The corona contracts in a black-hole transient

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 (that is, it 'turns on' after accreting material from its companion) it has a very hard (high-energy) X-ray spectrum produced by a hot corona^4,5 positioned above its accretion disk, and then transitions to a soft (lower-energy) spectrum dominated by emission from the geometrically thin accretion disk, which extends to the innermost stable circular orbit^6,7. Much debate persists over how this transition occurs and whether it is driven largely by a reduction in the truncation radius of the disk^8,9 or by a reduction 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 disk extends nearly to the central black hole^14,15. Observations of stellar-mass black holes, however, reveal equivalent (mass-scaled) reverberation lags that are much larger¹⁶, leading to the suggestion that the accretion disk in the hard-X-ray state of stellar-mass black holes is truncated at a few hundreds of gravitational radii from the black hole^17,18. Here we report X-ray observations of the black-hole transient MAXI J1820+070^19,20. We find that the reverberation time lags between the continuum-emitting corona and the irradiated accretion disk are 6 to 20 times shorter than previously seen. The timescale of the reverberation lags shortens by an order of magnitude over a period of weeks, whereas 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 disk.

Linear structures in the core of the Coma cluster of galaxies

The hot x-ray-emitting plasma in galaxy clusters is predicted to have turbulent motion, which can contribute around 10% of the cluster's central energy density. We report deep Chandra X-ray Observatory observations of the Coma cluster core, showing the presence of quasi-linear high-density arms spanning 150 kiloparsecs, consisting of low-entropy material that was probably stripped from merging subclusters. Two appear to be connected with a subgroup of galaxies at a 650-kiloparsec radius that is merging into the cluster, implying coherence over several hundred million years. Such a long lifetime implies that strong isotropic turbulence and conduction are suppressed in the core, despite the unrelaxed state of the cluster. Magnetic fields are presumably responsible. The structures seen in Coma present insight into the past billion years of subcluster merger activity.

Dissecting x-ray-emitting gas around the center of our galaxy

Most supermassive black holes (SMBHs) are accreting at very low levels and are difficult to distinguish from the galaxy centers where they reside. Our own Galaxy's SMBH provides an instructive exception, and we present a close-up view of its quiescent x-ray emission based on 3 megaseconds of Chandra observations. Although the x-ray emission is elongated and aligns well with a surrounding disk of massive stars, we can rule out a concentration of low-mass coronally active stars as the origin of the emission on the basis of the lack of predicted iron (Fe) Kα emission. The extremely weak hydrogen (H)-like Fe Kα line further suggests the presence of an outflow from the accretion flow onto the SMBH. These results provide important constraints for models of the prevalent radiatively inefficient accretion state.

Active galaxies and cluster gas

Two lines of evidence indicate that active galaxies, principally radio galaxies, have heated the diffuse hot gas in clusters. The first is the general need for additional heating to explain the steepness of the X-ray luminosity-temperature relation in clusters, the second is to solve the cooling-flow problem in cluster cores. The inner core of many clusters is radiating energy as X-rays on a time-scale much shorter than its likely age. Although the temperature in this region drops by a factor of about three from that of the surrounding gas, little evidence is found for gas much cooler than that. Some form of heating appears to be taking place, probably by energy transported outward from the central accreting black hole or radio source. How that energy heats the gas depends on poorly understood transport properties (conductivity and viscosity) of the intracluster medium. Viscous heating is discussed as a possibility. Such heating processes have consequences for the truncation of the luminosity function of massive galaxies.

X-rays from active galactic nuclei: relativistically broadened emission lines

The X-ray spectra of many Seyfert 1 galaxies show an iron-K emission line. Some of these lines are broad and skew, reflecting the rapid motion of the accreting gas together with the deep gravitational potential of the central black hole. The observational evidence for broad iron lines is reviewed, from ASCA to recent Chandra and XMM-Newton spectra. Examples are shown from MCG-6-30-15, for which line variability is discussed. Current results indicate that we are studying accretion flows well within 6GM/c(2).

Active galactic nuclei

Active galactic nuclei are the most powerful, long-lived objects in the Universe. Recent data confirm the theoretical idea that the power source is accretion into a massive black hole. The common occurrence of obscuration and outflows probably means that the contribution of active galactic nuclei to the power density of the Universe has been generally underestimated.

A Possible Intracluster Origin for the Excess Soft X-ray Component in Some Clusters

The soft x-ray spectra of the Virgo and Coma clusters cannot be modeled by emission from the well-known hot intracluster medium attenuated by photoelectric absorption along the line of sight in our galaxy. If the excess soft emission is due to thermal emission in the clusters from gas at about 1 million kelvin, then the gas should be rapidly cooling. The high rate of cooling poses problems for the origin of the gas and its sink. A model in which the component is due to turbulent mixing layers around clouds scattered throughout the intracluster medium may explain the excess emission. The gas in the mixing layer is reheated after cooling, so that the total mass remains relatively small.

ROSAT Observations of X-ray Emissions from Jupiter During the Impact of Comet Shoemaker-Levy 9

Röntgensatellit (ROSAT) observations made shortly before and during the collision of comet Shoemaker-Levy 9 with Jupiter show enhanced x-ray emissions from the planet's northern high latitudes. These emissions, which occur at System III longitudes where intensity enhancements have previously been observed in Jupiter's ultraviolet aurora, appear to be associated with the comet fragment impacts in Jupiter's southern hemisphere and may represent brightenings of the jovian x-ray aurora caused either by the fragment impacts themselves or by the passage of the fragments and associated dust clouds through Jupiter's inner magnetosphere.

Isoxicam and related 4-hydroxy-N-isoxazolyl-2H-1,2-benzothiazine-3-carboxamide 1,1-dioxides. Potent nonsteroidal antiinflammatory agents

A novel series of antiinflammatory agents, N-isoxazolyl-3-carboxamides of 4-hydroxy-2H-1,2-benzothiazine 1,1-dioxide, was synthesized and evaluated as antiinflammatory agents in the carrageenin-induced rat paw edema (CIRPE) assay and adjuvant-induced polyarthritis (AIP) assay. Several analogues were found to be equipotent or more potent than aspirin and phenylbutazone. Structure-activity relationships are discussed. One of the compounds, 4-hydroxy-2-methyl-N-(5-methyl-3-isoxazolyl)-2H-1,2-benzothiazine 3-carboxamide 1,1-dioxide (3a; isoxicam), was found to be 3 times as potent as phenylbutazone in the CIRPE and in the therapeutic AIP assays. Isoxicam (3a) is presently undergoing phase III clinical trial as an antiarthritic drug.