-thermal afterglow. We find that both FXRTs remain compatible with a compact binary merger progenitor, which produced low-mass ejecta and kilonova emission subdominant to the afterglow. However, alternative explanations such as a distant ($z\!\gtrsim\!1$) core-collapse supernova cannot be conclusively ruled out.

Figures (13)

• Figure 1: False-colour image of the field of LXT 240402A/GRB 240402B. The preliminary LEIA localisation (white circle) intercepts a bright galaxy, CGCG 138-001, at $\approx$210 Mpc. However, a fading X-ray and optical counterpart was localised further away from it. The inset zooms in on its position. A distant galaxy, partially blended with the nearby star, is seen at the position of the X-ray counterpart.
• Figure 2: Light curve and $E_{\rm peak}$ - $E_{\rm iso}$ function. Top panel: Background-subtracted 50 keV - 2 MeV light curve of GRB 240402B in 0.125-s bins as observed by Glowbug. The two primary peaks at T0+0.5 s and T0+3 s as described in 36030 are visible, with additional sub-structure in the light curve that is overall consistent with that observed by Konus-Wind 36041. Bottom panel: $E_{\rm peak}$ - $E_{\rm iso}$ diagram for long (grey) and short GRBs (blue), updated from Dichiara2021. The position of LXT 240402A/GRB 240402B is shown as a function of redshift, highlighting $z=$0.048 and 1.5. The Amati relation (dashed line; Amati2008) and its 2$\sigma$ scatter (grey area) are shown.
• Figure 3: Spectral energy distribution of CGCG 138-001 sdss2007. The optical spectrum was renormalised to match the photometric measurements. The most prominent features are identified.
• Figure 4: VLT/X-Shooter spectrum of the faint galaxy underlying the position of LXT240402A. The bottom panels show the one-dimensional spectrum (black points), smoothed with a three-pixel binning, along with its standard deviation (black error bars), zoomed in around the identified emission lines. The flux is normalised to its maximum value. The dashed lines mark the positions of the features, while the solid lines show their Lorentzian fits; from left to right: [O II] (purple), $H\beta$ (blue), [O III] (green), and $H\alpha$ (yellow). The top panels display the corresponding emission features in the two-dimensional spectrum (x-axis: wavelength; y-axis: spatial direction), highlighted by white circles.
• Figure 5: EP/WXT light curve of EP250207b in the 0.5-4.0 keV band, adaptively binned to achieve either a 3$\sigma$ significance per bin or a maximum bin size of 10 s. Error bars are at the 68% confidence level. The vertical dashed (dotted) lines mark the $T_{90}$ ($T_{50}$) interval.