Neutron stars are not only of astrophysical interest, but are also of great interest to nuclear physicists because their attributes can be used to determine the properties of the dense matter in their cores. One of the most informative approaches for determining the equation of state (EoS) of this dense matter is to measure both a star's equatorial circumferential radius R e and its gravitational mass M . Here we report estimates of the mass and radius of the isolated 205.53 Hz millisecond pulsar PSR J0030+0451 obtained using a Bayesian inference approach to analyze its energy-dependent thermal X-ray waveform, which was observed using the Neutron Star Interior Composition Ex-Corresponding author: M. C. Miller miller@astro.umd.edu a Einstein Fellow arXiv:1912.05705v1 [astro-ph.HE] 12 Dec 2019 Miller, Lamb, Dittmann, et al. plorer (NICER). This approach is thought to be less subject to systematic errors than other approaches for estimating neutron star radii. We explored a variety of emission patterns on the stellar surface. Our best-fit model has three oval, uniform-temperature emitting spots and provides an excellent description of the pulse waveform observed using NICER. The radius and mass estimates given by this model are R e = 13.02 +1.24 −1.06 km and M = 1.44 +0.15 −0.14 M (68%). The independent analysis reported in the companion paper by Riley et al. explores different emitting spot models, but finds spot shapes and locations and estimates of R e and M that are consistent with those found in this work. We show that our measurements of R e and M for PSR J0030+0451 improve the astrophysical constraints on the EoS of cold, catalyzed matter above nuclear saturation density.

The geometry of the accretion flow around stellar-mass black holes can change on timescales of days to months 1-3 . When a black hole emerges from quiescence it has a very hard X-ray spectrum produced by a hot corona 4, 5 , and then transitions to a soft spectrum dominated by emission from a geometrically thin accretion disc extending to the innermost stable circular orbit 6, 7 .Much debate, however, persists over how this transition occurs, whether it is driven largely by a reduction in the truncation radius of the disc 8, 9 or in the spatial extent of the corona 10, 11 . Observations of X-ray reverberation lags in supermassive black hole systems 12, 13 suggest that the corona is compact and that the disc extends in close to the central black hole 14, 15 . Observations of stellar mass black holes, however, reveal equivalent (mass-scaled) reverberation lags that are much larger 16 , leading to the suggestion that the accretion disc in the hard state of stellar mass black holes is truncated out to hundreds of gravitational radii 17, 18 . Here we report X-ray observations of the new black hole transient MAXI J1820+070 19, 20 . We find that the reverberation time lags between the continuum-emitting corona and the irradiated accretion disc are 6-20 times shorter than previously seen. The timescale of the reverberation lags shortens by an order of magnitude over a period of weeks, while the shape of the broadened iron K emission line remains remarkably constant. This suggests a reduction in the spatial extent of the corona, rather than a change in the inner edge of the accretion disc.MAXI J1820+070 19 (ASASSN-18ey 21 ) was discovered on 2018 March 11 with the Monitor of All-sky X-ray Image (MAXI) on board the International Space Station. The next day, the Neutron star Interior Composition Explorer (NICER) 22 started obtaining detailed observations and has continued observing since, at a cadence of 1-3 days 20 . The NICER X-ray Timing Instrument consists of an aligned collection of 52 active paired X-ray "concentrator" optics and silicon drift detectors, which record the arrival times and energies of individual X-ray photons. It provides a timing resolution of < 100 ns (25x faster than NASA's previous best X-ray timing instrument, the Rossi X-ray Timing Explorer) and the highest ever soft band peak effective area of 1900 cm 2 (nearly twice that of timing-capable EPIC-pn camera on XMM-Newton), all while providing good spectral resolution (145 eV at 6 keV), minimal pile-up on bright sources and very little deadtime. MAXI J1820+070 regularly reached 25000 counts/s in NICER's 0.2-12 2/23 keV band, while still providing high-fidelity spectral and timing products (for comparison, the XMM-Newton detectors become piled up for count rates of 600-800 counts/s 23 ). This high count rate allows us to probe timescales that are nearly an order of magnitude shorter than possible with XMM-Newton.Due to the enormity of the dataset, in this letter, we only describe the spectral-timing results of a subset of the total NICER observations ...

We present the set of deep Neutron Star Interior Composition Explorer (NICER) X-ray timing observations of the nearby rotation-powered millisecond pulsars PSRs J0437−4715, J0030+0451, J1231−1411, and J2124−3358, selected as targets for constraining the mass-radius relation of neutron stars and the dense matter equation of state via modeling of their pulsed thermal X-ray emission. We describe the instrument, observations, and data processing/reduction procedures, as well as the series of investigations conducted to ensure that the properties of the data sets are suitable for parameter estimation analyses to produce reliable constraints on the neutron star mass-radius relation and the dense matter equation of state. We find that the long-term timing and flux behavior and the Fourier-domain properties of the event data do not exhibit any anomalies that could adversely affect the intended measurements. From phase-selected spectroscopy, we find that emission from the individual pulse peaks is well described by a single-temperature hydrogen atmosphere spectrum, with the exception of PSR J0437−4715, for which multiple temperatures are required.

We report the discovery by the Nuclear Spectroscopic Telescope Array (NuSTAR) and the Neutron Star Interior Composition Explorer (NICER) of the accreting millisecond X-ray pulsar IGR J17591–2342. Coherent X-ray pulsations around 527.4 Hz (1.9 ms) with a clear Doppler modulation were detected. This implies an orbital period of ∼8.8 h and a projected semi-major axis of ∼1.23 lt-s. With the binary mass function, we estimate a minimum companion mass of 0.42 M⊙, obtained assuming a neutron star mass of 1.4 M⊙ and an inclination angle lower than 60°, as suggested by the absence of eclipses or dips in the light curve of the source. The broad-band energy spectrum, obtained by combining NuSTAR, swift and INTEGRAL observations, is dominated by Comptonisation of soft thermal seed photons with a temperature of ∼0.7 keV by electrons heated to 21 keV. We also detect black-body-like thermal direct emission that is compatible with an emission region of a few kilometers and a temperature compatible with the seed source of Comptonisation. A weak Gaussian line centred on the iron Kα complex can be interpreted as a signature of disc reflection. A similar spectrum characterises the NICER spectra, which was measured when the outburst faded.

PSR J0537−6910, also known as the Big Glitcher, is the most prolific glitching pulsar known, and its spin-induced pulsations are only detectable in X-ray. We present results from analysis of 2.7 years of NICER timing observations, from 2017 August to 2020 April. We obtain a rotation phase-connected timing model for the entire timespan, which overlaps with the third observing run of LIGO/Virgo, thus enabling the most sensitive gravitational wave searches of this potentially strong gravitational wave-emitting pulsar. We find that the short-term braking index between glitches decreases towards a value of 7 or lower at longer times since the preceding glitch. By combining NICER and RXTE data, we measure a long-term braking index n = −1.25 ± 0.01. Our analysis reveals 8 new glitches, the first detected since 2011, near the end of RXTE, with a total NICER and RXTE glitch activity of 8.88 × 10−7yr−1. The new glitches follow the seemingly unique time-to-next-glitch—glitch-size correlation established previously using RXTE data, with a slope of 5dμHz−1. For one glitch around which NICER observes two days on either side, we search for but do not see clear evidence of spectral nor pulse profile changes that may be associated with the glitch.