We present the results of deep Chandra, XMM–Newton and Suzaku observations of the nearby galaxy cluster Abell 85, which is currently undergoing at least two mergers, and in addition shows evidence ...for gas sloshing which extends out to r ≈ 600 kpc. One of the two infalling subclusters, to the south of the main cluster centre, has a dense, X-ray bright cool core and a tail extending to the south-east. The northern edge of this tail is strikingly smooth and sharp (narrower than the Coulomb mean free path of the ambient gas) over a length of 200 kpc, while towards the south-west the boundary of the tail is blurred and bent, indicating a difference in the plasma transport properties between these two edges. The thermodynamic structure of the tail strongly supports an overall north-westward motion. We propose, that a sloshing-induced tangential, ambient, coherent gas flow is bending the tail eastwards. The brightest galaxy of this subcluster is at the leading edge of the dense core, and is trailed by the tail of stripped gas, suggesting that the cool core of the subcluster has been almost completely destroyed by the time it reached its current radius of r ≈ 500 kpc. The surface-brightness excess, likely associated with gas stripped from the infalling southern subcluster, extends towards the south-east out to at least r
500 of the main cluster, indicating that the stripping of infalling subclusters may seed gas inhomogeneities. The second merging subcluster appears to be a diffuse non-cool-core system. Its merger is likely supersonic with a Mach number of ≈1.4.
We present the results from extensive, new observations of the Perseus Cluster of galaxies, obtained as a Suzaku Key Project. The 85 pointings analysed span eight azimuthal directions out to 2° = 2.6 ...Mpc, to and beyond the virial radius r
200 ∼ 1.8 Mpc, offering the most detailed X-ray measurements of the intracluster medium (ICM) at large radii in any cluster to date. The azimuthally averaged density profile for r > 0.4r
200 is relatively flat, with a best-fitting power-law index δ = 1.69 ± 0.13, significantly smaller than expected from numerical simulations. The entropy profile in the outskirts lies systematically below the power-law behaviour expected from large-scale structure formation models which include only the heating associated with gravitational collapse. Conversely, the pressure profile beyond ∼0.6r
200 shows an excess with respect to the best-fitting model describing the SZ measurements for a sample of clusters observed with the Planck satellite. The differences between the expected and measured density, entropy and pressure profiles can be explained by a systematic overestimation of the ICM density at large radii caused by homogeneous modelling of inhomogeneous gas distributions (i.e. gas clumping), with the density overestimates ranging from factors of ∼1.2 to 2 or more at r
200 along different directions. We find no evidence for a bias in the temperature measurements within the virial radius. Along the cluster minor axis, we find a flattening of the entropy profiles outside ∼0.6r
200, while along the major axis, the entropy rises all the way to the outskirts.
Astronomical Roentgen Telescope – X-ray Concentrator (ART-XC) is the hard X-ray instrument with grazing incidence imaging optics on board the Spektr-Roentgen-Gamma (SRG) observatory. The SRG ...observatory is the flagship astrophysical mission of the Russian Federal Space Program, which was successively launched into orbit around the second Lagrangian point (L2) of the Earth-Sun system with a Proton rocket from the Baikonur cosmodrome on 13 July 2019. The ART-XC telescope will provide the first ever true imaging all-sky survey performed with grazing incidence optics in the 4–30 keV energy band and will obtain the deepest and sharpest map of the sky in the energy range of 4–12 keV. Observations performed during the early calibration and performance verification phase as well as during the ongoing all-sky survey that started on 12 December 2019 have demonstrated that the in-flight characteristics of the ART-XC telescope are very close to expectations based on the results of ground calibrations. Upon completion of its four-year all-sky survey, ART-XC is expected to detect approximately 5000 sources (~3000 active galactic nuclei, including heavily obscured ones, several hundred clusters of galaxies, ~1000 cataclysmic variables and other Galactic sources), and to provide a high-quality map of the Galactic background emission in the 4–12 keV energy band. ART-XC is also well suited for discovering transient X-ray sources. In this paper, we describe the telescope, the results of its ground calibrations, the major aspects of the mission, the in-flight performance of ART-XC, and the first scientific results.
