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F. Bernardini

Publications and source records attributed to F. Bernardini.

At least 55 records · Page 3Linked to original sources
arXiv2015

Swift J0525.6+2416 and IGR J04571+4527: two new hard X-ray selected magnetic cataclysmic variables identified with XMM-Newton

IGR J04571+4527 and Swift J0525.6+2416 are two hard X-ray sources detected in the Swift/BAT and INTEGRAL/IBIS surveys. They were proposed to be magnetic cataclysmic variables of the Intermediate Polar (IP) type, based on optical spectroscopy. IGR J04571+4527 also showed a 1218 s optical periodicity, suggestive of the rotational period of a white dwarf, further pointing towards an IP classification. We here present detailed X-ray (0.3-10 keV) timing and spectral analysis performed with XMM-Newton, complemented with hard X-ray coverage (15-70 keV) from Swift/BAT. These are the first high signal to noise observations in the soft X-ray domain for both sources, allowing us to identify the white dwarf X-ray spin period of Swift J0525.6+2416 (226.28 s), and IGR J04571+4527 (1222.6 s). A model consisting of multi-temperature optically thin emission with complex absorption adequately fits the broad-band spectrum of both sources. We estimate a white dwarf mass of about 1.1 and 1.0 solar masses for IGR J04571+4527 and Swift J0525.6+2416, respectively. The above characteristics allow us to unambiguously classify both sources as IPs, confirming the high incidence of this subclass among hard X-ray emitting Cataclysmic Variables.

astro-ph.HE↗
arXiv2015

The phase diagrams of iron-based superconductors: theory and experiments

Phase diagrams play a primary role in the understanding of materials properties. For iron-based superconductors (Fe-SC), the correct definition of their phase diagrams is crucial because of the close interplay between their crystallo-chemical and magnetic properties, on one side, and the possible coexistence of magnetism and superconductivity, on the other. The two most difficult issues for understanding the Fe-SC phase diagrams are: 1) the origin of the structural transformation taking place during cooling and its relationship with magnetism; 2) the correct description of the region where a crossover between the magnetic and superconducting electronic ground states takes place. Hence a proper and accurate definition of the structural, magnetic and electronic phase boundaries provides an extremely powerful tool for material scientists. For this reason, an exact definition of the thermodynamic phase fields characterizing the different structural and physical properties involved is needed, although it is not easy to obtain in many cases. Moreover, physical properties can often be strongly dependent on the occurrence of micro-structural and other local-scale features (lattice micro-strain, chemical fluctuations, domain walls, grain boundaries, defects), which, as a rule, are not described in a structural phase diagram. In this review, we critically summarize the results for the most studied 11-, 122- and 1111-type compound systems, providing a correlation between experimental evidence and theory.

cond-mat.supr-con↗
arXiv2014

Long-term soft X-ray characterization of Supergiant Fast X-ray Transients: the cumulative luminosity distributions

We constructed the cumulative luminosity distributions of most supergiant fast X-ray transients (SFXTs) and the classical supergiant X-ray binary (SgXB) IGR J18027-2016 by taking advantage of the long term monitoring of these sources carried out with Swift/XRT (0.3-10 keV). Classical SgXBs are characterized by cumulative distributions with a single knee around $\sim$10$^{36}$-10$^{37}$ erg/s, while all SFXTs are found to be significantly sub-luminous and the main knee in their distributions is shifted at lower luminosities ($<$10$^{35}$ erg/s). As the latter are below the sensitivity limit of large field of view instruments operating in the hard X-ray domain ($>$15 keV), we show that a soft X-ray monitoring is required to reconstruct the entire profile of the SFXT cumulative luminosity distributions. The difference between the cumulative luminosity distributions of classical SgXBs and SFXTs is interpreted in terms of different wind accretion modes.

astro-ph.HE↗
arXiv2014

Investigating Variability of Quiescent Neutron Stars in the Globular Clusters NGC 6440 and Terzan 5

