Rm Bb Research Articles

Thermodynamic stability, compressibility matrices, and effects of mediated interactions in a strongly interacting Bose-Fermi mixture

We theoretically investigate the thermodynamic stability of a normal-state Bose-Fermi mixture, with a tunable Bose-Fermi pairing interaction $-U_{\rm BF}<0$ associated with a hetero-nuclear Feshbach resonance, as well as a weak repulsive Bose-Bose interaction $U_{\rm BB}\ge 0$. Including strong hetero-pairing fluctuations associated with the former interaction within the self-consistent $T$-matrix approximation, as well as the latter within the mean-field level, we calculate the compressibility matrix, to assess the stability of this system against density fluctuations. In the weak- and the intermediate-coupling regime with respect $-U_{\rm BF}$, we show that an effective attractive interaction between bosons mediated by density fluctuations in the Fermi component makes the system unstable below a certain temperature $T_{\rm clp}$ (leading to density collapse). When $U_{\rm BB}=0$, $T_{\rm clp}$ is always higher than the Bose-Einstein condensation (BEC) temperature $T_{\rm c}$. When $U_{\rm BB}>0$, the density collapse is suppressed, and the BEC transition becomes possible. It is also suppressed by the formation of tightly bound Bose-Fermi molecules when the hetero-pairing interaction $-U_{\rm BF}$ is strong; however, since the system may be viewed as a molecular Fermi gas in this case, the BEC transition does not also occur. Since quantum gases involving Bose atoms are known to be sensitive to inter-particle correlations, our results would be useful for the study of many-body properties of a Bose-Fermi mixture in a stable manner, without facing the unwanted density collapse.

The nature of X-ray spectral variability in SWIFT J2127.4+5654

We study the flux-flux plots (FFPs) of the Seyfert galaxy SWIFT J2127.4+5654 which was observed simultaneously by XMM-Newton and NuSTAR. The 0.7-2 keV FFPs show a non-linear behaviour, while they are well fitted by a straight line, in the $\sim 2-40$ keV range. Without any additional modelling, this result strongly suggests that neither absorption nor spectral slope variations can contribute significantly to the observed variability above 2 keV (on time scales $\lesssim$5 days, at least). The FFP method is capable of separating the variable and the constant X-ray spectral components in this source. We found that the low-, average- and high-flux spectra of the variable components are consistent with a power law varying in normalisation only plus a relativistic reflection component varying simultaneously with the continuum. At low energies, we identify an additional variable component, consistent with a neutral absorber with a variable covering fraction. We also detect two spectral components whose fluxes remain constant over the duration of the observations. These components can be well described by a blackbody emission of $kT_{\rm BB}\sim 0.18\, \rm keV$, dominating at soft X-rays, and a reflection component from neutral material. The spectral model resulting from the FFP analysis can fit well the energy spectra of the source. We suggest that a combined study of FFPs and energy spectra in the case of X-ray bright and variable Seyferts can identify, unambiguously, the various spectral components, and can lift the degeneracy that is often observed within different models fitting equally well time-averaged spectra of these objects.

Bulk Comptonization: new hints from the luminous blazar 4C+25.05

Blazars are often characterized by a spectral break at soft X-rays, whose origin is still debated. While most sources show a flattening, some exhibit a blackbody-like soft excess with temperatures of the order of $\sim$0.1 keV, similar to low-luminosity, non-jetted Seyferts. Here we present the analysis of the simultaneous XMM-Newton and NuSTAR observation of the luminous FSRQ 4C+25.05 ($z=2.368$). The observed 0.3-30 keV spectrum is best described by the sum of a hard X-ray power law ($\Gamma = 1.38_{-0.03}^{+0.05}$) and a soft component, approximated by a blackbody with $kT_{\rm BB} = 0.66_{-0.04}^{+0.05}$ keV (rest frame). If the spectrum of 4C+25.05 is interpreted in the context of bulk Comptonization by cold electrons of broad-line region photons emitted in the direction of the jet, such an unusual temperature implies a bulk Lorentz factor of the jet of $\Gamma_{\rm bulk}\sim 11.7$. Bulk Comptonization is expected to be ubiquitous on physical grounds, yet no clear signature of it has been found so far, possibly due to its transient nature and the lack of high-quality, broad-band X-ray spectra.

