We summarize basic observational results on Sagittarius~A* obtained from the radio, infrared and X-ray domain. Infrared observations have revealed that a dusty S-cluster object (DSO/G2) passes by SgrA*, the central super-massive black hole of the Mil
ky Way. It is still expected that this event will give rise to exceptionally intense activity in the entire electromagnetic spectrum. Based on February to September 2014 SINFONI observations. The detection of spatially compact and red-shifted hydrogen recombination line emission allows a us to obtain a new estimate of the orbital parameters of the DSO. We have not detected strong pre-pericenter blue-shifted or post-pericenter red-shifted emission above the noise level at the position of SgrA* or upstream the orbit. The periapse position was reached in May 2014. Our 2004-2012 infrared polarization statistics shows that SgrA* must be a very stable system - both in terms of geometrical orientation of a jet or accretion disk and in terms of the variability spectrum which must be linked to the accretion rate. Hence polarization and variability measurements are the ideal tool to probe for any change in the system as a function of the DSO/G2 fly-by. Due to the 2014 fly-by of the DSO, increased accretion activity of SgrA* may still be upcoming. Future observations of bright flares will improve the derivation of the spin and the inclination of the SMBH from NIR/sub-mm observations.
In the rotational sandpile model, either the clockwise or the anti-clockwise toppling rule is assigned to all the lattice sites. It has all the features of a stochastic sandpile model but belongs to a different universality class than the Manna class
. A crossover from rotational to Manna universality class is studied by constructing a random rotational sandpile model and assigning randomly clockwise and anti-clockwise rotational toppling rules to the lattice sites. The steady state and the respective critical behaviour of the present model are found to have a strong and continuous dependence on the fraction of the lattice sites having the anti-clockwise (or clockwise) rotational toppling rule. As the anti-clockwise and clockwise toppling rules exist in equal proportions, it is found that the model reproduces critical behaviour of the Manna model. It is then further evidence of the existence of the Manna class, in contradiction with some recent observations of the non-existence of the Manna class.
We present a detailed, photoionization modeling analysis of XMM-Newton/Reflection Grating Spectrometer observations of the Seyfert 2 galaxy NGC 1068. The spectrum, previously analyzed by Kinkhabwala et al. (2002), reveals a myriad of soft-Xray emissi
on lines, including those from H- and He-like carbon, nitrogen, oxygen, and neon, and M- and L-shell iron. As noted in the earlier analysis, based on the narrowness of the radiative recombination continua, the electron temperatures in the emission-line gas are consistent with photoionization, rather than collisional ionization. The strengths of the carbon and nitrogen emission lines, relative to those of oxygen, suggest unusual elemental abundances, which we attribute to star-formation history of the host galaxy. Overall, the emission-lines are blue-shifted with respect to systemic, with radial velocities ~ 160 km/s, similar to that of [O III] 5007, and thus consistent with the kinematics and orientation of the optical emission-line gas and, hence, likely part of an AGN-driven outflow. We were able to achieve an acceptable fit to most of the strong emission-lines with a two-component photoionization model, generated with Cloudy. The two components have ionization parameters and column densities of logU = -0.05 and 1.22, and logN(H) = 20.85 and 21.2, and covering factors of 0.35 and 0.84, respectively. The total mass of the X-ray gas is roughly of an order of magnitude greater than the mass of ionized gas determined from optical and near-IR spectroscopy, which indicates that it may be the dominant component of the narrow line region. Furthermore, we suggest that the medium which produces the scattered/polarized optical emission in NGC~1068 possesses similar physical characteristics to those of the more highly-ionized of the X-ray model components.