The quiescent spectrum of neutron star low-mass X-ray binaries typically consists of two components - a thermal component associated with emission from the neutron star surface, and a non-thermal power-law component whose origin is not well understood. Spectral fitting of neutron star atmosphere models to the thermal component is one of the leading methods for measuring the neutron star radius. However, it has been known for years that the X-ray spectra of quiescent neutron stars vary between observations. While most quiescent variability is explained through a variable power-law component, the brightest and best-studied object, Cen X-4, requires a change in the thermal component and such thermal variability could be a problem for measuring neutron star radii. In this paper, we significantly increase the number of sources whose quiescent spectra have been studied for variability. We examine 9 potential quiescent neutron stars with luminosities <1E34 erg/s over the course of multiple Chandra observations of the globular clusters NGC 6440 and Terzan 5 and find no strong evidence for variability in the effective temperature in 7 of the 9 sources. Two sources show a potential change in temperature, though this depends on the exact model fitted. CX1 in NGC 6440 is equally well fit by a variable thermal component or a variable power law. Therefore, the results are inconclusive and we cannot exclude or require thermal variability in that source. CX5 in NGC 6440 shows a potential change in temperature, though this depends on whether a power-law is included in the spectral fit or not. This suggests that thermal variability may not be widespread among quiescent neutron stars with luminosities < 1E34 erg/s, and hence thermal radiation remains a promising means to constraining neutron star radii.

astro-ph.HE↗
arXiv2014

Supergiant fast X-ray transients as an under-luminous class of supergiant X-ray binaries

The usage of cumulative luminosity distributions, constructed thanks to the long-term observations available through wide field hard X-ray imagers, has been recently exploited to study the averaged high energy emission (>17 keV) from Supergiant Fast X-ray Transients (SFXTs) and classical Supergiant High Mass X-ray Binaries (SgXBs). Here, we take advantage of the long term monitorings now available with Swift/XRT to construct for the first time the cumulative luminosity distributions of a number of SFXTs and the classical SgXB IGR J18027-2016 in the soft X-ray domain with a high sensitivity focusing X-ray telescope (0.3-10 keV). By complementing previous results obtained in the hard X-rays, we found that classical SgXBs are characterized by cumulative distributions with a single knee around $\sim$10$^{36}$-10$^{37}$ erg/s, while SFXTs are found to be systematically sub-luminous and their distributions are shifted at significantly lower luminosities (a factor of $\sim$10-100). As the luminosity states in which these sources spend most of their time are typically below the sensitivity limit of large field of view hard X-ray imagers, we conclude that soft X-ray monitorings carried out with high sensitivity telescopes are particularly crucial to reconstruct the complete profile of the SFXT cumulative luminosity distributions. The difference between the cumulative luminosity distributions of classical SgXBs and SFXTs is interpreted in terms of accretion from a structured wind in the former sources and the presence of magnetic/centrifugal gates or a quasi-spherical settling accretion regime in the latter.

astro-ph.HE↗
arXiv2014

The Large Observatory For x-ray Timing

The Large Observatory For x-ray Timing (LOFT) was studied within ESA M3 Cosmic Vision framework and participated in the final down-selection for a launch slot in 2022-2024. Thanks to the unprecedented combination of effective area and spectral resolution of its main instrument, LOFT will study the behaviour of matter under extreme conditions, such as the strong gravitational field in the innermost regions of accretion flows close to black holes and neutron stars, and the supra-nuclear densities in the interior of neutron stars. The science payload is based on a Large Area Detector (LAD, 10 m 2 effective area, 2-30 keV, 240 eV spectral resolution, 1 deg collimated field of view) and a WideField Monitor (WFM, 2-50 keV, 4 steradian field of view, 1 arcmin source location accuracy, 300 eV spectral resolution). The WFM is equipped with an on-board system for bright events (e.g. GRB) localization. The trigger time and position of these events are broadcast to the ground within 30 s from discovery. In this paper we present the status of the mission at the end of its Phase A study.

