ALBERT

Description: Using XMM-Newton spatially resolved X-ray imaging spectroscopy we obtain the temperature, density, entropy, gas mass, and total mass profiles for two groups of galaxies out to approximately 0.3 R(sub vir)(R(sub vir), the virial radius). Our density profiles agree well with those derived previously, and the temperature data are broadly consistent with previous results but are considerably more precise. Both of these groups are at the mass scale of 2x10(exp 13) M(solar mass), but have rather different properties. Both have considerably lower gas mass fractions at r 〈 0.3 R(sub vir), than the rich clusters. NGC2563, one of the least luminous groups for its X-ray temperature, has a very low gas mass fraction of approximately 0.004 inside 0.1 R(sub vir), which increases with radius. NGC4325, one of the most luminous groups at the same average temperature, has a higher gas mass fraction of 0.02. The entropy profiles and the absolute values of the entropy as a function of virial radius also differ, with NGC4325 having a value of approximately 100 keV cm(exp -2) and NGC2563 a value of approximately 300 keV cm(exp -2) at r approximately 0.1 R(sub vir). For both groups the profiles rise monotonically with radius and there is no sign of an entropy 'floor'. These results are inconsistent with pre-heating scenarios that have been developed to explain a possible entropy floor in groups, but are broadly consistent with models of structure formation that include the effects of heating and/or the cooling of the gas. The total entropy in these systems provides a strong constraint on all models of galaxy and group formation, and on the poorly defined feedback process that controls the transformation of gas into stars and thus the formation of structure in the universe.

Description: We present the average abundances of the intermediate elements obtained by performing a stacked analysis of all the galaxy clusters in the archive of the X-ray telescope AKA. We determine the abundances of Fe, Si, S, and Ni as a function of cluster temperature (mass) from 1 - 10 keV, and place strong upper limits on the abundances of Ca and Ar. In general, Si and Ni are overabundant with respect to Fe, while Ar and Ca are very underabundant. The discrepancy between the abundances of Si, S , Ar, and Ca indicate that the alpha-elements do not behave homogeneously as a single group. We show that the abundances of the most well-determined elements Fe, Si, and S in conjunction with recent theoretical supernovae yields do not give a consistent solution for the fraction of material produced by Type Ia and Type II supernovae at any temperature or mass. The general trend is for higher temperature clusters to have more of their metals produced in Type II supernovae than in Type Ias. The inconsistency of our results with abundances in the Milky Way indicate that spiral galaxies are not the dominant metal contributors to the intracluster medium (ICM). The pattern of elemental abundances requires an additional source of metals beyond standard SNIa and SNII enrichment. The properties of this new source are well matched to those of Type II supernovae with very massive, metal-poor progenitor stars. These results are consistent with a significant fraction of the ICM metals produced by an early generation of population III stars.

Description: We report on X-ray follow-up observations of the March 2018 nuclear transient event AT2018zf (ASASSN-18el) from an ongoing campaign that includes XMM-Newton, NuSTAR, Swift, and high cadence (daily-to-weekly) NICER observations. The event was associated with the Seyfert galaxy 1ES 1927+654, a "True Type-2" that, nevertheless, showed an X-ray spectrum typical of an unobscured Type 1 AGN. Optical monitoring revealed the emergence of broad Balmer emission lines following the outburst, suggesting a transition from a Type 2 to a Type 1 AGN on timescales consistent with the light-travel time between the central black hole and the broad line region. The optical outburst was followed by a dramatic shift in the X-ray spectrum as the hard X-ray luminosity of the corona plunged by 〉2 orders of magnitude and the spectrum became dominated by a 10^6 K thermal component. The subsequent X-ray evolution of 1ES 1927+654 includes an additional two-order of magnitude decline in overall luminosity, followed by an extended period of quiescence, and re-brightening to levels that make it the brightest AGN currently in the X-ray sky. Throughout its evolution, the source has displayed X-ray flux variability of factors of several on timescales less than hour, and of ~100 on timescales less than a day. The X-ray spectrum varies along a consistent luminosity-dependent track, wherein increases in flux are accompanied by a spectral hardening. We seem to be witnessing the onset of an instability in the pre-existing AGN disc and corona, followed by a still-continuing re-emergence of the corona. If the initial transient is identified as the tidal disruption of a star, it is possible that this was the instigator of this unique behavior.