Spectral and type I X-ray burst studies of M15 X-2 using NICER observations

Abstract

In this work, we present spectral and burst analyses of three thermonuclear type I X-ray bursts (B1, B2, and B3) detected from the ultracompact neutron star low-mass X-ray binary M15 X-2, using data from the Neutron Star Interior Composition Explorer (NICER). Time-averaged spectral fitting with the model $\mathrm{tbabs}\times (\mathrm{thcomp}\times \mathrm{diskbb})$ suggests that the source was in a soft or soft-intermediate spectral state, characterized by a photon power law index of $\Gamma \sim 2$ and an average mass accretion rate of ∼0.09 ${\dot{m}}_{Edd}$. The type I X-ray bursts exhibited rapid rise times of 1.25−2.75 s, followed by longer decay phases lasting 14.50−23.25 s, with characteristic burst timescales (τ) of ∼11 s, which are consistent with pure helium burning. Notably, burst B3 displayed a double-peaked profile indicative of a photospheric radius expansion event, from which we inferred the neutron star radius to be $10.8\begin{array}{c}+2.4\\ -2.2\end{array}$ km. Based on the peak flux of the burst, we estimated the source distance to be $10.54\begin{array}{c}+1.43\\ -1.26\end{array}$ kpc under the assumption of isotropic emission, and $14.06\begin{array}{c}+1.90\\ -1.68\end{array}$ kpc for anisotropic emission geometry. A strong ∼420 Hz burst oscillation candidate was detected in the cooling tail of burst B1.