Recent X-ray observations of galaxy clusters have shown that there is substructure present in the intracluster medium (ICM), even in clusters that are seemingly relaxed. This substructure is ...sometimes a result of sloshing of the ICM, which occurs in cool core clusters that have been disturbed by an off-axis merger with a sub-cluster or group. We present deep Chandra observations of the cool core cluster Abell 2029, which has a sloshing spiral extending radially outward from the center of the cluster to approximately 400 kpc at its fullest extent-the largest continuous spiral observed to date. We find a surface brightness excess, a temperature decrement, a density enhancement, an elemental abundance enhancement, and a smooth pressure profile in the area of the spiral. The sloshing gas seems to be interacting with the southern lobe of the central radio galaxy, causing it to bend and giving the radio source a wide-angle tail (WAT) morphology. This shows that WATs can be produced in clusters that are relatively relaxed on large scales. We explore the interaction between heating and cooling in the central region of the cluster. Energy injection from the active galactic nucleus is likely insufficient to offset the cooling, and sloshing may be an important additional mechanism in preventing large amounts of gas from cooling to very low temperatures.
Jupiter's ultraviolet (UV) aurorae, the most powerful and intense in the solar system, are caused by energetic electrons precipitating from the magnetosphere into the atmosphere where they excite the ...molecular hydrogen. Previous studies focused on case analyses and/or greater than 30‐keV energy electrons. Here for the first time we provide a comprehensive evaluation of Jovian auroral electron characteristics over the entire relevant range of energies (~100 eV to ~1 MeV). The focus is on the first eight perijoves providing a coarse but complete System III view of the northern and southern auroral regions with corresponding UV observations. The latest magnetic field model JRM09 with a current sheet model is used to map Juno's magnetic foot point onto the UV images and relate the electron measurements to the UV features. We find a recurring pattern where the 3‐ to 30‐keV electron energy flux peaks in a region just equatorward of the main emission. The region corresponds to a minimum of the electron characteristic energy (<10 keV). Its polarward edge corresponds to the equatorward edge of the main oval, which is mapped at M shells of ~51. A refined current sheet model will likely bring this boundary closer to the expected 20–30 RJ. Outside that region, the >100‐keV electrons contribute to most (>~70–80%) of the total downward energy flux and the characteristic energy is usually around 100 keV or higher. We examine the UV brightness per incident energy flux as a function of characteristic energy and compare it to expectations from a model.
Plain Language Summary
Aurorae, also commonly called Northern or Southern Lights, are among the most spectacular displays of nature. They are observed not only at Earth but at other planets too, such as Mars, Jupiter, and Saturn. In fact, Jupiter has the brightest aurora in the solar system. The aurora is created when electrons and/or ions in space precipitate into the atmosphere and excite the ambient gas. At Jupiter, they mostly shine in the ultraviolet which is invisible to our eyes but can be seen with suitable instrumentation. The faster the electrons, the deeper they go into the atmosphere, but also the more energy they carry, which eventually can be converted to create more light. This study is about characterizing the electrons that create Jupiter's aurora using many instruments from the National Aeronautics and Space Administration's Juno Mission. We find that different ultraviolet emissions correspond to different electron characteristics. Knowing the differences will help us to understand the bigger picture to explain the processes that create the aurora.
Key Points
We present a survey of Jovian auroral electrons characteristics from 50 eV to 1000 keV by Juno
We present a metric to identify main oval crossings in electron data using 3‐30 keV electrons energy flux
We estimate the UV brightness per incident electron energy flux as a function of characteristic energy
We present the results from new deep Chandra (∼410 ks) and Suzaku (∼180 ks) observations of the merging galaxy cluster ZwCl 0008.8+5215 (z = 0.104). Previous radio observations revealed the presence ...of a double radio relic located diametrically west and east of the cluster center. Using our new Chandra data, we find evidence for the presence of a shock at the location of the western relic, RW, with a Mach number from the density jump. We also measure and from the temperature jump, with Chandra and Suzaku, respectively. These values are consistent with the Mach number estimate from a previous study of the radio spectral index, under the assumption of diffusive shock acceleration ( ). Interestingly, the western radio relic does not entirely trace the X-ray shock. A possible explanation is that the relic traces fossil plasma from nearby radio galaxies that is reaccelerated at the shock. For the eastern relic we do not detect an X-ray surface brightness discontinuity, despite the fact that radio observations suggest a shock with . The low surface brightness and reduced integration time for this region might have prevented the detection. The Chandra surface brightness profile suggests , while the Suzaku temperature measurements found . Finally, we also detect a merger-induced cold front on the western side of the cluster, behind the shock that traces the western relic.