The nature of X-ray spectral variability in MCG–6-30-15

The flux-flux plot (FFP) method can provide model-independent clues regarding the X-ray variability of active galactic nuclei. To use it properly, the bin size of the light curves should be as short as possible, provided the average counts in the light curve bins are larger than $\sim 200$. We apply the FFP method to the 2013, simultaneous XMM-Newton and NuSTAR observations of the Seyfert galaxy MCG$-$6-30-15, in the 0.3-40 keV range. The FFPs above $\sim 1.6$ keV are well-described by a straight line. This result rules out spectral slope variations and the hypothesis of absorption driven variability. Our results are fully consistent with a power-law component varying in normalization only, with a spectral slope of $\sim 2$, plus a variable, relativistic reflection arising from the inner accretion disc around a rotating black hole. We also detect spectral components which remain constant over $\sim 4.5$ days (at least). At energies above $\sim 1.5$ keV, the stable component is consistent with reflection from distant, neutral material. The constant component at low energies is consistent with a blackbody spectrum of $kT_{\rm BB} \sim 100$ eV. The fluxes of these components are $\sim 10-20\%$ of the average continuum flux (in the respective bands). They should always be included in the models that are used to fit the spectrum of the source. The FFPs below 1.6 keV are non-linear, which could be due to the variable warm absorber in this source.

A broad-band self-consistent modelling of the X-ray spectrum of 4U 1626−67

The accretion-powered X-ray pulsar 4U 1626--67 is one of the few highly magnetized pulsars that accretes through Roche-lobe overflow from a low-mass companion. The characteristics of its broadband spectrum are similar to those of X-ray pulsars hosted in a high-mass X-ray binary systems, with a broad resonant cyclotron scattering feature (CRSF) at $\sim$37 keV. In this work, we examine the pulse-resolved and the pulse-averaged broadband spectrum using data from NuSTAR and Swift. We use the Becker & Wolff model of bulk+thermal Comptonization to infer key physical parameters of the accretion column flow and a broadband model for the disk reflected spectrum. In the softer X-ray band, we need to add a soft black-body component with $kT_{\rm bb}$ $\sim$0.5 keV, whose characteristics indicate a possible origin from the neutron star surface. Residuals suggest that the shape of the cyclotron line could be more satisfactorily fitted using a narrow core and broader wings and, at higher energies, a second harmonic could be present at $\sim$61 keV.

Revisiting the ultraluminous supersoft source in M 101: an optically thick outflow model

The M 101 galaxy contains the best-known example of an ultraluminous supersoft source (ULS), dominated by a thermal component at kT ≈ 0.1 keV. The origin of the thermal component and the relation between ULSs and standard (broad-band spectrum) ultraluminous X-ray sources are still controversial. We re-examined the X-ray spectral and timing properties of the M 101 ULS using archival Chandra and XMM–Newton observations. We show that the X-ray time-variability and spectral properties are inconsistent with standard-disc emission. The characteristic radius Rbb of the thermal emitter varies from epoch to epoch between ≈10 000 and ≈100 000 km; the colour temperature kTbb varies between ≈50 and ≈140 eV and the two quantities scale approximately as |$R_{\rm bb} \propto T_{\rm bb}^{-2}$|⁠. In addition to the smooth continuum, we also find (at some epochs) spectral residuals well fitted with thermal-plasma models and absorption edges: we interpret this as evidence that we are looking at a clumpy, multitemperature outflow. We suggest that at sufficiently high accretion rates and inclination angles, the supercritical, radiatively driven outflow becomes effectively optically thick and completely thermalizes the harder X-ray photons from the inner part of the inflow, removing the hard spectral tail. We develop a simple, spherically symmetric outflow model and show that it is consistent with the observed temperatures, radii and luminosities. A larger, cooler photosphere shifts the emission peak into the far-UV and makes the source dimmer in X-rays but possibly ultraluminous in the UV. We compare our results and interpretation with those of Liu et al.