Supernova fallback disks around neutron stars have been discussed to influence the evolution of the diverse neutron star populations. Slowly rotating neutron stars are most promising to find such disks. Searching for the cold and warm debris of old f
allback disks, we carried out Herschel PACS (70 $mu$m, 160 $mu$m) and Spitzer IRAC (3.6 $mu$m, 4.5 $mu$m) observations of eight slowly rotating ($Papprox 3 - 11$ s) nearby ($<1$ kpc) isolated neutron stars. Herschel detected 160 $mu$m emission ($>5sigma$) at locations consistent with the positions of the neutron stars RX J0806.4-4123 and RX J2143.0+0654. No other significant infrared emission was detected from the eight neutron stars. We estimate probabilities of 63%, 33% and 3% that, respectively, none, one, or both Herschel PACS 160 $mu$m detections are unrelated excess sources due to background source confusion or an interstellar cirrus. If the 160 $mu$m emission is indeed related to cold (10 K to 22 K) dust around the neutron stars, this dust is absorbing and re-emitting $sim 10$% to $sim 20$% of the neutron stars X-rays. Such high efficiencies would be at least three orders of magnitude larger than the efficiencies of debris disks around nondegenerate stars. While thin dusty disks around the neutron stars can be excluded as counterparts of the 160 $mu$m emission, dusty asteroid belts constitute a viable option.
We propose a variational approach for computing the macroscopic entanglement in a many-body mixed state, based on entanglement witness operators, and compute the entanglement of formation (EoF), a mixed-state generalization of the entanglement entrop
y, in single- and two-channel Kondo systems at finite temperature. The thermal suppression of the EoF obeys power-law scaling at low temperature. The scaling exponent is halved from the single- to the two-channel system, which is attributed, using a bosonization method, to the non-Fermi liquid behavior of a Majorana fermion, a half of a complex fermion, emerging in the two-channel system. Moreover, the EoF characterizes the size and power-law tail of the Kondo screening cloud of the single-channel system.
Lyman-Break Galaxy (LBG) samples observed during reionization ($zgtrsim6$) with Hubble Space Telescopes Wide Field Camera 3 are reaching sizes sufficient to characterize their clustering properties. Using a combined catalog from the Hubble eXtreme De
ep Field and CANDELS surveys, containing $N=743$ LBG candidates at z>6.5 at a mean redshift of $z=7.2$, we detect a clear clustering signal in the angular correlation function (ACF) at $sim4sigma$, corresponding to a real-space correlation length $r_{0}=6.7^{+0.9}_{-1.0}h^{-1}$cMpc. The derived galaxy bias $b=8.6^{+0.9}_{-1.0}$ is that of dark-matter halos of $M=10^{11.1^{+0.2}_{-0.3}}$M$_{odot}$ at $z=7.2$, and highlights that galaxies below the current detection limit ($M_{AB}sim-17.7$) are expected in lower-mass halos ($Msim10^{8}-10^{10.5}$M$_{odot}$). We compute the ACF of LBGs at $zsim3.8-zsim5.9$ in the same surveys. A trend of increasing bias is found from $z=3.8$ ($bsim3.0$) to $z=7.2$ ($bsim8.6$), broadly consistent with galaxies at fixed luminosity being hosted in dark-matter halos of similar mass at $4<z<6$, followed by a slight rise in halo masses at $zsim7$ ($sim2sigma$ confidence). Separating the data at the median luminosity of the $z=7.2$ sample ($M_{UV}=-19.4$) shows higher clustering at $z=5.9$ for bright galaxies ($r_{0}=5.5^{+1.4}_{-1.5}h^{-1}$cMpc, $b=6.2^{+1.2}_{-1.5}$) compared to faint galaxies ($r_{0}=1.9^{+1.1}_{-1.0}h^{-1}$cMpc, $b=2.7pm1.2$) implying a constant mass-to-light ratio $frac{dlogM}{dlogL}sim1.2^{+1.8}_{-0.8}$. A similar trend is present in the $z=7.2$ sample with larger uncertainty. Finally, our bias measurements allow us to investigate the fraction of dark-matter halos hosting UV-bright galaxies (the duty-cycle, $epsilon_{DC}$). At $z=7.2$ values near unity are preferred, which may be explained by the shortened halo assembly time at high-redshift.