astro-ph.IM↗
arXiv2014

Multiwavelength search for counterparts of supersoft X-ray sources in M31

We searched optical/UV/IR counterparts of seven supersoft X-ray sources (SSS) in M31 in the Hubble Space Telescope (HST) "Panchromatic Hubble Andromeda Treasury" (PHAT) archival images and photometric catalog. Three of the SSS were transient, the other four are persistent sources. The PHAT offers the opportunity to identify SSS hosting very massive white dwarfs that may explode as type Ia supernovae in single degenerate binaries, with magnitudes and color indexes typical of symbiotic stars, high mass close binaries, or systems with optically luminous accretion disks. We find evidence that the transient SSS were classical or recurrent novae; two likely counterparts we identified are probably symbiotic binaries undergoing mass transfer at a very high rate. There is a candidate accreting white dwarf binary in the error circle of one of the persistent sources, r3-8. In the spatial error circle of the best studied SSS in M31, r2-12, no red giants or AGB stars are sufficiently luminous in the optical and UV bands to be symbiotic systems hosting an accreting and hydrogen burning white dwarf. This SSS has a known modulation of the X-ray flux with a 217.7 s period, and we measured an upper limit on its derivative, 0.82 x 10(-11). This limit can be reconciled with the rotation period of a white dwarf accreting at high rate in a binary with a few-hours orbital period. However, there is no luminous counterpart with color indexes typical of an accretion disk irradiated by a hot central source. Adopting a semi-empirical relationship, the upper limit for the disk optical luminosity implies an upper limit of only 169 minutes for the orbital period of the white dwarf binary.

astro-ph.SR↗
arXiv2014

Understanding the $μ$SR spectra of MnSi without magnetic polarons

Transverse-field muon-spin rotation ($μ$SR) experiments were performed on a single crystal sample of the non-centrosymmetric system MnSi. The observed angular dependence of the muon precession frequencies matches perfectly the one of the Mn-dipolar fields acting on the muons stopping at a 4a position of the crystallographic structure. The data provide a precise determination of the magnetic dipolar tensor. In addition, we have calculated the shape of the field distribution expected below the magnetic transition temperature $T_C$ at the 4a muon-site when no external magnetic field is applied. We show that this field distribution is consistent with the one reported by zero-field $μ$SR studies. Finally, we present ab initio calculations based on the density-functional theory which confirm the position of the muon stopping site inferred from transverse-field $μ$SR. In view of the presented evidence we conclude that the $μ$SR response of MnSi can be perfectly and fully understood without invoking a hypothetical magnetic polaron state.

cond-mat.str-el↗
arXiv2013

X-ray and UV correlation in the quiescent emission of Cen X-4, evidence of accretion and reprocessing

We conducted the first long-term (60 days), multiwavelength (optical, ultraviolet, and X-ray) simultaneous monitoring of Cen X-4 with daily Swift observations, with the goal of understanding variability in the low mass X-ray binary Cen X-4 during quiescence. We found Cen X-4 to be highly variable in all energy bands on timescales from days to months, with the strongest quiescent variability a factor of 22 drop in the X-ray count rate in only 4 days. The X-ray, UV and optical (V band) emission are correlated on timescales down to less than 110 s. The shape of the correlation is a power law with index gamma about 0.2-0.6. The X-ray spectrum is well fitted by a hydrogen NS atmosphere (kT=59-80 eV) and a power law (with spectral index Gamma=1.4-2.0), with the spectral shape remaining constant as the flux varies. Both components vary in tandem, with each responsible for about 50% of the total X-ray flux, implying that they are physically linked. We conclude that the X-rays are likely generated by matter accreting down to the NS surface. Moreover, based on the short timescale of the correlation, we also unambiguously demonstrate that the UV emission can not be due to either thermal emission from the stream impact point, or a standard optically thick, geometrically thin disc. The spectral energy distribution shows a small UV emitting region, too hot to arise from the accretion disk, that we identified as a hot spot on the companion star. Therefore, the UV emission is most likely produced by reprocessing from the companion star, indeed the vertical size of the disc is small and can only reprocess a marginal fraction of the X-ray emission. We also found the accretion disc in quiescence to likely be UV faint, with a minimal contribution to the whole UV flux.