Cation Distribution and Temperature Dependence of Brillouin Function for Nickel-Substituted Manganese–Zinc Ferrites

Ni-substituted manganese–zinc (MnZn) ferrites with the composition of Mn0.506– x Zn0.244Ni x Fe2.250O4.0 ( $x = 0.066{\sim}0.122$ ) have been prepared by the solid-state reaction method. The cation distribution has been investigated by the Rietveld refinement of X-ray diffraction patterns, the microstructure has been observed using a scanning electron microscope, and the magnetic property has been measured using superconductor quantum interference devices and $B$ – $H$ analyzer. The results show that Zn2+ and Ni2+ ions prefer to occupy the tetrahedron site (A sublattice) and octahedron site (B sublattice), respectively. However, Mn2+ and Fe3+ ions can enter into A and B sublattices, where the ratio of Mn2+ ions occupying A and B sublattices is 4:1. The lattice parameter ( $a$ ) of the samples decreases with the increase of Ni-substituted content. Meanwhile, based on the Neel model of collinear-spin ferrimagnetism, the molecular-field coefficients $\omega _{\rm AA}$ , $\omega _{\rm BB}$ , and $\omega _{\rm AB}$ of the Ni-substituted MnZn ferrites have been calculated, and the magnetic moment of A and B sublattices versus temperature $T$ has also been investigated. The fitting results match well with the experimental data. Both $\omega _{\rm AB}$ and $\omega _{\rm BB}$ increase with the increase of the Ni-substituted content, but $\omega _{\rm AA}$ shows the opposite variation trend. The Curie temperature also increases with the increasing of the Ni-substituted content, which is attributed to the enhancement of superexchange interaction for A–B sublattice. In addition, the temperature dependence of initial permeability and core loss has been discussed.

XMM-Newtonand optical observations of the eclipsing polar CSS081231:071126+440405

Aims: We aim to study the temporal and spectral behaviour of the eclipsing polar CSS081231:071126+440405 from the infrared to the X-ray regime. Methods: We obtained phase-resolved XMM-Newton X-ray observations on two occasions in 2012 and 2013 in different states of accretion. In 2013 the XMM-Newton X-ray and UV data were complemented by optical photometric and spectroscopic observations. Results: CSS081231 displays two-pole accretion in the high state. The magnetic fields of the two poles are 36 and 69 MG, indicating a non-dipolar field geometry. The X-ray spectrum of the main accreting pole with the lower field comprises a hot thermal component from the cooling accretion plasma, $kT_{plas}$ of a few tens of keV, and a much less luminous blackbody-like component from the accretion area with $kT_{\rm bb} \sim$ 50-100\,eV. The high-field pole which was located opposite to the mass-donating star accretes at a low rate and has a plasma temperature of about 4\,keV. At both occasions the X-ray eclipse midpoint precedes the optical eclipse midpoint by 3.2 seconds. The center of the X-ray bright phase shows accretion-rate dependent longitudinal motion of ~20 degrees. Conclusions: CSS081231 is a bright polar that escaped detection in the RASS survey because it was in a low accretion state. Even in the high state it lacks the prominent soft component previously thought ubiquitous in polars. Such an excess may still be present in the unobserved extreme ultraviolet. All polars discovered in the XMM-Newton era lack the prominent soft component. The intrinsic spectral energy distribution of polars still awaits characterisation by future X-ray surveys such as eROSITA. The trajectory taken by material to reach the second pole is still uncertain.

CHANDRAOBSERVATIONS OF THE HIGH-MAGNETIC-FIELD RADIO PULSAR J1718–3718

High-magnetic-field pulsars represent an important class of objects for studying the relationship between magnetars and radio pulsars. Here we report on four Chandra observations of the high-magnetic-field pulsar J1718-3718 ($B=7.4\times10^{13}$\,G) taken in 2009 as well as on a re-analysis of 2002 Chandra observations of the region. We also report an improved radio position for this pulsar based on ATCA observations. We detect X-ray pulsations at the pulsar's period in the 2009 data, with pulsed fraction 52%$\pm$13% in the 0.8--2.0 keV band. We find that the X-ray pulse is aligned with the radio pulse. The data from 2002 and 2009 show consistent spectra and fluxes; a merged overall spectrum is well fit by a blackbody of temperature $186^{+19}_{-18}$\,eV, slightly higher than predicted by standard cooling models, however, the best-fit neutron star atmosphere model is consistent with standard cooling. We find the bolometric luminosity $L^{\infty}_{\rm bb}=4^{+5}_{-2} \times10^{32}$\,erg\,s$^{-1}\sim0.3\dot{E}$, for a distance of 4.5 kpc. We compile measurements of the temperatures of all X-ray detected high-$B$ pulsars as well as those of low-$B$ radio pulsars and find evidence for the former being on average hotter than the latter.

