Neutron Star Interior Composition Explorer has a comparatively low background rate, but it is highly variable, and its spectrum must be predicted using measurements unaffected by the science target. We describe an empirical, three-parameter model based on observations of seven pointing directions that are void of detectable sources. Two model parameters track different types of background events, while the third is used to predict a low-energy excess tied to observations conducted in sunlight. An examination of 3556 good time intervals (GTIs), averaging 570 s, yields a median rate (0.4–12 keV; 50 detectors) of 0.87 c s−1, but in 5% (1%) of cases, the rate exceeds 10 (300) c s−1. Model residuals persist at 20%–30% of the initial rate for the brightest GTIs, implying one or more missing model parameters. Filtering criteria are given to flag GTIs likely to have unsatisfactory background predictions. With such filtering, we estimate a detection limit, 1.20 c s−1 (3σ, single GTI) at 0.4–12 keV, equivalent to 3.6 × 10−12 erg cm−2 s−1 for a Crab-like spectrum. The corresponding limit for soft X-ray sources is 0.51 c s−1 at 0.3–2.0 keV, or 4.3 × 10−13 erg cm−2 s−1 for a 100 eV blackbody. These limits would be four times lower if exploratory GTIs accumulate 10 ks of data after filtering at the level prescribed for faint sources. Such filtering selects background GTIs 85% of the time. An application of the model to a 1 s timescale makes it possible to distinguish source flares from possible surges in the background.

We present a spectral analysis of NuSTAR and NICER observations of the luminous, persistently accreting neutron star (NS) low-mass X-ray binary Cygnus X-2. The data were divided into different branches that the source traces out on the Z-track of the X-ray color-color diagram; namely, the horizontal branch, the normal branch, and the vertex between the two. The X-ray continuum spectrum was modeled in two different ways that produced comparable quality fits. The spectra showed clear evidence of a reflection component in the form of a broadened Fe K line, as well as a lower-energy emission feature near 1 keV likely due to an ionized plasma located far from the innermost accretion disk. We account for the reflection spectrum with two independent models (relxillns and rdblur*rfxconv). The inferred inclination is in agreement with earlier estimates from optical observations of ellipsoidal lightcurve modeling (relxillns: i = 67 degrees +/- 4 degrees; rdblur*rfxconv: i = 60 degrees +/- 10 degrees). The inner disk radius remains close to the NS (R (in) <= 1.15 R (ISCO)) regardless of the source position along the Z-track or how the 1 keV feature is modeled. Given the optically determined NS mass of 1.71 +/- 0.21 M (circle dot), this corresponds to a conservative upper limit of R (in) <= 19.5 km for M = 1.92 M (circle dot) or R (in) <= 15.3 km for M = 1.5 M (circle dot). We compare these radius constraints to those obtained from NS gravitational wave merger events and recent NICER pulsar lightcurve modeling measurements.

In November 2019, MAXI detected an X-ray outburst from the known Be X-ray binary system RX J0209.6-7427 located in the outer wing of the Small Magellanic Cloud. We followed the outburst of the system with NICER which led to the discovery of X-ray pulsations with a period of 9.3 s. We analyzed simultaneous X-ray data obtained with NuSTAR and NICER allowing us to characterize the spectrum and provide an accurate estimate of its bolometric luminosity. During the outburst the maximum broadband X-ray luminosity of the system reached $1-2\times10^{39}$ erg/s, thus exceeding by about one order of magnitude the Eddington limit for a typical 1.4 $M_{\odot}$ mass neutron star (NS). Monitoring observations with Fermi/GBM and NICER allowed us to study the spin evolution of the NS and compare it with standard accretion torque models. We found that the NS magnetic field should be of the order of $3\times10^{12}$ G. We conclude that RX J0209.6-7427 exhibited one of the brightest outbursts observed from a Be X-ray binary pulsar in the Magellanic Clouds, reaching similar luminosity level to the 2016 outburst of SMC X-3. Despite the super-Eddington luminosity of RX J0209.6-7427, the NS appears to have only a moderate magnetic field strength.

We present Neutron Star Interior Composition Explorer X-ray and Arcminute Microkelvin Imager Large Array radio observations of a rapid hard-to-soft state transition in the black hole X-ray transient MAXI J1820+070. During the transition from the hard state to the soft state a switch between two particular types of quasiperiodic oscillations (QPOs) was seen in the X-ray power density spectra, from type-C to type-B, along with a drop in the strength of the broadband X-ray variability and a brief flare in the 7–12 keV band. Soon after this switch (∼2–2.5 hr) a strong radio flare was observed that corresponded to the launch of superluminal ejecta. Although hints of a connection between QPO transitions and radio flares have been seen in other black hole X-ray transients, our observations constitute the strongest observational evidence to date for a link between the appearance of type-B QPOs and the launch of discrete jet ejections.

