arXiv subjects

S. Allen

Publications and source records attributed to S. Allen.

28 records · Page 2Linked to original sources

arXiv2009

The growth and evolution of super massive black holes

We discuss the central role played by X-ray studies to reconstruct the past history of formation and evolution of supermassive Black Holes (BHs), and the role they played in shaping the properties of their host galaxies. We shortly review the progress in this field contributed by the current X-ray and multiwavelength surveys. Then, we focus on the outstanding scientific questions that have been opened by observations carried out in the last years and that represent the legacy of Chandra and XMM, as for X-ray observations, and the legacy of the SDSS, as for wide area surveys: 1) When and how did the first supermassive black holes form? 2) How does cosmic environment regulate nuclear activity (and star formation) across cosmic time? 3) What is the history of nuclear activity in a galaxy lifetime? We show that the most efficient observational strategy to address these questions is to carry out a large-area X-ray survey, reaching a sensitivity comparable to that of deep Chandra and XMM pointings, but extending over several thousands of square degrees. Such a survey can only be carried out with a Wide-Field X-ray Telescope (WFXT) with a high survey speed, due to the combination of large field of view and large effective area, i.e., grasp, and sharp PSF. We emphasize the important synergies that WFXT will have with a number of future groundbased and space telescopes, covering from the radio to the X-ray bands and discuss the immense legacy value that such a mission will have for extragalactic astronomy at large.

astro-ph.HE↗

arXiv2009

X-ray Cluster Cosmology

Sensitive, wide-area X-ray surveys which would be possible with the WFXT will detect huge samples of virialized objects spanning the mass range from sub-groups to the most massive clusters, and extending in redshift to beyond z=2. These samples will be an excellent dataset for carrying out many traditional cosmological tests using the cluster mass function and power spectrum. Uniquely, WFXT will be able not only to detect clusters but also to make detailed X-ray measurements for a large number of clusters and groups right from the survey data. Very high quality measurements of the cluster mass function and spatial correlation over a very wide range of masses, spatial scales, and redshifts, will be useful for expanding the cosmological discovery space, and in particular, in searching for departures from the "concordant" Lambda-CDM cosmological model. Finding such departures would have far-reaching implications on our understanding of the fundamental physics which governs the Universe.

astro-ph.CO↗

arXiv2009

The Very Local Universe in X-rays

While the exceptional sensitivity of Chandra and XMM-Newton has resulted in revolutionary studies of the Galactic neighborhood in the soft (<10 keV) X-ray band, there are many open questions. We discuss these issues and how they would be addressed by very wide-area (> 100 sq. deg.) X-ray surveys.

astro-ph.HE↗

arXiv2005

An X-ray Galaxy Cluster Survey for Investigations of Dark Energy

The amount and nature of dark energy (DE) can be tightly constrained by measuring the spatial correlation features and evolution of a sample of ~ 100,000 galaxy clusters over the redshift range 0<z < 1.5. Such an X-ray survey will discover all collapsed structures with mass above 3.5e14 solar masss at redshifts z<2 (i.e. the full range where such objects are expected) in the high Galactic latitude sky. Above this mass threshold the tight correlations between X-ray observables and mass allow direct interpretation of the data. We describe the constraints on Dark Energy that can be inferred from such a survey, using powerful self-calibration techniques to relate the X-ray observables (luminosity and temperature) to the underlying mass.

astro-ph↗

arXiv2003

Conserving approximations vs Two-Particle Self-Consistent Approach

The conserving approximation scheme to many-body problems was developed by Kadanoff and Baym using the functional-derivative approach. Another approach for the Hubbard model also satisfies conservation laws, but in addition it satisfies the Pauli principle and a number of sum rules. A concise formal derivation of that approach, using functional derivatives, is given in this conference paper to highlight formal analogies and differences with conserving approximations.

