Discovery of a Long Thermonuclear X-Ray Burst from the Ultracompact Binary 4U 1850-087

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 similar to 0.5 keV, a thermally Comptonized continuum, an asymptotic power-law photon index of Gamma similar to 2.2, and an electron temperature of kT e similar to 20-30 keV. The mean persistent flux was around 3.8 x 10-10 erg cm-2 s-1, corresponding to a local mass accretion rate of similar to 1 % m 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, tau approximate to 0.78 hr, the burst fluence, E b approximate to 4.1 x 1041 erg, and the ignition column depth, y ign approximate to 3.5 x 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 7 sigma significant similar to 1 keV emission lines in the burst spectra, which may originate from the surrounding accretion disk.