To investigate the relationship between thermal and non-thermal components in merger galaxy clusters, we present deep JVLA and Chandra observations of the HST Frontier Fields cluster MACS ...J0717.5+3745. The Chandra image shows a complex merger event, with at least four components belonging to different merging subclusters. Northwest of the cluster, ∼0.7 Mpc from the center, there is a ram-pressure-stripped core that appears to have traversed the densest parts of the cluster after entering the intracluster medium (ICM) from the direction of a galaxy filament to the southeast. We detect a density discontinuity north-northeast of this core, which we speculate is associated with a cold front. Our radio images reveal new details for the complex radio relic and radio halo in this cluster. In addition, we discover several new filamentary radio sources with sizes of 100-300 kpc. A few of these seem to be connected to the main radio relic, while others are either embedded within the radio halo or projected onto it. A narrow-angled-tailed (NAT) radio galaxy, a cluster member, is located at the center of the radio relic. The steep spectrum tails of this active galactic nucleus lead into the large radio relic where the radio spectrum flattens again. This morphological connection between the NAT radio galaxy and relic provides evidence for re-acceleration (revival) of fossil electrons. The presence of hot 20 keV ICM gas detected by Chandra near the relic location provides additional support for this re-acceleration scenario.
Abstract
Previous Juno mission event studies revealed powerful electron and ion acceleration, to 100s of kiloelectron volts and higher, at low altitudes over Jupiter's main aurora and polar cap (PC; ...poleward of the main aurora). Here we examine 30–1200 keV JEDI‐instrument particle data from the first 16 Juno orbits to determine how common, persistent, repeatable, and ordered these processes are. For the PC regions, we find (1) upward electron angle beams, sometimes extending to megaelectron volt energies, are persistently present in essentially all portions of the polar cap but are generated by two distinct and spatially separable processes. (2) Particle evidence for megavolt downward electrostatic potentials are observable for 80% of the polar cap crossings and over substantial fractions of the PC area. For the main aurora, with the orbit favoring the duskside, we find that (1) three distinct zones are observed that are generally arranged from lower to higher latitudes but sometimes mixed. They are designated here as the diffuse aurora (DifA), Zone‐I (ZI(D)) showing primarily downward electron acceleration, and Zone‐II (ZII(B)) showing bidirectional acceleration with the upward intensities often greater than downward intensities. (2) ZI(D) and ZII(B) sometimes (but not always) contain, respectively, downward electron inverted Vs and downward proton inverted Vs, (potentials up to 400 kV) but, otherwise, have broadband distributions. (3) Surprisingly, both ZI(D) and ZII(B) can generate equally powerful auroral emissions. It is suggested but demonstrated for intense portions of only one auroral crossing, that ZI(D) and ZII(B) are associated, respectively, with upward and downward electric currents.
Plain Language Summary
The science objectives of the Juno mission, with its spacecraft now orbiting Jupiter in a polar orbit, include understanding the space environments of Jupiter's polar regions and generation of Jupiter's uniquely powerful aurora. In Jupiter's polar cap regions (poleward of the main auroral oval encircling the northern and southern poles), we find here that (1) beams of electrons aligned with the upward magnetic field direction are ever‐present with energies extended to the 100s to 1,000s of kilo electron volts and (2) downward magnetic field‐aligned electrostatic potentials reaching greater than a million volts occur over broad regions for 80% of the polar cap crossings. For the main auroral oval, we find three distinct zones: designated here as diffuse aurora (DifA), Zone‐I (ZI(D)) showing downward electron acceleration to 100s of kiloelectron volts, and Zone‐II (ZII(B)) showing bidirectional acceleration with the upward intensities often greater than downward intensities. ZI(D) sometimes shows upward electrostatic potentials reaching 100s of kilovolts and is associated with upward magnetic field‐aligned electric currents. ZII(B) sometimes shows downward electrostatic potentials reaching 100s of kilovolts and is associated with downward electric currents. Unexpectedly from Earth studies, ZI(D) and ZII(B) are just as likely to generate the most intense auroral emissions.
Key Points
Jupiter's polar caps have upward electron beams essentially everywhere (100s of kiloelectron volts) and often downward megavolt electric potentials
Energetic particles reveal three main auroral acceleration zones: diffuse aurora (DifA), Zone‐I (downward), and Zone‐II (bidirectional)
ZI(D) and ZII(B) sometimes (but not always) contain, respectively, downward electron inverted Vs and downward proton inverted Vs
ABSTRACT We present an analysis of the complex gas hydrodynamics in the X-ray-luminous galaxy cluster RX J1347.5-1145 caught in the act of merging with a subcluster to its southeast using a combined ...186 ks Chandra exposure, 2.5 times greater than previous analyses. The primary cluster hosts a sloshing cold front spiral traced by four surface brightness edges west, southeast, east, and northeast from the primary central dominant galaxy, suggesting that the merger is in the plane of the sky. We measure temperature and density ratios across these edges, confirming that they are sloshing cold fronts. We observe the eastern edge of the subcluster infall shock, confirming that the observed subcluster is traveling from the southwest to the northeast in a clockwise orbit. We measure a shock density contrast of and infer a Mach number of 1.25 0.08 and a shock velocity of km s−1. Temperature and entropy maps show cool, low-entropy gas trailing the subcluster in a southwestern tail, consistent with core shredding. Simulations suggest that a perturber in the plane of the sky on a clockwise orbit would produce a sloshing spiral winding counterclockwise, opposite to that observed. The most compelling solution to this discrepancy is that the observed southeastern subcluster is on its first passage, shock-heating gas during its clockwise infall, while the main cluster's clockwise cold front spiral formed from earlier encounters with a second perturber orbiting counterclockwise.
