S. Ghizzardi

The European Photon Imaging Camera on XMM-Newton: The MOS cameras

The EPIC focal plane imaging spectrometers on XMM-Newton use CCDs to record the images and spectra of celestial X-ray sources focused by the three X-ray mirrors. There is one camera at the focus of each mirror; two of the cameras contain seven MOS CCDs, while the third uses twelve PN CCDs, dening a circular eld of view of 30 0 diameter in each case. The CCDs were specially developed for EPIC, and combine high quality imaging with spectral resolution close to the Fano limit. A lter wheel carrying three kinds of X-ray transparent light blocking lter, a fully closed, and a fully open position, is tted to each EPIC instrument. The CCDs are cooled passively and are under full closed loop thermal control. A radio-active source is tted for internal calibration. Data are processed on-board to save telemetry by removing cosmic ray tracks, and generating X-ray event les; a variety of dierent instrument modes are available to increase the dynamic range of the instrument and to enable fast timing. The instruments were calibrated using laboratory X-ray beams, and synchrotron generated monochromatic X-ray beams before launch; in-orbit calibration makes use of a variety of celestial X-ray targets. The current calibration is better than 10% over the entire energy range of 0.2 to 10 keV. All three instruments survived launch and are performing nominally in orbit. In particular full eld-of-view coverage is available, all electronic modes work, and the energy resolution is close to pre-launch values. Radiation damage is well within pre-launch predictions and does not yet impact on the energy resolution. The scientic results from EPIC amply full pre-launch expectations.

Cold fronts in galaxy clusters

Context. Cold fronts have been observed in several galaxy clusters. Understanding their nature and origin is extremely important for investigating the internal dynamics of clusters. Aims. To gain insight into the nature of these features, we carry out a statistical investigation of their occurrence in a sample of galaxy clusters observed with XMM-Newton and correlate this occurrence with different cluster properties. Methods. We selected a sample of 45 clusters starting from the B55 flux limited sample (Edge et al. 1990, MNRAS, 245, 559) and performed a systematic search for cold fronts. Results. We find that a large fraction of clusters host at least one cold front. Cold fronts are easily detected in all systems that are manifestly undergoing a merger event in the plane of the sky, while the presence of these features in the remaining clusters is related to a steep entropy gradient, in agreement with theoretical expectations. Assuming that cold fronts in cool core clusters are triggered by minor merger events, we estimate a minimum of 1/3 merging events per halo per Gyr.

RADIATIVE COOLING AND HEATING AND THERMAL CONDUCTION IN M87

The crisis of the standard cooling flow model brought about by Chandra and XMM-Newton observations of galaxy clusters has led to the development of several models that explore different heating processes in order to assess whether they can quench the cooling flow. Among the most appealing mechanisms are thermal conduction and heating through buoyant gas deposited in the intracluster medium (ICM) by active galactic nuclei (AGNs). We combine Virgo/M87 observations of three satellites (Chandra, XMM-Newton, and BeppoSAX) to inspect the dynamics of the ICM in the center of the cluster. Using the spectral deprojection technique, we derive the physical quantities describing the ICM and determine the extra heating needed to balance the cooling flow, assuming that thermal conduction operates at a fixed fraction of the Spitzer value. We assume that the extra heating is due to buoyant gas, and we fit the data using the model developed by Ruszkowski and Begelman. We derive a scale radius for the model of ~5 kpc, which is comparable with the M87 AGN jet extension, and a required luminosity of the AGN of a few × 1042 ergs s-1, which is comparable to the observed AGN luminosity. We discuss a scenario in which the buoyant bubbles are filled with relativistic particles and magnetic field, which are responsible for the radio emission in M87. The AGN is supposed to be intermittent and to inject populations of buoyant bubbles through a succession of outbursts. We also study the X-ray-cool component detected in the radio lobes and suggest that it is structured in blobs that are tied to the radio buoyant bubbles.

Abell 2142 at large scales: An extreme case for sloshing?

