ABSTRACT
We present results from a suite of binary merging cluster simulations. The hydrodynamical cluster simulations are performed employing a smoothed particle hydrodynamics formulation in which ...gradient errors are strongly reduced by means of an integral approach. We consider adiabatic as well as radiative simulations, in which we include gas cooling, star formation, and energy feedback from supernovae. We explore the effects of merging on the thermodynamic structure of the intracluster gas of the final merger remnant. In particular, we study how core entropy is generated during the merging and the stability properties of the initial cool-core profile against disruption. To this end, we consider a range of initial mass ratio and impact parameters. Final entropy profiles of our adiabatic merging simulations are in good accord with previous findings, with cool-cores being disrupted for all of the initial merging setups. For equal-mass off-axis mergers, we find that a significant contribution to the final primary core entropy is due to hydrodynamic instabilities generated by rotational motions, which are induced by tidal torques during the first pericentre passage. In radiative simulations, cool-cores are more resilient against heating processes; none the less, they are able to maintain their integrity only in the case of off-axis mergers with very unequal masses. We suggest that these results are robust against changes in the gas physical modelling, in particular to the inclusion of AGN thermal feedback.
Our findings support the view that the observed core cluster morphology emerges naturally in a merging cluster context, and conclude that the merging angular momentum is a key parameter in shaping the thermodynamical properties of the final merger remnant.
ABSTRACT
Using the Very Long Baseline Array and the European Very Long Baseline Interferometry Network, we have made a precise measurement of the radio parallax of the black hole X-ray binary MAXI ...J1820+070, providing a model-independent distance to the source. Our parallax measurement of (0.348 ± 0.033) mas for MAXI J1820+070 translates to a distance of (2.96 ± 0.33) kpc. This distance implies that the source reached (15 ± 3) per cent of the Eddington luminosity at the peak of its outburst. Further, we use this distance to refine previous estimates of the jet inclination angle, jet velocity, and the mass of the black hole in MAXI J1820+070 to be (63 ± 3)°, (0.89 ± 0.09) c, and (9.2 ± 1.3) M⊙, respectively.
Most of the baryons in galaxy clusters reside between the galaxies in a hot, tenuous gas. The densest gas in their centres should cool and accrete onto giant central galaxies at rates of 10-1,000 ...solar masses per year. No viable repository for this gas, such as clouds or new stars, has been found. New X-ray observations, however, have revealed far less cooling below X-ray temperatures than expected, altering the previously accepted picture of cooling flows. As a result, most of the gas must be heated to and maintained at temperatures above ∼2 keV (ref. 3). The most promising heating mechanism is powerful radio jets emanating from supermassive black holes in the central galaxies of clusters. Here we report the discovery of giant cavities and shock fronts in a distant (z = 0.22) cluster caused by an interaction between a radio source and the hot gas surrounding it. The energy involved is ∼6 × 1061 erg, the most powerful radio outburst known. This is enough energy to quench a cooling flow for several Gyr, and to provide ∼1/3 keV per particle of heat to the surrounding cluster.
ABSTRACT
During a 2018 outburst, the black hole X-ray binary MAXI J1820 + 070 was comprehensively monitored at multiple wavelengths as it underwent a hard to soft state transition. During this ...transition, a rapid evolution in X-ray timing properties and a short-lived radio flare were observed, both of which were linked to the launching of bi-polar, long-lived relativistic ejecta. We provide a detailed analysis of two Very Long Baseline Array observations, using both time binning and a new dynamic phase centre tracking technique to mitigate the effects of smearing when observing fast-moving ejecta at high angular resolution. We identify a second, earlier ejection, with a lower proper motion of 18.0 ± 1.1 mas d−1. This new jet knot was ejected 4 ± 1 h before the beginning of the rise of the radio flare, and 2 ± 1 h before a switch from type-C to type-B X-ray quasi-periodic oscillations (QPOs). We show that this jet was ejected over a period of ∼6 h and thus its ejection was contemporaneous with the QPO transition. Our new technique locates the original, faster ejection in an observation in which it was previously undetected. With this detection, we revised the fits to the proper motions of the ejecta and calculated a jet inclination angle of (64 ± 5)°, and jet velocities of $0.97_{-0.09}^{+0.03}c$ for the fast-moving ejecta (Γ > 2.1) and (0.30 ± 0.05)c for the newly identified slow-moving ejection (Γ = 1.05 ± 0.02). We show that the approaching slow-moving component is predominantly responsible for the radio flare, and is likely linked to the switch from type-C to type-B QPOs, while no definitive signature of ejection was identified for the fast-moving ejecta.
