On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ∼ 1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of 40 − 8 + 8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 M ⊙ . An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ∼ 40 Mpc ) less than 11 hours after the merger by the One-Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ∼10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position ∼ 9 and ∼ 16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC 4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta.

Studies were made of ASCA spectra of seven ultra-luminous compact X-ray sources (ULXs) in nearby spiral galaxies; M33 X-8 (Takano et al. two sources in NGC 4565 (Mizuno et al. 1999). With the 0.5-10 keV luminosities in the range 10 39−40 ergs s −1 , they are thought to represent a class of enigmatic X-ray sources often found in spiral galaxies. For some of them, the ASCA data are newly processed, or the published spectra are reanalyzed. For others, the published results are quoted. The ASCA spectra of all these seven sources have been described successfully with so called multi-color disk blackbody (MCD) emission arising from optically-thick standard accretion disks around black holes. Except the case of M33 X-8, the spectra do not exhibit hard tails. For the source luminosities not to exceed the Eddington limits, the black holes are inferred to have rather high masses, up to ∼ 100 solar masses. However, the observed innermost disk temperatures of these objects, T in = 1.1 − 1.8 keV, are too high to be compatible with the required high black-hole masses, as long as the standard accretion disks around Schwarzschild black holes are assumed. Similarly high disk temperatures are also observed from two Galactic transients with superluminal motions, GRO 1655-40 and GRS 1915+105. The issue of unusually high disk temperature may be explained by the black hole rotation, which makes the disk get closer to the black hole, and hence hotter.

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The present paper describes the analysis of multiple RXTE/PCA data of the black hole binary with superluminal jet, XTE J1550 − 564, acquired during its 1999-2000 outburst. The X-ray spectra show features typical of the high/soft spectral state, and can approximately be described by an optically thick disk spectrum plus a power-law tail. Three distinct spectral regimes, named standard regime, anomalous regime, and apparently standard regime, have been found from the entire set of the observed spectra. When the X-ray luminosity is well below ∼ 6 × 10 38 erg s −1 (assuming a distance of 5 kpc), XTE J1550 − 564 resides in the standard regime, where the soft spectral component dominates the power-law component and the observed disk inner radius is kept constant. When the luminosity exceeds the critical luminosity, the apparently standard regime is realized, where luminosity of the optically thick disk rises less steeply with the temperature, and the spectral shape is moderately distorted from that of the standard accretion disk. In this regime, radial temperature gradient of the disk has been found to be flatter than that of the standard accretion disk. The results of the apparently standard regime are suggestive of a slim disk (e.g., Abramowicz et al. 1988, Watarai et al. 2000 which is a solution predicted under high mass accretion rate. In the intermediate anomalous regime, the spectrum becomes much harder, and the disk inner radius derived using a simple disk model spectrum apparently varies significantly with time. These properties can

We report on the result of an X-ray observation of the X-ray Nova Velorum 1993 (GRS 1009–45), made with ASCA on 1993 November 10-11. The energy spectrum was extremely soft and accompanied by a hard tail, characteristics of the Galactic black-hole binaries in the soft state. It is well represented by a two-component model consisting of a multicolor disk model or a general relativistic accretion disk model, both describing emission from an optically thick accretion disk, and a power-law component with a photon index of ∼ 2.5. A spectral analysis of the soft component allows an estimation of the mass of the central object. For the optically-estimated distance (> 1 kpc) and inclination (> 37°), the mass is estimated to be $\gt 3.1 M_\odot$. These results strongly support that the compact object is a black hole.

We analyzed the ASCA X-ray data of 40 nearby clusters of galaxies, whose intraclustermedium temperature distributes in the range of 0.9-10 keV. We measured the Si and Fe abundances of the intracluster medium, spatially averaging over each cluster, but excluding the central ∼ 0.15h −1 50 Mpc region in order to avoid any possible abundance gradients and complex temperature structures. The Fe abundances of these clusters are 0.2-0.3 solar, with only weak dependence on the temperature of the intracluster medium, hence on the cluster richness. In contrast, the Si abundance is observed to increase from 0.3 to 0.6-0.7 solar from the poorer to richer clusters. These results suggest that the supernovae of both type-Ia and type-II significantly contribute to the metal enrichment of the intracluster medium, with the relative contribution of type-II supernovae increasing towards richer clusters. We suggest a possibility that a considerable fraction of type-II supernova products escaped from poorer systems.

The Ginga X-ray spectra of the two binary X-ray pulsars, 4U 1907]09 and Vela X-1, were analyzed for e †ects due to electron cyclotron resonance. For this purpose, a new continuum spectral model, called NPEX, was developed. The NPEX model, combined with the classical cyclotron scattering line proÐle, was Ðrst tested against the Ginga spectra (typically in 2È50 keV) of Her X-1, 4U 0115]63, 4U 1538[52, X0331]53, and Cep X-4 and was conÐrmed to reproduce successfully their overall spectra including the previously known cyclotron resonance features. Through application of the same model to the pulsephaseÈaveraged and phase-resolved Ginga spectra, it was conÐrmed that 4U 1907]09 and Vela X-1 exhibit fundamental cyclotron resonances at D20 and D25 keV, respectively. The data for both objects are also consistent with the presence of the second-harmonic resonances, which were discovered with hard X-ray experiments. Including these two examples, the cyclotron resonance e †ects are now established in about a dozen binary X-ray pulsars. Their surface magnetic Ðeld strengths, implied by their resonance energies, apparently distribute over a narrow range of (1È4) ] 1012 G. Although the fewer number of higher Ðeld objects may be an instrumental selection e †ect, the lack of objects with magnetic Ðelds of (0.2È1) ] 1012 G is concluded to be real. A limited number of ASCA data are utilized to reinforce this conclusion. These results suggest that the magnetic Ðeld of binary X-ray pulsars do not decay signiÐcantly at least in D108 yr.

X-ray spectra of the Galactic ridge emission in the Scutum arm region have been obtained with ASCA GIS and SIS in the energy range 0.7È10 keV. The observed spectra are basically of thermal emission from thin hot plasmas, and individual K emission lines from helium-like Mg, Si, S, and Fe ions are conÐrmed in both the GIS and SIS spectra. This means that the Galactic ridge X-ray emission cannot be explained by a single-temperature ionization-equilibrium plasma model. It cannot, however, be reproduced even if a nonequilibrium ionization model is introduced ; thus multiple plasma components are required. The GIS spectra are fairly well Ðtted by a double-temperature nonequilibrium ionization plasma model with temperatures of kT D 0.8 keV and kT D 7 keV. The softer component is found to be in an extremely low ionization state, with cm~3 s, while the harder component is in a relan e t D 109 tively high ionization state, though not yet in a full equilibrium. The GRXE properties obtained with the GIS are carefully reexamined by the highly resolved spectral-line features with the SIS.The soft and hard components are absorbed by equivalent hydrogen columns of 0.7 ] 1022 cm~2 and 4.6 ] 1022 cm~2, respectively. The surface brightness of the soft and hard components at b D 0¡ are estimated to be 1.9 ] 10~6 and 5.3 ] 10~7 ergs cm~2 s~1 sr~1 respectively, both in the 0.5È10 keV band. The surface brightness of the softer component extends toward signiÐcantly higher (D2¡) Galactic latitudes than the harder component, although their actual scale heights may be similar at D100 pc if the di †erences in their observable depths are taken into account. Spectral properties of the two components are seen to depend on the latitude ; the most noticeable e †ect is a rapid decrease in the Fe K line equivalent width seen in the hard component. Attempts are made to interpret the two components in terms of di †use hot plasmas Ðlling the interstellar space.