The black hole candidate and X-ray binary MAXI J1535-571 was discovered in 2017 September. During the decay of its discovery outburst, and before returning to quiescence, the source underwent at least four reflaring events, with peak luminosities of similar to 10(35-36) erg s(-1) (d/4.1 kpc)(2). To investigate the nature of these flares, we analysed a sample of NICER (Neutron star Interior Composition Explorer) observations taken with almost daily cadence. In this work, we present the detailed spectral and timing analysis of the evolution of the four reflares. The higher sensitivity of NICER at lower energies, in comparison with other X-ray detectors, allowed us to constrain the disc component of the spectrum at similar to 0.5 keV. We found that during each reflare the source appears to trace out a q-shaped track in the hardness-intensity diagram similar to those observed in black hole binaries during full outbursts. MAXI J1535 -571 transits between the hard state (valleys) and softer states (peaks) during these flares. Moreover, the Comptonized component is undetected at the peak of the first reflare, while the disc component is undetected during the valleys. Assuming the most likely distance of 4.1 kpc, we find that the hard-to-soft transitions take place at the lowest luminosities ever observed in a black hole transient, while the soft-to-hard transitions occur at some of the lowest luminosities ever reported for such systems.

Stuart J. Mumford∗1, 2, 3, Nabil Freij4, Steven Christe5, Jack Ireland5, Florian Mayer6, V. Keith Hughitt7, Albert Y. Shih5, Daniel F. Ryan8, 5, Simon Liedtke6, David Pérez-Suárez9, Pritish Chakraborty10, Vishnunarayan K I.6, Andrew Inglis11, Punyaslok Pattnaik12, Brigitta Sipőcz13, Rishabh Sharma6, Andrew Leonard3, David Stansby14, Russell Hewett15, Alex Hamilton6, Laura Hayes5, Asish Panda6, Matt Earnshaw6, Nitin Choudhary16, Ankit Kumar6, Prateek Chanda17, Md Akramul Haque18, Michael S Kirk11, Michael Mueller6, Sudarshan Konge6, Rajul Srivastava6, Yash Jain19, Samuel Bennett6, Ankit Baruah6, Will Barnes20, Michael Charlton6, Shane Maloney21, Nicky Chorley22, Himanshu6, Sanskar Modi6, James Paul Mason6, Naman96396, Jose Ivan Campos Rozo23, Larry Manley6, Agneet Chatterjee24, John Evans6, Michael Malocha6, Monica G. Bobra25, Sourav Ghosh24, Airmansmith976, Dominik Stańczak26, Ruben De Visscher6, Shresth Verma27, Ankit Agrawal6, Dumindu Buddhika6, Swapnil Sharma6, Jongyeob Park28, Matt Bates6, Dhruv Goel6, Garrison Taylor29, Goran Cetusic6, Jacob6, Mateo Inchaurrandieta6, Sally Dacie30, Sanjeev Dubey6, Deepankar Sharma6, Erik M. Bray6, Jai Ram Rideout31, Serge Zahniy5, Tomas Meszaros6, Abhigyan Bose6, André Chicrala32, Ankit6, Chloé Guennou6, Daniel D’Avella6, Daniel Williams33, Jordan Ballew6, Nick Murphy34, Priyank Lodha6, Thomas Robitaille6, Yash Krishan6, Andrew Hill6, Arthur Eigenbrot35, Benjamin Mampaey36, Bernhard M. Wiedemann6, Carlos Molina6, Duygu Keşkek6, Ishtyaq Habib6, Joseph Letts6, Juanjo Bazán37, Quinn Arbolante38, Reid Gomillion6, Yash Kothari6, Yash Sharma6, Abigail L. Stevens39, 40, Adrian Price-Whelan41, Ambar Mehrotra6, Arseniy Kustov6, Brandon Stone6, Trung Kien Dang42, Emmanuel Arias6, Fionnlagh Mackenzie Dover1, Freek Verstringe36, Gulshan Kumar43, Harsh Mathur44, Igor Babuschkin6, Jaylen Wimbish6, Juan Camilo Buitrago-Casas6, Kalpesh Krishna45, Kaustubh Hiware46, Manas Mangaonkar6, Matthew Mendero6, Mickaël Schoentgen6, Norbert G Gyenge47, Ole Streicher48, Rajasekhar Reddy Mekala6, Rishabh Mishra6, Shashank Srikanth43, Sarthak Jain6, Tannmay Yadav49, Tessa D. Wilkinson6, Tiago M. D. Pereira50, 51, Yudhik Agrawal12, jamescalixto6, yasintoda6, and Sophie A. Murray52

