Fabio Gastaldello

Abundance Gradients and the Role of Supernovae in M87

We make a detailed measurement of the metal abundance profiles and metal abundance ratios of the inner core of M87/Virgo observed by XMM-Newton during the performance verification phase. We use multitemperature models for the inner regions, and we compare the plasma codes APEC and MEKAL. We confirm the strong heavy-element gradient previously found by ASCA and BeppoSAX, but also find a significant increase in light elements, in particular O. This fact, together with the constant O/Fe ratio in the inner 9, indicates an enhancement of contribution in the core of the cluster, not only by Type Ia supernovae but also by Type II supernovae.

The Transient X-Ray Pulsar 4U 0115+63 from Quiescence to Outburst through the Centrifugal Transition

We report on a BeppoSAX observation of the transient X-ray pulsar 4U 0115+63, close to periastron. This led to the discovery of a dramatic luminosity variation from ~2 × 1034 to ~5 × 1036 ergs s -1 (a factor of 250) in less than 15 hr. The variation was accompanied by only minor (if any) changes in the emitted spectrum and pulse fraction. On the contrary, an observation near apastron detected the source in a nearly constant state at a level of ~2 × 1033 ergs s -1. Direct accretion onto the neutron star surface encounters major difficulties in explaining the source variability properties. When the different regimes expected for a rotating magnetic neutron star subject to a variable inflow of matter from its companion are taken into consideration, the results of the BeppoSAX observations of 4U 0115+63 can be explained naturally. In particular, close to apastron, the regime of centrifugal inhibition of accretion applies, whereas the dramatic source flux variability observed close to periastron is readily interpreted as the transition regime between direct neutron star accretion and the propeller regime. In this centrifugal transition regime, small variations of the mass inflow rate give rise to very large luminosity variations. We present a simple model for this transition, which we successfully apply to the X-ray flux and pulse fraction variations measured by BeppoSAX.

On the metal abundance in the core of M 87

We revisit the XMM-Newton observation of M 87 to make a new and more detailed measurement of the metal abundance prole. After having veried that spectral ts with single temperature models show a dramatic abundance drop within 1 arcmin of the cluster core, we show that more appropriate models, which include a multi-temperature component to account for the strong temperature gradient observed in M 87, and a power-law component to account for the emission of the nucleus and knot A, give a substantially flat abundance prole. The drastic abundance decrement found by tting a single temperature component to the data is an artifact known as Fe-bias (Buote 2000a,b) following from the application of an oversimplied spectral model to the data.

Ni ABUNDANCE IN THE CORE OF THE PERSEUS CLUSTER: AN ANSWER TO THE SIGNIFICANCE OF RESONANT SCATTERING

Using an XMM-Newton observation of the Perseus cluster we show that the excess in the flux of the 7-8 keV line complex previously detected by ASCA and BeppoSAX is due to an overabundance of nickel rather than to an anomalously high Fe Heβ/Fe Heα ratio. This observational fact leads to the main result that resonant scattering, which was assumed to be responsible for the supposed anomalous Fe Heβ/Fe Heα ratio, is no longer required. The absence of resonant scattering points toward the presence of significant gas motions (either turbulent or laminar) in the core of the Perseus Cluster.

XMM-Newton observation of the interacting cluster Abell 3528

We analyze the XMM-Newton dataset of the interacting cluster of galaxies Abell 3528 located westward in the core of the Shapley Supercluster, the largest concentration of mass in the nearby Universe. A3528 is formed by two interacting clumps (A3528-N at North and A3528-S at South) separated by 0.9 h −1 70 Mpc at redshift 0.053. XMM-Newton data describe these clumps as a relaxed structure with an overall temperature of 4.14 ± 0.09 and 4.29 ± 0.07 keV in A3528-N and A3528-S, respectively, and a core cooler by a factor 1.4-1.5 and super-solar metal abundance in the inner 30 arcsec. These clumps are connected by a X-ray soft, bridge-like emission and present an asymmetric surface brightness with significant excess in the North-West region of A3528-N and in the North-East area of A3528-S. However, we do not observe any evidence of shock heated gas, both in the surface brightness and in the temperature map. Considering also that the optical light distribution is more concentrated around A3528-N and makes A3528-S barely detectable, we do not find support for the originally suggested head-on pre-merging scenario and conclude that A3528 is in an off-axis post-merging phase, where the closest core encounter happened about 1-2 Gyrs ago.

Evaluation of the Athena/WFI instrumental background

The Wide Field Imager (WFI) is one of two focal plane instruments of the Advanced Telescope for High-Energy Astrophysics (Athena), ESA’s next large X-ray observatory, planned for launch in the early 2030’s. In the aimed orbit, a halo orbit around L2, the second Lagrange point of the Sun-Earth system the radiation environment, mainly consisting of solar and cosmic protons, electrons and He-ions, could affect the science performance. Furthermore as additional contribution the unfocused hard X-ray background is taken into account. It is important to understand and estimate the expected instrumental background and to investigate measures, like design modifications or analysis methods, which could improve the expected background level in order to achieve the challenging scientific requirement of < 5×10−3 cts/cm2/keV/s. For that purpose, the WFI background working group is investigating possible approaches, which will also be subject to technical feasibility studies. Finally an estimate of the WFI instrumental background for a proposed combination of design optimization and background rejection algorithm is given, showing that WFI is compliant with science background requirements.