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X-ray clusters

Group members: Silvano Molendi, Simona Ghizzardi, Fabio Gastaldello, Mariachiara Rossetti plus Sabrina De Grandi and Dominique Eckert

Cluster outer regions. We have a been working on this topic for quite some time, exploiting data from various X-ray satellites (ROSAT, Beppo-SAX and XMM-Newton) and very recently Planck. Our most important results may be summarized as follows:

  1. the metal abundance in cluster outskirts is roughly constant and independent of the thermodynamical state of the cluster (ie. cool core or non-cool core) (De Grandi & Molendi 2001, De Grandi et al. 2004, Leccardi & Molendi 2008b, Leccardi et al. 2010)
  2. Density profiles decrease rapidly in cluster outskirt; at large radii density profiles of CC appear to fall below density profiles of NCC systems (Eckert et al 2012).
  3. Temperature profiles decrease steadily and gently in cluster outskirts (Leccardi & Molendi 2008a).
  4. At variance with some recent Suzaku measurements we find no evidence of entropy flattening and inversions in cluster outskirts (Leccardi + Molendi 2010, Eckert et al. 2013a)

Cool-core/Non Cool-Core Clusters. The CC/NCC dichotomy is another topic we have been working on for some time. Our most important results may be summarized as follows:

  1. CC systems are generally relaxed system, while NCC systems often show evidence of being morphologically disturbed and in some instances of major mergers (Leccardi, Molendi & Rossetti 2010).
  2. at variance with previous claims the majority of clusters in the local Universe are not cool core, once the X-ray survey bias favoring CC systems is properly taken into account the CC fraction is estimated to be about 40%;
  3. cluster cores feature metal abundance excesses which correlate with the entropy decrement in the core, i.e. the stronger the entropy decrement the more significant the abundance excess (De Grandi & Molendi 2001, De Grandi et al. 2004, Leccardi & Molendi 2008b, Leccardi, Molendi & Rossetti 2010).
  4. THe majority of NCC systems has a relatively low entropy core, characterized by a metal abundance similar to the one found in the core of CC systems, we interpret this as evidence of a past CC phase in the life of most NCC systems.
  5. Cluster wide Diffuse radio emission is always found in NCC systems, indicating that the non-thermal electrons responsible for the radio emission are accelerated through processes associate to a merger event sufficiently strong to disrupt a pre-existing cool core (Rossetti et al. 2011).

Cold Fronts. In systems lacking evidence of major mergers cold fronts have been found in core regions where the entropy gradient is sufficiently developed to allow their formation and prevent their break-up (Ghizzardi et al 2010). These cold fronts are believed to result from tidal effects (dubbed sloshing) associated to minor merger events. A detailed analysis of a the entropy and metal abundance in a long observation of the core of Abell496 is allowing us to trace the sloshing of the ICM in unprecedented detail (Ghizzardi et al. in prep). Recently we have found evidence that extreme sloshing might be responsible for the disruption of cool cores (Rosssetti et al. in prep).

NuSTAR. NuSTAR is an explorer class NASA mission launched on June 13th 2012. NuSTAR carries the fist experiment capable of focusing X-rays above 10 keV. The mission is expected to produce a significant improvement in our undersatanding of various classes of astrophysical sources. The italian partecipation includes use of the Malindi ground station, software developement at ASDC-ASI and scientific support to the mission through a number of INAF scientists, including Silvano Molendi, who co-chairs the Galaxy Cluster working group.