Abstract Black Hole X-ray binaries are known to exhibit variability in their light curves on timescales as short as milliseconds and as long as months. The short-timescale variability can be in the form of quasi-periodic oscillations (QPOs), which may be produced by general relativistic effects. In this work, we looked at low-frequency QPOs from an exciting recent black hole transient, MAXI J1820+070, which first went into outburst in 2018. This source was observed in a multi-wavelength campaign that included the NICER mission, a soft X-ray telescope attached to the International Space Station. In our analysis, we applied X-ray timing and spectral-timing techniques to help place constraints on the QPO emission mechanism in the extreme environment surrounding the black hole. Although the QPO amplitude was too low to carry out phase-resolved spectroscopy, our work indicates that weak QPOs such as this one may be present in the light curves of many sources.

NICER has observed the new X-ray transient MAXI J1631-479 (Kobayashi et al., ATel #12320; Miyasaka et al., ATel #12340) on a daily basis since January 15, 2019.

In this paper we describe the potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission for studies related to accretion flows in the strong field gravity regime around both stellar-mass and supermassive black-holes. eXTP has the unique capability of using advanced “spectral-timing-polarimetry” techniques to analyze the rapid variations with three orthogonal diagnostics of the flow and its geometry, yielding unprecedented insight into the inner accreting regions, the effects of strong field gravity on the material within them and the powerful outflows which are driven by the accretion process. X-spinmeasurements

We present the Spectroscopic Time-Resolving Observatory for Broadband Energy X-rays (STROBE-X), a probe-class mission concept selected for study by NASA. It combines huge collecting area, high throughput, broad energy coverage, and excellent spectral and temporal resolution in a single facility. STROBE-X offers an enormous increase in sensitivity for X-ray spectral timing, extending these techniques to extragalactic targets for the first time. It is also an agile mission capable of rapid response to transient events, making it an essential X-ray partner facility in the era of time-domain, multi-wavelength, and multi-messenger astronomy. Optimized for study of the most extreme conditions found in the Universe, its key science objectives include: (1) Robustly measuring mass and spin and mapping inner accretion flows across the black hole mass spectrum, from compact stars to intermediate-mass objects to active galactic nuclei. (2) Mapping out the full mass-radius relation of neutron stars using an ensemble of nearly two dozen rotation-powered pulsars and accreting neutron stars, and hence measuring the equation of state for ultradense matter over a much wider range of densities than explored by NICER. (3) Identifying and studying X-ray counterparts (in the post-Swift era) for multiwavelength and multi-messenger transients in the dynamic sky through cross-correlation with gravitational wave interferometers, neutrino observatories, and high-cadence time-domain surveys in other electromagnetic bands. (4) Continuously surveying the dynamic X-ray sky with a large duty cycle and high time resolution to characterize the behavior of X-ray sources over an unprecedentedly vast range of time scales. STROBE-X's formidable capabilities will also enable a broad portfolio of additional science.

We demonstrate the enormous progress in our understanding of accretion onto compact objects that can be readily achieved by a next generational large-area X-ray timing instrument.

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Kilohertz quasi-periodic oscillations (kHz QPOs) are the most rapid (quasi-)coherent kind of variability that have been detected in the light curves of accreting neutron star X-ray binaries. Previous spectral-timing work using the rms spectrum revealed that the lower kHz QPO emission is a Comptonized blackbody, consistent with that expected from the boundary layer between the accretion flow and neutron star surface. To better interpret the spectral variability, we present phase-resolved spectroscopy of a kHz QPO for the first time, using a method based on the energy-dependent cross-correlation function. We find that the Comptonized spectral shape changes as a function of QPO phase, and the variations of the spectral parameters must intrinsically lag one another. These spectral variations could be explained by radial oscillations in the boundary layer caused by unstable accretion onto the neutron star, which could be due to plasma instabilities, asteroseismic modes, or an opacity-radiation trade-off like in the variable star mechanism. These possibilities can be explored in greater detail with current and future X-ray missions such as AstroSat, NICER, eXTP, and STROBE-X.