We present results obtained with a new XMM-Newton observation of A2142, a famous textbook example of cluster with multiple cold fronts, which has been studied in detail with Chandra but whose large scale properties are presented here for the first time. We report the discovery of a a new cold front, the most distant one ever detected in a galaxy cluster, at about one Mpc from the center to the SE. Residual images, thermodynamics and metal abundance maps are qualitatively in agreement with predictions from numerical simulations of the sloshing phenomenon. However, the scales involved are much larger, similarly to what recently observed in the Perseus cluster. These results show that sloshing is a cluster-wide phenomenon, not confined in the cores, which extends well beyond the cooling region involving a large fraction of the ICM up to almost half of the virial radius. The absence of a cool core and a newly discovered giant radio halo in A2142, in spite of its relaxed X-ray morphology, suggest that large scale sloshing, or the intermediate merger which caused it, may trigger Mpc-scale radio emission and may lead to the disruption of the cluster cool core

Back and forth from cool core to non-cool core: clues from radio halos

X-ray astronomers often divide galaxy clusters into two classes: “cool core” (CC) and “non-cool core” (NCC) objects. The origin of this dichotomy has been the subject of debate in recent years, between “evolutionary” models (where clusters can evolve from CC to NCC, mainly through mergers) and “primordial” models (where the state of the cluster is fixed “ab initio” by early mergers or preheating). We found that in a well-defined sample (clusters in the GMRT Radio halo survey with available Chandra or XMM-Newton data), none of the objects hosting a giant radio halo can be classified as a cool core. This result suggests that the main mechanisms that can start a large-scale synchrotron emission (most likely mergers) are the same as those that can destroy CC, which therefore strongly supports “evolutionary” models of the CC-NCC dichotomy. Moreover, combining the number of objects in the CC and NCC state with the number of objects with and without a radio-halo, we estimated that the time scale over which a NCC cluster relaxes to the CC state should be larger than the typical life-time of radio-halos and likely shorter than � 3 Gyr. This suggests that NCC transform into CC more rapidly than predicted from the cooling time, which is about 10 Gyr in NCC systems, allowing the possibility of a cyclical evolution between the CC and NCC states.

A cluster in a crowded environment: XMM-Newton and Chandra observations of A3558

Combining XMM-Newton and Chandra data, we have performed a detailed study of Abell 3558. Our analysis shows that its dynamical history is more complicated than previously thought. We have found some traits typical of cool core clusters (surface brightness peaked at the center, peaked metal abundance profile) and others that are more common in merging clusters, like deviations from spherical symmetry in the thermodynamic quantities of the ICM. This last result has been achieved with a new technique for deriving temperature maps from images. We have also detected a cold front and, with the combined use of XMM-Newton and Chandra, we have characterized its properties, such as the speed and the metal abundance profile across the edge. This cold front is probably due to the sloshing of the core, induced by the perturbation of the gravitational potential associated with a past merger. The hydrodynamic processes related to this perturbation have presumably produced a tail of lower entropy, higher pressure and metal rich ICM, which extends behind the cold front for 500 kpc. The unique characteristics of A3558 are probably due to the very peculiar environment in which it is located: the core of the Shapley supercluster.

XMM-Newton first-light observations of the Hickson galaxy group 16

This paper presents the XMM-Newton first-light observations of the Hickson-16 compact group of galaxies. Groups are possibly the oldest large-scale structures in the Universe, pre-dating clusters of galaxies, and are highly evolved. This group of small galaxies, at a redshift of 0.0132 (or 80 Mpc) is exceptional in the having the highest concentration of starburst or AGN activity in the nearby Universe. So it is a veritable laboratory for the study of the relationship between galaxy interactions and nuclear activity. Previous optical emission line studies indicated a strong ionising continuum in the galaxies, but its origin, whether from starbursts, or AGN, was unclear. Combined imaging and spectroscopy with the EPIC X-ray CCDs unequivocally reveals a heavily obscured AGN and a separately identified thermal (starburst) plasma, in NGC 835, NGC 833 and NGC 839. NGC 838 shows only starburst thermal emission. Starbursts and AGN can evidently coexist in members of this highly evolved system of merged and merging galaxies, implying a high probability for the formation of AGN as well as starbursts in post-merger galaxies.