We present a detailed Chandra, XMM-Newton, Very Large Array (VLA) and Hubble Space Telescope analysis of one of the strongest cool core clusters known, RX J1532.9+3021 (z = 0.3613). Using new, deep ...90 ks Chandra observations, we confirm the presence of a western X-ray cavity or bubble, and report on a newly discovered eastern X-ray cavity. Based on the distribution of the optical filaments, as well as a jet-like structure seen in the 325 MHz VLA radio map, we suggest that the cluster harbors older outflows along the north to south direction. The jet of the central AGN is therefore either precessing or sloshing-induced motions have caused the outflows to change directions. We confirm that the central AGN is highly sub-Eddington and conclude that a > 10 super(10) M sub(middot in circle) or a rapidly spinning black hole is favored to explain both the radiative-inefficiency of the AGN and the powerful X-ray cavities.
ABSTRACT
We present a new statistical method for constructing background subtracted measurements from event list data gathered by X-ray and gamma-ray observatories. This method was initially ...developed specifically to construct images that account for the high background fraction and low overall count rates observed in survey data from the Mikhail Pavlinsky ART-XC telescope aboard the Spektrum Röntgen Gamma (SRG) mission, although the mathematical underpinnings are valid for data taken with other imaging missions and analysis applications. This method fully accounts for the expected Poisson fluctuations in both the sky photon and non-X-ray background count rates in a manner that does not result in unphysical negative counts. We derive the formulae for arbitrary confidence intervals for the source counts and show that our new measurement converges exactly to the standard background subtraction calculation in the high signal limit. Utilizing these results, we discuss several variants of images designed to optimize different science goals for both pointed and slewing telescopes. Using realistic simulated data of a galaxy cluster as observed by ART-XC, we show that our method provides a more significant and robust detection of the cluster emission as compared to a standard background subtraction. We also demonstrate its advantages using real observations of a point source from the ART-XC telescope. These calculations may have widespread applications for a number of source classes observed with high energy telescopes.
ABSTRACT
We report on the Imaging X-ray Polarimetry Explorer (IXPE) observation of the closest and X-ray brightest Compton-thick active galactic nucleus (AGN), the Circinus galaxy. We find the source ...to be significantly polarized in the 2–6 keV band. From previous studies, the X-ray spectrum is known to be dominated by reflection components, both neutral (torus) and ionized (ionization cones). Our analysis indicates that the polarization degree is 28 ± 7 per cent (at 68 per cent confidence level) for the neutral reflector, with a polarization angle of 18° ± 5°, roughly perpendicular to the radio jet. The polarization of the ionized reflection is unconstrained. A comparison with Monte Carlo simulations of the polarization expected from the torus shows that the neutral reflector is consistent with being an equatorial torus with a half-opening angle of 45°–55°. This is the first X-ray polarization detection in a Seyfert galaxy, demonstrating the power of X-ray polarimetry in probing the geometry of the circumnuclear regions of AGNs, and confirming the basic predictions of standard Unification Models.
ABSTRACT
We present an analysis of the X-ray active galactic nucleus (AGN) population in a sample of seven massive galaxy clusters in the redshift range 0.35 < z < 0.45. We utilize high-quality ...Chandra X-ray imaging to robustly identify AGN and precisely determine cluster masses and centroids. Follow-up VIsible Multi-Object Spectrograph optical spectroscopy allows us to determine which AGN are cluster members. Studying the subset of AGN with 0.5–8 keV luminosities >6.8 × 1042 erg s−1, within r ≤ 2r500 (approximately the virial radius), we find that the cluster AGN space density scales with cluster mass as $\sim M^{-2.0^{+0.8}_{-0.9}}$. This result rules out zero mass dependence of the cluster X-ray AGN space density at the 2.5σ level. We compare our cluster X-ray AGN sample to a control field with identical selection and find that the cluster AGN fraction is significantly suppressed relative to the field when considering the brightest galaxies with V < 21.5. For fainter galaxies, this difference is not present. Comparing the X-ray hardness ratios of cluster member AGN to those in the control field, we find no evidence for enhanced X-ray obscuration of cluster member AGN. Lastly, we see tentative evidence that disturbed cluster environments may contribute to enhanced AGN activity.
In this article we address the issue of denial of service attacks targeting the hardware and software of voice over IP servers or by misusing specific signaling protocol features. As a signaling ...protocol we investigate here the session initiation protocol. In this context we mainly identify attacks based on exhaustion of the memory of VoIP servers, or attacks that incur high CPU load. We deliver an overview of different attack possibilities and explain some attacks in more detail, including attacks utilizing the DNS system and those targeting the parser. A major conclusion of the work is the knowledge that SIP provides a wide range of features that can be used to mount DoS attacks. Discovering these attacks is inherently difficult, as is the case with DoS attacks on other IP components. However, with adequate server design, efficient implementation, and appropriate hardware, the effects of a large portion of attacks can be reduced
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