Mass-Radius Constraints for 2S 0918-549 from an RXTE Superexpansion Burst: A Direct Cooling-Tail Analysis

Abstract

Thermonuclear (Type I ) X-ray bursts from accreting neutron stars offer a means to determine neutron-star (NS) mass ($M$) and radius ($R$) and thereby probe the properties of matter at supranuclear density. A subset of these events, photospheric radius-expansion (PRE) bursts, provide a particularly powerful tool to constrain the neutron-star $M$ and $R$. Here, we apply the direct cooling-tail method to 2S~0918$-$549, using a rare superexpansion burst observed by \emph{RXTE}. We fit only the post-touchdown data within \(F/F_{\rm td}\in[0.6,0.95]\), employing modern atmosphere models (pure He and metal-enriched). The pure-He atmosphere yields a good description of the cooling tail (\(χ^{2}/ν=18.12/14\)), whereas metal-rich models fail; information-criterion tests (AIC/BIC) disfavor adding a free absorption edge in every time bin, indicating that heavy-element ashes are unnecessary. The joint fit gives a distance \(d=4.1-5.3\) kpc and mass-radius constraints \(M=1-2\,M_\odot\) and \(R=9.7-11.9\) km (99\% confidence). These results suggest that representative families of both gravity-bound and self-bound equations of state remain viable at the $1\sigma$ confidence level.

12 pages, 5 figures, 2 tables, published in ApJ

References in corpus (19)

• Gaia Data Release 3: Summary of the content and survey properties
• Estimating distances from parallaxes. V: Geometric and photogeometric distances to 1.47 billion stars in Gaia Early Data Release 3
• PSR J0030+0451 Mass and Radius from NICER Data and Implications for the Properties of Neutron Star Matter
• A NICER View of PSR J0030+0451: Millisecond Pulsar Parameter Estimation
• Neutron Star Observations: Prognosis for Equation of State Constraints
• A NICER View of the Massive Pulsar PSR J0740+6620 Informed by Radio Timing and XMM-Newton Spectroscopy
• Thermonuclear (type-I) X-ray bursts observed by the Rossi X-ray Timing Explorer
• Timing Measurements of the Relativistic Binary Pulsar PSR B1913+16
• The Mass and Radius of the Neutron Star in 4U 1820-30
• Neutron star mass and radius measurements from atmospheric model fits to X-ray burst cooling tail spectra
• Pulse profiles of millisecond pulsars and their Fourier amplitudes
• X-ray bursting neutron star atmosphere models using an exact relativistic kinetic equation for Compton scattering
• Multi-telescope timing of PSR J1518+4904
• The effect of accretion on the measurement of neutron star mass and radius in the low-mass X-ray binary 4U 1608-52
• Evidence for enhanced persistent emission during sub-Eddington thermonuclear bursts
• The direct cooling tail method for X-ray burst analysis to constrain neutron star masses and radii
• Biases for neutron-star mass, radius and distance measurements from Eddington-limited X-ray bursts
• NICER views moderate, strong, and extreme photospheric expansion bursts from the ultracompact X-ray binary 4U 1820$-$30
• Relativistic outflow from two thermonuclear shell flashes on neutron stars