astro-ph.HE↗
arXiv2013

Daily, multiwavelength Swift monitoring of the neutron star low-mass X-ray binary Cen X-4: evidence for accretion and reprocessing during quiescence

We conducted the first long-term (60 days), multiwavelength (optical, ultraviolet, and X-ray) simultaneous monitoring of Cen X-4 with daily Swift observations from June to August 2012, with the goal of understanding variability in the low mass X-ray binary Cen X-4 during quiescence. We found Cen X-4 to be highly variable in all energy bands on timescales from days to months, with the strongest quiescent variability a factor of 22 drop in the X-ray count rate in only 4 days. The X-ray, UV and optical (V band) emission are correlated on timescales down to less than 110 s. The shape of the correlation is a power law with index gamma about 0.2-0.6. The X-ray spectrum is well fitted by a hydrogen NS atmosphere (kT=59-80 eV) and a power law (with spectral index Gamma=1.4-2.0), with the spectral shape remaining constant as the flux varies. Both components vary in tandem, with each responsible for about 50% of the total X-ray flux, implying that they are physically linked. We conclude that the X-rays are likely generated by matter accreting down to the NS surface. Moreover, based on the short timescale of the correlation, we also unambiguously demonstrate that the UV emission can not be due to either thermal emission from the stream impact point, or a standard optically thick, geometrically thin disc. The spectral energy distribution shows a small UV emitting region, too hot to arise from the accretion disk, that we identified as a hot spot on the companion star. Therefore, the UV emission is most likely produced by reprocessing from the companion star, indeed the vertical size of the disc is small and can only reprocess a marginal fraction of the X-ray emission. We also found the accretion disc in quiescence to likely be UV faint, with a minimal contribution to the whole UV flux.

astro-ph.HE↗
arXiv2013

On the nature of the hard X-ray sources SWIFTJ1907.3-2050, IGRJ12123-5802 and IGRJ19552+0044

The INTEGRAL and Swift hard X-ray surveys have identified a large number of new sources, among which many are proposed as Cataclysmic Variables (CVs). Here we present the first detailed study of three X-ray selected CVs, Swift J1907.3-2050, IGRJ12123-5802, and IGRJ19552+0044 based on XMM-Newton, Suzaku, Swift observations and ground based optical and archival nIR/IR data. Swift J1907.3-2050 is highly variable from hours to months-years at all wavelengths. No coherent X-ray pulses are detected but rather transient features. The X-ray spectrum reveals a multi-temperature optically thin plasma absorbed by complex neutral material and a soft black body component arising from a small area. These characteristics are remarkably similar to those observed in magnetic CVs. A supra-solar abundance of nitrogen could arise from nuclear processed material from the donor star. Swift J1907.3-2050 could be a peculiar magnetic CV with the second longest (20.82 h) binary period. IGRJ12123-5802 is variable in the X-rays on a timescale of ~7.6 h. No coherent pulsations are detected, but its spectral characteristics suggest that it could be a magnetic CV of the Intermediate Polar (IP) type. IGRJ19552+0044 shows two X-ray periods, 1.38 h and 1.69 h and a X-ray spectrum characterized by a multi-temperature plasma with little absorption.We derive a low accretion rate, consistent with a CV below the orbital period gap. Its peculiar nIR/IR spectrum suggests a contribution from cyclotron emission. It could either be a pre-polar or an IP with the lowest degree of asynchronism.

astro-ph.HE↗
arXiv2013

Role of Dirac cones in magnetotransport properties of REFeAsO (RE=rare earth) oxypnictides

In this work we study the effect of the rare earth element in iron oxypnictides of composition REFeAsO (RE=rare earth). On one hand we carry out Density Functional Theory calculations of the band structure, which evidence the multiband character of these compounds and the presence of Dirac cones along the Y-Γ and Z-R directions of the reciprocal space. On the other hand, we explore transport behavior by means of resistivity, Hall resistance and magnetoresistance measurements, which confirm the dominant role of Dirac cones. By combining our theoretical and experimental approaches, we extract information on effective masses, scattering rates and Fermi velocities for different rare earth elements.