Preparation and Properties of Novel Triphenylpyridine‐Containing Hyperbranched Polyimides Derived from 2,4,6‐Tris(4‐aminophenyl)pyridine under Microwave Irradiation

AbstractA novel ${\rm BB'}_{{\rm 2}} $‐type triamine was synthesized for the first time and employed to prepare a series of amine‐ and anhydride‐terminated triphenylpyridine‐containing hyperbranched polyimides under microwave irradiation. The structure of the triamine monomer and polymers were verified by FTIR and 1H NMR spectroscopy. The predominantly amorphous polyimides were soluble in m‐cresol, NMP, DMSO, DMAc and DMF at room temperature or upon heating. The polymers showed glass transition temperatures above 310 °C and a 5 and 10% weight‐loss temperature above 482 and 559 °C. Tensile strength and modulus were higher than 53.1 MPa and 1.01 GPa, respectively. AM‐HBPI‐2 and AD‐HBPI‐2 showed UV‐Vis absorptions in the region 200–400 and 200–350 nm and displayed strong purple and blue fluorescence.magnified image

Rectilinear Spanning Trees Versus Bounding Boxes

For a set $P\subseteq {\Bbb R}^2$ with $2\leq n=|P| &lt; \infty$ we prove that ${{\rm mst}(P)\over{\rm bb}(P)} \leq{1\over\sqrt{2}}\sqrt{n}+{3\over2}$ where ${\rm mst}(P)$ is the minimum total length of a rectilinear spanning tree for $P$ and ${\rm bb}(P)$ is half the perimeter of the bounding box of $P$. Since the constant ${1\over\sqrt{2}}$ in the above bound is best-possible, this result settles a problem that was mentioned by Brenner and Vygen (Networks 38 (2001), 126-139).

Quantum topological excitations: from the sawtooth lattice to the Heisenberg chain

The recently elucidated structure of the delafossite YCuO$_{2.5}$ reveals a Cu-O network with nearly independent $\Delta$ chains having different interactions between the $s=1/2$ spins. Motivated by this result, we study the $\Delta$ chain for various ratios $J_{\rm bb}/J_{\rm bv}$ of the base-base and base-vertex interactions. By exact diagonalization and extrapolation, we show that the elementary excitation spectrum, which (within numerical error) is the same for total spins $S_{\rm tot}=0$ and 1, has a gap only in the interval $0.4874(1) \leq J_{\rm bb}/J_{\rm bv} \leq 1.53(1)$. The gap is dispersionless for $J_{\rm bb}/J_{\rm bv}=1$, but has increasing $k$-dependence as $J_{\rm bb}/J_{\rm bv}$ moves away from unity, related to the instability of dimers in the ground state.

The soft X-ray properties of the polar EK UMa

We present the first extensive soft X-ray observation of the polar EK UMa |$(1{\rm E}\, 1048.5 + 5241)$|⁠. The complete orbital light curve obtained from this ROSAT observation shows a single bright phase of width |$\Delta \phi = 0.63.$| EK UMa is not detected outside this phase interval. During the bright phase a number of deep dips in the flux are seen. These dips can be caused by an additional absorption of |$8 \times 10^{18} \lt \Delta N_H \lt 3 \times 10^{20} {\rm atom\, cm}^{-2}$|⁠. The soft X-ray spectrum has an unusually high temperature of |$50 \lt kT_{\rm BB} \lt 62\rm {eV}$|⁠. No hard X-ray flux is detected, and EK UMa is shown to have a large soft X-ray excess, with a soft-to-hard X-ray luminosity ratio of 7.7–12.2.