2024ApJ...969...15L

We report the detection of a long X-ray burst from the ultracompact binary 4U 1850-087. The type I X-ray burst was observed on MJD 60171.65 by the Monitor of All-sky X-ray Image and Neutron Star Interior Composition Explorer (NICER). We analyze the NICER data between MJD 60095.19 and 60177.43, which includes the observation covering parts of the long X-ray burst decay phase, i.e., 0.15-3.8 hr after the trigger. The persistent spectra are well described by a multicolor disk blackbody, with an inner temperature of ∼0.5 keV, a thermally Comptonized continuum, an asymptotic power-law photon index of Γ ∼ 2.2, and an electron temperature of kT_e ∼ 20-30 keV. The mean persistent flux was around 3.8 × 10^–10 erg cm^–2 s^–1, corresponding to a local mass accretion rate of $\sim 1 \% \,{\dot{m}}_{\mathrm{Edd}}$. Part of the time-resolved burst spectra show a clear deviation from the blackbody model, which can be improved by considering the enhanced persistent emission due to the Poynting-Robertson drag or the reflected disk emission illuminated by the burst. From the burst flux during the cooling phase, we estimate the burst duration, τ ≈ 0.78 hr, the burst fluence, E_b ≈ 4.1 × 10⁴¹ erg, and the ignition column depth, y_ign ≈ 3.5 × 10¹⁰ g cm^–2. These long X-ray burst parameters from 4U 1850-087 suggest a regime of unstable burning of a thick, pure helium layer slowly accreted from a helium donor star. Moreover, we identify 7σ significant ∼1 keV emission lines in the burst spectra, which may originate from the surrounding accretion disk.