We analyze five epochs of Neutron star Interior Composition Explorer (NICER) data of the black hole X-ray binary MAXI J1820+070 during the bright hard-to-soft state transition in its 2018 outburst with both reflection spectroscopy and Fourier-resolved timing analysis. We confirm the previous discovery of reverberation lags in the hard state, and find that the frequency range where the (soft) reverberation lag dominates decreases with the reverberation lag amplitude increasing during the transition, suggesting an increasing X-ray emitting region, possibly due to an expanding corona. By jointly fitting the lag-energy spectra in a number of broad frequency ranges with the reverberation model reltrans, we find the increase in reverberation lag is best described by an increase in the X-ray coronal height. This result, along with the finding that the corona contracts in the hard state, suggests a close relationship between spatial extent of the X-ray corona and the radio jet. We find the corona expansion (as probed by reverberation) precedes a radio flare by ∼5 days, which may suggest that the hard-to-soft transition is marked by the corona expanding vertically and launching a jet knot that propagates along the jet stream at relativistic velocities.Unified Astronomy Thesaurus concepts: Low-mass x-ray binary stars (939); Stellar mass black holes (1611); High energy astrophysics (739); Black hole physics (159); X-ray transient sources (1852)

We analyze 11 Nuclear Spectroscopic Telescope Array and Swift observations of the black hole X-ray binary GX339-4in the hard state, 6 of which were taken during the end of the 2015 outburst and 5 during a failed outburst in 2013. These observations cover luminosities from 0.5% to 5% of the Eddington luminosity. Implementing the most recent version of the reflection model relxillCp, we perform simultaneous spectral fits on both data sets to track the evolution of the properties in the accretion disk, including the inner edge radius, the ionization, and the temperature of the thermal emission. We also constrain the photon index and electron temperature of the primary source (the "corona"). We observe a maximum truncation radius of 37R g in the preferred fit for the 2013 data set, and a marginal correlation between the level of truncation and luminosity. We also explore a self-consistent model under the framework of coronal Comptonization, and find consistent results regarding the disk truncation in the 2015 data, providing a more physical preferred fit for the 2013 observations.

We analyze seven Neutron Star Interior Composition Interior Explorer (NICER) and Nuclear Spectroscopic Telescope Array epochs of the black hole X-ray binaryGX339-4in the hard state during its two most recent hard-only outbursts in 2017 and 2019. These observations cover the 1-100 keV unabsorbed luminosities between 0.3% and 2.1% of the Eddington limit. WithNICER's negligible pileup, high count rate, and unprecedented time resolution, we perform a spectral-timing analysis and spectral modeling using relativistic and distant reflection models. Our spectral fitting shows that as the inner disk radius moves inward, the thermal disk emission increases in flux and temperature, the disk becomes more highly ionized, and the reflection fraction increases. This coincides with the inner disk increasing its radiative efficiency around∼1% Eddington. We see a hint of the hysteresis effect at∼0.3% of Eddington; the inner radius is significantly truncated during the rise (>49R g), while only a mild truncation (∼5R g) is found during the decay. At higher frequencies (2-7 Hz) in the highest-luminosity epoch, a soft lag is present whose energy dependence reveals a thermal reverberation lag with an amplitude similar to previous findings for this source. We also discuss the plausibility of the hysteresis effect and the debate of the disk truncation problem in the hard state. Unified Astronomy Thesaurus concepts: Black holes (162); High energy astrophysics (739); Astrophysical black holes (98); Stellar mass black holes (1611); Low-mass X-ray binary stars (939); X-ray astronomy (1810) BH   the distance to GX339-4is difficult to accurately measure, and only a lower limit of ∼5kpc can be derived. In this work, to estimate the

We perform the first systematic search of all NICER archival observations of black hole (and candidate) low-mass X-ray binaries for signatures of reverberation. Reverberation lags result from the light travel time difference between the direct coronal emission and the reflected disk component, and therefore their properties are a useful probe of the disk-corona geometry. We detect new signatures of reverberation lags in eight sources, increasing the total sample from three to 11, and study the evolution of reverberation lag properties as the sources evolve in outbursts. We find that in all of the nine sources with more than one reverberation lag detection, the reverberation lags become longer and dominate at lower Fourier frequencies during the hard-to-soft state transition. This result shows that the evolution in reverberation lags is a global property of the state transitions of black hole low-mass X-ray binaries, which is valuable in constraining models of such state transitions. The reverberation lag evolution suggests that the corona is the base of a jet that vertically expands and/or gets ejected during state transition. We also discover that in the hard state, the reverberation lags get shorter, just as the quasiperiodic oscillations (QPOs) move to higher frequencies, but then in the state transition, while the QPOs continue to higher frequencies, the lags get longer. We discuss the implications of the coronal geometry and physical models of QPOs in light of this new finding.

Abstract:Einstein's theory of general relativity was proposed over 100 years ago and has successfully passed a large number of observational tests in the weak field regime. However, the strong field regime is largely unexplored, and there are many modified and alternative theories that have the same predictions as Einstein's gravity for weak fields and present deviations when gravity becomes strong. RELXILL_NK is the first relativistic reflection model for probing the spacetime metric in the vicinity of astrophysical black holes and testing Einstein's gravity in the strong field regime. Here, we present our current constraints on possible deviations from Einstein's gravity obtained from the black holes in 1H0707-495, Ark 564, GX 339-4, and GS 1354-645.