We present multifrequency monitoring observations of the black hole X-ray binary V404 Cygni throughout its 2015 June outburst. Our data set includes radio and mm/sub-mm photometry, taken with the ...Karl G. Jansky Very Large Array, Arc-Minute MicroKelvin Imager Large Array, Sub-millimeter Array, James Clerk Maxwell Telescope, and the Northern Extended Millimetre Array, combined with publicly available infrared, optical, UV, and X-ray measurements. With these data, we report detailed diagnostics of the spectral and variability properties of the jet emission observed during different stages of this outburst. These diagnostics show that emission from discrete jet ejecta dominated the jet emission during the brightest stages of the outburst. We find that the ejecta became fainter, slower, less frequent, and less energetic, before the emission transitioned (over 1–2 d) to being dominated by a compact jet, as the outburst decayed towards quiescence. While the broad-band spectrum of this compact jet showed very little evolution throughout the outburst decay (with the optically thick to thin synchrotron jet spectral break residing in the near-infrared/optical bands; ∼2–5 × 10^14 Hz), the emission still remained intermittently variable at mm/sub-mm frequencies. Additionally, we present a comparison between the radio jet emission throughout the 2015 and previous 1989 outbursts, confirming that the radio emission in the 2015 outburst decayed significantly faster than in 1989. Lastly, we detail our sub-mm observations taken during the 2015 December mini-outburst of V404 Cygni, which demonstrate that, similar to the main outburst, the source was likely launching jet ejecta during this short period of renewed activity.
We use the largest complete sample of 64 galaxy clusters (HIghest X-ray FLUx Galaxy Cluster Sample) with available high-quality X-ray data from Chandra, and apply 16 cool-core diagnostics to them, ...some of them new. In order to identify the best parameter for characterizing cool-core clusters and quantify its relation to other parameters, we mainly use very high spatial resolution profiles of central gas density and temperature, and quantities derived from them. We also correlate optical properties of brightest cluster galaxies (BCGs) with X-ray properties. To segregate cool core and non-cool-core clusters, we find that central cooling time, tcool, is the best parameter for low redshift clusters with high quality data, and that cuspiness is the best parameter for high redshift clusters. 72% of clusters in our sample have a cool core (tcool < 7.7 h71-1/2 Gyr) and 44% have strong cool cores (tcool < 1.0 h71-1/2 Gyr). We find strong cool-core clusters are characterized as having low central entropy and a systematic central temperature drop. Weak cool-core clusters have enhanced central entropies and temperature profiles that are flat or decrease slightly towards the center. Non-cool-core clusters have high central entropies. For the first time we show quantitatively that the discrepancy in classical and spectroscopic mass deposition rates can not be explained with a recent formation of the cool cores, demonstrating the need for a heating mechanism to explain the cooling flow problem. We find that strong cool-core clusters have a distribution of central temperature drops, centered on 0.4Tvir. However, the radius at which the temperature begins to drop varies. This lack of a universal inner temperature profile probably reflects the complex physics in cluster cores not directly related to the cluster as a whole. Our results suggest that the central temperature does not correlate with the mass of the BCGs and weakly correlates with the expected radiative cooling only for strong cool-core clusters. Since 88% of the clusters in our sample have a BCG within a projected distance of 50 h71-1 kpc from the X-ray peak, we argue that it is easier to heat the gas (e.g. with mergers or non-gravitational processes) than to separate the dense core from the brightest cluster galaxy. Diffuse, Mpc-scale radio emission, believed to be associated with major mergers, has not been unambiguously detected in any of the strong cool-core clusters in our sample. Of the weak cool-core clusters and non-cool-core clusters, most of the clusters (seven out of eight) that have diffuse, Mpc-scale radio emission have a large (> 50 h71-1 kpc) projected separation between their BCG and X-ray peak. In contrast, only two of the 56 clusters with a small separation between the BCG and X-ray peak (<50 h71-1 kpc) show large-scale radio emission. Based on this result, we argue that a large projected separation between the BCG and the X-ray peak is a good indicator of a major merger. The properties of weak cool-core clusters as an intermediate class of objects are discussed. Finally we describe individual properties of all 64 clusters in the sample.
We present an intensive radio and X-ray monitoring campaign on the 2009 outburst of the Galactic black hole candidate X-ray binary H1743−322. With the high angular resolution of the Very Long ...Baseline Array, we resolve the jet ejection event and measure the proper motions of the jet ejecta relative to the position of the compact core jets detected at the beginning of the outburst. This allows us to accurately couple the moment when the jet ejection event occurred with X-ray spectral and timing signatures. We find that X-ray timing signatures are the best diagnostic of the jet ejection event in this outburst, which occurred as the X-ray variability began to decrease and the Type C quasi-periodic oscillations disappeared from the X-ray power density spectrum. However, this sequence of events does not appear to be replicated in all black hole X-ray binary outbursts, even within an individual source. In our observations of H1743−322, the ejection was contemporaneous with a quenching of the radio emission, prior to the start of the major radio flare. This contradicts previous assumptions that the onset of the radio flare marks the moment of ejection. The jet speed appears to vary between outbursts, with a possible positive correlation with outburst luminosity. The compact core radio jet reactivated on transition to the hard intermediate state at the end of the outburst, and not when the source reached the low hard spectral state. Comparison with the known near-infrared behaviour of the compact jets suggests a gradual evolution of the compact jet power over a few days near the beginning and end of an outburst.
