Simultaneous NICER and NuSTAR observations of the Ultracompact X-ray Binary 4U 0614+091

We present the first joint NuSTAR and NICER observations of the ultracompact X-ray binary 4U 0614+091. This source shows quasiperiodic flux variations on the timescale of ∼days. We use reflection modeling techniques to study various components of the accretion system as the flux varies. We find that the flux of the reflected emission and the thermal components representing the disk and the compact object trend closely with the overall flux. However, the flux of the power-law component representing the illuminating X-ray corona scales in the opposite direction, increasing as the total flux decreases. During the lowest flux observation, we see evidence of accretion disk truncation from roughly 6 gravitational radii to 11.5 gravitational radii. This is potentially analogous to the truncation seen in black hole low-mass X-ray binaries, which tends to occur during the low/hard state at sufficiently low Eddington ratios

Constraining the Dense Matter Equation of State with Joint Analysis of NICER and LIGO/Virgo Measurements

The Neutron Star Interior Composition Explorer collaboration recentlypublished a joint estimate of the mass and the radius of PSR J0030+0451,derived via X-ray pulse-profile modeling. Raaijmakers et al. exploredthe implications of this measurement for the dense matter equation ofstate (EOS) using two parameterizations of the high-density EOS: apiecewise-polytropic model, and a model based on the speed of sound inneutron stars (NSs). In this work we obtain further constraints on theEOS following this approach, but we also include information about thetidal deformability of NSs from the gravitational wave signal of thecompact binary merger GW170817. We compare the constraints on the EOS tothose set by the recent measurement of a 2.14 M⊙ pulsar,included as a likelihood function approximated by a Gaussian, and find asmall increase in information gain. To show the flexibility of ourmethod, we also explore the possibility that GW170817 was a NS-blackhole merger, which yields weaker constraints on the EOS

NICER–NuSTAR Observations of the Neutron Star Low-mass X-Ray Binary 4U 1735–44

International audienceWe report on the first simultaneous Neutron Star Interior Composition Explore (NICER) and Nuclear Spectroscopic Telescope Array (NuSTAR) observations of the neutron star (NS) low-mass X-ray binary 4U 1735−44, obtained in 2018 August. The source was at a luminosity of ∼1.8 (D/5.6 kpc)2 × 1037 erg s−1 in the 0.4–30 keV band. We account for the continuum emission with two different continuum descriptions that have been used to model the source previously. Despite the choice in continuum model, the combined passband reveals a broad Fe K line indicative of reflection in the spectrum. In order to account for the reflection spectrum we utilize a modified version of the reflection model relxill that is tailored for thermal emission from accreting NSs. Alternatively, we also use the reflection convolution model of rfxconv to model the reflected emission that would arise from a Comptonized thermal component for comparison. We determine that the innermost region of the accretion disk extends close to the innermost stable circular orbit (R ISCO) at the 90% confidence level regardless of reflection model. Moreover, the current flux calibration of NICER is within 5% of the NuSTAR/FPMA(B)