Ergodic to Non-ergodic Behavior Transitions and Hysteresis in Ecosystem Models

A widely used concept in natural sciences is the ergodic principle stating that the temporal average state of system equals the average of single states of an ensemble of the system. Originally formulated by Boltzmann to describe the physics of an ideal gas, the ergodic principle was and still is applied in, for example, the assessment of developmental aspects of individuals, but also in the growth series concept of whole ecosystems. Hysteresis on the other hand describes the observable contrary of the ergodic principle, that is, that the current state of a system strictly depends on the individual temporal development steps, or that individual history is unequivocally important. This work will provide evidence for ergodic to non-ergodic transitions in the application of biogeochemical ecosystem models using the showcase of Congo Basin rainforests. Using a climate gradient from west to east, ergodic model behavior is shown for a virgin forest refuge, non-ergodic behavior for a current forest savannah mosaic and ergodic behavior again for large open savannahs. Additionally, the occurrence of hysteresis related to the prevailing initial vegetation will be demonstrated, whereby the non-ergodic phase along the climate gradient is shown to be more extensive if rainforest was the original vegetation, as opposed to rainforest establishing on non-forest sites. The combination of ergodic to non-ergodic transitions—sometimes referred to as catastrophic shifts in ecosystems—will be put into context with the changing distribution patterns of rainforest and savannah over the course of the Holocene

Swift J053041.9-665426, a new Be/X-ray binary pulsar in the Large Magellanic Cloud

We observed the newly discovered X-ray source Swift J053041.9-665426 in the X-ray and optical regime to confirm its proposed nature as a high mass X-ray binary. We obtained XMM-Newton and Swift X-ray data, along with optical observations with the ESO Faint Object Spectrograph, to investigate the spectral and temporal characteristics of Swift J053041.9-665426. The XMM-Newton data show coherent X-ray pulsations with a period of 28.77521(10) s (1 sigma). The X-ray spectrum can be modelled by an absorbed power law with photon index within the range 0.76 to 0.87. The addition of a black body component increases the quality of the fit but also leads to strong dependences of the photon index, black-body temperature and absorption column density. We identified the only optical counterpart within the error circle of XMM-Newton at an angular distance of ~0.8 arcsec, which is 2MASS J05304215-6654303. We performed optical spectroscopy from which we classify the companion as a B0-1.5Ve star. The X-ray pulsations and long-term variability, as well as the properties of the optical counterpart, confirm that Swift J053041.9-665426 is a new Be/X-ray binary pulsar in the Large Magellanic Cloud.Comment: 10 pages, 8 figures, accepted for publication in A&

Science with the EXTraS Project: Exploring the X-ray Transient and variable Sky

The EXTraS project (Exploring the X-ray Transient and variable Sky) will characterise the temporal behaviour of the largest ever sample of objects in the soft X-ray range (0.1-12 keV) with a complex, systematic and consistent analysis of all data collected by the European Photon Imaging Camera (EPIC) instrument onboard the ESA XMM-Newton X-ray observatory since its launch. We will search for, and characterize variability (both periodic and aperiodic) in hundreds of thousands of sources spanning more than nine orders of magnitude in time scale and six orders of magnitude in flux. We will also search for fast transients, missed by standard image analysis. Our analysis will be completed by multiwavelength characterization of new discoveries and phenomenological classification of variable sources. All results and products will be made available to the community in a public archive, serving as a reference for a broad range of astrophysical investigations.Comment: 4 pages, 1 figure. Refereed Proceeding of "The Universe of Digital Sky Surveys" conference held at the INAF - Observatory of Capodimonte, Naples, on 25th-28th November 2014, to be published in the Astrophysics and Space Science Proceedings, edited by Longo, Napolitano, Marconi, Paolillo, Iodic

A new super-soft X-ray source in the Small Magellanic Cloud: Discovery of the first Be/white dwarf system in the SMC?

The Small Magellanic Cloud (SMC) hosts a large number of Be/X-ray binaries, however no Be/white dwarf system is known so far, although population synthesis calculations predict that they might be more frequent than Be/neutron star systems. XMMUJ010147.5-715550 was found as a new faint super-soft X-ray source (SSS) with a likely Be star optical counterpart. We investigate the nature of this system and search for further high-absorbed candidates in the SMC. We analysed the XMM-Newton X-ray spectrum and light curve, optical photometry, and the I-band OGLE III light curve. The X-ray spectrum is well represented by black-body and white dwarf atmosphere models with highly model-dependent temperature between 20 and 100 eV. The likely optical counterpart AzV 281 showed low near infrared emission during X-ray activity, followed by a brightening in the I-band afterwards. We find further candidates for high-absorbed SSSs with a blue star as counterpart. We discuss XMMUJ010147.5-715550 as the first candidate for a Be/white dwarf binary system in the SMC.Comment: 6 pages, 4 figures, accepted by A&

Probabilistic Spatial and Temporal Resilience Landscapes for the Congo Basin

Recent research by Hirota et al. (2011) introduced the concept of resilience landscapes for tropical forests and savannahs. Basically, the approach statistically relates the probability of current forest/savannah occurrence with the concept of tipping points, at which the ecosystem has no other choice except to switch from on stable state (e.g., forest) to its alternative stable state (e.g., savannah) or vice versa. This work will use a biogeochemical modelling approach to establish such probabilistic resilience landscapes for the Congo Basin rainforest biome. In a first step, the occurrence of tipping points will be related to climate features like annual precipitation, dry season length, occurrence of startiform non-precipitating cloud cover and the inter-annual variation in precipitation. In the second, spatial resilience landscapes for the Congo Basin will be provided using present climate conditions. Their relation to current forest/savannah distribution will be assessed and evident congruencies and discrepancies will be discussed. In a third step, the concept of temporal resilience landscapes will be developed along the patch-level life cycle dynamics of the Congo Basin rainforest biome. In a final step, the implications of results for ecosystem management decision will be assessed and possible implications on policy and land-use decisions will be presented

Ferromagnetism or slow paramagnetic relaxation in Fe-doped Li3_3N?

We report on isothermal magnetization, M\"ossbauer spectroscopy, and magnetostriction as well as temperature-dependent alternating-current (ac) susceptibility, specific heat, and thermal expansion of single crystalline and polycrstalline Li$_2$(Li$_{1-x}$Fe$_x$)N with $x = 0$ and $x \approx 0.30$. Magnetic hysteresis emerges at temperatures below $T \approx 50\,$K with coercivity fields of up to $\mu_0H = 11.6\,$T at $T = 2\,$K and magnetic anisotropy energies of $310\,$K ($27\,$meV). The ac susceptibility is strongly frequency dependent ($f\,=\,10$--$10,000\,$Hz) and reveals an effective energy barrier for spin reversal of $\Delta E \approx 1100\,$K. The relaxation times follow Arrhenius behavior for $T > 25\,$K. For $T < 10\,$K, however, the relaxation times of $\tau \approx 10^{10}\,$s are only weakly temperature-dependent indicating the relevance of a quantum tunneling process instead of thermal excitations. The magnetic entropy amounts to more than $25\,$J mol$^{-1}_{\rm Fe}\,$K$^{-1}$ which significantly exceeds $R$ln2, the value expected for the entropy of a ground state doublet. Thermal expansion and magnetostriction indicate a weak magneto-elastic coupling in accordance with slow relaxation of the magnetization. The classification of Li$_2$(Li$_{1-x}$Fe$_x$)N as ferromagnet is stressed and contrasted with highly anisotropic and slowly relaxing paramagnetic behavior.Comment: 12 pages, 10 figure