Two short (< 2 s) γ-ray bursts (GRBs) have recently been localized and fading afterglow counterparts detected. The combination of these two results left unclear the nature of the host galaxies of the ...bursts, because one was a star-forming dwarf, while the other was probably an elliptical galaxy. Here we report the X-ray localization of a short burst (GRB 050724) with unusual γ-ray and X-ray properties. The X-ray afterglow lies off the centre of an elliptical galaxy at a redshift of z = 0.258 (ref. 5), coincident with the position determined by ground-based optical and radio observations. The low level of star formation typical for elliptical galaxies makes it unlikely that the burst originated in a supernova explosion. A supernova origin was also ruled out for GRB 050709 (refs 3, 31), even though that burst took place in a galaxy with current star formation. The isotropic energy for the short bursts is 2-3 orders of magnitude lower than that for the long bursts. Our results therefore suggest that an alternative source of bursts-the coalescence of binary systems of neutron stars or a neutron star-black hole pair-are the progenitors of short bursts.
We present new results on the shock around the south-west radio lobe of Centaurus A using data from the Chandra Very Large Programme observations (740 ks total observing time). The X-ray spectrum of ...the emission around the outer south-western edge of the lobe is well described by a single power-law model with Galactic absorption – thermal models are strongly disfavoured, except in the region closest to the nucleus. We conclude that a significant fraction of the X-ray emission around the south-west part of the lobe is synchrotron, not thermal. We infer that in the region where the shock is strongest and the ambient gas density lowest, the inflation of the lobe is accelerating particles to X-ray synchrotron emitting energies, similar to supernova remnants such as SN1006. This interpretation resolves a problem of our earlier, purely thermal, interpretation for this emission, namely that the density compression across the shock was required to be much larger than the theoretically expected factor of 4. We describe a self-consistent model for the lobe dynamics and shock properties using the shell of thermal emission to the north of the lobe to estimate the lobe pressure. Based on this model, we estimate that the lobe is expanding to the south-west with a velocity of ∼2600 km s−1, roughly Mach 8 relative to the ambient medium. We discuss the spatial variation of spectral index across the shock region, concluding that our observations constrain γmax for the accelerated particles to be ∼108 at the strongest part of the shock, consistent with expectations from diffusive shock acceleration theory. Finally, we consider the implications of these results for the production of ultra-high energy cosmic rays (UHECRs) and TeV emission from Centaurus A, concluding that the shock front region is unlikely to be a significant source of UHECRs, but that TeV emission from this region is expected at levels comparable to current limits at TeV energies, for plausible assumed magnetic field strengths.
We have measured half-light radii, r sub(h), for thousands of globular clusters (GCs) belonging to the 100 early-type galaxies observed in the ACS Virgo Cluster Survey and the elliptical galaxy NGC ...4697. An analysis of the dependencies of the measured half-light radii on both the properties of the GCs themselves and their host galaxies reveals that, in analogy with GCs in the Galaxy but in a milder fashion, the average half-light radius increases with increasing galactocentric distance or, alternatively, with decreasing galaxy surface brightness. For the first time, we find that the average half-light radius decreases with the host galaxy color. We also show that there is no evidence for a variation of r sub(h) with the luminosity of the GCs. Finally, we find in agreement with previous observations that the average r sub(h) depends on the color of GCs, with red GCs being 617% smaller than their blue counterparts. We show that this difference is probably a consequence of an intrinsic mechanism, rather than projection effects, and that it is in good agreement with the mechanism proposed by Jordan. We discuss these findings in light of two simple pictures for the origin of the r sub(h) of GCs and show that both lead to a behavior in rough agreement with the observations. After accounting for the dependencies on galaxy color, galactocentric radius, and underlying surface brightness, we show that the average GC half-light radii <r sub(h)> can be successfully used as a standard ruler for distance estimation. We outline the methodology, provide a calibration for its use, and discuss the prospects for this distance estimator with future observing facilities. We find <r sub(h)> = 2.7 c 0.35 pc for GCs with (g - z) = 1.2 mag in a galaxy with color (g - z) sub(gal) = 1.5 mag and at an underlying surface z-band brightness of k sub(z) = 21 mag arcsec super(-2). Using this technique, we place an upper limit of 3.4 Mpc on the 1s line-of-sight depth of the Virgo Cluster. Finally, we examine the form of the r sub(h) distribution for our sample galaxies and provide an analytic expression that successfully describes this distribution.