cond-mat.str-el↗

arXiv2000

Non-perturbative approach to the attractive Hubbard model

A non-perturbative approach to the single-band attractive Hubbard model is presented in the general context of functional derivative approaches to many-body theories. As in previous work on the repulsive model, the first step is based on a local-field type ansatz, on enforcement of the Pauli principle and a number of crucial sum-rules. The Mermin-Wagner theorem in two dimensions is automatically satisfied. At this level, two-particle self-consistency has been achieved. In the second step of the approximation, an improved expression for the self-energy is obtained by using the results of the first step in an exact expression for the self-energy where the high- and low-frequency behaviors appear separately. The result is a cooperon-like formula. The required vertex corrections are included in this self-energy expression, as required by the absence of a Migdal theorem for this problem. Other approaches to the attractive Hubbard model are critically compared. Physical consequences of the present approach and agreement with Monte Carlo simulations are demonstrated in the accompanying paper (following this one).

cond-mat.str-el↗

arXiv2000

Pairing fluctuations and pseudogaps in the attractive Hubbard model

The two-dimensional attractive Hubbard model is studied in the weak to intermediate coupling regime by employing a non-perturbative approach. It is first shown that this approach is in quantitative agreement with Monte Carlo calculations for both single-particle and two-particle quantities. Both the density of states and the single-particle spectral weight show a pseudogap at the Fermi energy below some characteristic temperature T*, also in good agreement with quantum Monte Carlo calculations. The pseudogap is caused by critical pairing fluctuations in the low-temperature renormalized classical regime $ω< T$ of the two-dimensional system. With increasing temperature the spectral weight fills in the pseudogap instead of closing it and the pseudogap appears earlier in the density of states than in the spectral function. Small temperature changes around T* can modify the spectral weight over frequency scales much larger than temperature. Several qualitative results for the s-wave case should remain true for d-wave superconductors.

cond-mat.str-el↗

arXiv2000

Many-body Theory vs Simulations for the pseudogap in the Hubbard model

The opening of a critical-fluctuation induced pseudogap (or precursor pseudogap) in the one-particle spectral weight of the half-filled two-dimensional Hubbard model is discussed. This pseudogap, appearing in our Monte Carlo simulations, may be obtained from many-body techniques that use Green functions and vertex corrections that are at the same level of approximation. Self-consistent theories of the Eliashberg type (such as the Fluctuation Exchange Approximation) use renormalized Green functions and bare vertices in a context where there is no Migdal theorem. They do not find the pseudogap, in quantitative and qualitative disagreement with simulations, suggesting these methods are inadequate for this problem. Differences between precursor pseudogaps and strong-coupling pseudogaps are also discussed.

cond-mat.str-el↗

arXiv1999

Role of symmetry and dimension on pseudogap phenomena

The attractive Hubbard model in d=2 is studied through Monte Carlo simulations at intermediate coupling. There is a crossover temperature $T_X$ where a pseudogap appears with concomitant precursors of Bogoliubov quasiparticles that are not local pairs. The pseudogap in $A(k,ω)$ occurs in the renormalized classical regime when the correlation length is larger than the direction-dependent thermal de Broglie wave length, $ξ_{th}=\hbar v_{F}(k)/k_{B}T.$ The ratio $T_{X}/T_{c}$ for the pseudogap may be made arbitrarily large when the system is close to a point where the order parameter has SO(n) symmetry with n>2. This is relevant in the context of SO(5) theories of high $T_c$ but has more general applicability.

cond-mat.str-el↗

arXiv1999

Attractive Hubbard model and single-particle pseudogap due to classical pairing fluctuations in two dimensions

It is shown that in the two-dimensional attractive Hubbard model, the mean-field phase transition is replaced by a renormalized classical regime of fluctuations where a pseudogap opens up in the single-particle spectral weight. It is argued that this pseudogap and precursors of the ordered state quasiparticles can occur only in strongly anisotropic quasi two-dimensional materials. This precursor phenomenon differs from preformed local pairs. Furthermore, while critical antiferromagnetic fluctuations would also lead to a pseudogap in the repulsive model, there are some important differences between the superconducting and magnetic pseudogap.

cond-mat.str-el↗