X-ray reflection models are used to constrain the properties of the accretion disk, such as the degree of ionization of the gas and the elemental abundances. In combination with general relativistic ray tracing codes, additional parameters like the spin of the black hole and the inclination to the system can be determined. However, current reflection models used for such studies only provide angle-averaged solutions for the flux reflected at the surface of the disk. Moreover, the emission angle of the photons changes over the disk due to relativistic light bending. To overcome this simplification, we have constructed an angle-dependent reflection model with the XILLVER code and self-consistently connected it with the relativistic blurring code RELLINE. The new model, relxill, calculates the proper emission angle of the radiation at each point onï£ij the accretion disk, and then takes the corresponding reflection spectrum into account. We show that the reflected spectra from illuminated disks follow a limb-brightening law highly dependent on the ionization of disk and yet different from the commonly assumed form I ∝ ln(1 + 1/µ). A detailed comparison with the angle-averaged model is carried out in order to determine the bias in the parameters obtained by fitting a typical relativistic reflection spectrum. These simulations reveal that although the spin and inclination are mildly affected, the Fe abundance can be over-estimated by up to a factor of two when derived from angle-averaged models. The fit of the new model to the Suzaku observation of the Seyfert galaxy Ark 120 clearly shows a significant improvement in the constrain of the physical parameters, in particular by enhancing the accuracy in the inclination angle and the spin determinations.

We present the first extensive radio to γ-ray observations of a fast-rising blue optical transient (FBOT), AT 2018cow, over its first ∼100 days. AT 2018cow rose over a few days to a peak luminosity L pk ∼ 4 × 10 44 erg s −1 exceeding those of superluminous supernovae (SNe), before declining as L ∝ t −2 . Initial spectra at δt 15 days were mostly featureless and indicated large expansion velocities v ∼ 0.1 c and temperatures arXiv:1810.10720v1 [astro-ph.HE] 25 Oct 2018 2 MARGUTTI ET AL. reaching T ∼ 3 × 10 4 K. Later spectra revealed a persistent optically-thick photosphere and the emergence of H and He emission features with v ∼ 4000 km s −1 with no evidence for ejecta cooling. Our broad-band monitoring revealed a hard X-ray spectral component at E ≥ 10 keV, in addition to luminous and highly variable soft X-rays, with properties unprecedented among astronomical transients. An abrupt change in the X-ray decay rate and variability appears to accompany the change in optical spectral properties. AT 2018cow showed bright radio emission consistent with the interaction of a blastwave with v sh ∼ 0.1 c with a dense environment (Ṁ ∼ 10 −3 − 10 −4 M yr −1 for v w = 1000 km s −1 ). While these properties exclude 56 Ni-powered transients, our multi-wavelength analysis instead indicates that AT 2018cow harbored a "central engine", either a compact object (magnetar or black hole) or an embedded internal shock produced by interaction with a compact, dense circumstellar medium. The engine released ∼ 10 50 − 10 51.5 erg over ∼ 10 3 − 10 5 s and resides within lowmass fast-moving material with equatorial-polar density asymmetry (M ej,fast 0.3 M ). Successful SNe from low-mass H-rich stars (like electron-capture SNe) or failed explosions from blue supergiants satisfy these constraints. Intermediate-mass black-holes are disfavored by the large environmental density probed by the radio observations.

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 ...

The spins of ten stellar black holes have been measured using the continuumfitting method. These black holes are located in two distinct classes of X-ray binary systems, one that is persistently X-ray bright and another that is transient. Both the persistent and transient black holes remain for long periods in a state where their spectra are dominated by a thermal accretion disk component. The spin of a black hole of known mass and distance can be measured by fitting this thermal continuum spectrum to the thin-disk model of Novikov and Thorne; the key fit parameter is the radius of the inner edge of the black hole's accretion disk. Strong observational and theoretical evidence links the inner-disk radius to the radius of the innermost stable circular orbit, which is trivially related to the dimensionless spin parameter a * of the black hole (|a * | < 1). The ten spins that have so far been measured by this continuum-fitting method range widely from a * ≈ 0 to a * > 0.95. The robustness of the method is demonstrated by the dozens or hundreds of independent and consistent measurements of spin that have been obtained for several black holes, and through careful consideration of many sources of systematic error. Among the results discussed is a dichotomy between the transient and persistent black holes; the latter have higher spins and larger masses. Also discussed is recently discovered evidence in the transient sources for a correlation between the power of ballistic jets and black hole spin.