We present results from deep (380 ks) Chandra observations of the active galactic nucleus (AGN) outburst in the massive early-type galaxy NGC 4472. We detect cavities in the gas coincident with the ...radio lobes and estimate the eastern and western lobe enthalpy to be erg and erg and the average power required to inflate the lobes to be erg s−1 and erg s−1, respectively. We also detect enhanced X-ray rims around the radio lobes with sharp surface brightness discontinuities between the shells and the ambient gas. The temperature of the gas in the shells is less than that of the ambient medium, suggesting that they are not AGN-driven shocks but rather gas uplifted from the core by the buoyant rise of the radio bubbles. We estimate the energy required to lift the gas to be up to erg and erg for the eastern and western rims, respectively, constituting a significant fraction of the total outburst energy. A more conservative estimate suggests that the gas in the rim was uplifted at a smaller distance, requiring only 20%-25% of this energy. In either case, if a significant fraction of this uplift energy is thermalized via hydrodynamic instabilities or thermal conduction, our results suggest that it could be an important source of heating in cool core clusters and groups. We also find evidence for a central abundance drop in NGC 4472. The iron abundance profile shows that the region along the cavity system has a lower metallicity than the surrounding undisturbed gas, similar to the central region. This also shows that bubbles have lifted low-metallicity gas from the center.
The M49 group, residing outside the virial radius of the Virgo cluster, is falling onto the cluster from the south. We report results from deep XMM-Newton mosaic observations of M49. Its hot gas ...temperature is 0.8 keV at the group center and rises to 1.5 keV beyond the brightest group galaxy (BGG). The group gas extends to radii of ∼300 kpc to the north and south. The observations reveal a cold front ∼20 kpc north of the BGG center and an X-ray-bright stripped tail 70 kpc long and 10 kpc wide to the southwest of the BGG. We argue that the atmosphere of the infalling group was slowed by its encounter with the Virgo cluster gas, causing the BGG to move forward subsonically relative to the group gas. We measure declining temperature and metallicity gradients along the stripped tail. The tail gas can be traced back to the cooler and enriched gas uplifted from the BGG center by buoyant bubbles, implying that active galactic nucleus outbursts may have intensified the stripping process. We extrapolate to a virial radius of 740 kpc and derive a virial mass of 4.6 × 1013 M for the M49 group. Its group atmosphere appears truncated and deficient when compared with isolated galaxy groups of similar temperatures. If M49 is on its first infall to Virgo, the infall region of a cluster could have profound impacts on galaxies and groups that are being accreted onto galaxy clusters. Alternatively, M49 may have already passed through Virgo once.
We present the first results from a very deep (~650 ks) Chandra X-ray observation of A2052, as well as archival Very Large Array radio observations. The data reveal detailed structure in the inner ...parts of the cluster, including bubbles evacuated by radio lobes of the active galactic nucleus (AGN), compressed bubble rims, filaments, and loops. Two concentric shocks are seen, and a temperature rise is measured for the innermost one. On larger scales, we report the first detection of an excess surface brightness spiral feature. The spiral has cooler temperatures, lower entropies, and higher abundances than its surroundings, and is likely the result of sloshing gas initiated by a previous cluster-cluster or sub-cluster merger. Initial evidence for previously unseen bubbles at larger radii related to earlier outbursts from the AGN is presented.
While the terrestrial aurorae are known to be driven primarily by the interaction of the Earth's magnetosphere with the solar wind, there is considerable evidence that auroral emissions on Jupiter ...and Saturn are driven primarily by internal processes, with the main energy source being the planets' rapid rotation. Prior observations have suggested there might be some influence of the solar wind on Jupiter's aurorae and indicated that auroral storms on Saturn can occur at times of solar wind pressure increases. To investigate in detail the dependence of auroral processes on solar wind conditions, a large campaign of observations of these planets has been undertaken using the Hubble Space Telescope, in association with measurements from planetary spacecraft and solar wind conditions both propagated from 1 AU and measured near each planet. The data indicate a brightening of both the auroral emissions and Saturn kilometric radiation at Saturn close in time to the arrival of solar wind shocks and pressure increases, consistent with a direct physical relationship between Saturnian auroral processes and solar wind conditions. At Jupiter the correlation is less strong, with increases in total auroral power seen near the arrival of solar wind forward shocks but little increase observed near reverse shocks. In addition, auroral dawn storms have been observed when there was little change in solar wind conditions. The data are consistent with some solar wind influence on some Jovian auroral processes, while the auroral activity also varies independently of the solar wind. This extensive data set will serve to constrain theoretical models for the interaction of the solar wind with the magnetospheres of Jupiter and Saturn.