Apparent high metallicity in 3-4 keV galaxy clusters: the inverse iron-bias in action in the case of the merging cluster Abell 2028

Context. Recent work based on a global measurement of the ICM properties finds evidence for an increase in the iron abundance in galaxy clusters with temperatures around 2−4 keV up to a value about 3 times higher than is typical of very hot clusters ZFe � 0.25 Z� . Aims. We have started a study of the metal distribution in nearby X-ray luminous poor galaxy clusters in the temperature range 3−4 keV with measured high abundances. Our study aims at spatially resolving the metal content of the central regions of the ICM, in particular characterizing how our measurements are biased by the diagnostics adopted and by the possible multi-temperature nature of the projected observed spectra. We report here on a 42 ks XMM-Newton observation of the first object in the sample, the cluster Abell 2028. Methods. We selected interesting regions of the ICM to analyze the spatially resolved structure of projected temperature and abundance, thereby producing two-dimensional maps. Results. The XMM-Newton observation of the first object of the sample, the cluster Abell 2028, reveals the complex structure of the cluster over a scale of ∼300 kpc, showing an interaction between two subclusters in cometary-like configurations. Cold fronts have been detected at the leading edges of the two substructures. The core of the main subcluster is likely hosting a cool corona. We show that a one-component fit for this region returns a biased high metallicity. The inverse iron bias is caused by the behavior of the fitting code in shaping the Fe-L complex. In the presence of a multi-temperature structure of the ICM, the best-fit metallicity turns out to be artificially higher when the projected spectrum is modeled with a single temperature component, while it is not related to the presence of both Fe-L and Fe-K emission lines in the spectrum. After accounting for the inverse iron bias, the overall abundance of the cluster is consistent with the one typical of hotter, more massive clusters. Conclusions. We caution against interpreting high abundances inferred when fitting a single thermal component to spectra derived from relatively large apertures in 3−4 keV clusters, because the inverse iron bias can be present. Most of the inferences trying to relate high abundances in 3−4 keV clusters to fundamental physical processes will probably have to be revised.

Smaug: A New Technique for the Deprojection of Galaxy Clusters

This paper presents a new technique for reconstructing the spatial distributions of hydrogen, temperature, and metal abundance of a galaxy cluster. These quantities are worked out from the X-ray spectrum, modeled starting from few analytical functions describing their spatial distributions. These functions depend on some parameters, determined by fitting the model to the observed spectrum. We have implemented this technique as a new model in the XSPEC software analysis package. We describe the details of the method and apply it to work out the structure of the cluster A1795. We combine the observation of three satellites, exploiting the high spatial resolution of Chandra for the cluster core, the wide collecting area of XMM-Newton for the intermediate regions, and the large field of view of BeppoSAX for the outer regions. We also make a threefold test on the validity and precision of our method by (1) comparing its results with those from a geometrical deprojection, (2) examining the spectral residuals at different radii of the cluster, and (3) reprojecting the unfolded profiles and comparing them directly to the measured quantities. Our analytical method yields the parameters defining the spatial functions directly from the spectra. Their explicit knowledge allows a straightforward derivation of other indirect physical quantities like the gravitating mass, as well as a fast and easy estimate of the profile uncertainties.

Dark matter-baryons separation at the lowest mass scale: The bullet group

We report on the X-ray observation of a strong lensing selected group, SL2S J08544-0121, with a total mass of 2.4±0.6×1014 M⊙ which revealed a separation of 124±20 kpc between the X-ray emitting collisional gas and the collisionless galaxies and dark matter (DM), traced by strong lensing. This source allows to put an order of magnitude estimate to the upper limit to the interaction cross-section of DM of 10 cm2 g-1. It is the lowest mass object found to date showing a DM-baryons separation, and it reveals that the detection of bullet-like objects is not rare and confined to mergers of massive objects opening the possibility of a statistical detection of DM-baryons separation with future surveys.© 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.