cond-mat.supr-con↗
arXiv2013

Playing quantum hide-and-seek with the muon: localizing muon stopping sites

One of the most fundamental limitations of a muon-spin relaxation experiment can be the lack of knowledge of the implantation site of the muon and the uncertainty about the muon's perturbation of its host. Here we review some of the work done on the `muon site problem' in the solid state and highlight some recent applications of electronic structure calculations that have successfully characterized the quantum states of muons in a number of insulating compounds containing fluorine, in a number of pnictide superconductors, and in ZnO.

cond-mat.str-el↗
arXiv2013

Magnetic glassy phase in FeSeTe single crystals

The evolution of the magnetic order in FeSeTe crystals as a function of Se content was investigated by means of ac/dc magnetometry and muon-spin spectroscopy. Experimental results and self-consistent DFT calculations both indicate that muons are implanted in vacant iron-excess sites, where they probe a local field mainly of dipolar origin, resulting from an antiferromagnetic (AFM) bicollinear arrangement of iron spins. This long-range AFM phase disorders progressively with increasing Se content. At the same time all the tested samples manifest a marked glassy character that vanishes for high Se contents. The presence of local electronic/compositional inhomogeneities most likely favours the growth of clusters whose magnetic moment "freezes" at low temperature. This glassy magnetic phase justifies both the coherent muon precession seen at short times in the asymmetry data, as well as the glassy behaviour evidenced by both dc and ac magnetometry.

cond-mat.supr-con↗
arXiv2013

Ab initio strategy for muon site assignment in wide band gap fluorides

We report on an ab initio strategy based on Density Functional Theory to identify the muon sites. Two issues must be carefully addressed, muon delocalization about candidate interstitial sites and local structural relaxation of the atomic positions due to $μ^+$-sample interaction. Here, we report on the validation of our strategy on two wide band gap materials, LiF and YF3, where localization issues are important because of the interplay between muon localization and lattice relaxation

cond-mat.mtrl-sci↗
arXiv2013

Theoretical investigation of FeTe magnetic ordering under hydrostatic pressure

We investigate the pressure phase diagram of FeTe, predicting structural and magnetic properties in the normal state at zero temperature within density functional theory (DFT). We carefully examined several possible different crystal structures over a pressure range up to $\approx 30 $ GPa: simple tetragonal (PbO type), simple monoclinic, orthorhombic (MnP type), hexagonal (NiAs and wurzite type) and cubic (CsCl and NaCl type). We predict pressure to drive the system through different magnetic ordering (notably also some ferromagnetic phases) eventually suppressing magnetism at around 17GPa. We speculate the ferromagnetic order to be the reason for the absence of a superconducting phase in FeTe at variance with the case of FeSe.

cond-mat.supr-con↗
arXiv2013

Effects of isoelectronic Ru substitution at the Fe site on the energy gaps of optimally F-doped SmFeAsO

We studied the effects of isoelectronic Ru substitution at the Fe site on the energy gaps of optimally F-doped SmFeAsO by means of point-contact Andreev reflection spectroscopy. The results show that the SmFe_{1-x}Ru_{x}AsO_{0.85}F_{0.15} system keeps a multigap character at least up to x=0.50, and that the gap amplitudes Delta_1 and Delta_2 scale almost linearly with the local critical temperature TcA. The gap ratios 2Delta_i/(kB Tc) remain approximately constant only as long as Tc > 30 K, but increase dramatically when Tc decreases further. This trend seems to be common to many Fe-based superconductors, irrespective of their family. Based on first-principle calculations of the bandstructure and of the density of states projected on the different bands, we show that this trend, as well as the Tc dependence of the gaps and the reduction of Tc upon Ru doping, can be explained within an effective three-band Eliashberg model as being due to a suppression of the superfluid density at finite temperature that, in turns, modifies the temperature dependence of the characteristic spin-fluctuation energy.

cond-mat.supr-con↗