We present magnetization, specific heat, resistivity, and Hall effect measurements on the cubic B20 phase of MnGe and CoGe and compare to measurements of isostructural FeGe and electronic structure calculations. In MnGe, we observe a transition to a
magnetic state at $T_c=275$ K as identified by a sharp peak in the ac magnetic susceptibility, as well as second phase transition at lower temperature that becomes apparent only at finite magnetic field. We discover two phase transitions in the specific heat at temperatures much below the Curie temperature one of which we associate with changes to the magnetic structure. A magnetic field reduces the temperature of this transition which corresponds closely to the sharp peak observed in the ac susceptibility at fields above 5 kOe. The second of these transitions is not affected by the application of field and has no signature in the magnetic properties or our crystal structure parameters. Transport measurements indicate that MnGe is metal with a negative magnetoresistance similar to that seen in isostructural FeGe and MnSi. Hall effect measurements reveal a carrier concentration of about 0.5 carriers per formula unit also similar to that found in FeGe and MnSi. CoGe is shown to be a low carrier density metal with a very small, nearly temperature independent diamagnetic susceptibility.
We investigate the electrical conductance and thermopower of a quantum dot tunnel coupled to external leads described by an extension of the Anderson impurity model which takes into account the assisted hopping processes, i.e., the occupancy-dependen
ce of the tunneling amplitudes. We provide analytical understanding based on scaling arguments and the Schrieffer-Wolff transformation, corroborated by detailed numerical calculations using the numerical renormalization group (NRG) method. The assisted hopping modifies the coupling to the two-particle state, which shifts the Kondo exchange coupling constant and exponentially reduces or enhances the Kondo temperature, breaks the particle-hole symmetry, and strongly affects the thermopower. We discuss the gate-voltage and temperature dependence of the transport properties in various regimes. For a particular value of the assisted hopping parameter we find peculiar discontinuous behaviour in the mixed-valence regime. Near this value, we find very high Seebeck coefficient. We show that, quite generally, the thermopower is a highly sensitive probe of assisted hopping and Kondo correlations.
Since the experimental realisation of the integer quantised Hall effect in a two dimensional electron system subject to strong perpendicular magnetic fields in 1980, a central question has been the interrelation between the conductance quantisation a
nd the topological properties of the system. It is conjectured that if the electron system is described by a Bloch hamiltonian, then the system is insulating in the bulk of the sample throughout the quantised Hall plateau due to magnetic field induced energy gap. Meanwhile, the system is conducting at the edges resembling a 2+1 dimensional topological insulator without the time-reversal symmetry. However, the validity of this conjecture remains unclear for finite size, non-periodically bounded real Hall bar devices. Here we show experimentally that the close relationship proposed between the quantised Hall effect and the topological bulk insulator is prone to break for specific magnetic field intervals within the plateau evidenced by our magneto-transport measurements performed on GaAs/AlGaAs high purity Hall bars with two inner contacts embedded to bulk. Our data presents a similar behaviour also for fractional states, in particular for 2/3, 3/5 and 4/3.
We present studies of thermal entanglement of a three-spin system in triangular symmetry. Spin correlations are described within an effective Heisenberg Hamiltonian, derived from the Hubbard Hamiltonian, with super-exchange couplings modulated by an
effective electric field. Additionally a homogenous magnetic field is applied to completely break the degeneracy of the system. We show that entanglement is generated in the subspace of doublet states with different pairwise spin correlations for the ground and excited states. At low temperatures thermal mixing between the doublets with the same spin destroys entanglement, however one can observe its restoration at higher temperatures due to the mixing of the states with an opposite spin orientation or with quadruplets (unentangled states) always destroys entanglement. Pairwise entanglement is quantified using concurrence for which analytical formulae are derived in various thermal mixing scenarios. The electric field plays a specific role -- it breaks the symmetry of the system and changes spin correlations. Rotating the electric field can create maximally entangled qubit pairs together with a separate spin (monogamy) that survives in a relatively wide temperature range providing robust pairwise entanglement generation at elevated temperatures.
